What should I do if the size of CNC machining is unstable?

Knowledge sharing by Guangdong Shunde Teamwork Model Technology Co., Ltd, whom with over 20 years rapid prototyping experience.

cause of issue:

1) The tool tip is damaged and not sharp

2) The machine tool generates resonance and is placed unevenly

3) Mechanical crawling

4) The processing technology is not good

Solution (with comparison):

1) If the tool is not sharp after being worn or damaged, re-grind the knife or choose a better tool to re-tool

2) The machine tool generates resonance or is not placed smoothly, adjusts the level, lays the foundation, and fixes it smoothly.

3) The reason why the machine generates crawling is that the carriage guide rail is worn hard, and the screw ball is worn or loose. The machine tool should pay attention to maintenance. After going to work, the wire should be cleaned and the lubricant should be added in time to reduce friction.

4) Select the coolant suitable for the machining of the workpiece; if it can meet the processing requirements of other processes, try to use a higher spindle speed

2. The workpiece produces a taper size

cause of issue:

1) The level of machine tool placement is not adjusted well, one high and one low, resulting in uneven placement

2) When turning the long axis, the workpiece material is relatively hard, and the tool is deeper, causing the knife phenomenon.

3) The tailstock thimble is not the same as the spindle

solution

1) Use the level to adjust the level of the machine, lay a solid foundation, and fix the machine to improve its toughness.

2) Choose a reasonable process and appropriate cutting feed to avoid the tool force to make the knife

3) Adjust the tailstock

3. The driver phase lamp is normal, and the processed workpiece size is large and small.

cause of issue

1) Machine tool pallets run at high speed for a long time, causing wear on the screw and bearing

2) The repeat positioning accuracy of the tool holder is deviated in long-term use.

3) The carriage can return to the machining starting point accurately every time, but the size of the machined workpiece still changes. This phenomenon is generally caused by the main shaft, and the high-speed rotation of the main shaft causes the bearing to wear seriously, resulting in a change in the machining size.

Solution (with comparison)

1) Use the dial gauge to lean against the bottom of the tool holder, and edit a fixed cycle program through the system, check the repeat positioning accuracy of the carriage, adjust the screw clearance, and replace the bearing.

2) Check the repeatability of the tool holder with a dial indicator, adjust the machine or replace the tool holder

3) Use a dial indicator to check whether the workpiece can be accurately returned to the starting point of the program after processing the workpiece; if possible, repair the spindle and replace the bearing

4. The workpiece size differs from the actual size by a few millimeters, or a certain axial change

cause of issue

1) The speed of rapid positioning is too fast, and the drive and motor reaction are not generated.

2) After long-term friction and wear, the mechanical lead screw and bearing are too tightly stuck

3) The knife holder is too loose after the tool change

4) The program error of editing ends, the head and tail do not respond or end without canceling the tool.

5) The system's electronic gear ratio or step angle setting is incorrect.

Solution (with comparison)

1) If the rapid positioning speed is too fast, adjust the speed of G0, cutting acceleration and deceleration and time to make the drive and motor operate normally at the rated operating frequency.

2) After the machine wear occurs, the carriage, the screw and the bearing are too tightly stuck, and the repair must be re-adjusted.

3) If the tool holder is too loose after the tool change, check whether the tool holder reversal time is satisfied, check whether the turbine vortex inside the tool holder is 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 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 dimensional deviation is too large, check whether the system parameters are set properly, especially if the parameters such as the electronic gear ratio and the step angle are damaged. This phenomenon can be measured by playing the meter.

5. The processing arc effect is not ideal, the size is not in place

cause of issue

1) The overlap of the vibration frequencies causes resonance

2) Processing technology

3) The parameter setting is unreasonable, the feed rate is too large, and the arc machining is out of step.

4) Looseness caused by large screw clearance or out of step caused by excessive screw tightening

5) Timing belt wear

solution

1) Find the part that produces resonance, change its frequency, avoid resonance

2) Consider the processing technology of the workpiece material and make a reasonable program

3) For stepper motors, the processing rate F cannot be set too large

4) Whether the machine tool is installed firmly, placed stably, the pallet is too tight after wear, the gap is increased or the tool holder is loose, etc.

