9 Major Causes of Unstable CNC Lathe Machining Precision

CNC machine tools are fast, efficient, and cost-effective production tools. They are easy to operate, streamline processes, and significantly reduce labor waste. However, internal or external factors can sometimes cause unstable machining precision, making troubleshooting more difficult. CNC lathe manufacturers have summarized nine causes of unstable CNC lathe machining precision to help you address them.

1. Accurate workpiece dimensions, poor surface finish

Causes: Damaged or blunt tool tip; machine vibration and unstable positioning; machine creep; poor machining process.

Solution: If the tool is worn or damaged and not sharp, resharpen it or select a better tool for recalibration. If the machine tool resonates or is unstable, adjust the level, lay a solid foundation, and secure it securely. Machine creep can be caused by severe wear on the carriage guides or wear or looseness on the ball screw. The machine tool should be maintained regularly, with the wire cleaned between shifts and lubricated to reduce friction. Choose a coolant suitable for the workpiece being machined. While meeting the requirements of other processes, try to use a higher spindle speed.

2. Workpiece Taper

Cause: The machine tool is not properly leveled, with one side higher than the other, resulting in unstable positioning. When turning long shafts, the material is relatively hard, requiring the tool to engage deeply, causing tool deflection. The tailstock thimble is not concentric with the spindle.

Solution: Use a spirit level to adjust the machine tool’s level, lay a solid foundation, and secure the machine tool to improve its toughness. Choose a suitable process and appropriate cutting feed rate to prevent tool deflection. Adjust the tailstock.

3. The driver phase indicator is normal, but the machined workpiece size fluctuates.

Cause: The machine tool carriage has been operating at high speed for extended periods, causing wear on the lead screw and bearings. The toolholder’s repeatability has deteriorated over time. The carriage accurately returns to the starting point each time, but the workpiece dimensions still vary. This phenomenon is generally caused by the spindle, whose high-speed rotation causes severe bearing wear, leading to dimensional variations. (This section discusses metalworking and is worth noting.)

Solution: Use a dial indicator to rest against the bottom of the toolholder. Simultaneously, program a fixed cycle in the system to check the carriage’s repeatability, adjust the lead screw clearance, and replace the bearings. Use a dial indicator to check the toolholder’s repeatability, adjust the mechanism, or replace the toolholder. Use a dial indicator to verify that the workpiece returns accurately to the program starting point after machining. If so, inspect the spindle and replace the bearings.

4. The workpiece dimensions differ by several millimeters from the actual dimensions, or there is a significant change in one axis.

Causes: Rapid positioning is too fast, preventing the drive and motor from responding; the mechanical carriage lead screw and bearings are overtightened and stuck due to long-term friction wear; the tool holder is too loose after a tool change and does not lock securely; the program is incorrectly edited, with the start and end points not aligning, or tool compensation is terminated without canceling; the system’s electronic gear ratio or step angle is incorrectly set.

Solution: If the rapid positioning speed is too fast, adjust the GO speed, cutting acceleration and deceleration, and time appropriately to ensure the drive and motor operate normally at the rated operating frequency. If the machine tool wears and the carriage, lead screw, or bearings become stuck due to overtightening, they must be readjusted and repaired. If the tool holder is too loose after a tool change, check whether the tool holder reversal time is sufficient, check the worm gear inside the tool holder for wear, excessive clearance, or loose installation. If the cause is a programming issue, modify the program according to the workpiece drawing requirements, select a suitable machining process, and write the correct program according to the instructions in the manual. If dimensional deviation is excessive, check whether the system parameters are set correctly, especially whether parameters such as the electronic gear and step angle are corrupted. This can be measured with a dial indicator.

5. Unsatisfactory arc processing results and dimensions are not in place. Small and medium-sized CNC lathes

Causes: Overlapping vibration frequencies leading to resonance; machining process; improper parameter settings; excessive feed rate causing arc processing to lose steps; looseness due to excessive lead screw clearance or loss of steps due to overtightening; worn timing belt.

Solution: Identify the component causing resonance and change its frequency to avoid it; consider the workpiece material’s processing technology and develop a reasonable program; for stepper motors, do not set the machining speed F too high; ensure the machine tool is securely installed and positioned stably; check if the carriage is worn, tight, has increased clearance, or is the tool holder loose; and replace the timing belt. Good content on metalworking, worth checking.

6. Occasionally, workpieces may be out of tolerance during mass production.

Cause: The tooling must be carefully inspected, taking into account the operator’s operating method and the reliability of the clamping. Due to dimensional changes caused by clamping, the tooling must be improved to minimize errors caused by human error. The CNC system may be affected by external power supply fluctuations or interference, automatically generating interference pulses. These pulses are then transmitted to the driver, causing the driver to receive excess pulses and drive the motor to lose or reduce power.

Solution: Understand the underlying mechanisms and implement anti-interference measures as much as possible. For example, isolate high-voltage cables that are susceptible to strong electric field interference from weak-voltage signal cables, add anti-interference absorption capacitors, and use shielded cables for isolation. Additionally, check that the ground wire is securely connected and that the ground contact is closest. Take all anti-interference measures to prevent system interference.

7. The workpiece has variations during one machining process, while the dimensions of all other processes are accurate.

Cause: Are the parameters in the program segment correct? Are they within the predetermined trajectory? Does the programming format comply with the manual?

Solution: If the threading program segment has misaligned threads or incorrect pitch, immediately consider the peripheral configuration (encoder) of the thread machining process and the objective factors of the function.

8. The workpiece shows incremental or decremental changes at each process step.

Causes: Programming errors; improper system parameter settings; improper configuration settings; mechanical transmission components exhibiting regular, periodic changes.

Solution: Check whether the program instructions are executed according to the specified trajectory specified in the manual. This can be verified by using a dial indicator. Position the dial indicator at the program’s starting point to see if the carriage returns to the starting position after the program completes. Repeat the program and observe the results to identify patterns. Check whether the system parameters are set appropriately or have been modified. Check whether the relevant machine tool configuration meets the requirements for connection and coupling parameters, and whether the pulse equivalent is accurate. Check the machine tool’s transmission components for damage and gear coupling. Check for periodic or regular faults. If present, inspect the key components and eliminate them.

9. System-induced dimensional instability.

Causes: Improper system parameter settings; unstable operating voltage; external interference causing the system to lose step; capacitors installed, but impedance mismatch between the system and the driver, resulting in loss of useful signals; abnormal signal transmission between the system and the driver; system damage or internal fault.

Solution: Check whether the speed and acceleration time are too large, whether the spindle speed and cutting speed are reasonable, whether the operator’s parameter modification has caused the system performance to change; install voltage stabilizing equipment; make sure the ground wire is reliably connected, add anti-interference absorption capacitors at the driver pulse output contacts; select the appropriate capacitor model; check whether the signal connection line between the system and the driver is shielded and the connection is reliable; check whether the system pulse generation signal is lost or increased; send to the factory for repair or replace the motherboard