Drilling and tapping 200+ 3/8" holes in 3/4" aluminum

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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?

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

Hey Karl,

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

Brian Lawson

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

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

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

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.

Good night,

Wes

PS Day 16 of my work week

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.