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General Mach Discussion / Re: whats happening with threading
« on: December 04, 2008, 12:53:12 PM »
Here are some further observations I have made today.
If I use auto speed control (PWM) my speed is unstable 285 to 315 at nominal 300 RPM (varies with feedrate on Z axis)
In this condition I get erratic movement of x axis on almost every pass of the thread and variation in pitch. This is the same on G32 and G76.
If I use manual speed control, the speed is quite stable (298 to 300). In this condition most passes of the thread are correct, but 1 or sometimes 2 passes result in movement of the X axis and or variation in feedrate.
After several more tests I was able to observe that once or twice during the complete thread my spindle DRO speed would jump to 1000 RPM for a second or less, then settle back at 300. When this happened, the following pass would have the problem with movement on the X axis and/or change of feedrate, then continue normally. This also happens in both G32 and G76.
It therefore seems that the problem is speed related. I can understand that with the variation in feedrate, but the strange movement of the X axis was not so apparent.
The OP says he has a very steady spindle speed. I thought I did, but it wasn't until I watched the DRO throughout the entire thread that I noticed the momentary change.
I have played with index debounce, speed averaging and step/direction pulse in motor tuning, cleaned the optical switch on the indexer but these don't appear to affect it. I tried reducing the acceleration of the Z axis and this appears to have helped. It may just be a fluke, but I will try some more tests at different levels.
It wasn't my intention to gatecrash the OP's original message, but I believe from todays tests that there is a clear link between the spindle speed feedback and the strange movements on the X axis.
If I use auto speed control (PWM) my speed is unstable 285 to 315 at nominal 300 RPM (varies with feedrate on Z axis)
In this condition I get erratic movement of x axis on almost every pass of the thread and variation in pitch. This is the same on G32 and G76.
If I use manual speed control, the speed is quite stable (298 to 300). In this condition most passes of the thread are correct, but 1 or sometimes 2 passes result in movement of the X axis and or variation in feedrate.
After several more tests I was able to observe that once or twice during the complete thread my spindle DRO speed would jump to 1000 RPM for a second or less, then settle back at 300. When this happened, the following pass would have the problem with movement on the X axis and/or change of feedrate, then continue normally. This also happens in both G32 and G76.
It therefore seems that the problem is speed related. I can understand that with the variation in feedrate, but the strange movement of the X axis was not so apparent.
The OP says he has a very steady spindle speed. I thought I did, but it wasn't until I watched the DRO throughout the entire thread that I noticed the momentary change.
I have played with index debounce, speed averaging and step/direction pulse in motor tuning, cleaned the optical switch on the indexer but these don't appear to affect it. I tried reducing the acceleration of the Z axis and this appears to have helped. It may just be a fluke, but I will try some more tests at different levels.
It wasn't my intention to gatecrash the OP's original message, but I believe from todays tests that there is a clear link between the spindle speed feedback and the strange movements on the X axis.