And, you won't believe what a MESS this will make, if you ever mess up
and have the spindle running at the wrong speed for the calculated
feedrate. been there, done that!

What will happen, it will tear the tap out of the collet?

Anyway, obviously rigid tapping, optionally with the floating holder,
is the way to go.

i

I predict that rigid tapping will be a priority for you AFTER you've
broken a few taps off and ruined the part. BTDT

Karl

On 2010-09-27, Karl Townsend wrote:

And, you won't believe what a MESS this will make, if you ever mess up
and have the spindle running at the wrong speed for the calculated
feedrate. been there, done that!

What will happen, it will tear the tap out of the collet?

Anyway, obviously rigid tapping, optionally with the floating holder,
is the way to go.

i

I predict that rigid tapping will be a priority for you AFTER you've
broken a few taps off and ruined the part. BTDT

I think that I understand the basics of rigid tapping: the Z axis
follows the measured rotation of the spindle. When the spindle rotates
steadily, I would think that it can be done pretty well easily.

But how does rigid tapping handle reversal of the spindle, when the
spindle speed changes very rapidly, from forward to stop and to
reverse? The Z speed also needs to be adjusted super sensitively?

i

On Mon, 27 Sep 2010 21:00:46 -0500, Ignoramus21149
wrote:

On 2010-09-27, Karl Townsend wrote:

And, you won't believe what a MESS this will make, if you ever mess up
and have the spindle running at the wrong speed for the calculated
feedrate. been there, done that!

What will happen, it will tear the tap out of the collet?

Anyway, obviously rigid tapping, optionally with the floating holder,
is the way to go.

i

I predict that rigid tapping will be a priority for you AFTER you've
broken a few taps off and ruined the part. BTDT

I think that I understand the basics of rigid tapping: the Z axis
follows the measured rotation of the spindle. When the spindle rotates
steadily, I would think that it can be done pretty well easily.

But how does rigid tapping handle reversal of the spindle, when the
spindle speed changes very rapidly, from forward to stop and to
reverse? The Z speed also needs to be adjusted super sensitively?

i

Its called electronic gearing. For every X encoder counts, move Z axis
Y encoder counts. Or the Z axis is slaved to the spindle. So, when you
stop the spindle, Z motion stops. When you revese the spindle Z moves
negative.

Karl

BIG SNIP

So, when you stop the spindle, Z motion stops.
When you revese the spindle Z moves negative.

Karl

Hey Karl,

Hmmmmm.....wouldn't that drive the tap INTO the work? VBG

Brian Lawson

"Ignoramus21149" wrote in message
...
On 2010-09-27, Mike Henry wrote:

"Ignoramus21149" wrote in message
...
On 2010-09-27, Karl Townsend wrote:

How bad were the chips?

I had no trouble with my mystery metal AL. Some AL is a stone bitch to
feed the chips up the drill bit, others work great. As nearly all my
metal falls in the mystery metal class, i can't tell you which grade
is best.

You should feed maybe twice as fast as my small run.

Karl


Karl, thanks. I will try on some aluminum junk that I have, first.

Tapping, as Richard noted, may be a challenge.

What tap would you recommend for this (tapping aluminum)?

Thanks

i

If they are through holes, I would go with a gun tap - they push the
chips
ahead of the tap. I have a Procunier tapping head you could borrow for a
week or so if that would help. I've tapped a bunch of 0-80 and 4-40
holes
with nary a broken tap so 5/16" shouldn't be a problem for that style of
head.


Mike, did you use that head on a CNC mill?

Not yet, but I know of someone else that has used the same head hundreds of
times on a CNC mill.

On 09/27/2010 03:15 PM, Ignoramus21149 wrote:


And, you won't believe what a MESS this will make, if you ever mess up
and have the spindle running at the wrong speed for the calculated
feedrate. been there, done that!

What will happen, it will tear the tap out of the collet?

