Experimental Method

TiN, TiAlN, TiN-MoS?, and CrTiAlN composite coatings were deposited on YT14 cemented carbide cutters using a closed-field unbalanced magnetron sputtering ion plating equipment from Teer Company. The nano hardness and elastic modulus of the coatings were measured using a Nano Test 600 nano hardness tester. To reduce experimental errors, the hardness and elastic modulus values were the average of five measurements. The hardness of the coatings was also verified using a Vickers microhardness tester, and the morphology and phase structure of the cutter coatings were observed and analyzed using a Quanta 200 scanning electron microscope (SEM) and an Advance 8 X-ray diffractometer (XRD). The cutting tests of the coated cutters were conducted on a CNC machining center, with PCNiMoVA steel as the cutting material. The flank wear was observed and measured using a 30x tool microscope. The cutting time when the wear strip width on the flank face exceeded 0.6mm was used as the basis for evaluating the tool’s life, and the cutting life of the tools was compared.

Experimental Results and Analysis

Coating Hardness and Elastic Modulus Testing

Figure 1 shows the loading and unloading curves obtained during the nano hardness measurement process of the CrTiAlN composite coating. The loading and unloading curves not only provide the hardness of the CrTiAlN film but also its elasticity. Define R = (ha – h) / h as the elastic recovery coefficient, where ha is the indentation depth at maximum load, and h is the residual depth of the indentation after unloading. According to the definition of R, the greater the value of R, the greater the elasticity of the film. Therefore, based on the nano indentation curve in Figure 1, the hardness of the CrTiAlN film is 33 GPa, and the elastic modulus is 675 GPa. Figure 2 is a comparative analysis chart of the nano hardness of TiN, TiAlN, TiN-MoS?, and CrTiAlN coatings. It can be seen from the figure that the nano hardness measurement values of the four coatings are 18 GPa, 30 GPa, 15 GPa, and 33 GPa, respectively. The nano hardness ranking is as follows: CrTiAlN > TiAlN > TiN > TiN-MoS?.

Figure 3 shows the measured elastic modulus values of each coating. It can be seen from the figure that the elastic modulus of the four coatings are 214 GPa, 346 GPa, 164 GPa, and 675 GPa, respectively. The ranking of the elastic modulus is as follows: CrTiAlN > TiAlN > TiN > TiN-MoS?. This indicates that the elastic modulus of the coatings is directly proportional to their hardness. However, the CrTiAlN coating shows the greatest relative increase in elastic modulus, with a value significantly higher than other coatings, reaching 675 GPa, which suggests that the deposited CrTiAlN coating has both high hardness and high elasticity.

Meanwhile, the Vickers microhardness tester was used to perform microhardness verification tests on each cutter coating, with a indentation load of 15g and a duration of 10 seconds. The measurement results are shown in Figure 4. By comparing the coating nano hardness in Figure 2 with the coating microhardness values in Figure 4, it can be found that the microhardness variation trend of each coating is the same as the nano hardness variation trend, with the CrTiAlN coating having the relatively highest Vickers microhardness at HV1560.

Analysis of Coating Surface Morphology

The surface morphologies of the TiN, TiAlN, TiN-MoS?, and CrTiAlN coated cutters are shown in Figure 5. It can be observed from the figure that there is a significant difference in the surface morphology of the four coatings, indicating that the addition of composite elements has caused a great change in the crystalline state of the TiN compounds. Among them, the TiN coating surface phase tissue is uniform, with relatively fine particles, while the TiAlN coating surface morphology is relatively rough, with coarser particle tissue. The TiN-MoS? coating surface is distributed with a large amount of flaky mixed structure, mainly due to the uniform distribution of MoS? phase in the TiN coating, tending towards a composite structure in the mixed state, which serves a self-lubricating function. The CrTiAlN coating surface grains are relatively fine, the coating is dense and uniform, and the surface is distributed with a large number of hard points.

Drilling Test

The four types of coated cemented carbide cutters were used to machine PCNiMoVA steel, and the wear condition of the cutters was inspected to compare the durability of different coated cutters.

The cutting test conditions for the coated cutters were as follows: the cutting method was external cylindrical cutting, the cutting speed was 160 m/min, the feed rate was 0.15 mm/r, the cutting depth was 0.5 mm, and dry cutting was performed. The cutting time when the wear strip width on the flank face exceeded 0.6 mm was used as the basis for evaluating the tool’s life, and the cutting life of the tools was compared.

A comparison of the cutting life of the coated cutters is shown in Figure 6.

From the figure, it can be seen that under dry cutting conditions, the cutting life of the uncoated cutters was the shortest, and the service life of the coated cutters was significantly better than that of the uncoated cutters. Among them, the CrTiAlN coated cutter had the longest cutting life. The ranking of the cutting life of the four coated cutters was as follows: CrTiAlN > TiN-MoS? > TiAlN > TiN. This indicates that Cr and Al elements form hard phases in the TiN coating, and the addition of Al elements is beneficial for the formation of Al oxides, which avoids further oxidation during the cutting process, improves the oxidation resistance of the cutters, and is conducive to increasing the cutting life of the tools. Meanwhile, the MoS? lubricating phase helps to reduce the friction coefficient of the cutters, improve the anti-wear capability of the tools, and also extends the service life of the cutters.

In summary, due to the comprehensive utilization of the advantages of various coating components in the multi-component composite coatings, they achieve better comprehensive performance, ensuring excellent wear resistance and toughness, reducing the formation of built-up edge, and possessing mechanical shock and thermal shock resistance, which can greatly improve the tool life.

XRD Test Analysis

XRD analysis method was used to characterize the phase structure of the CrTiAlN tool coating with the best cutting performance, and the results are shown in Figure 7. The XRD spectrum analysis indicates that the crystal phases of the coating are mainly composed of Cr, CN, CEN, and TiN at room temperature, and the amorphous phase in the coating was not detected. At the same time, high magnification scanning analysis of the tool coating revealed a large number of hard phase particles distributed on the coating surface. Combined with X-ray diffraction analysis, it is known that these hard phases are mainly CN, CrN, TiN, and AlN phases. These hard phases are beneficial for improving the cutting life of the coated cutters.

Conclusion

The author has prepared TiN, TiAlN, TiN-MoS?, and CrTiAlN composite coatings using a closed-field unbalanced magnetron sputter ion plating PVD coating process. Comparative tests on the mechanical properties and cutting performance of the coatings show that:

The nanoindentation analysis obtained the nano hardness ranking of the four types of cutter coatings as follows: CrTiAlN > TiAlN > TiN > TiN-MoS?. The elastic modulus of the coatings is directly proportional to their hardness.

Under dry cutting conditions, when drilling PCNiMoVA steel, the cutting life of the coated cutters is ranked as: CrTiAlN > TiN-MoS? > TiAlN > TiN. This indicates that the cutting performance of the multi-component composite coatings is significantly better than that of the simple TiN coating, suggesting a direction for the future development of coated cutters.

