Beginning of article
I would like to ask you whether the dimensions of the parts recently processed by the CNC lathe you are operating often exceed the tolerance range. Is the smoothness of its surface not as good as before? Or worse, the machine tool will occasionally make abnormal noises and then return to normal on its own. These problems may seem scattered, but their roots often lie in the same place. This article will use my first-person perspective to guide you step by step to discover where the problem lies, how to repair it, and how to prevent such problems from happening again after the repair is completed. Let’s leave aside those arcane control theories and focus on practical situations you encounter every day.
1. Stop for a moment and don’t rush to change the tool.
You realize that the first part is out of tolerance, what should you do? My advice is don't replace the blade right away. Replacing the blade is the simplest operation, but it cannot solve the underlying problem. You need to conduct a comprehensive visual inspection first. Shine a flashlight through the spindle chuck to see if there are any iron filings. Check the tool holder on the tool turret for signs of looseness. Check the direction of the coolant nozzle to see if the chips are flushed back into the cutting area.
Many times, I encountered what was called an accuracy problem, but was actually chips getting wrapped around the workpiece. One of the common situations is that errors occur after a shift change, and the new operator does not pay attention to the chip pile left by the previous shift. This inspection only takes thirty seconds. If you save these thirty seconds, it may take an additional thirty minutes to perform the knife calibration again.
2. The cold engine state is different from the hot engine state
After turning on the phone in the morning, have you done a warm-up procedure? Let me ask you. If not, then the data on the first twenty parts cannot be trusted. The spindle bearings of CNC lathes will physically expand when the temperature changes. The screw will also physically expand when the temperature changes. This amount of expansion is fatal to precision parts.
Before proceeding with formal processing, what should be done first is to write a simple heat engine program. This program requires the spindle to run back and forth within a common speed range, and also enables each axis to reciprocate, and lasts about five minutes. Only when the temperature of the equipment stabilizes can the formal processing be started. You may think this is a waste of time, but if you carefully calculate the time required to rework a part, you will find that five minutes is actually quite cost-effective. The heat engine step can directly solve about 30% of the random accuracy problem.
3. Use your tool setter
Many operators are reluctant to use tool setters and think that manual tool setting is more direct. Let me tell you why this is wrong. Manual tool setting relies on your feeling, but the feeling is unstable. You didn't sleep well late last night and felt sleepy and tired. Your feeling has changed. You drank coffee during lunch and your feeling has changed again.
The tool setting instrument gives a stable value. All you need to do is to clean the knife before setting it. Use an airflow tool to blow the tool handle and the knife sheath clean. This action must form a muscle memory. Every time you install a knife, whether it is a brand new knife or a regrinded knife, it must go through The reason why the tool setter works like this is because you don’t know the level of the knife sharpener. A common situation is that the blades are ground externally. There is a difference of 0.05 millimeters in the height of the tool tip between the two batches. If no measurement is performed, this error will be directly transferred to the parts.
4. The material is not something you can control, but the process is
Hello, you have received a new batch of materials. This batch of new materials looks the same as the previous batch, but in fact the hardness may be different, and the internal stress distribution may also be different. So how should we deal with this situation? My response is to reduce the depth of cut and feed. When using the original program to process new materials, leave enough margin for the first part to be able to make adjustments.
The following are the specific steps. First, reduce the depth of cut by 30% and reduce the feed by 20%. Measurements are taken after processing is complete. If the size is normal, it means there is not much difference between the new material and the old material. If the dimensions are abnormal, you have to readjust the tool compensation value in the program. Don't expect to get it right the first time. I am used to making 20% compensation each time and adjusting it in five times. In this way, even if it is over-adjusted, it will not hit the knife.
When it comes to adjusting the compensation value, this leads to a very common technique in CNC torno diagnostics, which is to observe the wear trend. If the compensation value slowly changes in the same direction every day, it indicates normal tool wear. If the compensation value suddenly jumps, there must be something wrong somewhere, such as the material has changed, or the coolant concentration is inappropriate.
5. The secret of coolant
Cooling is not the only purpose of coolant. It is responsible for chip removal, lubrication, and rust prevention. Do you remember the last time you checked the coolant concentration? Most people can't tell. If the coolant concentration is too low, tool wear will accelerate. Once the concentration is too high, too much foam will be produced, which will affect the observation.
What to do? Buy a refractometer, then turn on the machine to measure once a day, record the measurement results and archive them. When the concentration changes by more than 5%, you need to make adjustments. A common situation is that the evaporation rate is fast in the summer, and the concentration will naturally rise. Many people mistakenly believe that as long as the concentration reaches the standard, they don’t need to worry about it. In fact, after the water evaporates, the additives will also produce precipitation. In this case, pure water needs to be added according to the corresponding proportion, rather than adding water randomly.
6. Don’t be superstitious about original factory parameters.
The parameter table given when the equipment is launched on the market is measured under ideal conditions. And the workshop you're in isn't ideal. Temperature and humidity vary within a workshop, and the foundation's shock-absorbing properties vary. So those parameters can only be used as a starting point, not as the final point.
