Re: [CAD_CAM_EDM_DRO] Re: General Tool Life Question

"Fitch R. Williams" wrote:

> On Thu, 17 Jan 2002 07:10:27 +0000, Jon Elson <elson@...>
> wrote:
>
> >I use a Procunier "CNC" tapping head, that has clutches and
> >gears to drive the tap forward when down feed pressure is applied,
> >and reverses the tap at 2 X spindle speed when upward pressure
> >is applied. The spindle of the CNC machine controls depth.
> >I use Alum-Tap when tapping in aluminum, it works amazingly
> >well.
>
> Jon,
>
> This sounds like a wonderful capability. I have some projects coming up
> where that capability would be a huge help. However I get nervous
> speculating on what happens if there is a mismatch between feed rate and
> rpm? I don't have a good accurate way to set spindle rpm - although I
> could probably adapt one of Sphero's digital tachs to my spindle some
> way if I had to. The mill spindle has a variable speed pulley with hand
> crank, and to complicate things (add uncertainty) it is fed from a VFD,
> and the speed has to be manually adjusted each time it is turned on -
> the Anilam control I have doesn't have a spindle on-off output, not even
> a contact closure other than from the big red panic button (darn it).
>
> I can do the basic arithmetic to calculate what tap rpm and feed rates
> would go together, and what the hole depth would be, etc., that's not
> the issue.
>
> My concern is for the following situations when using a tapping
> attachment like you listed on The Animal:
>
> Case 1: This seems like the worst case. Spindle RPM is slower (for
> whatever reason) than the tap needs to keep up with the selected feed
> rate. In this case the spindle will over run the tap and push (hard) on
> the tap. It might damage something.

Yup, this is the case that the CNC tapping head cannot accomodate.
I've never had this happen. And, I just read the spindle speed off the
belt chart and plugged it into a program to write the G code for the
sequence, and it worked beautifully. There is SOME compliance in
the tap spindle, but it isn't much.

> Case 2: RPM is faster than the tap needs to keep up with the feed rate.
> In this condition, the tap will gain on the spindle - the tap will pull.
> I would think that if this is the case the tapping head would reverse or
> disengage (?) momentarily then re-engage and keep tapping.

The forward clutch will slip - not a real big problem.

> Case 3 is when spindle speed and feed match, but that one takes care of
> itself of course.
>
> With that as a background, I have some question and would be interested
> in your experience with this. The questions are: Does case 2 work as I
> described? If so, do you always make sure your spindle is a bit faster
> than required so that you are operating as described in case 2? If so,
> how much faster?

I did not compensate for a slow spindle at all, or clock the spindle with a
tach
before using the standard RPM off the chart. And, it just WORKS! I HAVE
seen the clutch slip just a little bit - if i watch closely, I can
occasionally
see the tap hesitate for a tiny moment and then continue turning. If you
have
no idea of your spindle speeds (ie. a vari-speed drive that is no longer
in precise calibration) then you probably neet to time it with a tach.
As I said, I just used the numbers off the speed chart, and it just works
beautifully. The only way I've broken taps is to program the machine to
feed
too deeply. There is no overtorque clutch on the CNC tapping heads.

If you are going to program the spindle RPM to go faster than the quill
feed,
I would keep it to a VERY small number. You don't want to burn up the
cork clutches in the tapping head. The acceleration of the quill feed
can allow the tap to start into the work before the quill feed is up to 100%

of programmed feedrate, and that provides a little compliance in the
spindle for the tapping operation. Oh, and make sure feedrate override
is at 100% (or use the program to disallow override) or it will be a mess!

Jon