This paper aims to perform high-speed machining of TC4 titanium alloy using a vibration-damping end mill (end mill with unequal tooth pitch angles). The MATLAB software is used to perform a Fast Fourier Transform (FFT) on the milling force, with a focus on analyzing the impact of chatter on the machining of titanium alloys. The objective is to optimize the cutting speed while ensuring the quality of the machined surface and low cutting force, thereby improving cutting efficiency.

Dynamic Milling Force Model

To study the cutting stability, a reasonable dynamic model needs to be established. Compared to the workpiece with higher stiffness, the end mill can be considered as an elastic body. Since the end mill has a very high stiffness in the axial (z-direction) direction, the milling machining system can be simplified into a “spring-damping” system with two mutually perpendicular degrees of freedom in the x-direction (feed) and y-direction, as shown in Figure 1. Φj(t) represents the rotation angle of the end mill; hi(t) represents the dynamic chip thickness; fz is the feed per tooth.

Establishing the equations of motion for 2 degrees of freedom?movement:

In the formula: m, s, c, k represent the mass, displacement, damping coefficient, and elastic coefficient of the cutting system, respectively; Fx(t) and Fy(t) are the dynamic milling forces in the x and y directions, respectively. The milling forces studied in this paper are all Fy, and the motion equation is simplified to a single degree of freedom motion equation:

The milling force Fy(t) is related to the dynamic chip thickness hi(t) during the milling process and is a function of the end mill’s rotation angle φj(t), making it a periodic function. The variation frequency of the cutting force is the tool tooth passing frequency (TPF). Due to manufacturing and clamping errors causing the tool system to be asymmetric, its variation frequency is the spindle rotation frequency (SF).

The spindle rotation frequency (SF) is defined as:

In the formula: ω is the angular velocity of the spindle rotation, in radians per minute; v is the linear velocity of the spindle, in meters per minute; D is the diameter of the end mill, in millimeters; the unit of spindle rotation frequency is Hz.

The tool tooth passing frequency (TPF) is defined as:

In the formula: N represents the number of teeth on the end mill.

Milling Experiment

Experimental Setup, Workpiece Material, and Cutting Tool

The milling experiment setup is shown in Figure 2. The machine used is a DAEWOO ACE-V500 machining center with a rated power of 15 kW and a maximum torque of 286.2 N?m. The milling force is measured by a Kistler 9257B three-coordinate force measuring instrument with a sampling frequency of 7,000 Hz. The cutting tool used is a 4-fluted variable pitch solid carbide end mill, and the distribution of the tooth pitch angles at the bottom is shown in Figure 3. The diameter of the end mill is 20 mm, and the overhang length is 74.3 mm. The workpiece material is TC4 titanium alloy.

Section 2.2: Experimental Design

The experiment was conducted using dry cutting and conventional milling. The cutting speed v, spindle rotation frequency (SF), and tool tooth passing frequency (TPF) are listed in Table 1. The feed per tooth fz, axial cutting depth ap, and radial cutting depth ae were kept constant at 0.08 mm/tooth, 20 mm, and 0.5 mm, respectively. The range of cutting speed varied from 80 to 360 m/min. Figure 4 shows the milling sequence, where the sequence numbers correspond to those in Table 1.

Experimental Results

Time-Domain Analysis of Cutting Forces

Figure 5 shows the relationship between the maximum amplitude of Fy and the cutting speed (v). From the figure, it can be observed that when the cutting speed is in the range of 80~160 m/min, the maximum milling force remains essentially unchanged. When the cutting speed reaches 200 m/min, the milling force suddenly increases, and when the cutting speed reaches 240 m/min, the milling force reaches its first peak. Subsequently, as the cutting speed increases,The cutting force significantly decreases until the cutting speed increases to 320 m/min, at which point the milling force reaches its minimum. When the cutting speed is 360 m/min, the milling force reaches its maximum value.

Analysis of Milling Force Spectrum

The frequency domain provides more information about the cutting process than the time domain. As the direct responder of the tool and workpiece, the milling force can describe the vibration condition of the cutting system. From the collected y-direction milling force Fy signal, a 4-second segment of data in the middle was selected and processed using MATLAB software for Fast Fourier Transform (FFT) to analyze the milling force spectrum and determine the machining condition.

Since the variation frequency of the milling force is TPF, the peaks of the milling force generally appear at TPF and its integer multiples (n·TPF) in the frequency domain. However, due to tool manufacturing and mounting errors, the variation frequency of the milling force is often SF, meaning the spectrum peaks of the milling force appear at SF and its integer multiples (n·SF).

Figures 6(a) to (h) show the milling force spectrum analysis at different cutting speeds. From the figures, it can be seen that when the cutting speed is 80 and 120 m/min, the peaks of the milling force spectrum appear at the spindle rotation frequency (SF) and the tool tooth passing frequency (TPF). When the cutting speed is 160, 280, and 320 m/min, the peaks of the milling force appear not only at SF and TPF but also at high frequencies. The peak at the spindle rotation frequency (SF) is larger at higher cutting speeds than at lower cutting speeds, which is due to the mass imbalance caused by tool eccentricity. As the cutting speed increases, the centrifugal force increases, leading to an increase in the y-direction milling force peak. When the cutting speed is 240 and 360 m/min, the larger peaks at high frequencies do not appear at SF and its integer multiple frequencies, indicating that chatter occurs in the cutting system at these times, with chatter frequencies of 734 Hz (v=240 m/min), 730 Hz, 1,111 Hz, 1,268 Hz, 1,363 Hz, and 1,459 Hz (v=360 m/min). Combined with Figure 5, it can be determined that when chatter occurs, the radial milling force increases significantly.

Machined Surface Quality

The Wyko NT9300 white light interferometer was used to measure the machined surface topography. This instrument uses the principle of optical interference and can achieve nanometer precision in topography measurement.

Figure 7 shows the machined surface topography measured by the white light interferometer. From the figure, it can be seen that when v=160 m/min, the surface topography is the best. When v=240 and 360 m/min, there is a large area of material removal on the machined surface. This is partly due to chatter, which increases friction, extrusion, and tearing between the tool and the workpiece, causing the tool to produce unstable vibrations on the workpiece surface, leading to overcutting in some areas. On the other hand, these violent movements cause the cutting temperature to rise, resulting in adhesion between the flank of the tool and the workpiece. As the tool moves, most of the adhered workpiece material is carried away by the tool.

By comparing Figure 5 with Figure 8, it is found that the roughness curve follows a similar trend to the cutting force curve. When chatter occurs (at cutting speeds of 240 and 360 m/min), both the cutting force and roughness reach their maximum values, proving that chatter not only increases the cutting force but also increases the surface roughness.

From Figure 8, it can also be seen that for stable cutting (cutting speeds of 80 to 160 m/min), the higher the cutting speed, the lower the surface roughness. Although stable cutting occurs at cutting speeds of 200 and 280 m/min, Figures 6(d) and (f) show that the high-frequency peaks of the milling force are significant, which affects the surface quality. When the cutting speed is 320 m/min, the cutting force is small, and the spectrum peaks are also small, but the surface roughness is poor. This is due to the tool eccentricity (larger peak at SF) having a significant impact on surface roughness at high cutting speeds.

