How to judge the accuracy of a machining center

- Mar 25, 2021-

Everyone's workshop is estimated to have a machining center, the accuracy of a machining center is very important, because the accuracy of the machining center affects the processing quality, so about the accuracy of the machining center, engaged in machining also has been studying ways to reduce errors as much as possible. So how to judge the accuracy of a machining center?

1. Positioning of vertical machining center specimen

The test piece should be located in the middle of the x stroke and placed along the Y and Z axes at the appropriate position suitable for the positioning of the test piece and fixture and the length of the tool. When there are special requirements for the positioning position of the specimen, it shall be specified in the agreement between the manufacturer and the user.

2. Fixation of specimen

The specimen should be easily installed on a special fixture to achieve the maximum stability of the tool and fixture. The mounting surface of fixture and test piece shall be straight.

The parallelism between the mounting surface of the test piece and the clamping surface of the fixture shall be inspected. Appropriate clamping methods should be used to allow the tool to penetrate and machine the full length of the central hole. It is suggested to use countersunk screws to fix the specimen to avoid the interference between the tool and the screw, and other equivalent methods can also be used. The total height of the specimen depends on the fixing method used.

3. Material, tool and cutting parameters of the specimen

The material, cutting tools and cutting parameters of the test piece shall be 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 for iron castings and 300m / min for aluminum castings.

2) Feed rate: about (0.05 ~ 0.10) mm / tooth.

3) Cutting depth: the radial cutting depth of all milling processes should be 0.2mm.

4. Size of specimen

If the test piece is cut several times, the overall dimension decreases and the aperture increases. When it is used for acceptance inspection, it is recommended to select the final contour machining test piece whose dimension is consistent with that specified in this standard, so as to truly reflect the cutting accuracy of the machining center. The test piece can be used repeatedly in the cutting test, and its specification shall be kept within 10% of the characteristic size given in this standard. When the test piece is used again, a thin layer cutting shall be carried out to clean all the surfaces before the new finish cutting test.

It is estimated that you will encounter another problem in the process of using the machining center. Why does the accuracy of the machining center deteriorate when it is used? Did we buy parallel products?

The poor machining accuracy of parts in machining center is generally due to that the feed dynamic of each axis is not adjusted properly according to the error during installation and adjustment, or that the transmission chain of each axis of the machine tool has changed after use and wear (such as the change of lead screw clearance, pitch error, axial movement, etc.). It can be solved by readjusting and modifying the gap compensation.

When the dynamic tracking error is too large and gives an alarm, it can check whether the servo motor speed is too high; whether the position detection element is in good condition; whether the position feedback cable connector is in good contact; whether the corresponding analog output latch and gain potentiometer are in good condition; whether the corresponding servo drive device is normal.

If the machining accuracy is not good due to overshoot when the machine tool is moving, it may be that the acceleration and deceleration time is too short, and the speed change time can be appropriately extended; it may also be that the connection between the servo motor and the lead screw is loose or the rigidity is too poor, and the gain of the position ring can be appropriately reduced; it may be that the roundness of the two axis linkage is out of tolerance, and this kind of deformation may be caused by the improper adjustment of the machine. If the positioning accuracy of the shaft is not good or the clearance compensation of the lead screw is improper, the roundness error will occur when the image limit is over.

But there is another folk saying: high precision is guaranteed by people. It is said that the machine tools with the highest machining accuracy in the world are calibrated by people. In Japan, there are special personnel to calibrate the machining accuracy of machine tools. Such people are priceless in every machine tool manufacturer. According to this statement, it is possible for us to use parts with relatively low accuracy to assemble machine tools with amazing machining accuracy.