36
« on: October 03, 2009, 12:15:23 AM »
Hello all:
I have read the entire thread this afternoon, and marvel at the efforts to solve this problem. The electronic/technical skills of all involved is certainly high, and a great deal of work has been put forth by all.
However, this problem appears to be escalating to higher and higher levels of complexity, with computer clock microsecond timing and motor speed variables shifting about seemingly at random. I wholly agree with the post's of Simpson36. Perhaps it is time to rethink the problem, i.e. think "simple."
Engine lathes have been cutting threads for a long, long time, and the problem of synchronizing the spindle/carriage is done with a precision lead screw, geared to the spindle, which moves the carriage by means of a bronze half-nut, engaged at the appropriate time as indicated by a carriage dial geared to the lead screw. The machinist was the computer, watching the carriage dial to engage the half-nut at the exact moment, to synchronize spindle and threading tool exactly, after selecting the gearbox change gears for the thread pitch.
The spindle index-pulse signal has now replaced carriage dial and half-nut, and the machinist's eye. The lead screw is geared to the carriage motor and motor encoder. But, the old school gear train between spindle and lead screw is gone; replaced by microsecond time clocking counters, motor speed and torque guesstimates, number averaging, and much more things that this writer does not understand at all. However, it does seem akin to the old-school gearing system, except rubber bands are used where gears and keyways used to be! And, it is not simple.
May I suggest using an encoder on the spindle, with the index-pulse retained of course. The spindle encoder signals become the baseline source of all timing for carriage motor drive synchronization. These spindle encoder signals become an electronic gearbox, if linked correctly with the carriage drive encoder counts. Spindle speed can vary all over the place, as well as the Computer frequency clock ticks, and will in no way affect the "electric gearbox" ratio between spindle and carriage.
Example 1:
500 count/rev encoder on spindle, carriage encoder .0001" linear travel per encoder pulse (2000 count/revolution , .1/5 inch pitch leadscrew)
Cutting a 1/4-20 thread, the pitch is .05" per 500 pulses per revolution of spindle. .05 linear travel = 500 linear motor pulses, per 500 pulses from spindle encoder.
Example 2:
Cutting a 1/2-13 thread, the pitch is .076923077" linear travel. This = 769.23077 pulses per 500 spindle pulses per one spindle rotation. It would take 4 revolutions, or 4 full pitches before a full .0001 pulse step had to be added, if the ratio was 500/769.
With a 500 count spindle encoder, at 600 RPM, the pulses per second are only 5000 pulses per second. If I read the thread correctly, the number of 25,000 PPS is the clock speed of Mach3. I seriously doubt anyone is going to want to do threading over 1200 RPM, and the coarser the thread, the lower the spindle speed becomes. If the spindle speed must be high, i.e. fine pitch, small diameter, the Mach threading program should be able to post an upper limit spindle speed for a given thread pitch.
I am sure there are elegant, mathematical routines to synchronize the lathe spindle and carriage, for any thread pitch. Machining threads to a known standard of precision is fundamental, in the machine industry.
Regards,
John