Conclusion

Under dry cutting conditions, when high-speed milling TC4 titanium alloy, chatter occurs at cutting speeds of 240 m/min and 360 m/min, with chatter frequencies around 730, 1,111, 1,363, and 1,459 Hz.

For stable cutting, the cutting speed has little effect on the magnitude of the milling force. However, when chatter occurs, the cutting force significantly increases, and the surface quality of the machining deteriorates.

At a cutting speed of 160 m/min, the surface roughness is low, and the cutting force is small, so 160 m/min can be recommended as the optimal cutting speed.

How to Solve Burr Issues

Chemical Deburring

Chemical deburring utilizes chemical energy for processing. Chemical ions adhere to the surface of the parts, forming a film with high electrical resistance and low conductivity, which protects the workpiece from corrosion. Since metalworking burrs are higher than the surface, they can be removed through chemical action. This method of deburring is widely used in fields such as pneumatic, hydraulic, and construction machinery.

High-Temperature Deburring

First, place the parts that need deburring into a tightly sealed chamber, then introduce the chamber into a hydrogen-oxygen mixed gas environment with a certain pressure. Ignite the mixed gas to cause an explosion, releasing heat to burn off the burrs without damaging the parts.

Roll Grinding Deburring

Place the parts and abrasive materials into a closed drum. As the drum rotates, a dynamic torque sensor, along with the parts and abrasive, generates a grinding action to remove metalworking burrs. Abrasives can be made of quartz sand, wood chips, alumina, ceramics, and metal rings, among others.

Manual Deburring

This method is more traditional and also the most time-consuming and labor-intensive. It mainly involves manually grinding with tools such as steel files, sandpaper, and grinding heads. The most commonly used tool in production now is the trimming knife.

Process Deburring – Edge Rounding

Edge rounding can refer to any action that removes the sharpness from the edges of metal components. However, it is typically associated with creating a radius on the edges of parts.

Edge rounding is not simply about removing sharpness or deburring, but about breaking the edges of metal components to improve their surface coating coverage and protect them from corrosion.

Removing metalworking burrs in Milling Parts

In milling parts, deburring is more complex and costly, as multiple metalworking burrs are formed at different positions with varying sizes during milling. It is particularly important to select the correct process parameters to minimize metalworking burr size.

Main Factors Affecting Burr Formation in End Milling

① Milling parameters, milling temperature, and cutting environment, among others, have a certain impact on burr formation. Some key factors, such as feed rate and milling depth, are reflected through the plane cutting angle theory and the Edge Engagement Sequence (EOS) theory.

② The better the plasticity of the workpiece material, the more likely it is to form Type I burrs. In the processing of brittle materials with end milling, if the feed rate or the plane cutting angle is large, it is conducive to the formation of Type III burrs (deficits).

③ When the angle between the end face of the workpiece and the machined plane is greater than a right angle, the increased support stiffness of the end face can suppress machining burr formation.

④ The use of cutting fluid is beneficial for extending tool life, reducing tool wear, lubricating the milling process, and thereby reducing the size of burrs.

⑤ Tool wear has a significant impact on machining burr formation. When the tool wears to a certain extent and the tip radius increases, not only does the burr size in the tool retracting direction increase, but type burrs may also form in the tool cutting-in direction.

⑥ Other factors, such as tool material, also have a certain impact on burr formation. Under the same cutting conditions, diamond tools are more effective in suppressing metalworking burr formation than other types of tools.

How to Effectively Handle Burrs Generated During Tool Retraction

To suppress burrs generated during tool retraction, eliminating the space where burrs are produced is an effective method. For example, measures such as chamfering can be taken to reduce the space before retracting the tool.

Using appropriate cutting conditions to suppress machining burrs should aim to minimize the amount of cutting residue, and it is necessary to select the most suitable tool and cutting conditions. Use tools with a large rake angle and sharp cutting edges. Increase cutting speed to improve cutting characteristics. Especially during finish cutting, it is essential to use the minimum cutting depth and feed rate.

The size of the space between the tool and the workpiece determines the size of the burrs. Let’s take a look at the relationship diagram below.

Summary

In fact, during the machining process, burrs are inevitable, so it is best to address the metalworking burr issue through process improvements, avoiding excessive manual intervention. Using chamfering end mills can reduce the space where burrs are produced, effectively remove burrs, and is also a very suitable method for clearing burrs.

Mechanism of Tool Breakage

Abrasive Wear by HardParticles

This is mainly caused by mechanical wear from impurities in the workpiece material, hard particles such as carbides, nitrides, and oxides contained in the material matrix, and fragments of built-up edge. These hard particles scratch grooves on the tool surface. Abrasive wear by hard particles occurs in tools at all cutting speeds, but it is the main cause of wear in low-speed steel tools. At low cutting temperatures, other forms of wear are not significant. It is generally believed that the amount of wear caused by abrasive wear by hard particles is proportional to the relative sliding distance or cutting distance between the tool and the workpiece.

Adhesive Wear

Adhesion refers to the bonding phenomenon that occurs when the tool and workpiece material come into contact at atomic distances. It is a so-called cold welding phenomenon that occurs due to plastic deformation on the actual contact surface of the friction surfaces under sufficient pressure and temperature. It is the result of the adhesive force between the fresh surface atoms formed by the plastic deformation of the two friction surfaces. The adhesive points on the two friction surfaces are sheared or stretched and carried away by the opposite surface due to relative motion, which causes adhesive wear.

Adhesive wear can occur on the contact surfaces between two materials, whether on the soft material side or the hard material side. Generally, the breakdown of adhesive points occurs more frequently on the side with lower hardness, i.e., the workpiece material. However, the tool material often has defects such as uneven structure, internal stresses, microcracks, pores, and local soft spots, so the carbide tool surface also frequently breaks and is carried away by the workpiece material, forming adhesive wear. High-speed steel, ceramic, cubic boron nitride, and diamond tools can all experience wear due to adhesion. The size of the carbide grains in cemented carbide has a significant impact on the speed of adhesive wear. The temperature at which the tool material and workpiece material adhere to each other greatly affects the severity of adhesive wear. Other factors such as the hardness ratio of the carbide tool to the workpiece material, the shape and structure of the tool surface, as well as cutting conditions and the stiffness of the process system, all affect adhesive wear.

Diffusion Wear

Due to the high temperatures during cutting, and the fact that the carbide tool surface is constantly in contact with the freshly cut surface, which has high chemical reactivity, the chemical elements of the two friction surfaces may diffuse into each other. This results in a change in the chemical composition of both, weakening the properties of the tool material and exacerbating the wear process. Diffusion wear increases with the rise in cutting temperature. For a given tool material, as the temperature increases, the rate of diffusion initially increases slowly and then accelerates. Different elements have different diffusion rates, so the severity of diffusion wear is greatly related to the chemical composition of the carbide tool material. Additionally, the rate of diffusion is also related to the flow velocity of the chip layer on the tool surface, which corresponds to the velocity of the chip flowing over the rake face. Slower flow velocity results in slower diffusion.

