Re: EMC and threading on mill

Roger,

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.

Feed the tap in to depth, just like you were drilling a hole.
Reverse spindle rotation when depth is reached and feed back out of
hole.

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

--- In CAD_CAM_EDM_DRO@y..., "Roger Swift" <vrsculptor@h...> wrote:
> I was trying to understand how to tap small holes (1/4-20) using
EMC.
>
> Hardware: Servo mill, a tap holder that is essentially a collet
mill holder that telescopes and a 4HP spindle controlled by a +/- 10
volt servo board and a a tach. There is also a low resolution single
channel spindle encoder. The spindle speed is currently manual (set
with a wall wart and a 5K pot).
>
> How I think it should work: Start spindle at low speed, feed quill
to engage tap to start treading, rotate spindle 20 turns clockwise,
tool self feeds on telescoping holder, lift quill a bit and rotate
spindle counter clockwise to disengage. I'm pretty sure this is how
the original controller (long gone, never witnessed) handled it.
>
> Any thoughts on how to configure this? Is this possible with any
other PC based G-code drivers?
>
> Roger
>
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