Using carbide drill bit to make holes on nodular cast iron is a?commonly taken choice?when?automobile engine crankshaft?is manufactured.?Although nodular cast iron has good ductility and fatigue strength, due to the?long chips?and poor performance?under processing , especially?under?drilling, nodular cast iron’s?characteristics,including?high unit cutting heat and difficult chip removal,?lead to the service life of the carbide drilling bit lower than that of other cast iron materials. In this article, the wear morphology of carbide drill bit in processing nodular cast iron is observed, the main wear forms are summarized, and the prediction formula of bit life is established through our experiment stats, which is convenient for carbide drill bits’ parameter adjustment and life setting.

 

Main wear forms of carbide drilling bits

Drill angle wear and drill tip wear are common wear forms of carbide drilling bits. The failure form after drill angle wear is usually rough hole wall or bit edge collapse, but the drill angle wear can be effectively restrained by passivation. The drill angle wear is light after the test. The drill bit tip?plays an important role in guiding and centering in the drilling process, and 80% of the drilling axial force is generated at the drill tip (transverse edge). When the drill tip is worn, the axial force increases, the centering is unstable, and the carbide drilling bit swing increases, resulting in large hole diameter, outlet burr and even carbide drilling bit fracture. Drill tip usually?seriously worn after working, and the transverse edge?on the tip is?almost ground flat.

Taking carbide drill bit?on the nodular cast iron QT600 crankshaft production line as an example, the failure form of most carbide drilling bits is that the hole diameter is too large. Therefore, the wear of drill tip is the main cause of carbide drilling bit?failure when machining nodular cast iron, which should be used to judge the rationality of tool life setting.

Carbide Drill bits life formula

In practical application, in addition to observing the wear morphology of the bit, there are two common methods to measure whether the setting of the bit life is reasonable. One is to consider the total depth of the holes processed within the bit life (referred to as the total depth), and the other is to consider the processing distance of the bit shoulder (referred to as the total distance). The former is easy to calculate, and the total depth can be calculated by multiplying the number of pieces by the hole depth, but the influence of cutting speed and feed rate on tool life is not considered.?Although the cutting speed and feed rate are considered in the calculation of the latter, it is still unable to quantify the impact of the change of cutting parameters on the tool life. By changing cutting parameters and tracking the change of tool life, the formula of bit life is established.

According to the classical cutting theory, the most influential factors on tool life are cutting speed, feed rate and cutting depth. In drilling, the cutting depth is determined by the cutting speed and feed rate. The formula of bit tool life only needs to consider the cutting speed and feed rate as variables, which is expressed as

T = aVF

In order to facilitate the calculation, the equation is transformed into

lnT = lna + blnV?+ clnF

In which T?is the tool life defined by the crossing distance of the bit shoulder (m), V is the cutting speed defined by the linear speed of the bit shoulder (M / min), and F is the feed rate (mm / min)

Test scheme and formula solution of carbide drill bits on ductile cast iron

The dynamic balance station of crankshaft production line is to meet the requirements of crankshaft dynamic balance by drilling a series of balance holes with different angles (phases) and depths (balance amount) on crankshaft blades. In order to accurately control the depth of the balance hole, the equipment monitors the axial force received by the carbide drilling bit?in real time. This function also plays a role in monitoring the tool wear. When the axial force exceeds the set alarm value, it will prompt to replace the tool and display the total depth of the working hole within the tool life. The balance hole station bit not only has constant tool wear standard, but also can accurately record the service life (total depth) of the test bit. Therefore, it is selected as the test tool. Combined with the application practice, the alarm value of drilling axial force is set to 5500no

According to the processing beat of the equipment and the use of tools, set the test scheme: the rotating speed is gradually reduced from 3300r / min to 2750r / min and 2250r / min, and the feed rate is reduced from 660mm / min to 600mm / min. the test scheme and results are shown in Table 1.