Figure 1 shows the diffusion wear of WC-Co cemented carbide, indicating that tungsten carbide (WC) and cobalt (Co) have dissolved into the surface layer of the steel, and this surface layer has also melted at the interface. In the figure, the upper layer is steel, the lower layer is cemented carbide, and the middle white layer is the melted layer, which is in a locally melted area, with WC grains surrounded within it. Because the temperature is higher at the crescent-shaped depression on the rake face, the diffusion rate is high and wear occurs quickly. At the same time, since adhesion occurs when the temperature rises to a certain degree, diffusion wear and adhesive wear often occur simultaneously, easily forming a crescent-shaped depression.

Chemical Wear

Chemical wear occurs at certain temperatures where the tool material reacts chemically with surrounding media (such as oxygen in the air, extreme pressure additives like sulfur and chlorine in cutting fluids, etc.), forming a layer of compounds with lower hardness on the tool surface, which is then carried away by the chips, accelerating tool wear; or because the tool material is corroded by a certain medium, causing tool wear.

The main types of normal tool wear include hard particle wear, adhesive wear, diffusion wear, and chemical wear, and there are interactions among them. For different tool materials, under different cutting conditions, and when machining different workpiece materials, the primary cause of wear may be one or two of these types.

Methods to Improve Tool Performance

Because it is difficult to balance the wear resistance and toughness of cemented carbide tool materials, users can only select suitable tool materials from various grades of cemented carbide based on specific machining objects and conditions, which brings inconvenience to the selection and management of cemented carbide tools. To further improve the comprehensive cutting performance of cemented carbide tool materials, current research focuses include the following aspects:

Grain Refinement

By refining the grain size of the hard phase, increasing the intergranular surface area of the hard phase, and enhancing the intergranular bonding strength, the strength and wear resistance of cemented carbide tool materials can be improved. When the WC grain size is reduced to sub-micron levels, the hardness, toughness, strength, and wear resistance of the material can all be enhanced, and the temperature required for full densification can also be reduced. The grain size of conventional cemented carbide is 3~5μm, fine-grained cemented carbide has a grain size of 1~1.5μm (micron level), and ultra-fine-grained cemented carbide can have a grain size below 0.5μm (sub-micron, nano-level). Common grain refinement processes include physical vapor deposition, chemical vapor deposition, plasma deposition, and mechanical alloying. Since these grain refinement processes are not yet mature, nano-grains easily grow into coarse grains during the sintering of cemented carbide, and the general growth of grains will lead to a decrease in material strength. Individual coarse WC grains are often a significant factor causing material fracture. On the other hand, fine-grained cemented carbide is relatively expensive, which also restricts its promotion and application.

Surface, Overall Heat Treatment, and Cycle Heat Treatment for Carbide tools

Surface treatments such as nitriding and boriding on the surfaces of cemented carbide with good toughness can effectively improve their surface wear resistance. For cemented carbide with good wear resistance but poor toughness, overall heat treatment can change the bonding composition and structure in the material, reduce the adjacency of the WC hard phase, thereby improving the strength and toughness of the cemented carbide. Using cycle heat treatment processes to alleviate or eliminate stress at the grain boundaries can comprehensively improve the overall performance of cemented carbide materials.

Addition of Rare Metals

Adding rare metal carbides such as TaC and NbC to cemented carbide materials can form complex solid solution structures with the original hard phases WC and TiC, further strengthening the hard phase structure. At the same time, it can inhibit the growth of hard phase grains and enhance the uniformity of the structure, which is greatly beneficial to improving the comprehensive performance of cemented carbide. In the standard P, K, M grades of cemented carbide, there are grades that have added Ta(Nb)C (especially more in the M grade).

2 The forming process and main characteristics of boundary notch in cemented carbide cutting tools

The boundary notch of cemented carbide cutting tools is a wear area,which is?relatively large,resulting from friction between main cutting edge and the surface of?the workpiece as the following Fig.1.Fig.1(a)shows a traditional wearing type of the?flank.The rake face A,and flank face Aa?are also shown.Fig.1 (b)shows the main?dimension of boundary notch of the lathe tool,in which VN represented the height of?boundary notch and C refers to the width.It is apparent that the greater the?dimensions of VN and C are,the greater it destroys the performance of tools and influences the machining quality.

By experiment,the forming process of the boundary?notch can be divided into the?following three steps:firstly,several micro cracks are produced at main cutting edge.Secondly,the mesh fractures are found in the boundary areas and they will spread.

Finally,the piece material will be denuded and the boundary notch is formed.In the?subsequent cutting process,the dimension of the boundary becomes bigger and?bigger.

Fig.2 shows the forming process of boundary notch of the cemented carbide?cutting?tools.

Main factors to influence boundary notch are mechanical performance of the?piece material,the cutter material,and geometry parameter of the cutter.The?following experiments were carried out in order to expound the forming mechanism?and evolution rules of the boundary notch.

3 Experiment conditions and testing measures for assessing boundary notch in cemented carbide cutting tools

The lathe C6130 and reversible cutting tool are used in the experiment.Five?cutter materials are employed.Main mechanical parameters of cutter material are?shown in Table 1.

The machining piece is the friction-welded line of the single hydraulic pillar.The?width of the welded line is 15mm and the machining allowance is 5.5mm.Besides,the above pillar is welded with 270SiMn and 45#steel.The relatively mechanical?performances of the welded line are shown in Table 2.

Based on manufacturing experience and relative information in China and other?countries about similar machining process,the chosen machining and tool geometry?parameters are shown in Table 3.

The boundary notch dimensions of the cemented carbide cutting tools (boundary?notch height VN and width C are directly attained by tool microscope.In order to?ensure reliability of the results,repeated experiments are carried out.The recurrent?performance is good.

Influences of Cutting angles

Influences of Cutting Edge Angle

The results of the variety boundary notch are shown as in Fig.4 when the cutting?edge angle is changed.From Fig.4 we can find that,with the lessening of the cutting?edge angle Kr,the dimensions of the boundary notch decrease.The reason is that?with the lessening of the cutting edge angle Kr,the length of the cutting edge that?acts on cutting becomes larger and the average loads on the cutting edge be?come?lighter.

Influences of Cutter Corner Radius r

The results of the variety boundary notch with the cutter corner changing are?shown as Fig.5.The boundary notch dimension decreases with the cutter corner radius?are becoming lesser.The reason is that with the increasing of the cutter corner radius,the impact-resistance performance.

Therefore,under the same cutting conditions,boundary notch dimensions (VN,C)decrease?when the cutter corner radius becomes lesser.

Influences of Negative Chamfer bal in cutting tool

The experiment results of the variety boundary notch are shown as in Fig.6 when?the width of the negative chamfer is changed.The dimension of the boundary notch?will decrease when the width of the negative chamfer ba decreases.Therefore,in?order to resist or decrease the cutter boundary notch,the lesser negative chamfer bal should be chosen.

