On 2010-12-18, Wild_Bill wrote:
As I was about ready to send the following statements as a follow-up post to
earlier questions, I think I determined a basic need for a signal to
monitor/track the rotation of a tap.
If the tap needs to re-enter the hole during the tapping operation, knowing
the rotational position of the tap would matter, so the tap is in sync with
the previously-cut thread.
Since CNC tapping isn't generally a pecking-type operation AFAICT, how often
would the signals from a high count encoder be any more practical than a low
count model?

EMC waits for an index pulse to start tapping.

My curiousity was wrt the importance of one count out of 4096 being of any
significance to the rotational position of a tap.. in real life.

I wasn't trying to suggest that tapping should be done at higher speeds in a
home shop.

If a large diameter (12"+) workpiece such as a division plate for a critical
timing or positioning application were being machined, then a 4096 count
encoder (on a rotary table, for example) would be more practical, for radial
positional accuracy.

My question, is why would anyone care if a tap's rotational position was
within 1/4096th or even 1/1024th out of position?

I doubt anyone would.

For testing, it's good to know if the encoder signals are being
accurately counted, but in terms of the depth of thread or almost
any other tapped hole parameters, the high count encoder rate is not
significant.

If the controller undercounts counts, then the tap would not be in
sync with the spindle.

It's not as if a thread needs to start at a specific
position, generally. Tapping a blind hole could require an
incremental change in where the tap is stopped, I suppose.

I could see having a relatively high count encoder on a CNC lathe
spindle, for example, to allow incremental stops for a thread or
groove, but not for a tap in a mill. This example would be where a
cutting tool on a lathe is started or retracted from the workpiece
at a specific rotational position.

As far as encoder count rates, a lower count per rotation would be more
desirable for very high speeds.. low enough as to not exceed the CNC
circuit's effective counting rate (input signal frequency).

Having such a high count for a mill spindle's rotational position doesn't
seem to have much significance (to me), as far as milling, tapping or
drilling are concerned.

As I suggested earlier.. wouldn't a low count (100 or even less) encoder
serve adequately for a mill spindle's rotational position?


Say, 5/8-10 thread, has 1/10" per revolution. An accuracy of one count
out of 100, would be 0.001". Not completely insignificant for a tap,
and maybe enough to trip a following error.

Plug in your TPI and your count per revolution, to get the vertical
positioning accuracy.

i