You have to find out the processing parameters that suit your workshop by yourself, so how should you find out? Three sets of comparative tests are to be done. The first set uses 80% of the factory data, the second set uses 100%, and the third set uses 120%. Process five parts respectively, record the surface quality and dimensional stability, and then select the best performing set as your benchmark parameters.
7. Three methods of accuracy recovery
Assuming that several steps have been tried but the accuracy is still not stable, then mechanical problems need to be taken into consideration. I have divided solutions related to mechanical problems into three levels.
The first layer is to clean and lubricate it, remove the protective cover, clean the old grease and chips on the screw and guide rail, and then add the specified grease again. This action can solve about half of the structural problems.
The second level is to adjust the gap. Use a dial indicator to measure the backlash of the screw. If the gap exceeds the allowable value range specified in the equipment manual, then compensation measures must be implemented in the system parameters. When performing this operation, someone else needs to assist in shaking the handwheel, and one person must be responsible for checking the dial indicator.
The level of disassembly and maintenance is the third level, which covers matters related to replacing the display with spindle bearings or screw nuts. Once you reach such a step, my advice to you is to find a professional maintenance person. Doing it yourself is risky, and you do not have the necessary tools to ensure installation accuracy.
8. A list to be done every day
Maybe your problem is not a technical issue, but a management issue. All you need is a simple checklist. The table I'm using only has three rows of information. The first line covers the coolant concentration and liquid level. The second line involves the cleaning status and value zeroing check of the tool setter. The third line contains the running record of the heat engine program.
Before starting each day, take three minutes to fill out this form. Before leaving get off work, take another two minutes to check. This form cannot guarantee that problems will not arise, but it can ensure that you will have clues to follow if problems arise. You know that the last item yesterday was good and the first item today is broken, then the problem lies in the evening shift or morning preparation.
9. How to balance efficiency and quality
Your boss may urge you to do it quickly, but the cost is an increase in the scrap rate. I think this balance point can be calculated by calculating the material cost and tool cost of a part, and then calculating your per-piece labor cost, multiplying the scrap rate by the part cost, and then subtracting the labor time saved due to the increase in speed.
A common situation is that stainless steel parts are processed. If the speed is slightly faster, the tool life will be reduced from fifty pieces to thirty pieces. However, five more pieces can be made every day, which can actually be calculated as a loss. This is because the time spent on tool replacement and the material cost of scrap products exceed the profit brought by those five pieces. You should use actual data to explain, rather than just relying on your feelings.
10. Optimize your program structure
When evaluating the quality of the program, you need to pay attention to three points. First, whether the auxiliary time will be consumed for no reason, such as whether the tool change position is the closest distance. Secondly, whether the cutting path reaches the optimal state and whether there is no idle cutting. Third, whether the safety distance is set properly. If the distance is too far, the time will be increased. If the distance is too close, there is a risk of knife collision. Punctuation mark.
I will give you a specific optimization idea, measured by the CNC Torno Efficiency indicator. It mainly focuses on the proportion of actual cutting time from the start to the stop of the spindle. If the proportion is less than 70%, it means that your program has a lot of idling without actual processing. So how to improve it? Adjust the rapid movement magnification to the maximum, but the premise of this operation is that you must ensure that there are no obstacles on the path and cancel the pause time between each process.
FAQQ/A
Q: The spindle makes abnormal noise but the bearings are fine after checking. Why.
A: Generally speaking, it is a problem with the belt tensioner. Adjust the tension of the belt or replace the tensioner bearing. Then the abnormal noise will disappear.
Q: What should I do if the processed roundness is out of tolerance and the program adjustment is useless.
A: Check the spindle encoder. Once the encoder becomes loose, it will cause angle feedback errors, so the encoder fixing screws must be tightened.
Q: There is a sense of lag in the interpolation motion of the X-axis and Z-axis. How to solve it.
A: Enter the servo parameters and increase the position loop gain by 5% each time until the movement can follow the hand.
Q: The turret locking sound is loud and the positioning is inaccurate. What’s wrong?
Because the locking sprocket is worn or stuck with iron filings, it is necessary to disassemble the sprocket, clean it, and check the wear of the tooth surface.
Conclusion and suggestions for action
Let me repeat the core points again. Eighty percent of CNC lathe precision problems are management problems rather than equipment problems. Has the heating process been carried out, has the cleaning work been carried out, has the inspection operation been carried out, and has the compensation tracking been implemented. If these things are properly completed, most faults will not occur.
Starting tomorrow, you can do three things. First, when the morning light first appears every day, the machine is forced to run a five-minute warm-up program when it is turned on; second, a refractometer is purchased to measure the coolant concentration every day and record it; third, a trend chart of the tool compensation value is constructed to observe whether the direction of daily compensation is consistent.
These three things do not cost money, you just need to develop a habit. When you look back a month later, the scrap rate should have dropped a lot. You are not repairing the equipment, but operating the equipment. Your goal is not to wait for the equipment to break down and then repair it, but to make the equipment always run in the state where CNC torno productivity is highest. That state is stable, predictable, and free of accidents.