Deburring Machining Process

The burrs have some influences on cutter boundary notch in metal machining?process.A deburring cutter is chosen to decrease the adverse influence on cutter.A?different result between deburring machining process and common machining process?is shown as in Fig.7.It can be seen that about 75%of the boundary notch is?decreased.So,burr is a main factor to produce and increase the boundary notch of the?cutter.

Conclusion

From above experimental research and theoretical analysis,the following?conclusions are attained:

1)Boundary notch of the cutting tool can be expressed by boundary notch height?VN and boundary notch width C.The forming processes of boundary notch can be?divided into three steps:micro-tipping appears firstly;Then,mesh fractures expand;Finally,boundary notch results.

(2)Main factors that influence boundary notch of cemented carbide cutter are?piece material,cutter material and cutter geometry parameters.

(3)Deburring machining process and adjusting cutting tool geometry parameters(to reduce edge angle K,and width of negative chamfer ba,to increase cutter corner?radius re)can be chosen to decrease effectively boundary notch,which ensures the?quality of workpiece and cutting?performances of cutting tool.

Three-Axis Machining Center

The three-axis machining center is the most widely used, including the X, Y, and Z axes, also known as three-axis simultaneous machining centers. The three-axis machining center can perform simple plane machining, but it can only machine one side at a time. It can effectively machine materials such as metal, aluminum, wood, etc.

Functions and Advantages of Three-Axis Machining Centers

The most effective machining surface of a vertical machining center (three-axis) is the top surface of the workpiece. A horizontal machining center, with the aid of a rotating table, can only complete the machining of the four sides of the workpiece. Currently, high-end machining centers are developing towards five-axis control, allowing the completion of five-sided machining in one setup. With the configuration of a high-end five-axis simultaneous CNC system, high-precision machining of complex spatial surfaces can also be achieved.

Four-Axis Machining Center

The four-axis machining center adds an additional rotation axis to the three-axis, usually the A-axis. The rotation of the A-axis allows the workpiece to rotate around the vertical axis on the horizontal plane, enabling multi-face machining. The four-axis machining center is suitable for situations where machining is required on different faces of the workpiece, such as inclined surfaces, oblique holes, etc.

Features of Four-Axis Simultaneous Machining

(1) Machining that cannot be achieved by three-axis simultaneous machining machines or requires overly complex fixturing.

(2) Improvement of the precision, quality, and efficiency of free-form surfaces.

(3) The difference between four-axis and three-axis is the addition of one rotation axis. The establishment of the four-axis coordinates and their code representation:

Determination of the Z-axis: The direction of the machine spindle axis or the vertical direction of the workpiece fixture is the Z-axis.

Determination of the X-axis: The horizontal plane parallel to the workpiece mounting surface or the direction perpendicular to the workpiece’s rotation axis within the horizontal plane is the X-axis, with the direction away from the spindle axis being the positive direction.

Five-Axis Machining Center

The five-axis machining center adds another rotation axis to the four-axis, usually the C-axis. The rotation of the C-axis allows the workpiece to rotate around an axis perpendicular to the table, enabling more complex multi-angle and surface machining. The five-axis machining center is suitable for complex shapes, multi-angle machining, including spatial surface machining, special-shaped machining, hollowing-out machining, drilling, oblique holes, oblique cutting, etc. It is a means to solve the machining of impellers, blades, ship propellers, heavy-duty generator rotors, steam turbine rotors, large diesel engine crankshafts, etc.

What are the 5 axis? in a five-axis machining center?

X-axis: The X-axis is the horizontal axis of the machining center, controlling the movement of the tool in the horizontal direction. The movement of the X-axis affects the lateral position of the workpiece, determining its position and shape on the horizontal plane.

Y-axis: The Y-axis is the longitudinal axis of the machining center, controlling the movement of the tool in the longitudinal direction. The movement of the Y-axis affects the longitudinal position of the workpiece, determining its position and shape on the longitudinal plane.

Z-axis: The Z-axis is the vertical axis of the machining center, controlling the movement of the tool in the vertical direction. The movement of the Z-axis affects the height position of the workpiece, determining its position and shape on the vertical plane.

A-axis: The A-axis is the rotation axis of the four-axis and five-axis machining centers, controlling the rotation of the workpiece on the horizontal plane. The rotation of the A-axis allows the workpiece to rotate around the vertical axis on the horizontal plane, enabling multi-face machining.

C-axis: The C-axis is the rotation axis of the five-axis machining center, controlling the rotation of the workpiece around an axis perpendicular to the table. The rotation of the C-axis allows the workpiece to rotate around the rotation axis on the vertical plane, enabling more complex multi-angle and surface machining.

Types of Five-Axis Machining Centers

Vertical Five-Axis Machining Center

This type of machining center has two methods for the rotation axis: one is the worktable rotation axis.

The worktable, which is set on the bed, can rotate around the X-axis, defined as the A-axis. The general working range of the A-axis is +30 degrees to -120 degrees. There is also a rotating table in the middle of the worktable, which rotates around the Z-axis at the position shown in the illustration, defined as the C-axis. The C-axis can rotate 360 degrees. With the combination of the A-axis and C-axis, the workpiece fixed on the worktable can be machined on all five sides except the bottom surface by the vertical spindle. The minimum indexing value for the A-axis and C-axis is usually 0.001 degrees, which allows the workpiece to be subdivided into any angle for machining inclined surfaces and oblique holes.

If the A-axis and C-axis are linked with the XYZ three linear axes, complex spatial surfaces can be machined. Of course, this requires the support of high-end CNC systems, servo systems, and software. The advantage of this setup is that the structure of the spindle is relatively simple, the spindle rigidity is very good, and the manufacturing cost is relatively low.

Spindle Rotation Vertical Five-Axis Machining Center

The vertical machining center’s spindle has gravity acting downward, and the radial force on the bearings during high-speed idle operation is equal, resulting in good rotation characteristics. Therefore, the speed can be increased, with a general high speed reaching over 12,000 rpm, and the practical maximum speed has reached 40,000 rpm. The spindle system is equipped with a circulating cooling device, where the circulating cooling oil carries away the heat generated by high-speed rotation, is cooled to an appropriate temperature through a chiller, and then flows back into the spindle system.

The X, Y, Z three linear axes can also use linear encoders for feedback, with bidirectional positioning accuracy within the micron level. Since the rapid feed reaches 40-60 m/min or more, the ball screws for the X, Y, Z axes mostly adopt central cooling. Similar to the spindle system, the heat is carried away by the circulating oil that flows through the center of the ball screws after being chilled.

Summary

In the field of CNC machine tools, three-axis machining centers have relatively simple processing capabilities, four-axis machining centers can achieve multi-face machining, and five-axis machining centers have a higher level of multi-angle and surface machining capabilities. The main difference between three-axis, four-axis, and five-axis machining centers lies in the number of axes and processing capabilities. The choice of a suitable machining center should be determined based on specific processing requirements and complexity.

Original Quality Issues of the Cutting Tools

Due to the weak quality awareness of some cutting tool manufacturers, the tools produced have some quality issues, such as:

(1) The most obvious quality issue is the unexpected fracture of the tool body due to poor quality of the tool body material and non-standard heat treatment.

