9 Reasons Affecting the Accuracy of CNC Machine Tools (Analysis and Solutions)

In this post, we will analyze 9 different reasons which affect the accuracy of the CNC machine, and the related solutions will also be provided accordingly.

Let’s dive into it.

Reasons Affecting The Accuracy Of CNC Machine Tools

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

Cause of failure:

1. The tool tip is damaged and not sharp;

2. The machine tool resonates and is placed unsteadily;

3. The machine tool crawls; Poor processing technology.

Solution:

1. If the tool is not sharp after being worn or damaged, regrind the tool or select a better tool to set the tool again;

2. If the machine tool resonates or is not placed stably, adjust the level, lay the foundation and fix it stably;

3. The reasons for the creeping of the machine are the severe wear of the carriage guide rail and the wear or looseness of the lead screw ball. The machine tool should be maintained.

4. After going to and from work, the iron wire should be cleaned and lubricating oil should be added in time to reduce friction;

Select the coolant suitable for workpiece processing, and try to select a higher spindle speed when it can meet the processing requirements of other processes.

Reasons Affecting The Accuracy Of CNC Machine Tools

2. Large and small taper head of workpiece

Cause of failure:

1. The placement level of the machine tool is not adjusted well, one high and one low, resulting in unstable placement;

2. When turning a long shaft, the contributing material is relatively hard and the cutting tool is relatively deep, resulting in the phenomenon of cutting;

3. The ejector pin of tailstock is not concentric with the main shaft.

Solution:

1. Use the level gauge to adjust the levelness of the machine tool, lay a solid foundation, fix the machine tool and improve its toughness;

2. Select a reasonable process and appropriate cutting feed to avoid the tool from being forced to yield;

3. Adjust the tailstock.

3. The phase light of the driver is normal, while the size of the processed workpiece is large and small.

Cause of failure:

1. The machine tool carriage runs at high speed for a long time, resulting in the wear of screw rod and bearing;

2. The repeated positioning accuracy of the tool holder has deviation in long-term use;

3. The carriage can accurately return to the processing starting point every time, but the size of the processed workpiece still changes.

4. 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:

1. Lean the dial indicator against the bottom of the tool holder and edit a fixed cycle program through the system to check the repeated positioning accuracy of the carriage, adjust the screw clearance and replace the bearing;

2. Check the repeated positioning accuracy of the tool holder with a dial indicator, adjust the machinery or replace the tool holder;

3. Use a dial indicator to check whether the processed workpiece returns to the starting point of the program accurately.

If it is possible, overhaul the spindle and replace the bearing.

Reasons Affecting The Accuracy Of CNC Machine Tools

4. The size of the workpiece differs from the actual size by a few millimeters, or there is a great change in an axial direction.

Cause of failure:

1. The speed of fast positioning is too fast, and the drive and motor cannot react;

2. After long-term friction loss, the mechanical carriage screw rod and bearing are too tight and stuck;

3. The tool holder is too loose and cannot be locked after changing the tool;

4. The editing program is wrong, and the head and tail end are not echoed or the cutter compensation is not cancelled;

5. The electronic gear ratio or step angle of the system is set incorrectly.

Solution:

1. If the fast positioning speed is too fast, properly adjust the speed of go, cutting acceleration and deceleration speed and time to make the driver and motor work normally under the rated operating frequency;

2. If the carriage and screw crane bearings are too tight and stuck after the wear of the machine tool, they must be readjusted and repaired;

3. 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;

4. If it is caused by the program, it is necessary to modify the program, improve it according to the requirements of the workpiece drawing, select a reasonable processing technology, and write the correct program according to the instructions of the manual;

5. If the size deviation is too large, check whether the system parameters are set reasonably, especially whether the parameters such as electronic gear and step angle are damaged.

