Lathe spindle control - accuracy for threading

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08 Nov 2011 17:24 #14682 by phidauex
I'm retrofitting an Emco 120P lathe for EMC2, and have some questions about my spindle control method. I have a 3HP induction motor with a VFD (GE AF-300 E11), which appears to support a form of sensorless vector control. The lathe spindle has a built-in 100ppr encoder. I'm considering controlling the machine's IO with a 5i25/7i76 mesa card combo, which gives a spindle encoder input, as well as fwd/rev and a 0-5V output for spindle speed. I'm hoping to be able to tap and thread on this machine, but I'm concerned about the accuracy of the feedback loop from the encoder to the 0-5V speed control. Is that stable enough for threading? Or should I look into using the Modbus input on the VFD to send it more precise speed information?

-Sam

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08 Nov 2011 17:59 #14687 by cncbasher
it should work fine for threading , you will also need a second sensor on the spindle as an index pulse , this should give a single pulse per rev , if your existing encoder has no index pulse,
this is easily achived , either by a single photo sensor or proximity sensor .

also check if the spindle encoder is quadrature output as this would give you 4 x 100 pulses .
but either way for threading it should be fine .

you will also need to bring the encoder into Emc as well as the speedcontroller , or direct the the encoder to Emc and then on an output pin to the speed controler , this will give Emc more control of the spindle

good luck in your conversion .

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08 Nov 2011 18:50 #14688 by PCW
It should perhaps be pointed out that accurate spindle speed control is not required for spindle synchronized motions in EMC.

Threading and tapping basically electronically gear the motion axis with the spindle
so the spindle speed need only be as accurate as required to maintain proper cutting speeds.

Here's a nice example video/

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08 Nov 2011 18:55 #14689 by phidauex
Great video, thanks!

That helps me understand it a bit better, the speed can vary a bit, but as long as EMC is getting the encoder output, it can still sync the axis motion. Is 100ppr + index pulse high enough resolution? That is fully 3.6 degrees between pulses, which seems good, but I'm lacking context with this type of machine.

-Sam

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08 Nov 2011 20:13 #14697 by PCW
100 PPR should be fine
If you have quadrature and 400 PPR so much the better
also quadrature would be required for tapping
(since you need controlled motion in both directions)

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14 Nov 2011 02:27 #14802 by phidauex
Did some probing around with the scope, and found that the encoder does have an index pulse, but does NOT have quadrature, just a single 100ppr output. I suppose this means that threading will still be possible (though less accurately?), but not tapping.

Peter from Mesa thinks that this will still work with the encoder interface on the 7i76 card.

I suppose I could live with that for now, until I get around to finding a good replacement encoder that does full quadrature and index. Any recommendations for generic encoders that are easy to integrate with spindles?

-Sam

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14 Nov 2011 12:58 #14806 by andypugh
phidauex wrote:

Did some probing around with the scope, and found that the encoder does have an index pulse, but does NOT have quadrature, just a single 100ppr output.

That seems odd. Are there any gears in the headstock that you could use to give you quadrature?

Any recommendations for generic encoders that are easy to integrate with spindles?

Because lathe spindles typically have a big hole in the middle, it is a bit of a challenge. I ended up making my own, and many others have done the same. A made a vaguely adequate one by wrapping some laser-printed stripes on a piece of paper round the spindle shaft, but when that started to peel off I made an aluminium disc one.

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15 Nov 2011 13:13 - 15 Nov 2011 13:15 #14832 by ArcEye

Any recommendations for generic encoders that are easy to integrate with spindles?

If you have a keyed shaft or other means of positive location the below works well.
The disc has 120 holes read by an optical switch and the single embedded magnet provides the index pulse via a hall effect switch.

The second pic shows a 60 hole disc I made first, but found 120 was better.

regards
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Last edit: 15 Nov 2011 13:15 by ArcEye.

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15 Nov 2011 13:16 #14833 by ArcEye
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13 Dec 2011 19:24 - 13 Dec 2011 19:30 #15642 by Harry_Y
phidauex wrote:

Did some probing around with the scope, and found that the encoder does have an index pulse, but does NOT have quadrature, just a single 100ppr output. I suppose this means that threading will still be possible (though less accurately?), but not tapping.

Peter from Mesa thinks that this will still work with the encoder interface on the 7i76 card.

I suppose I could live with that for now, until I get around to finding a good replacement encoder that does full quadrature and index. Any recommendations for generic encoders that are easy to integrate with spindles?

-Sam


I got one of these:

www.ebay.com/itm/RENCO-MODULAR-ENCODER-O...&hash=item3c9aed3e71

In testing it looks good, I am in the process of mounting it to my lathe

100 pulses per rev
A, B, Index, Inverse A, B, Index
5v
Mounts onto a 3/8 shaft

From what I could find it looks good to
6,000 RPM and life or 2 billion revolutions
Last edit: 13 Dec 2011 19:30 by Harry_Y.

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