(2) Issues such as chip adhesion, thermal cracking of the cutting edge, and severe tool breakage due to improper selection of the insert grade. Each insert grade is suitable for specific machining materials and cutting conditions, and generally, they can be selected based on the performance and application of the carbide grade. However, in practice, the selection often does not follow the regulations, allowing for a great deal of flexibility.

(3) Structural problems. Improper cutting angles, poor chip evacuation, too many teeth, poor strength, and unbalanced cutting forces are all hidden dangers that can lead to abnormal damage of the cutting tools. These factors are closely related to the manufacturing experience and technical level of the tool manufacturers.

Issues Related to Tool Usage

When indexable carbide cutting tools are used for certain finishing operations (such as precision boring or precision milling), the experience and technical level of the user are not highly demanding. However, when used for rough machining, higher requirements are needed. Tool-using manufacturers may cause abnormal damage to the tools, increase tool costs, and affect machining efficiency due to a lack of corresponding experience and technology. For example:

Tool breakage due to machine stalling

If the tool diameter is too large, the number of teeth is too dense, or the feed rate is too high, it can cause the machine’s power to be insufficient, leading to machine stalling and tool breakage, which is one form of abnormal tool damage. Therefore, considering the machine’s power capacity is a primary condition for cutting operations.

Cutting Fluid Issues

Cutting fluid plays a role in cooling, lubricating, cleaning, chip evacuation, and improving the quality of the machined surface during the machining process. However, cutting operations have different requirements for the properties, flow rate, and pressure of the cutting fluid. Whether to use cutting fluid depends on the machining conditions; it is not correct to assume that cutting fluid always has a beneficial effect. For example, if cutting fluid is added in the middle of cutting with an indexable carbide end mill, it will certainly damage the insert.

Poor rigidity of the process system

Due to the vibration of the process system, the loosening between the connecting components can occur, and if not inspected and discovered in a timely manner, it can lead to tool breakage. If the user is unaware that the insert is damaged and continues to use it, it will inevitably result in damage to the tool body.

Using a milling cutter with unequal pitch is one method to reduce vibration and improve cutting stability.

Improper selection of cutting parameters

Indexable carbide cutting tools require different cutting speeds and feed rates under different cutting conditions. Generally, tool manufacturers will provide recommended values that have been proven reasonable and reliable through cutting experiments. Blind use will inevitably lead to abnormal damage to the tools. For example, when a milling cutter engages with the workpiece, the cutting parameters should be appropriately reduced to prevent sudden impacts that could cause the milling cutter’s edge to break.

Issues caused by fastening screws

Because screws are small and relatively inexpensive to replace, their importance in the machining process is often not given sufficient attention by users. Some users, when installing inserts, use sleeves or other means to increase the torque, resulting in excessive pre-stress on the screws, making it impossible to loosen and replace the insert again, leading to the scrapping of the tool body. Sometimes, users mix up screw models, resulting in inappropriate clamping force. Others fail to conduct periodic inspections or replace damaged screws in a timely manner, creating potential safety hazards.

Exceeding the Scope of Use

Different types of indexable carbide cutting tools have their respective scope of use. Users must follow the guidelines to avoid abnormal damage. For example, indexable carbide spot drills are generally suitable for machining holes with a depth less than three times the diameter. If the depth is too great, it can damage the tool due to poor chip evacuation. Moreover, the surface to be machined must be flat, without grooves or protrusions; otherwise, vibration will occur at the moment of drilling, leading to edge breakage or even damage to the tool body. If there is a pre-drilled hole in the part, its diameter should not exceed 1/4 of the drill bit’s diameter; otherwise, the feed rate should be reduced, and so on.

Issues with the Workpiece Material

In addition to considering the above factors, sometimes there is still a high rate of tool breakage. At this point, one should consider whether the condition of the blank is poor, or if there is a severe sand inclusion situation, and methods to improve the condition of the blank should be considered.

Some Suggestions in Conclusion

(1) For users, it is advisable to purchase indexable carbide cutting tools produced by regular manufacturers, as these manufacturers provide technical documentation, usage instructions, and comprehensive after-sales service, generally ensuring that there are no original quality issues with the products.

(2) The using manufacturers should organize professionals to provide technical training on the use of indexable carbide cutting tools for operators, and constantly inspect and remedy any unfavorable factors that may arise during the entire machining process to avoid unnecessary losses and improve production efficiency.

The successful use of indexable carbide cutting tools not only depends on the quality of the tools themselves but is also related to every aspect of the entire machining system; for example, the performance of the machine tool, the reliability of workpiece clamping, the rigidity of the tool holder system, the rationality of blade selection, and the correct selection of cutting parameters all affect the machining results. Only by using the cutting tools correctly and reasonably can one avoid abnormal damage, improve machining efficiency, and achieve the maximum economic benefits.

The wear on the back face of the cutting tool has a more significant impact on machining accuracy and cutting force than the wear on the front face, and it is easier to control and measure. Therefore, the back face wear land width VB is usually used as the standard for tool bluntness. The wear process generally consists of three stages: the initial wear stage, the normal wear stage, and the severe wear stage. As shown in Figure 1, during the initial wear stage, due to the rough surface of the tool, the contact area between the cutting tool surface and the workpiece is small, resulting in higher compressive stress and faster wear, with a larger slope in the schematic curve. In the normal wear stage, after the initial wear, the tool surface has been smoothed, the contact area between the tool and the workpiece increases, and the pressure decreases, so the amount of wear increases slowly and relatively stably with the extension of cutting time. When the tool wears to a certain extent, the cutting force and cutting temperature increase rapidly, the wear accelerates drastically, and the cutting tool fails.

Tool Wear Mechanism

Tool wear is often the combined result of mechanical, thermal, and chemical actions. Generally, the following mechanisms are recognized for tool wear:

Abrasive Wear

This type of wear occurs due to the presence of hard particles between contact surfaces. The mechanism of abrasive wear mainly involves the continuous micro-cutting and scoring actions of the abrasive particles on the friction surface, resulting in the formation of grooves parallel to the direction of relative motion on the friction surface. The rate of abrasive wear is directly proportional to the normal load Fn and the wear coefficient K, and inversely proportional to the material hardness H. Generally, the higher the hardness of the cutting tool material, the better its resistance to abrasive wear.

Therefore, according to the theory of friction and wear, increasing the hardness and wear resistance of the cutting tool material, achieving an appropriate hardness/toughness ratio, and reducing the surface roughness of the cutting part of the cutting tool can all decrease the abrasive wear of the tool.

Adhesive Wear

This type of wear is caused by the attraction between molecules and atoms on the contact surfaces. Under sufficient pressure and temperature, plastic deformation occurs, leading to a phenomenon known as cold welding, which is the result of the adhesive forces between the fresh surface atoms formed by the plastic deformation of the friction surfaces. An ideal adhesive friction surface is not necessarily the harder the better; it should have a surface characteristic that combines soft and hard features, as shown in Figure 2, with a soft surface (I), a hard subsurface (II), and a gentle transition zone (III) below. In other words, from the perspective of adhesive wear, the tool surface should have good lubricity, the subsurface layer should have high hardness to provide support, and the gentle transition zone below prevents the occurrence of layer-by-layer spalling of the cutting tool material.

