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Author Topic: Opinion and advice - explain relationship of encoder to kernel speed  (Read 17034 times)

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Offline simpson36

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I have a stepper powered 4th axis (shown in the show and tell section) that I am considering converting to servo power so that it can be used on the mill as a pseudo lathe.

Currently I have a 10:1 reduction and a 10,000 RPM DC motor to use which would net a max 1,000 RPM which is adequate for my purpose. I can install the encoder wither on the motor or on the belt drive idler.

First, I wonder if anyone is doing something similar and if it is considered viable.

Second, if I put a 200 step encoder on the motor, there would be a max of 2,000,000 pulses per minute. What kernel speed would be needed for that and in general what is the relationship between the encoder steps and kernel speed?

Offline Hood

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #1 on: April 09, 2009, 10:16:20 AM »
A 200 line encoder would be 800 pulses per encoder rev so at 10,000 rpm that would be  8,000,000 pulses per min . Divide that by 60 and it is 133,333.3333333 pulses per second or 133.33333333 KHz kernel you would need. Only way would be with a SmoothStepper or some other external motion controller.
Hood

vmax549

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #2 on: April 09, 2009, 11:01:02 AM »
BUT if you mounted the encoder on the spindle shaft it only turns a MAX of 1000 RPM

SAY 13khz or so (;-) DOABLE

(;-) TP

Offline Hood

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #3 on: April 09, 2009, 11:03:33 AM »
BUT if you mounted the encoder on the spindle shaft it only turns a MAX of 1000 RPM

SAY 13khz or so (;-) DOABLE

(;-) TP

Depends on backlash, motor might not be too happy if there is even just a little
Hood

Offline simpson36

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #4 on: April 10, 2009, 05:17:46 PM »
Timing belt drive with idler, so no backlash, but i can see the idea needs more thinking through.

A 200 line sensor puts out 800 pulses . .  interesting . . quadrature?  The formula seems pretty straightforward, thanks!

Putting a sensor on the axis is not practical, but I could either use an encoder with less steps . . say 50, and put the encoder on the motor, or run off a larger idler and gear down to the encoder. I have seen huge machine tools that wof that way with the encoder driven off the lead screws with tiny timing belts.

From the speculative responses so far, I take it that a servo driven 4th axis/pseudo lathe is not common?

vmax549

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #5 on: April 10, 2009, 09:42:27 PM »
THe other problem will be that the 4th axis "as is" cannot continously spin like a lathe . You will have to swapaxis with the spindle in order to make it work (;-)

(;-) TP

Offline simpson36

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #6 on: April 10, 2009, 11:58:36 PM »
There are a couple of ways around the continuous spin issue. As I recall, though, one method got broken in .043 and is probably still broken since the rev number has not changed.

Basically, you just imput an outrageous number of degrees to rotate and then reset to zero for the next cut. There is likely some maximum number of degrees you can command, but I didn't fool with it long enough to discover that and other details.

Perhaps Mach will add a 'run continuously' type of command to the program or smooth stepper migt accomplish the same thing. It is a common function in machining centers, so the capability exists, just not in Mach at the moment, I suppse.

Offline Hood

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #7 on: April 11, 2009, 04:54:15 AM »
Commanding a large number I suspect wouldnt work , if for example you said G1 A1000000000000000000X2Z3 your X and Z moves would be extremely slow and not complete until the spindle had reached its position.
 Swap axis should be easy enough to accomplish although at this point in time I have never tried it I will be using it for rigid tapping if I ever get the mill completed.
Hood

Offline simpson36

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #8 on: April 11, 2009, 11:21:25 AM »
The moves would happen simultaneously, unless I am missing something. It seems to me that for turning, feed per rev is the way to go, so it would be a simple calc.

Say you want to program a 1" length of cut at .003" per rev. You simply program 1/.003 turns for the move.

  Set depth of cut increment

  Call sub

  G1 A120240 (334 turns x 360 degrees)  X1

  Back out of the material a bit

  G0 X-1

  Reset A to zero (I don't recall the code off hand).

  Return from sub.


Note: it's been a while, but I seem to recall that rev .043 is broken and A will not reset to zero. I posted that info, but I don't know if this is fixed yet.

Threading is similarly simple, you simply set the 'feed' to the desired pitch.  I've already done that . . . just didn't have enough power with a 600oz-in Stepper to drive the 4th axis, hence the need for a servo motor.


Just to add one more thought: a servo driven 4th axis would (theoretically) solve the threading problems Mach seems to be having using a single pulse per rev indexer. 
« Last Edit: April 11, 2009, 11:37:53 AM by simpson36 »

Offline Hood

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Re: Opinion and advice - explain relationship of encoder to kernel speed
« Reply #9 on: April 11, 2009, 11:56:54 AM »
Yes that is doable and is what I will be doing to get rigid tapping if the SS doesnt support it when I eventually get the mill finished. It would however be a lot of working out and coding. The easiest option would be to set up two profiles, one a mill and the other a Turn and when you wanted to do lathe work just start up the Turn profile, the A axis would be set as a spindle in it.
Hood