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How to determine the accuracy of a processing center?

2019-08-05

Reprinted from:Smart Manufacturing IMS

It is estimated that there will be a processing center in everyone’s workshop. The accuracy of a processing center is of great importance because the accuracy of the processing center affects the quality of the processing. Therefore, the accuracy of the processing center and the machining process have also been studying ways to reduce the error as much as possible.. So how to judge the accuracy of a processing center? Let us now turn to four aspects.

1. Positioning of vertical processing center specimens:
The specimen shall be located in the middle of the X stroke and placed along the Y and Z axes in an appropriate position suitable for the fitting and fixture positioning and tool length. When there are special requirements for the location of the specimen, it should be stipulated in the agreement between the manufacturer and the user.
2. The fixation of the sample:
The specimen shall be installed conveniently on a special fixture to achieve maximum stability of the cutter and fixture. The mounting surface of the fixture and specimen shall be straight.
The parallelism between the mounting surface of the specimen and the clamp holder should be checked. Appropriate gripping methods should be used to allow the tool to penetrate and process the full length of the central hole. It is recommended to fix the specimen with a headrest screw to avoid interference between the cutter and the screw. Other equivalent methods can also be used. The total height of the sample depends on the fixed method chosen.

3. The material, knife and cutting parameters of the specimen:
The materials and cutting tools and cutting parameters of the specimen are selected according to the agreement between the manufacturer and the user and shall be recorded. The recommended cutting parameters are as follows:
1) Cutting speed: about 50m/min of cast iron; Aluminium is about 300m/min.
2) Feed: approximately(0.05 ~ 0.10) mm/tooth.
3) Cutting depth: The radial cutting depth of all milling processes should be 0.2 mm.
4. The size of the sample:
If the sample has been cut several times, the shape size is reduced, and the aperture is increased. When used for acceptance inspection, it is recommended to use the final outline processing sample size in accordance with this standard so as to accurately reflect the cutting accuracy of the processing center. The sample can be used repeatedly in the cutting test, and its specifications should be kept within 10 % of the feature size taxi given in this standard. When the specimen is re-used, a thin cutting shall be performed before a new precision cutting test is performed to clean up all surfaces and then test.


There is another problem with the use of the processing center. Why does the accuracy of the processing center become worse when it is used? Did we buy parallel?
The machining accuracy of the parts in the processing center is generally due to the fact that when the installation is adjusted, the feed dynamics between the axes are not adjusted according to the error, or due to the wear and tear of the use. There are changes in the transmission chain of each shaft of the machine tool(such as the gap of the wire bar, the change of pitch error, the axial movement, etc.). It can be resolved by adjusting and modifying the gap compensation amount.


When the dynamic tracking error is too large and the alarm, it can be checked: whether the servo motor speed is too high; Position check element is good; Whether the position feedback cable connector contacts well; Whether the corresponding analog output latch and gain potentiometer are good; Whether the corresponding servo drive device is normal.
If the machining precision is not good when the machine tool is moving, the increase and decrease time may be too short, and the speed change time can be extended appropriately. It may also be that the connection between the servo motor and the wire bar is too loose or rigid, and the gain of the position ring can be appropriately reduced. It may be that the roundness of the two-axis linkage is extremely poor. This deformation may be caused by mechanical adjustments. The accuracy of Axis positioning is not good, or the miscompensation of the gap of the screw will lead to the error of roundness.


The original machine tool has so much knowledge of accuracy.
However, there is also a saying in the folk: high accuracy is guaranteed by people. It is said that the most accurate machine tools in the world are calibrated by people. In Japan, there are people who specialize in calibrating the machining accuracy of machine tools. Such people are priceless in every machine tool manufacturer. According to this, it is possible to use relatively less accurate parts to assemble machine tools with amazing machining accuracy.

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