6. If this phenomenon occurs, it can be measured by playing a dial indicator.

5. The effect of machining circular arc is not ideal, and the size is not in place.

Cause of failure:

1. The overlapping of vibration frequencies leads to resonance;

2. Processing technology;

3. The parameter setting is unreasonable and the feed speed is too large, which makes the arc machining out of step;

4. Looseness caused by large screw clearance or out of step caused by too tight screw;

5. The synchronous belt is worn.

Reasons Affecting The Accuracy Of CNC Machine Tools

Solution:

1. Find out the components that produce resonance and change their frequency to avoid resonance;

2. Consider the processing technology of workpiece materials and reasonably prepare procedures;

3. For stepper motor, the processing rate f cannot be set too large;

4. Whether the machine tool is firmly installed and placed stably, whether the carriage is too tight after wear, the gap increases or the tool holder is loose;

5. Replace the timing belt.

6. In batch production, the workpiece is out of tolerance occasionally.

Cause of failure:

1. The tooling and fixture must be carefully checked, and considering the operation method of the operator and the reliability of the fixture, the tooling must be improved due to the dimensional change caused by the fixture, so that the workers can avoid misjudgment due to human negligence as far as possible;

2. The numerical control system may automatically generate interference pulse after being affected by the fluctuation of external power supply or interference, which is transmitted to the drive, causing the drive to accept redundant pulse and drive the motor to take away or leave less.

Solution:

1. Understand and master the rules, and try to take some anti-interference measures, such as isolating the strong current cable interfered by strong electric field from the signal line of weak current signal, adding anti-interference absorption capacitance and using shielded wire for isolation;

2. In addition, check whether the ground wire is firmly connected and the grounding contact is nearest.

3. Take all anti-interference measures to avoid interference to the system.

7. The processing of one process of the workpiece changes, and the dimensions of other processes are accurate.

Cause of failure:

Whether the program parameters of the program section are reasonable, whether they are within the predetermined track, and whether the programming format meets the requirements of the manual

Solution:

When the thread program section has disordered teeth and wrong pitch, it is immediately associated with the peripheral configuration (encoder) of processing thread and the objective factors of this function.

8. Each working procedure of the workpiece has the phenomenon of increasing or decreasing.

Cause of failure:

1. Programming error; Unreasonable setting of system parameters;

2. Improper configuration and setting;

3. The mechanical transmission parts have regular and periodic change faults.

Solution:

1. Check whether the instructions used in the program are executed according to the required track specified in the manual.

2. You can judge by playing the dial indicator.

3. Locate the dial indicator at the starting point of the program, and let the carriage return to the starting point after the program is completed, and then repeat the execution, even if you observe the results and master the law;

4. Check whether the system parameters are set reasonably or considered to be changed;

5. Whether the single calculation on the coupling parameters of connection calculation of relevant machine tool configuration meets the requirements and whether the pulse equivalent is accurate;

6. Check whether the transmission part of the machine tool is damaged, whether the gear coupling is uniform, and whether there are periodic and regular faults.

If so, check the key parts and eliminate them.

Reasons Affecting The Accuracy Of CNC Machine Tools

9. The dimensional change caused by the system is unstable

Cause of failure:

1. Unreasonable system parameter setting;

2. Unstable working voltage; The system is subject to external interference, resulting in system out of step;

3. The capacitor has been added, but the impedance between the system and the driver does not match, resulting in the loss of useful signals;

4. The signal transmission between the system and the driver is abnormal;

5. System damage or internal failure.

Solution:

1. Whether the speed and acceleration time are too large, whether the spindle speed and cutting speed are reasonable, and whether the operator’s parameter modification leads to the change of system performance;

2. Install voltage stabilizing equipment;

3. The grounding wire shall be reliably connected, and anti-interference absorption capacitance shall be added at the pulse output contact of the driver;

4. Select appropriate capacitor model;

5. Check whether the signal connecting line between the system and the driver is shielded, whether the connection is reliable, and whether the system pulse generation signal is lost or increased;

6. Send to the factory for repair or replacement of the main board.

Reasons Affecting The Accuracy Of CNC Machine Tools

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