How to improve the accuracy and efficiency of CNC machining

How to improve the accuracy and efficiency of CNC machining?

Under the condition that the stiffness allows, rough machining to take a larger depth of cut to reduce the number of tool walks and improve the productivity of the workpiece; finish machining generally take a smaller depth of cut to obtain a higher surface quality. Affect the final machining accuracy and processing efficiency of the workpiece, in addition to the reasons for the CNC machine tool itself, but also from a reasonable processing route settings, tool selection and correct installation, a reasonable choice of cutting volume, programming skills and rapid control of dimensional accuracy and other aspects of comprehensive consideration.

Programming skills

CNC programming is the most basic work of CNC machining, workpiece processing program preparation directly affects the final machining accuracy and processing efficiency of the machine tool. You can start from the clever use of the inherent program, reduce the cumulative error of the CNC system, flexible use of the main program and subroutines and other aspects.

Flexible use of the main program and subroutines

In the complex mold processing, generally use a mold in the form of multiple parts for processing. If there are several identical shapes on the mold, the relationship between main program and subroutines should be flexibly used, and subroutines should be called repeatedly in the main program until the processing is completed. Not only can ensure the consistency of the processing size can also improve its processing efficiency.

Reduce the cumulative error of the CNC system

Generally use incremental way to program the workpiece, is the previous point as the benchmark for processing, so the continuous execution of multiple programs is bound to produce a certain cumulative error, so try to use the absolute way of programming in the preparation of the program, so that each program segment to the workpiece origin as the benchmark, so that the cumulative error of the CNC system can be reduced to ensure machining accuracy.

Machining accuracy is mainly used to produce the degree of the product, and both machining accuracy and machining error are terms used to evaluate the geometric parameters of the machined surface. But the actual parameters obtained by any processing method will not be absolutely accurate, from the function of the part, as long as the machining error is within the tolerance range required by the part diagram, it is considered that the machining accuracy is guaranteed.

Machining accuracy is the degree to which the actual geometric parameters (size, shape and position) of the part after machining conform to the ideal geometric parameters. The difference between them is called machining error. The size of the machining error reflects the level of machining accuracy. The larger the error, the lower the machining accuracy, and the smaller the error, the higher the machining accuracy.
The following is a brief introduction to improve the machining accuracy of the workpiece what are the methods.

Adjustment of the process system

(1) Trial cutting method adjustment by trial cutting – measuring the size – adjust the tool draft – go tool cutting – trial cutting again, and so on until the required size. This method has low production efficiency and is mainly used for single-piece small batch production.
(2) Adjustment method by pre-adjusting the machine tool, fixture, workpiece and the relative position of the tool to obtain the required size. This method of high productivity, mainly for mass production.

Reduce machine errors

(1) Should improve the rotary accuracy of the bearing:

• Selection of high-precision rolling bearings;
• The use of high-precision multi-oil wedge dynamic pressure bearings;
• The use of high-precision hydrostatic bearings.

(2）Improve the precision of the bearing parts:

• Improve the machining accuracy of the box support hole and spindle journal;
• Improve the machining accuracy of the mating surface with the bearing;
• Measure and adjust the radial runout range of the corresponding parts, so that the error compensation or offset.

(3) Appropriate preload for rolling bearings.

• Can eliminate the clearance;
• Increase the bearing stiffness;
• Homogenize the rolling body error.

(4) So that the spindle rotation accuracy is not reflected to the workpiece.

Reduce the transmission chain transmission error

• (1) The number of transmission parts is small, the transmission chain is short, the transmission accuracy is high;
• (2) The use of reduced speed transmission is an important principle to ensure transmission accuracy, and the closer to the end of the drive vice, the smaller the transmission ratio should be;
• (3) The accuracy of the end pieces should be higher than other transmission parts.

Reduce tool wear

• (1) In the tool size wear to reach a sharp wear stage before the tool must be re-sharpened;
• (2) The use of special cutting oil for adequate lubrication;
• (3) Tool material should meet the process requirements.

Reduce the force deformation of the process system

• (1) Improve the stiffness of the system, especially to improve the stiffness of the weak link in the process system;
• (2) Reduce the load and its variation.

Reduce the thermal deformation of the process system

• (1) Reduce the heat source of heat and isolate the heat source;
• (2) Balanced temperature field;
• (3) The use of reasonable machine tool components structure and assembly benchmark;
• (4) Accelerate the heat transfer equilibrium;
• (5) Control the ambient temperature.

Reduce residual stress

• (1) Increase the heat treatment process to eliminate internal stress;
• (2) Reasonable arrangement of the process.

The above is the method to reduce the error of processing workpiece, reasonable arrangement of the process can effectively improve the accuracy of the workpiece.

Reasonable setting of processing route

Reasonable setting of machining route and machining sequence is an important basis for optimizing the workpiece machining program. It can be considered from the aspect of machining trajectory and tool feeding method.
In the CNC milling of workpieces, it is necessary to combine the workpiece’s technological requirements to select the appropriate tool feed method to ensure the cutting accuracy and efficiency of the workpiece. When milling the outer contour of the flat workpiece, the cutter entry and exit routes should be arranged. Try to cut in and out along the extension line of the contour curve to avoid tool marks at the intersection. At the same time in the milling process should be based on the workpiece situation to choose the smooth milling or reverse milling.

Tool selection and correct installation

Whether it is CNC machining or general machining, the tool is directly acting on the workpiece, so its selection and installation is the most important factor in the workpiece machining accuracy and surface quality. In particular, the workpiece is processed on the CNC machining center, the tool is stored in the tool magazine in advance, and once the processing begins, it must not be replaced at will. So the general principle of tool selection is: easy to install and adjust, good rigidity, durability and accuracy.

Cutting amount of reasonable choice

Cutting amount is determined by the CNC machining process is an important element, its size is the main motion of the machine tool and feed movement of important parameters, the workpiece processing accuracy, processing efficiency and tool wear has an important impact. The selection of the cutting amount includes cutting speed, back draft and feed amount. The basic selection principle is: under the condition that the rigidity allows, rough machining to take a larger depth of cut, in order to reduce the number of tool walking, improve the productivity of the workpiece; finish machining generally take a smaller depth of cut, in order to obtain a higher surface quality.