5) Replace the timing belt

6. Occasionally, the workpiece is out of tolerance in mass production.

1) Occasionally, there is a change in the size of the mass production, and then the processing is resumed without modifying any parameters.

2) Occasionally, a size is inaccurate in mass production, and then the processing size is still unsatisfactory, and it is accurate after re-aligning the knife.

solution

1) The fixture must be carefully inspected, taking into account the operator's method of operation, and the reliability of the clamping; due to the dimensional changes caused by the clamping, the tooling must be improved to minimize the misjudgment of the worker.

2) The numerical control system may be subject to fluctuations or interference from external power sources, and automatically generate interference pulses, which are transmitted to the drive to cause the drive to accept excess pulse-driven motor to walk more or less; understand the rules and try to use some anti-interference measures. For example, a strong electric cable with strong electric field interference is isolated from a signal line of a weak electric signal, and an anti-interference absorption capacitor is added and isolated by a shielded wire. In addition, check whether the ground wire is firmly connected, the grounding contact is the closest, take all anti-interference measures to avoid system interference.

7. There is a change in the processing of one part of the workpiece, and the other processes are accurate in size.

cause of issue

Whether the parameters of the block program are reasonable, whether it is within the predetermined track, and whether the programming format meets the requirements of the specification.

solution

When there is a chaotic tooth in the thread block, the pitch is wrong, it immediately associates with the peripheral configuration of the machining thread (encoder) and the objective factors of the function, such as: spindle speed, thread lead and feed speed (928 TC with DY3) When the thread is machined, the spindle speed X thread lead is ≤1700 mm/min), whether the number of encoders is consistent with the computer setting; when the size of the circular block is found to be incorrect, check whether the programming track of the arc is on the same arc. , is there any special relationship between the circle and the circle?

8. Each process of the workpiece has an increasing or decreasing phenomenon

cause of issue

1) Program writing error

2) System parameter settings are unreasonable

3) Improper configuration settings

4) Mechanical transmission components have regular periodic changes

solution

1) Check whether the instructions used by the program are executed according to the required trajectory specified in the manual. You can judge by dialing the hundred tables. Position the dial indicator at the starting point of the program to let the board return to the starting position after the program ends, and then repeat the execution. Observe the results and master the rules

2) Check if the system parameters are set properly or artificially changed

3) Whether the calculation of the relevant machine tool configuration on the connection calculation coupling parameters meets the requirements, and whether the pulse equivalent is accurate

4) Check whether the transmission part of the machine tool is damaged or not, and whether the gear coupling is uniform, check whether there is periodicity or regular failure. If there are, check the key parts and give them exclusion

9. The workpiece size is only a few different from the actual size.

cause of issue

1) The machine tool is rubbed and worn during long-term use. As the clearance of the screw increases, the backlash of the machine tool is too large, so that the size of the machining process floats, so the error of the workpiece always changes within the gap.

2) The tool used for machining the workpiece is incorrectly selected, vulnerable, and the tool is not clamped or not tight.

3) Process selection of reasonable spindle speed, cutting feed rate and cutting amount according to the workpiece material

4) related to the balance and stability of the machine tool placement

5) When the CNC system generates out-of-step or drive selection, the power is insufficient, the torque is small, etc.

6) Is the lock locked after the tool change?

7) Whether the spindle has a beating motion and a poor coaxiality of the tailstock

8) In some special processing occasions, the backlash cannot be filled in, resulting in always deviation in processing.

Solution (with comparison)

1) After the machine tool wear screw gap becomes larger, reduce the gap by adjusting the screw nut and tightening the middle carriage line, or obtain the gap value by the dialing table (the general clearance is within 0.15 mm), which can be replenished into the computer. Replace the gap with the gap compensation function of the computer to make the workpiece size meet the requirements

2) Since the tool material changes the size of the machined workpiece, the tool is reasonably selected according to the requirements, and the tool angle and the fixture are reasonably selected according to the process requirements of the workpiece due to the incorrect tool clamping.

