The 9 Causes Of Tolerance In CNC Machining

(STL.News) CNC machining tolerance refers to the degree of deviation between the actual geometric parameters (geometric size, geometric shape, and mutual position) of the part after processing and the ideal geometric parameters.  The degree between actual geometrical parameters and the ideal geometrical parameters after machining is the machining accuracy.  The smaller the tolerance, the higher the machining accuracy is.  Machining accuracy and machining tolerance are two references to the same issue.  Therefore, machining tolerance reflects the level of machining accuracy.

There are 9 main causes of machining tolerance.

Manufacturing tolerance of the machines

The manufacturing tolerance of machines mainly includes spindle rotation tolerance, guide rail tolerance, and transmission chain tolerance.  The rotation tolerance of the spindle means the variation of the actual rotation axis of each spindle between its average rotation axis, which will directly affect the accuracy of the workpiece being processed.  The main reasons for the rotation tolerance of the main shaft are the coaxiality tolerance of the main shaft, the tolerance of the bearing itself, the coaxiality tolerance between the bearings, and the main shaft winding.

The guide rail is a reference for determining the relative positional relationship of each component on the machines, and also a reference for the machine movement.  The manufacturing tolerance of the guide rail itself, uneven wear of the guide rail, and the installation quality are important factors that cause the guide rail tolerance.  The transmission chain tolerance refers to the tolerance of the relative motion between the transmission elements at the two ends of the transmission chain.  It is caused by the manufacturing and assembly tolerance of the various components in the transmission marrow and wears during use.

Machining tool(cutter)

It is inevitable that any cutter will wear during the cutting process, this will cause a change of the workpiece’s size and shape.  The impact of cutter geometric tolerance on machining tolerance varies with the cutter type.  When using a fixed-size cutter, the manufacturing tolerance of the cutter will directly affect the machining accuracy of the workpiece.  While for general cutters such as CNC turning cutters, milling cutters, etc, the manufacturing tolerance has no direct effect on machining tolerance.

Clamping fixture

The role of the clamping fixture is to make the workpiece equivalent to the correct position of the cutter and machines, so it has a great influence on the machining tolerance, especially the position tolerance.

Positioning tolerance

Positioning tolerance mainly includes reference misalignment tolerance and positioning auxiliary inaccurate tolerance.  When processing the workpiece on the machines, you must select several geometric elements on the workpiece as the positioning benchmark during processing.  If you choose the positioning benchmark and design benchmark (the reference used to determine the size and position of a surface on the part drawing) does not coincide, a reference misalignment tolerance will occur.  The positioning surface of the workpiece and the positioning element of the fixture form the positioning auxiliary.  The maximum positional variation of the workpiece caused by the inaccurate manufacturing of the positioning auxiliary and the cooperation between the positioning auxiliary is called the inaccurate tolerance of the positioning auxiliary.  Inaccurate tolerance in positioning auxiliary will only occur when the adjustment method is used for processing, and will not occur during trial cutting processing.

Force deformation of the process system

Workpiece rigidity: If the rigidity of the workpiece is relatively lower compared to the machine, cutter, and fixture in the process system, under the cutting force the deformation will have a great impact on the machining tolerance.

Cutter rigidity: The external turning cutter has high rigidity in the normal Y direction of the machining surface, and its deformation is negligible.  For the inner hole with a smaller diameter, the rigidity of the shank is very poor, and the deformation of the shank’s force will greatly affect the accuracy of the hole processing.

Rigidity of the machine components: The machine is composed of many components.  So far, there is no suitable simple calculation method for the rigidity of the machine components. Experimental methods are mainly used to determine the rigidity of the machine components. The factors that affect the rigidity of machine components include contact surface deformation, friction, low-rigidity parts, and clearance.  For example, in injection molding, the mold cavity has to bear high pressure, if the mold stiffness is not enough, it will make the mold deformation and thus reduce the standard injection molding tolerances, so the influence of the cavity structure on the stiffness needs to be fully considered.  It is necessary to overcome the wear and deformation of the mold, in which the manufacturing accuracy of the mold is the main factor that affects the injection molding tolerances.

Thermal deformation of the process system

The thermal deformation of the process system has a great influence on the machining tolerance, especially in precision machining and large parts machining, the processing tolerance caused by thermal deformation can sometimes account for 50% of the total workpiece tolerance.

Adjustment tolerance

In each process of CNC machining, the process system must always be adjusted in one way or another.  Since the adjustment cannot be absolutely accurate, an adjustment tolerance occurs.  In the process system, the mutual position accuracy of the workpiece and cutter on the machine is guaranteed by adjusting the machine, cutter, fixture, or workpiece.  When the original accuracy of the machine, cutter, fixtures, and workpiece blanks meets the process requirements without considering dynamic factors, adjustment tolerance plays a decisive role in machining tolerance.

Measurement tolerance

When the parts are measured during or after machining, the measurement accuracy is directly affected by the measurement method, the accuracy of the measuring tool, and subjective or objective factors of the workpiece.

Residual stress

Residual stress is the stress that remains in a solid material after the primary source of stress generation has been removed from the material.  Or the stress existing inside the part without external force is called residual stress.  Once the residual stress is generated on the workpiece, it will make the workpiece metal in a high-energy unstable state, which instinctively needs to be converted to a stable state at a low energy level, and accompanied by deformation, so that the workpiece loses its original machining accuracy.

Conclusion

CNC machining tolerances are an important aspect of ensuring the quality and functionality of the parts produced by setting limits on up to what extent the dimensional variations are allowed.  We should consider some factors like work holders, material, tooling performance, and the selection of preferred machining facility when working with tolerances.  If you want to know more about CNC machining, please subscribe to Wayken or send an inquiry.