[sansbury]

I'd like to add an encoder (e.g. a basic single-pulse type) to my spindle for realtime RPM and maybe rigid tapping. Wondering if anyone has done one of these and if so how they went about it--phototransistor type, Hall-effect sensor, etc.? I've done a simple one to enable RPM monitoring and threading on a lathe with LinuxCNC, but not sure about best approach for this, particularly something that won't affect spindle balance.

[keebler303]

A single pulse encoder will NOT work for rigid tapping. When the tap is coming to a stop at the bottom of the hole, how could the Z axis accurately match the screw pitch when it is receiving no spindle feedback for the last .99 revolutions (worst case) of the spindle before it reverses? Then it reverses and the Z axis does not know it has reversed for another .99 revolutions. On a 3/8-16 thread this could add up to .125" of error! I would suggest a real quadrature encoder for rigid tapping support.

If you just wish to monitor speed, you could paint half of the periphery of your drive pulley white and the other half black, then use an infrared emitter/detector pair to see the white/black transition. Optical methods will have a minimal effect on balance as you are only putting on a little bit of paint. If your spindle goes really fast, hat becomes important. The infrared sensors I like to use for tachometers are part number QRD1114. You can google that part for sources and circuits.

Matt

[sansbury]

If you just wish to monitor speed, you could paint half of the periphery of your drive pulley white and the other half black, then use an infrared emitter/detector pair to see the white/black transition. Optical methods will have a minimal effect on balance as you are only putting on a little bit of paint. If your spindle goes really fast, hat becomes important. The infrared sensors I like to use for tachometers are part number QRD1114. You can google that part for sources and circuits.

Ah, interesting idea. Or, if I did two smallish white bars 180° apart, that would essentially work the same (except 2 PPR) and eliminate any balance issue, right? Or is the weight of paint unlikely to be an issue on a 5k spindle?

I thought 1PPR could do rigid tapping since I use that for threading on a lathe, but I see now that's simpler than rigid tapping. I can do just fine with T/C tapping I think, don't need to do tiny threads or blind holes.

[keebler303]

I'd not worry about the weight of paint until you were around 20k or so. Of course it also depends on the weight of the paint compared to the weight of your spindle. A tiny little spindle at 50k RPM would probably get beat up by the imbalance of a dab of paint on it.

2 marks 180 apart, 4 marks 90 apart, doesn't really matter too much. Just be careful that your sensor has the bandwidth to see each pulse. If you run it too fast then white and black will all start to look like gray and you will not get a good number. 5000 rpm is only 83 Hz with a 1 ppr sensor so I'd not worry too much about that.

Lathe threading doesn't have to speed up, slow down or change directions while it is in the cut. Rigid tapping requires all three. Not to mention absolute angular position if you want to do peck rigid tapping or repeat rigid tapping. You have to have an index pulse for that.

Matt