Well, depends on the tap and the material. I was not using a compliant
holder, but a Procunier 15000 "CNC" tapping head. So, there was only
about .020" clearance in the tapping head. I had the belt in the wrong
groove so the spindle was turning too slow by at least 30%. it just
mashed a 6-32 tap through the holes, without breaking it! Basically
like a drill with WAY too mugh feed. I was amazed later when I figured
out what had gone wrong.

Jon

On 09/27/2010 09:00 PM, Ignoramus21149 wrote:

I think that I understand the basics of rigid tapping: the Z axis
follows the measured rotation of the spindle. When the spindle rotates
steadily, I would think that it can be done pretty well easily.

But how does rigid tapping handle reversal of the spindle, when the
spindle speed changes very rapidly, from forward to stop and to
reverse? The Z speed also needs to be adjusted super sensitively?
I thought I replied to this, but don't see it.

So, the idea is the spindle encoder is zeroed at the index pulse, and
then counts up from there. The scale parameter sets it so one rev gives
a position count of 1.00, each turn counts up by exactly 1.00 output units.

Once the encoder counter has sync'ed to the index pulse, then the Z axis
is slaved to that position divided by the thread pitch. So, whatever
the spindle rotation is, the Z axis follows it.

You need to adjust the spindle reversal so the Z axis can follow it.
If you are just using relays to command the VFD forward and reverse,
then you need to set parameters in the VFD to control the accel/decel
rate. If you are using a DAC channel to control the VFD, you can have
EMC give the VFD a speed ramp.
I had to put a lowpass filter on the spindle speed command so the
reversal was not too abrupt. The VFD could reverse just fine, but it
could exceed the ability of the Z axis to follow it. After putting in
the filter, it does fine. (EMC2 also has a limit component that limits
change to a linear slew rate, and that has been suggested as the right
way to do this. But, i had used lowpass before and knew it.)

A corollary of this is that coarser taps need to be run at lower spindle
speed because the Z moves more for each rev than a fine-pitch tap. You
can use Halscope to watch Z axis following error during the reversal to
decide where to make these tradeoffs.

Jon

Ignoramus24898 wrote:

I bought a couple of 3/4" thick aluminum fixture plates.

http://igor.chudov.com/tmp/Fixture-Plate.jpg

It was a local sale.

I would like to drill and tap them with 5/16" drill and 3/8" tap, say
spaced at 1" interval. That makes for about 200 holes to be drilled
and tapped on my CNC mill.

Some questions.


It is getting late but the threads I read so far didn't mention chamfering the hole before
tapping.

Link to one of our posters.

http://www.spaco.org/taptips.htm

See item #6

Good night,

Wes

PS Day 16 of my work week

On 2010-09-28, Ignoramus21149 wrote:

[ ... ]

I think that I understand the basics of rigid tapping: the Z axis
follows the measured rotation of the spindle. When the spindle rotates
steadily, I would think that it can be done pretty well easily.

But how does rigid tapping handle reversal of the spindle, when the
spindle speed changes very rapidly, from forward to stop and to
reverse? The Z speed also needs to be adjusted super sensitively?

Essentially -- make sure that the encoder on the spindle can
create directional information as well as angular motion. For every
output pulse from the spindle in a clockwise direction, move down an
amount calculated from the number of pulses per rotation from the
encoder, and the pitch of the tap. If you start getting pulses in the
reverse direction -- move the spindle up the same amount per pulse.

So -- let the spindle slow down and speed up in reverse at
whatever speed it will do it -- and only worry about keeping the
vertical position correct for the accumulated rotation.

Let's say you have a 16 TPI tap (0.0625"/revolution) and you
have 360 pulses per revolution, you move down (or up) 0.000174" per
pulse. (You'll need to accumulate the pulses and calculate at a higher
resolution, and then command the vertical motion to a position within
the resolution of the vertical position encoder.

Enjoy,
DoN.

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