Therefore, according to the theory of friction and wear, appropriately improving the hardness/toughness ratio can reduce the cutting force and cutting temperature, which is beneficial for reducing adhesive wear of the tool.

Diffusion Wear

Diffusion wear generally occurs during high-temperature machining processes. When cutting metal, the chemical elements of both the chip, workpiece, and tool diffuse into each other in the solid state during contact, altering the original material composition and structure, which makes the tool material become brittle and accelerates cutting tool wear. This type of wear caused by the migration of elements in the solid state is called diffusion wear. It is a type of wear characterized by chemical properties.

Oxidation Wear on Cutting tool

During the cutting process, due to the increase in cutting speed or the poor thermal conductivity of the work material, the temperature between the tool and the chip reaches high levels (700~1000℃). The tool material or the chip reacts with oxygen to form corresponding oxide films. This is also a type of wear characterized by chemical properties. Generally, during cutting, the cutting tool is in close contact with the chip and workpiece, where oxygen content is low, while in areas away from the contact zone, oxygen content is higher, leading to severe oxidation wear. During high-speed cutting, where cutting temperatures are high, oxidation wear is more likely to occur. The degree of oxidation wear is determined by the cutting speed, the amount of oxygen, and the oxidation resistance of the tool material. Therefore, selecting cutting tool materials with better high-temperature stability and a smaller wear coefficient, or using physical methods to reduce cutting temperatures, such as coolant, can reduce the occurrence of oxidation wear.

Study on the Cutting Performance and Wear Mechanism of Coated Cemented Carbide Cutting Tools

Hard coatings have advantages such as high hardness, wear resistance, low friction coefficient, high resistance to high-temperature oxidation, and good chemical stability. They are widely used in tool machining. By applying surface coating technology to deposit a layer of high-performance coating on the tool substrate, the machining efficiency and cutting tool life can be improved. Under different machining conditions, coated tools exhibit different machining performances. In some cases, the cutting performance of coated cemented carbide tools may even be lower than that of uncoated cemented carbide tools. Therefore, by studying the cutting performance and wear mechanism of coated cemented carbide cutting tools, the modification mechanism of coated cemented carbide tools can be clarified, allowing for a reasonable selection of coating materials and structures, optimization of machining parameters and tool geometric parameters, and truly achieving the goals of high-quality, efficient, and low-cost machining.

Some scholars have studied the turning of 42CrMo4V steel with TiAlN coated cemented carbide inserts at different cutting speeds. The cutting speed significantly affects the failure mode of the tool. As the speed increases, the cutting force increases, and the stress on the coating increases, leading to earlier failure of the coating. Figure 3 shows the time at which failure occurs in the TiAlN coated cemented carbide insert at different cutting speeds. The coating failure process occurs because the stress on the coating exceeds the cohesive strength of the coating or the bonding force between the coating and the substrate, resulting in the formation of fragments. Subsequently, abrasive wear dominates the cutting tool failure process.

We have studied the wear mechanism of TiC coated cemented carbide cutting tools under different cutting speeds. The wear process of the coated inserts can be divided into three stages: the initial wear stage, the normal wear stage, and the final wear stage, as shown in Figure 5. As the cutting speed increases and the cutting time extends, the main wear mechanisms are diffusion wear, plastic deformation wear, and plastic fatigue spalling wear. In the initial stage of wear, severe friction occurs between the tool and the chips on the front and back surfaces of the cutting tool, respectively, resulting in plastic slip of the surface coating material in the opposite direction of cutting tool feed. This leads to plastic fracture of the coating material on the front and back surfaces, which is plastic fatigue spalling wear, and the coatings on the front and back surfaces are worn through at points R and F as shown in Figure 4. In the normal wear stage, although the coatings at points R and F are worn through, the surrounding coatings play a supporting role, thus delaying the expansion of the worn-through areas on the front and back surfaces of the tool.

?Names of the Various Parts of an End Mill

Names of the various parts of an end mill:

Structural Types of End Mills

Classified by the Number of Cutting Edges:

The cylindrical surface and end face of an end mill usually have cutting edges distributed on them, which can engage in cutting simultaneously or individually. Based on the number of blades, end mills can be categorized into double-edge, triple-edge, quadruple-edge, and multi-edge types.

The fewer the number of blades, the larger the chip flute, but the worse the rigidity.

Comparison of the advantages and disadvantages of end mills with different numbers of blades:

According to the type of bottom end tooth shape

According to the shape of the cutting edge

General-purpose end mill

Widely used, applicable to slot machining, side machining, and step surface machining, etc. In addition, it can be used in all situations of rough machining, semi-finishing, and finishing.

Tapered end mill

Used for conical surface machining after general cutting, such as mold draft angle machining and concave portion machining, etc.

Gear-type end mill

The cutting edge is wavy, producing fine chips, with low cutting force, suitable for rough machining, not suitable for finish machining.

Forming end mill

A forming blade is a cutting edge that is shaped to match the contours of the workpiece being machined. Special shapes typically need to be custom-made based on the product’s shape and dimensions.

According to the blade arrangement of end mills

By tool material

End mills can be classified by material into: high-speed steel, solid carbide, carbide with coating, CNB, PCD, etc.

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Key Points for Selecting End Mills

When using an end mill, multiple factors need to be considered to ensure machining efficiency, precision, and tool life. Here are some key considerations:

① Material of the workpiece: Different materials (such as steel, cast iron, aluminum alloy, plastic, composite materials, etc.) require tools with different characteristics. For example, when machining aluminum alloy, a specialized aluminum end mill can be chosen, which typically has good chip evacuation and heat resistance; when machining high-hardness materials, a carbide tool with a high-wear-resistant coating should be selected.

② Machining form and precision requirements: Choose the shape and number of cutting edges based on the shape of the machining surface (plane, slot, contour, etc.) and the required surface roughness. For instance, a ball nose end mill is suitable for complex surface machining, while a flat or rounded end mill is suitable for plane and edge machining. For high-precision machining, choose an end mill with higher arc precision.

③ Helix angle of the end mill: The helix angle affects cutting efficiency and tool life. When machining materials with poor thermal conductivity (such as stainless steel), a large helix angle can improve chip evacuation and heat dissipation, extending tool life. For thin-walled workpieces or machining with poor rigidity, a small helix angle can reduce cutting forces and avoid workpiece deformation.

④ Tool material and coating: Carbide is the most commonly used tool material. For different working conditions, selecting the appropriate carbide grade and coating (such as TiCN, TiAlN, etc.) can enhance tool performance. High-speed steel (HSS) is suitable for low-speed and low-hardness material machining, while carbide is more suitable for high-speed and high-hardness material machining.

⑤ Tool structure: Solid, brazed, and indexable tools each have their advantages. Solid tools have good rigidity and are suitable for precision machining; brazed and indexable tools are convenient for blade replacement and are suitable for mass production.

