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Common Faults and Analysis of Simple CNC Lathe (Part 2)

Sep. 19, 2020

5. Offside when tool returns to zero

Generally it is caused by too much running resistance of the mechanical transmission system. When cutting feed, the tool post runs at low speed, is driven by low voltage, and the stepper motor has a small running torque, which is not enough to overcome resistance and cause lost steps. When returning to zero, the stepper motor is driven by high voltage, with high running speed, large torque, and no cutting resistance, so the stepper motor does not lose steps. In this way, losing steps when going and returning to normal will cause the phenomenon of not returning to zero. At this time, you can check whether there is any iron filings or foreign matter on the transmission gear in the stepper motor reducer or the transmission gear between the stepper motor and the lead screw, or whether the slide plate is too tight to increase the running resistance.

6. The size of the workpiece after processing has a large error

One possibility is that the screw or nut is loosely connected to the lathe machine. There is no tool resistance during idling, and the slide plate operates normally. Due to the increase in cutting resistance during processing, the connection between the lead screw or nut and the lathe becomes loose, causing the size of the workpiece to drift. The fault can be eliminated by tightening the connection part. Another possibility is caused by the electric tool post. If the tool post cannot be automatically locked after the tool is changed, and the tool deviates from the machining point during cutting, the above phenomenon will also be caused. At this time, check the knife holder locking device and the knife holder control box.

Lathe Machine

Lathe Machine

7. Large part size error of workpiece

Mainly caused by the excessive clearance between the nut and the screw. Because the screw nut and the lead screw run in a certain section for a long time, the gap between this section is increased. At the beginning of the program, the measured screw clearance is compensated into the program, but it cannot be compensated in the wear section, so that the partial size of the workpiece is out of tolerance. The solution is to repair or replace the lead screw.

8. The electric tool post cannot be positioned and rotated when changing tools

This is because when the program asks for a certain tool, the electric tool post is transferring the tool. When the tool is rotated to the tool, there is no response signal, so that the tool post cannot be rotated and cannot be positioned. The Hall element on the electric tool post should be checked. When the Hall element is damaged, the signal output will not be detected when the tool is in place, causing the above phenomenon. Just replace the Hall element of the knife.

9. Return to monitoring status and stop work during program execution

It is usually caused by a failure of the monitoring program or strong magnetic interference. The strong magnetic interference can be solved by grounding or shielding. If the program is not executed or the execution instruction is not followed when the program is started, it will immediately return to the monitoring state. Generally, the monitoring program or computer hardware fails, and the suspicious chip can be replaced, such as the off-chip program memory chip, the programmable interface chip or the single-chip microcomputer itself. Sometimes an off-chip data memory failure can also cause this phenomenon. Otherwise, we have to find the manufacturer to re-debug.

10. Processing programs are often lost

If the machining program is lost after the control system is powered off, and the machining program is re-entered after the machine is powered on, the machine can process normally, it may be that the backup battery voltage is reduced or disconnected, causing the machining program in the data memory to not be maintained after the machine is powered off And lost. Just replace the backup battery. If the processing program is often partly or completely lost during processing, it is most likely that the data memory is faulty. At this time, the off-chip data memory or the microcontroller itself can be replaced.

In short, the simple CNC lathe is composed of mechanical and electrical parts. After a failure occurs, it must be analyzed from both mechanical and electrical aspects to determine whether it is a mechanical failure or an electrical failure, and then in-depth analysis to find the fault point.

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