cnc tapping Re: General Tool Life Question

Smoke,

What you gain by "differential threading" as Ian explained is that
the hole can be tapped without reversing the headstock spindle to get
the tap out of the hole. The main spindle can continue running at
very high speed, 5000+ rpm would not be unusual. This method is
widely used in multi-axis swiss screw machines where several tools
are cutting simultaneously. The other tools can continue their
cutting operations on the OD of the part as the tapping operation is
happening. The concept was perfected years ago on cam operated swiss
machines and continues on CNC swiss type machines. It not unusual
for a swiss type machine to have as many as 5 tools cutting at the
same time.

The cycle times of high production parts made on swiss (sliding
headstock) machines are measured in fractions of a second, time
wasted in spindle reversal is not productive.

I have never heard this referred to as "rigid" tapping, though.
Rigid tapping is a different concept. And, most CNC lathes don't use
differential threading.

Doug

--- In CAD_CAM_EDM_DRO@y..., "Smoke" <smoke@t...> wrote:
> I don't believe you gain a thing by rotating both the headstock and
the
> tool. Your procedure will work just as well without rotating the
tool at
> all. Slow the headstock speed all the way down to the required
difference
> and you'll accomplish the same thing. The procedures works just as
well in
> a manual lathe with a stationary tool as well as in the mill with
the work
> being stationary.
>
> There are several 'secrets' to this which eliminate the need for
exact speed
> matching:
> 1. Rotate the spindle at a slower rate...about 60 RPM or less.
This will
> reduce overtravel when forward feed is stopped.
> 2. Sliding movement of the tap compensates for mismatch between feed
> rate/RPM mismatch.
> 3. Stop the forward feed before reversing the spindle rotation.
> 4. Use a properly designed tool holder.
>
> Smoke
>
> > Rigid tapping as it is done on most cnc lathes relies on speed
> > differentials. The work is spinning in the chuck but the tap is
also
> > spinning in a powered spindle in the 'tailstock'. They both
revolve in the
> > same direction but to tap a hole, the tailstock spindle speed is
increased
> > and to back the tap out, the tailstock spindle speed is reduced.
This is a
> > bit safer method than trying to adjust feed of the tap towards
the work
> (in
> > this method the tap is free to slide in and out of its spindle
and the
> only
> > feed is to bring the tip of the tap into contact with the work)
especially
> > if there is friction control on the tailstock spindle so that it
can slip
> if
> > the tap bites too hard.
> >
> > Ian
> >