What is the reason for the dimensional instability of NC milling machine?

In the maintenance of NC milling machine, fault descriptions such as "inaccurate machining size", "incorrect machining size" and "dimensional deviation" are often heard (described as "inaccurate machining size" below), which are generally accidental faults. When the maintenance service personnel encounter such problems, they replace the motherboard, upgrade the software or replace the whole machine. They do not find the root cause of the fault. Other products encounter such faults and cannot provide reference.

1. The workpiece size is accurate and the surface finish is poor

Fault cause: the tool tip is damaged and not sharp; The machine tool resonates and is placed unsteadily; The machine tool crawls; Poor processing technology.

Solution: if the tool is not sharp after being worn or damaged, regrind the tool or select a better tool to set the tool again; If the machine tool resonates or is not placed stably, adjust the level, lay the foundation and fix it stably; The causes of mechanical crawling are severe wear of carriage guide rail and wear or looseness of lead screw ball. Pay attention to the maintenance of the machine tool. Clean the iron wire after work and add lubricating oil in time to reduce friction.

2. The phase light of the driver is normal, and the size of the machined workpiece is large and small

Fault cause: the machine tool carriage runs at high speed for a long time, resulting in the wear of screw rod and bearing; The repeated positioning accuracy of the tool holder has deviation in long-term use; The carriage can accurately return to the processing starting point every time, but the size of the processed workpiece still changes. This phenomenon is generally caused by the spindle. The high-speed rotation of the spindle makes the bearing wear seriously, resulting in the change of machining size.

Solution: lean the dial indicator against the bottom of the tool holder, edit a fixed cycle program through the system, check the repeated positioning accuracy of the carriage, adjust the screw clearance, and replace the bearing; Check the repeated positioning accuracy of the tool rest with a dial indicator, adjust the machinery or replace the tool rest.

3. The difference between the workpiece size and the actual size is a few millimeters or the axial change is large

Fault cause: the speed of fast positioning is too fast, and the drive and motor cannot react; After long-term friction loss, the mechanical carriage screw rod and bearing are too tight and stuck; The tool holder is too loose and cannot be locked after changing the tool; The electronic gear ratio or step angle of the system is set incorrectly.

Solution: the fast positioning speed is too fast. Properly adjust the go speed, cutting acceleration and deceleration speed and time to make the driver and motor work normally under the rated operating frequency. If the carriage and screw crane bearing are too tight and stuck after the machine tool is worn, it needs to be readjusted and repaired. If the tool holder is too loose after tool change, check whether the reversing time of the tool holder is met, whether the worm and worm inside the tool holder are worn, whether the clearance is too large, whether the installation is too loose, etc. If it is caused by the program, it is necessary to modify the program, improve it according to the requirements of the workpiece drawing, and write the correct program according to the instructions of the manual.