⑥ Number of flutes and shank structure: The number of flutes affects the tool rigidity and chip flute size. When the workpiece rigidity is low, it is advisable to choose a tool with fewer flutes to improve chip evacuation; the shank design (standard, long neck, tapered neck) needs to be selected based on the machining depth and workpiece shape. Tapered neck end mills provide better rigidity and machining accuracy.

⑦ Tool length: Under the condition of meeting the machining requirements, choose the shortest tool length as much as possible to increase stability, reduce vibration, and thereby improve the quality of machining.

⑧ Cost-effectiveness: Consider the tool cost and machining efficiency comprehensively and choose a cost-effective solution.

In summary, the selection of end mills is a process of comprehensive consideration, involving workpiece material, machining requirements, tool performance, cost-effectiveness, and other aspects. Correct selection can greatly improve machining efficiency, reduce costs, and ensure machining quality.

A notable property of wood is its anisotropy, which leads to distinctions between longitudinal, transverse, end-wise, and transitional cutting during the machining of solid wood. Wood-based panels and wood composite materials are made from wood monomers, such as veneers, shavings, or fibers, which are combined with adhesives under specific temperatures and pressures to form a composite material. The properties of these materials are determined by the wood monomers, their arrangement, and the characteristics of the adhesive. The machinability also varies due to the structure and the proportion and nature of the additives. For example, medium-density fiberboard (MDF) is nearly isotropic, with almost equal cutting resistance across the board; particleboard, due to uneven density distribution and differences between the surface and internal structure, has significant variations in cutting resistance; and blockboard, composed of glued solid wood strips, has differences in grain direction between the strips, thus exhibiting both similar and different properties compared to solid wood during machining.

Selection of Wood Milling Cutter Structure

Based on the nature and requirements of the cutting object, a comprehensive consideration from both technical and economic perspectives is necessary to select the appropriate milling cutter structure. Options include solid wood milling cutters (Figure 3), welded solid milling cutters (Figure 4), assembled milling cutters (Figure 5), and combined milling cutters.

Selection of Milling Cutter Cutting Parameters (Table 1)

The cutting parameters of a milling cutter include the cutting speed of the cutter, the feed rate of the workpiece, and the milling depth. The cutting speed of the milling cutter depends on the cutter’s rotational speed and diameter. The feed rate of the workpiece depends on the requirements for the surface quality of the machined surface. The surface roughness of the workpiece is largely determined by the feed per tooth during the cutting process. If the feed per tooth is too large, the machined surface will be too rough; if the feed per tooth is too small, the machined surface may exhibit burn marks.

Depending on the different milling objects and surface quality requirements, the recommended feed per tooth for wood products processing is generally as follows: for rough machining, Uz = 0.8~1.5mm; for finish machining, Uz = 0.4~0.8mm. If the feed per tooth is within 0.1~0.3mm, there is a risk of the machined surface being burned. For all wood products, the common feed per tooth is Uz = 0.3~1.5mm. For a smooth surface, the feed per tooth Uz = 0.3～0.8mm; for a medium surface, Uz = 0.8~2.5mm; for a rough surface or when surface quality is not a concern, the feed per tooth Uz = 2.5-5.0mm.

Stability of Milling Cutter Operation

The stability of the milling cutter during operation is the basis for ensuring machining accuracy and surface quality. This includes two aspects: first, the vibration of the milling cutter during cutting due to external force excitation; and second, the deformation of the milling cutter under the action of external forces. Vibration is related to the structure of the milling cutter, the method of installation, and unbalanced mass. When the frequency of the external excitation force is close to the natural frequency of the milling cutter,the wood milling cutter will resonate, and the amplitude will increase significantly. A considerable portion of the milling cutters used for wood cutting are installed in a cantilevered manner. In this case, the static force due to the cutter’s own weight and the dynamic force generated by high-speed rotation act in combination on the cutter shaft. Under the action of this force, the cutter shaft will deform, producing a certain degree of deflection.

Since tool vibration and cutter shaft deformation will ultimately severely affect the surface quality of the workpiece, it is necessary to limit the unbalanced mass of the milling cutter and to avoid the cutter’s rotational frequency from matching its natural frequency. Additionally, for cantilevered cutter shafts, it is essential to limit the mass of the wood milling cutter, which means restricting the length and diameter of the cutter.

Safety of Milling Cutter Processing

The safety of wood milling cutter processing includes restrictions on the rotational speed of the milling cutter, limitations on chip thickness (Figure 6，7), restrictions on the profile height of shaped wood milling cutters, and limitations on the thickness and projection length of the blades for assembled wood milling cutters.

The characteristic of wood milling processing is high speed, with the rotational speed of the milling cutter often exceeding 3000rpm. High-speed cutting brings a series of safety issues. When the spindle speed of the milling machine reaches 9000rpm, the use of assembled milling cutters should be prohibited except for shank cutters with a diameter less than 16mm; strict non-destructive testing of the welds on welded solid milling cutters should also be conducted. When the milling cutter leaves the factory, the manufacturer has marked the maximum allowable speed on the body of the cutter, and the user must strictly adhere to this regulation; under no circumstances should the maximum allowable speed be exceeded.

The limitation on chip thickness is a necessary measure to prevent severe overload of the milling cutter due to excessive feed. According to the regulations of the German Association of Woodworking Machinery and Tool Manufacturers, for manually fed machines, the thickness of the milling chips should not exceed 1.1mm, and there are certain requirements for the width of the chip flute for different cutting milling cutters. For semi-mechanically fed machines, the maximum thickness of the milling chips should not exceed 10mm. For fully automatic, mechanically fed machines, there are no restrictions on the thickness of the milling chips and the chip flute, but general safety regulations must be observed.

For shaped milling cutters, the profile height of the shaping contour is closely related to the clamping method of the cutter, the thickness of the workpiece being cut, and the diameter of the cutter. Once the thickness of the workpiece, the diameter of the cutter, and the diameter of the center hole are determined, the profile height of the cutter reflects the cutter’s own strength and rigidity, as well as its ability to withstand cutting resistance. Therefore, there must be limitations on the profile height to ensure safety when using the cutter. On multi-axis milling machines (four-sided planers) and double-end wood milling machines or mortising machines, the spindle shaft diameter must not be less than 30mm. Moreover, due to the space limitation for installing the cutter, the height of the shaped contour cannot be too high.

When designing the body of an assembled wood milling cutter, the issue of blade clamping must be considered. Whether it is a cylindrical or disc-shaped body, the blade clamping form must ensure that it can provide a sufficiently large clamping force to counteract the rotational centrifugal force. For the integral blades or inserted welded blades clamped radially by the pressure plate, there must be a minimum limit on the projection length of the blade, as well as the thickness and length of the blade. When the thickness and length of the blade are less than this minimum limit, it means that the use of the wood milling cutter should be strictly prohibited. Otherwise, there will be safety risks. Figure 8 shows the projection length and the thickness and length of the blade for assembled wood milling cutters.