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General Mach Discussion / Re: Problems threading on the lathe
« on: October 04, 2009, 02:11:03 PM »
Some time ago I read the thread in it's entirety. I have since followed it closely. It have been very interesting reading and I,m very impressed on the effort laid down by everybody in trying to get the threading to be as good as possible. I have thought a few times on saying something but have decided to keep shut as I probably not have had something useful to say.
The last post by RICH where he says the lead is decreasing is however particularly interesting. I have been thinking since some time ago on the fact that Mach can't speed up if that was necessary in order to catch up on an increasing spindle speed. The thought I have had is what if the spindle speed measurement done just prior to each pass is slightly slower than the actual spindle speed when threading occur. Then the pitch will be to large. I understand the spindle speed is measured in only one revolution prior to the threading on each pass. It places a huge demand on the exact measurement of the time between those two index pulses on that one revolution. When threading at say 500rpm, that is 3,000 degrees/second or 333 µs/degree. I have been thinking on what is the repeatability of the index trigger. An error of a couple hundred µs of the exact time of the trigger signal and the wrong way may cause a noticeably deviation of the pitch on long threads, especially at high rpm's.
I hope I remember the previous posts by ART correctly as what I have said now else definitely would be incorrect. It may very well be incorrect even if I remember ART's previous explanations correctly. In that case disregard what I have said here.
Steinar
The last post by RICH where he says the lead is decreasing is however particularly interesting. I have been thinking since some time ago on the fact that Mach can't speed up if that was necessary in order to catch up on an increasing spindle speed. The thought I have had is what if the spindle speed measurement done just prior to each pass is slightly slower than the actual spindle speed when threading occur. Then the pitch will be to large. I understand the spindle speed is measured in only one revolution prior to the threading on each pass. It places a huge demand on the exact measurement of the time between those two index pulses on that one revolution. When threading at say 500rpm, that is 3,000 degrees/second or 333 µs/degree. I have been thinking on what is the repeatability of the index trigger. An error of a couple hundred µs of the exact time of the trigger signal and the wrong way may cause a noticeably deviation of the pitch on long threads, especially at high rpm's.
I hope I remember the previous posts by ART correctly as what I have said now else definitely would be incorrect. It may very well be incorrect even if I remember ART's previous explanations correctly. In that case disregard what I have said here.
Steinar