Basic knowledge of machining precision of CNC machine tools(1) Factory Suppliers Manufacturers Quotes

Basic knowledge of machining precision of CNC machine tools(1)

Machining accuracy is mainly used to produce product grades, and machining accuracy and machining tolerance are terms used to evaluate the geometric parameters of the machined surface. The machining accuracy is measured by the tolerance level. The smaller the grade value is, the higher the accuracy is. The machining error is represented by a numerical value. The larger the value, the larger the error. High machining accuracy means that the machining error is small and vice versa. There are 20 tolerance levels from IT01, IT0, IT1, IT2, IT3 to IT18. If IT01 indicates that the part has the highest machining precision, IT18 indicates that the machining accuracy of the part is the lowest. Generally, IT7 and IT8 are medium machining precision. level.


The actual parameters obtained by any processing method are not absolutely accurate. From the function of the parts, as long as the machining error is within the tolerance range required by the part drawing, it is considered that the machining accuracy is guaranteed.


The quality of the machine depends on the quality of the part and the quality of the machine. The quality of the part includes the machining accuracy and surface quality of the part.


Machining accuracy refers to the degree to which the actual geometric parameters (size, shape, and position) of the part after machining are in accordance with the ideal geometric parameters. The difference between them is called the machining error. The size of the machining error reflects the level of machining accuracy. The greater the error, the lower the machining accuracy, and the smaller the error, the higher the machining accuracy.


First, the adjustment method of processing accuracy


1, reduce machine error

(1) Improve the manufacturing precision of the spindle components

1) The rotation accuracy of the bearing should be improved:

1 select high-precision rolling bearings;

2 using high-precision multi-oil hydraulic bearing;

3 uses high precision hydrostatic bearings.


2) The accuracy of the parts with the bearing should be improved:

1 Improve the processing precision of the box support hole and the spindle journal;

2 Improve the machining accuracy of the surface matched with the bearing;

3 Measure and adjust the radial runout range of the corresponding part to compensate or cancel the error.


(2) Appropriate preloading of rolling bearings

1 can eliminate the gap;

2 increase the bearing stiffness;

3 Homogenize rolling element error.


(3) The spindle rotation accuracy is not reflected on the workpiece.

2. Adjust the process system

(1) Trial adjustment

By trial cutting - measuring the size - adjusting the tool's amount of knife - cutting the knife - then try again, and so on until the desired size is reached. This method has low production efficiency and is mainly used for single-piece small batch production.


(2) Adjustment method

The required dimensions are obtained by pre-adjusting the relative positions of the machine tool, fixture, workpiece and tool. This method has high productivity and is mainly used for mass production.


3, reduce tool wear

The tool must be reground before the tool size wear reaches a sharp wear stage.


4, reduce the transmission chain transmission error

(1) The number of transmission parts is small, the transmission chain is short, and the transmission precision is high;

(2) Adopting the speed reduction transmission is an important principle to ensure the transmission accuracy, and the closer to the end of the transmission pair, the smaller the transmission ratio should be;

(3) The accuracy of the end piece should be higher than other transmission parts.


5, reduce the deformation of the process system

(1) Improve the rigidity of the system, especially the stiffness of the weak link in the process system

1) Reasonable structural design

1 Minimize the number of joint faces;

2 to prevent the occurrence of local low stiffness;

3 The basic part, the structure and cross-sectional shape of the support should be selected reasonably.


2) Improve the contact stiffness of the joint surface

1 to improve the quality of the joint surface between the parts of the machine tool;

2 preloading the machine components;

3 Improve the accuracy of the workpiece positioning datum and reduce its surface roughness value.


3) Adopt reasonable clamping and positioning methods


(2) Reduce the load and its changes

1) Reasonably select the tool geometry parameters and cutting amount to reduce the cutting force;

2) Grouping of embryos, try to make the processing margin of the embryo in the adjustment uniform.


6, reduce residual stress

(1) adding a heat treatment process for eliminating internal stress;

(2) Reasonably arrange the process.

7, reduce the thermal deformation of the process system

(1) Adopt reasonable machine tool component structure and assembly standard

1) Adopting a thermal symmetrical structure - in the gearbox, symmetrically arranging the shaft, bearing, transmission gear, etc., the temperature of the box wall can be evenly increased, and the deformation of the box body is reduced;

2) Reasonable selection of assembly standards for machine tool components.


(2) Reduce heat generation and isolation heat source of heat source

1) use a smaller amount of cutting;

2) When the precision of parts is high, the rough finishing process is separated;

3) Separate the heat source from the machine as much as possible to reduce the thermal deformation of the machine tool;

4) For the heat source that cannot be separated, such as the spindle bearing, the screw nut pair, and the high-speed moving guide pair, improve the friction characteristics from the aspects of structure, lubrication, etc., and reduce the heat or use the heat insulating material;

5) Use forced air cooling, water cooling and other heat dissipation measures.


(3) Equilibrium temperature field


(4) Accelerate to achieve heat transfer balance


(5) Control the ambient temperature

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