Re: [CAD_CAM_EDM_DRO] Re: Thread milling

Peter Renolds wrote:

> Hi Guys,
>
> Am I missing something? Don't you need full servo control of your
> spindle in order to perform thread milling? My understanding is the
> tool actually runs in a helix. The other thing that stuck me with the
> initial thread and the machine pictured (with limited Z clearence) was
> also the limited torque available from a small spindle - particularly at
> low RPM's - unless geared. I don't have any experience with thread
> milling so I'm interested to learn more from those in the know.
>
> Jon, you mentioned you perform thread milling - I assume you have a
> servo controlled spindle via EMC?
>
> My experience with CNC threading has been with a Tapmatic threading unit
> that reverses direction when the feed direction changes. Once I got the
> hang of it, it has worked very well - but I still need to be watching it
> to clear chips periodically.
>
> Cheers, Peter
>
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Peter,

Thread milling requires only a control that will move the Z during a
circular move, and can be done with stepper motors.

I did a bit of thread milling on a prototype part. Cut up a 5/8-11 tap
to leave a single cutting point. Relieved the 'uphill' side of the crest
flank for clearance.

Thread mill crests are ground perpendicular to the shank, taps have a
spiral. Taps rotate in the center of the hole and advance into the hole
at the thread pitch, thus the spiral is correct.
Thread mills rotate about the tool axis, offset from the hole axis, thus
the high side of the crest will rub. It might not be terribly critical
for hobby work, I've read posts from people that use taps and don't
relieve them. But it won't pass for aerospace quality work...

Jon