Shape tolerance refers to the allowable variation of the shape of a single material element, including roundness, cylindricity, planarity, line profile, face profile and straightness. It is the geometrical tolerance of the tested element, which is independent without any error to the datum.

Selection principle

On the premise of meeting the functional requirements of parts, considering the process economy and detection conditions, the most economical tolerance value is selected. It is also necessary to consider the functional requirements, structure and rigidity of the parts, determine the tolerance value according to the tolerance table, and consider the coordination relationship between the tolerance values.

The given shape tolerance of the same element shall be less than the position tolerance, and the shape tolerance of cylindrical parts shall be less than the dimension tolerance. Generally, 30% of the dimension zone shall be selected in the sample stage, 40% in the small batch stage, and 60% to 80% in the large batch stage, depending on the difficulty of processing the workpiece.

Coordinate the relationship with the surface roughness. The surface roughness value is 20% to 30% of the shape error.

Flatness tolerance level shall be lower than perpendicularity and parallelism tolerance level; roundness tolerance level shall be lower than radial circular run out tolerance level.

The problem composition of shape tolerance:

1. Cylindricity out of tolerance

• Straightness and flatness out of tolerance

3.Out of tolerance of line profile and surface profile

Cause analysis:

Irregular operation of employees

1. The installation error of the tool and workpiece is not verified when the tool is replaced

2. Rough operation when changing workpieces

Caused by machine

1. When the spindle bearing of the machine tool is worn or loose due to the rotation error of the machine tool, the out of tolerance of roundness, cylindricity and flatness will occur;

2. If the bearing of the lathe machining center is worn or loose, the roundness and cylindricity will be out of tolerance;

3. The geometric relationship between the feed motion and the main motion is not correct: if the tool’s motion path is not vertical to the workpiece’s rotation axis, cylindricity error will be generated, and if the uneven line, flatness error will be generated;

4. The straightness error of machine tool guide rail in horizontal or vertical plane causes roundness or flatness error in machining indication;

Material cause

1. The shape and size change caused by the eccentric workpiece processing, and the cylindricity is mostly out of tolerance;

2. Because of the change of workpiece material, the hardness of material is mostly related to heat treatment;

3. When the residual stress of workpiece is released, the thickness of workpiece is not consistent, which has a great influence, and the majority of workpiece is blank;

Caused by process design system

1. The mechanical deformation of the process system and the thermal deformation of the process system are not fully considered in the process design.

2. It is difficult to machine materials, and the tool durability is not fully considered.

3. The tool is not rigid enough.

4. Environmental temperature difference is also a major factor.

Develop Interim Measures

Out of tolerance of shape and size caused by improper operation of operators

On site training and guiding the operation methods of the processing personnel, so that the operators can operate correctly and complete the parts processing

Shape and size out of tolerance caused by machine

1. If the order is urgent and there is no maintenance time, the dial indicator can be used to adjust the bearing clearance and complete the emergency production order.

2. Check whether the temperature rise of each part of the machine tool is consistent, whether it is due to the change of the temperature rise of the machine tool, the change of the original static geometric dimension of the machine tool, and the change of the processing track. The machine tool can reach the heat balance state by preheating the machine tool. If not, change the program control value first to complete the emergency order.

3. The straightness error of machine tool guide rail in horizontal or vertical plane causes roundness or flatness error in machining indication, and the relative displacement direction is adjusted from tangent direction to normal direction.

Develop permanent measures

The shape and size out of tolerance caused by the operator’s irregular operation.

Standardize the operation steps, standardize the placement standards of workpieces, and form the operation instructions to guide the standardized production of workers.

Shape and size out of tolerance caused by machine

1. Check the main shaft bearing, correct or replace the high-precision bearing, and improve the assembly quality of the main engine;

2. 2. Correct the static geometric dimension of machine tool installation;

Out of tolerance of shape and size caused by materials

1. Calculation of workpiece eccentricity and reasonable counterweight

2. Whether the heat treatment process of workpiece material is reasonable and in accordance with the standard

3. Improve workpiece clamping conditions through tooling design

Shape and size out of tolerance caused by process system

Standardized control of documents: tool design, whether machining error calculation is reasonable, whether cutting parameters are reasonable, tool geometry, whether stress calculation is accurate in machining process, whether static stress of process system is reasonable, whether arrangement of process route is reasonable, whether machining structure of parts has concentration of residual stress, whether adopting reasonable heat treatment process, workpieces Whether there is residual stress in the shaping process.