There are three test results, and the following three equations can be obtained

InT1= Ina + blnV?4- c ×?lnF_l?（ 1）

Equation 1

lnT₂?=?Ina + blnV₂?+ clnF₂?（2）

Table 1 test scheme and results

Test scheme	Tool?speed n（ rpm）	cutting speedV(?mm/min)	Feed speed F(?mm/min)	Total processing depth	Cutting depth
S(mm)	H(mm)
1	3300	104	660	25	3864
2	2750	86	600	34	4845
3	2250	71	600	43	5046

InT3?= Ina + blnV₃?+ clnF_3???(3)

To facilitate the solution, define

A₂=lnT₁-lnT₂,

A₃=lnT₁-lnT₃?,

B₂=lnV₁-lnV₂?,

B₃₌lnV_1-lnV₃

C₂=lnF₁?-lnF₂

C3=lnF₁– lnF₃.

Feed index?c = (A₂B₃-a₃b₂)/(c₂b₃?-c₃b₂), Speed index?b = (A₂-C₂?×?c)/B₂,Formula coefficient?a =lnT₁?– b?V₁-c?f₁。

After substituting the test data,?a = 3959619943, b = -0. 20209,c= -1.98728,?Therefore, the tool life formula should be

T = 3959619943 V^－0.20209F^-1.98728

Formula extension and application limitations of carbide drill bits

(1) Derivation of the carbide drill bit working time formula?

Because only three groups of representative tool data are needed (if the service life exceeds the set tool life, the hole diameter will be too large), the life prediction formula of drill tools on the same production line can be calculated. Therefore, this method has strong popularization. Taking other curved axis tools as an example, the cutting parameters are substituted into the above formula, the theoretical tool tip distance is calculated, and the theoretical life is calculated according to the machining depth of a single piece.

As shown in Table 2, there is little difference between the theoretical and actual life of most tools. It can be seen that the tool life still has high accuracy within a certain range.

Table 2 theoretical and actual life of cutting tools

Tool number	切割速度	Feed speed	Drillspeed	Actual tool tip distance	Theoretical tool tip distance	Error ratio	Current life
	( mm/min)	( mm/min)	( mm/min)
OP050ST02	78	450	104	8673	8757	-1%	480
OP050ST04	52	720	176	3548	3739	-5%	280
OP010ST04	118	700	32	3227	3351	-4%	600
OP010ST09	50	500	71	7138	7766	-9%	1088
OP010ST15	65	650	91.5	4599	4373	5%	500
OP010ST22	68	500	48	6840	7309	-7%	1050
OP100STO2	79	900	50	2182	2207	-1%	500

(2) Formula limitations

In the process of popularization, it is found that the formula has large deviation for the prediction results of large length diameter ratio and large diameter bits. Tools with large aspect ratio generally refer to tools with the ratio of length to diameter greater than 12, such as crankshaft oil passage hole drill. Due to the large aspect ratio and insufficient rigidity, it is more sensitive to the swing in the drilling process. In addition, the long drilling time and poor chip removal effect lead to the burning of the cutting edge after the accumulation of cutting heat. Therefore, the predicted life is higher than the actual life. Large diameter bit usually refers to the bit with a diameter greater than 12mm, such as the bit for drilling the center hole at the flange end of the crankshaft. The reason for the nonconformity of large diameter bit is not clear and needs to be studied in the future.

結(jié)論

1. When machining nodular cast iron with carbide drill bits, the main wear form is drill tip wear. The common failure form after drill tip wear is large aperture or burr of machining port. The most serious case is broken blade or broken blade
2. The wear degree of drill tip can be monitored by detecting the axial force of drilling. When machining QT600 nodular cast iron crankshaft, the limit value of axial force is recommended to be 5500n.
3. The life prediction formula of carbide drilling bit?is applicable to the carbide drilling bit?used in most mechanical crankshaft production lines. However, for bits with aspect ratio above 12 and diameter above 12mm, the prediction error of bit life is large.