Re: EMC and threading on mill

Jon,

If Roger is speaking of a conventional non-clutched
tension/compression tap holder, that method won't work well (if at
all). For one thing, the compression spring is much too strong to
allow itself to compress without starting the tap into the hole
(compression spring is harder than tension in all the units I've
seen/used). Secondly, the thread will get badly shaved on small
taps in soft material if you allow the holder to extend much using
tap lead. The shaving will happen because the self feed of the small
tap will be overcome by the pull of the tension spring.

Also, what if the tapped hole is deeper than the holder's extension?

The holder I'm looking at right now is made by Tapmatic called a
tapping spindle, not to be confused with their clutch type, self-
reversing tapping heads. It has about 1/8" compression with a very
hard spring. And about 1/2" extension with a much weaker spring.
This unit would be limited to approximately 1/2" thread depth if the
only downfeed was via holder extension, we use this all the time to
do 7/8" deep 10-32's (with form taps).

The operation is much simpler than you and Roger are making it. Do
it as if you were drilling a hole at low rpm, except for the spindle
reversal at bottom. Set feedrate by calculation, then fine tune by
observing the extension/compression action of the holder, adjust so
that the holder remains slightly extended from it's relaxed
position . Avoid compression, a little extension is OK.

Doug

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> What you need to do is advance the spindle until the tap holder
> has no compression left with the tap touching the work, and spindle
> rotating at known RPM (roughly). Then, feed the spindle into the
work
> at a rate determined by feedrate = RPM / TPI. This will cause the
tap
> to be started into the work at approximately the correct feed.
After
> perhaps .05 - .1", stop forward feed, and let the tap holder extend.
> When tapped to depth, stop the spindle and reverse, and feed the
> spindle back out so the tap holder won't be totally comressed as
> the tap exits the hole. You will want to use the "exact stop" mode
> command, G61, to make sure the tap actually achieves full depth.
> You may need a dwell at the bottom, as the spindle will be reversing
> and coming up to speed, before you start to back out the spindle.
>
> I do something like this on my system, with a Procunier CNC tapping
> head. it has no axial compliance, so I have to feed in and out
matching
> the spindle rotation. It has clutches such that if there is no
infeed or
> outfeed force axially on the tap, then it is in neutral. When the
spindle
> is feeding forward, the clutch will slip a little if the spindle
rotation
> gets ahead of the linear feed. Conversely, if the linear feed gets
> ahead of the rotation, it breaks the tap. but, I've never had that
> happen.
>
> Jon