Identifying the Five Main Causes of Machine Tool Inaccuracy and Solutions

For a machine tool, accuracy is the most important criterion for evaluating its quality. In fact, accuracy can be considered the essence of the machine tool. However, during actual operation, various factors can lead to a decrease in this accuracy, which can be quite frustrating. Today, we will discuss five situations that can cause a reduction in the accuracy of machine tools and their corresponding solutions.

 

Situation 1: The machine tool can process and the workpiece size is relatively accurate, but the surface finish of the workpiece is poor

There are generally four reasons for this situation:
The tip of the machine tool is dull; the machine tool resonates during operation; the machine tool exhibits a creeping phenomenon; and the processing technology is faulty.

Solution:
- If a tool is dull due to wear or damage, either resharpen the knife directly or select a better tool to realign it.

- The resonance of a machine tool is often caused by uneven placement. This can be resolved by adjusting the machine tool to a horizontal position and securing it firmly.

- Machine tool crawling is typically due to wear on the carriage guide rail or wear or loosening of the lead screw ball. This occurs when maintenance is insufficient. It is recommended to clean the lead screw and apply lubricating oil regularly to reduce friction. Increasing the frequency of machine tool maintenance is advised. For instance, conducting idle maintenance for 2 to 3 days is beneficial. If there is a high processing volume, it’s best to clean the lead screw daily and add lubricating oil. Do not be overly concerned about lubricating oil consumption, as its cost is much lower compared to the output.

- If there are defects during the machining process, ensure the spindle operates at the highest suitable speed to meet the requirements of other processes. Additionally, select a coolant that is more appropriate for the workpiece being machined.

Situation 2: The workpiece produced has different taper sizes

There are generally 3 reasons for this:
- The machine tool is not positioned correctly, resulting in an unstable and non-horizontal placement.
- When rotating the long axis, the workpiece material is quite hard, causing the tool to dig in too deeply and leading to potential damage to the tool.
- The tailstock ejector pin is not aligned with the spindle.

Solution:

- Use a spirit level to ensure the machine tool is properly leveled. Establish a solid foundation for the machine tool and secure it in place to enhance its stability.
- Select an appropriate processing method and cutting feed rate to minimize the stress on the tool.
- Adjust the position of the tailstock so that its ejector pin aligns perfectly with the spindle.

 

Situation 3: The driver phase light is normal, but the size of the processed workpiece is different, which causes this situation to be generally caused by three reasons:

- The long-term high-speed operation of the machine tool carriage leads to significant wear on the screw rod and bearings.
- There is a deviation in the tool holder after prolonged processing and use.
- Although the carriage can accurately return to the starting point of machining each time, the dimensions of the machined workpiece still show deviations.

Solution:
- Use a dial gauge to press against the bottom of the tool holder. Simultaneously, create a fixed cycle program through the system to verify the repeated positioning accuracy of the carriage. Adjust the screw clearance as needed, and replace the bearing if necessary.
- Check the repeatability of the tool holder using the dial gauge. Adjust the machinery or replace the tool holder if needed.
- Use the dial gauge to ensure that the workpiece accurately returns to the starting point of the program after machining. If there are any discrepancies, consider overhauling the spindle and replacing the bearing.

Situation 4: There are generally 5 reasons for the difference between the size of the workpiece and the actual size by a few millimeters, or the huge change in a certain axial direction:

- The positioning speed is too fast, causing the drive and motor to be unable to react in time.
- After prolonged friction, the carriage screw and bearings of the CNC machinery parts have become too tight and are stuck.
- The knife holder is loose, and the locking mechanism is not secure.
- There is an error in the editing program, resulting in the head and tail not aligning or the knife repair function not canceling properly.
- The system’s electronic gear ratio or step angle is incorrectly set.

Solution:
- If the rapid positioning speed is too high, adjust the G0 speed accordingly, as well as the cutting acceleration and deceleration rates and times, to ensure that the driver and motor operate normally within their rated operating frequency.
- If the carriage, screw, and bearings become too tight or stuck due to wear on the machine tool, they may need to be readjusted or repaired.
- If the tool holder is too loose after replacement, check whether the tool holder’s reversal time matches the specific tool. Also, inspect the worm gear and worm for wear and clearance to confirm proper installation.
- If the issue is caused by the program, it must be modified and improved according to the specifications in the workpiece drawing. Select a suitable processing technology and write the correct program following the manual’s instructions.
- If there is a significant size deviation, verify that the system parameters are set correctly. Pay particular attention to parameters such as electronic gears and step angles, as any damage can be assessed using a dial gauge.

 

Situation 5: The processing arc effect is not ideal, and the size is not in place, which causes this situation to be generally 5 reasons:

- Resonance occurs due to the overlap of vibrational frequencies.
- The processing technology used is inadequate.
- The parameter settings are inappropriate, and the feed speed is excessive, causing the arc processing to fall out of synchronization.
- Loosening can occur due to a significant gap in the screw rod; alternatively, loss of synchronization may result from the screw rod being overly tight.
- The timing belt may be worn.

Solution:

- Identify the components that are causing resonances, adjust their frequencies, and avoid these issues.
- Enhance the processing technology for CNC components materials and carefully develop efficient procedures.
- For stepper motors, avoid setting the processing rate too high.
- Ensure that the machine tool is securely installed, positioned horizontally, and leveled properly. Additionally, address any issues such as excessive tightness from carriage wear, increasing gaps, or looseness in the tool holder.
- Replace the timing belt directly if it shows signs of wear.

Before repairing and maintaining the machine tool, it is essential to thoroughly observe its specific condition. Understanding the machine’s issues completely and comprehensively is crucial before proceeding with any actions. Failing to do so may not only lead to production and processing difficulties but could also compromise the safety of the operators. Therefore, gathering detailed information about the machine tool is the first step in resolving its problems.

 

 

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