Nine errors in CNC machining

1、Machine manufacturing errors

The manufacturing error of a machine tool mainly includes spindle rotation error, guideway error and drive chain error.

Spindle rotation error refers to the spindle of the actual axis of rotation at each instant relative to its average rotation axis of change, it will directly affect the accuracy of the workpiece being processed. The main reasons for the spindle slew error are spindle coaxiality error, bearing error itself, the coaxiality error between the bearings, spindle winding and so on. Guide rail is a machine tool to determine the relative position of the machine parts of the relationship between the benchmark, but also the machine tool movement benchmark. The manufacturing error of the guide rail itself, the uneven wear and tear of the guide rail and the quality of the installation is an important factor that causes the guide rail error. Transmission chain error refers to the beginning and end of the transmission chain between the two ends of the relative movement of the transmission element error. It is caused by the manufacturing and assembly errors of each component link in the transmission chain, as well as the wear and tear in the process of using.

2、Geometric error of the tool

Any tool in the cutting process is unavoidable to produce wear, and the resulting size and shape of the workpiece to change. Tool geometry error on the impact of machining error with different types of tools and different: the size of the tool processing, tool manufacturing error will directly affect the workpiece machining accuracy; and the general tool (such as turning tools, etc.), the manufacturing error on the machining error has no direct impact.

3, the geometric error of the fixture

The role of the fixture is to make the workpiece is equivalent to the tool and machine with the correct position, so the geometric error of the fixture on the machining error (especially the position error) has a great impact.

4, positioning error

Positioning error mainly includes the benchmark does not coincide with the error and the manufacturing inaccuracy of positioning vice. When machining the workpiece on the machine, some geometric elements of the workpiece must be selected as the machining positioning benchmark, if the chosen positioning benchmark does not coincide with the design benchmark (the benchmark used to determine the size and position of a surface on the parts map), it will produce the benchmark non-coincidence error.

Workpiece positioning surface and fixture positioning elements together constitute the positioning vice, due to the positioning vice manufacturing inaccuracy and positioning vice with the gap between the maximum position change caused by the workpiece, known as the positioning vice manufacturing inaccuracy error. Positioning manufacturing inaccuracy error will only arise when using the adjustment method of processing, not in the test cutting method of processing.

5, the error caused by the deformation of the force of the process system

Workpiece stiffness: If the stiffness of the workpiece in the process system is relatively low compared to the machine, tool and fixture, the deformation of the workpiece caused by the lack of stiffness under the action of the cutting force will have a greater impact on the machining error.

Tool stiffness: the outer turning tool in the machining surface normal (y) direction of the stiffness, the deformation is negligible. Boring smaller diameter holes, toolholder stiffness is very poor, the toolholder force deformation on the hole machining accuracy will have a great impact.

Stiffness of machine tool components: machine tool components consist of many parts, the rigidity of machine tool components has yet to be appropriate simple calculation method, the current method is mainly used to determine the rigidity of machine tool components experimental methods. Factors affecting the stiffness of machine tool components are combined with the impact of surface contact deformation, the impact of friction, the impact of low stiffness parts, the impact of clearance.

6, the error caused by heat deformation of the process system

Thermal deformation of the process system has a greater impact on the processing error, especially in precision machining and large parts processing, the processing error caused by thermal deformation can sometimes account for 50% of the total error of the workpiece.

7、Adjustment error

In every machining process, the process system is always adjusted in one way or another. Since the adjustment cannot be absolutely accurate, the adjustment error arises. In the process system, the mutual position accuracy of workpieces and tools on the machine is ensured by adjusting the machine, tool, fixture or workpiece. When the machine tool, tool, fixture and workpiece blank and other original accuracy of the process requirements and do not take into account the dynamic factors, the adjustment error on the machining error plays a decisive role.

8、Measuring error

Parts in the processing or measurement after processing, as a result of measurement methods, gauge accuracy and workpiece and subjective and objective factors directly affect the measurement accuracy.

9、Internal stress

The stress that exists inside the part without external force is called internal stress. Once the internal stress on the workpiece, it will make the workpiece metal in a high energy level of instability, it instinctively to the low energy level of the stable state, and accompanied by deformation, so that the workpiece loss of the original machining accuracy.