Rigid Tapping - VFD and needed encoder ppm

Hello Everyone,

I am planning to convert a BF20 Optimum Mill to CNC and want use rigid tapping.
I am currently redesigning the spindle. For rigid tapping, a encoder with A, B and Z pulse is needed, atleast that is what my reserch got me.
I would like to have the encoder directly on the spindleshaft, so no beld needs to be used.

So my question is, how many slots does this "encoder disc" need to have?
(1 slot would equal 2 pulses right? Rising edge and falling edge?)

And would it be possible and practical to use a ac motor with a vfd to do rigid tapping?

Any input is highly appreciated.

Thanks in advance!

Depends also on the number of sensors. If you increase the number of sensors it will give you more pulses per revolution. ( I think to put 4 sensors on the thing is a realistic approach)

The hardware is also a factor. How many ppr can your system keep track of?

Then (i did not try it) a floating tap holder can help compensate small deviations when it is not super accurate.

I was thinking of drilling 2mm holes on a 70mm diameter. So there could be 55 holes.

The amount of sensors is a good idea for scaling this up!

The Hardware will be a Mesa 7i96, i think it has a 1mhz max input frequency.

But whats the minimum pulses per rotation that is going to still give good results? That would be very interesting to me.

Greetings from germany!
 

You can make or purchase a quadrature encoder with index that uses three detectors. With quadrature encoding the counter increments each time either the A or B detector transitions so you get 4X the number of teeth or holes. You also get direction and diagnostics if the pulses transition incorrectly. The index signal Z is generated once per rev and can be implemented with a deeper tooth or another hole offset from your main pattern. There is no need for excessive resolution for threading. I have no problems with 65 teeth/260 pulses per rev on my lathe or rigid tapping  using a 100ppr encoder on my mill.

With 55 holes and two detectors (A and B ) you would get 220ppm. To achieve this you'd want to get approx 50% duty on/off from each detector (controlled by size of hole or teeth and how the detector aligns. You may want more or slighty larger holes to get approx 50% duty. You would want the A and B signals offset by approx 25%. You should use halscope to display the signals as you adjust the detectors. The index detector can be adjusted independently as it is normally aligned to a different radius. 

If you have enough holes/teeth (55 should be fine) you can also use "missing tooth" detection to index.

I have implemented and run a homebuilt quad encoder with index on my lathe for several years and through a couple upgrades. I also use an Omron optical encoder to rigid tap on my mill. I suggest paying $20 for an omron 1000ppr encoder and 3d printing GT2 timing pulleys. You can use a GT2 belt or just use a rubber band. 

If you really want to get crazy you can take a old printer apart. Most old printers use plastic optical wheels with an easily accessible encoders all over the place. The problem I ran into was cutting the wheel to fit around the shaft. These will have crazy resolution but the mesa encoder can probably handle that. (check ppr for the rpm you want vs maximum mesa encoder frequency)

Your 55 holes should be fine, in quadrature that would give 220 counts/rev.
This is a little blurb I wrote about putting a spindle encoder on a Bridgeport head, using the bull gear as the encoder "disc". Pretty much similar to what you are suggesting.
Jon