3) When it is suspected that it is a process problem in processing, according to the nature of the material, the machining process is reasonably programmed to select the appropriate spindle speed, cutting feed rate and cutting amount.

4) Due to the resonance of the machine tool, the machine tool is placed stably, the level is adjusted, the foundation is laid down if necessary, and the installation is stable.

5) The dimensional change produced by the CNC system, first determine whether the program is programmed according to the size requirements of the paper, and then check whether the set parameters are reasonable according to the selected configuration (such as: G0 fast positioning speed and acceleration/deceleration time constant during cutting). Whether someone deliberately changes, and secondly, consider whether the selected driver power is reasonable, and judge whether the pulse sent by the computer to the driver is out of step by judging the phase lamp.

6) Check that the tool holder has a reversal time after the tool change. Is there enough time for the tool holder to lock, check the positioning of the tool holder and loosen the locking screw?

7) Check whether the coaxiality of the main shaft and the tailstock are pulsating or striking.

8) Use programming skills to eliminate gaps

10. The drive causes dimensional instability

cause of issue

1) The signal sent by the driver is lost, resulting in loss of drive

2) The servo drive parameters are not set properly and the gain factor setting is unreasonable.

3) The driver sends a signal interference, resulting in out of step

4) The drive is in a high temperature environment, and no good heat dissipation measures are taken, resulting in unstable dimensions, and may also cause changes in internal parameters of the drive, causing malfunctions.

5) Insufficient drive torque or insufficient motor torque

6) The drive current of the driver is not enough

7) Drive damage

Solution (corresponding to the above)

1) First determine whether to use stepper drive or servo drive: Stepper motor drive can judge whether there is out-of-synchronization by phase lamp or dial dial. The servo drive can be judged by the number of pulses on the drive or by the dial indicator.

2) Modify the gain parameters with reference to the DA98 manual.

3) Install shielded cable and install anti-interference capacitor

4) Guarantee a good heat dissipation and ventilation environment, the proper temperature is an important factor to ensure the processing performance

5) Replace the drive or motor to make the torque meet the actual needs

6) If the drive current is still not enough, the drive needs to be replaced.

7) Drive to the factory for repair

11. The dimensional change caused by the system is unstable

cause of issue

1) System parameter settings are unreasonable

2) The operating voltage is unstable

3) The system is subject to external interference, resulting in system out of step

4) The capacitor has been added, but the impedance between the system and the driver does not match, resulting in loss of useful signal.

5) The signal transmission between the system and the drive is incorrect.

6) System damage or internal failure

Solution (with comparison)

1) Whether the speed is fast, the acceleration time is too large, the spindle speed, the cutting speed is reasonable, whether the system performance changes due to the operator's parameter modification

2) Install voltage regulator equipment

3) Grounding wire and make sure that it is connected reliably. Add anti-interference absorption capacitor at the pulse output contact of the driver. Under normal circumstances, the interference of the inverter is large. Please judge with load, because the larger the load will be Let the inverter load current increase, the greater the interference generated

4) Select the appropriate capacitor model

5) Check whether the signal connection between the system and the driver is shielded, whether the connection is reliable, and check whether the pulse signal of the system is lost or increased.

6) Send the factory for repair or replace the motherboard

12. Mechanically unstable processing dimensions

cause of issue

1) Is the stepper motor damping plate too tight or too loose?

2) Insulation performance is reduced due to water in the motor plug, motor damage

3) The size of the workpiece is not properly clamped

4) The workpiece has an ellipse

5) The screw backlash is too large

6) The mechanical screw is installed too tightly

Solution (with comparison)

1) Adjust the damping disc to make the motor non-resonant

2) Replace the motor plug, protect it, or replace the motor

3) Check if the feed amount is too large or too fast, and check the workpiece clamping. The chuck should not extend too long to avoid the knife.

4) Check the spindle's runout, overhaul the spindle, and replace the bearing

5) Check the backlash of the screw by dialing the dial indicator, whether the clearance has been added from the system, and whether the clearance is too large after the filling

6) Check if there is crawling on the screw, if there is a slow response