Re: [CAD_CAM_EDM_DRO] Re: EMC and threading on mill

I'll have to give tapping a try on the MAXNC...in wax first, of course.
Have to be plenty careful calculating the RPM/feed ratio as the on screen
RPM indication never matches the figure in the gcode.
For example...it reads 1950 RPM when the gcode says 2000.

This is a very good reason to have a spring loaded tapping spindle.

An even better tapping spindle would be of the type used on transfer
machines in the auto industry. These are not spring loaded but telescope on
a splined shaft. The assembly is encased in a bushing which is threaded to
match the thread on the tap. This way, the spindle length is free to change
if necessary due to differences in RPM / feed rate, etc. and no "thread
shaving" takes place.

Maybe I can find the time to draw one (or two) us hobbyists could build for
use in small machines?

On a Bridge port all you need to do is set the RPM as low as possible, use a
spiral pointed tap, pull the quill down until the tap starts in. The thread
on the tap will pull the tap into the hole at the correct rate. When you
get to the desired tap depth just reverse the spindle and the tap will back
out of the hole.
This is one advantage of having manual feed available.

Smoke

> Here's how I've always done it on machines without rigid tapping as a
> control feature. I don't know EMC, but I assume it doesn't have
> rigid tapping.
>
> Calculate a feedrate based on the on the thread pitch and spindle
> rpm. Example, 1/4-20, 1000rpm-50 ipm, 500rpm-25 ipm, 250 rpm-12.5
> ipm, etc.

> Telescoping tap holders usually have a spring loaded telescoping
> (extension) and compression feature to compensate for difference
> between your programmed feedrate and the actual thread pitch. Among
> other reasons, that difference can be caused by inexact spindle speed
> control and issues involving how quickly the spindle stops/reverses
> at the bottom of the hole.
>
> Generally, the telescoping feature would not be used to feed the tap
> in with the quill not feeding. I suppose you could do that for
> shallow threads, but you risk "shaving" the thread profile due to the
> self feeding action of the tap fighting against the telescoping
> spring's force. The telescoping position should remain as close as
> possible to it's relaxed position during the tapping operation. You
> may find this can be accomplished best by slightly varying the
> feedrate as needed from the calculated value on both the infeed and
> outfeed rates.
>
> Whenever I do this operation, if possible, I try to "tune" the
> feedrate by test running the tap in soft material (plastic, wood,
> foam) to verify everything. If the rpm is low enough it's very easy
> to see the amount of extension or compression you're getting with
> your holder. That way I'm not so likely to break a tap or ruin the
> part if I've miscalculated something.
>
> Doug
>
>