Poor man's rigid tapping?

[CRM]

I have a 3 axis bed mill with a manual quill. I don't have a spindle encoder, nor do I have the funds at this time for the rigid tapping option for the Centroid control. If the quill were equipped with a glass scale, would it be possible to configure the PLC program when it encounters a G84 command to:
1) start the spindle clockwise at the programmed speed
2) position X &Y to the programmed location with the Z axis at the programmed R-plane
3) monitor the quill (W axis) for movement toward the programmed tapping depth
4) reverse spindle (possibly even at a speed greater than 100% of programmed) and monitor W axis, waiting for it to return to the R-plane (W "home")
5) return spindle to clockwise and programmed speed and position to next XY location (as a safety measure, monitoring W to make sure it doesn't move below it's "home" position by more than X.xxxx amount during the XY positioning move, else feedhold until it again is above the established limit)

In other words, I want to automate what I have done on 2 axis CNC Bridgeports for 20 years: Utilize CNC axis positioning to power tap holes. I got the idea from a DoAll drill press we had that had a tapping mode, wherein when spindle movement reached the bottom of the stroke, it tripped a microswitch that reversed the spindle. When you returned the spindle to the top of travel, it tripped another microswitch which set the spindle rotating clockwise again.
Wondering if the concept could be applied to a CNC mill that had a manual quill? It would be assumed that the quill is fully retracted and locked to run the CNC program like normal, but when it encountered a G84 tapping canned cycle, you would have operator intervention to unlock the quill and "run the handle" when you need to tap. For me, it wouldn't be a big deal since I'm already there manually applying tap lube during the operation anyway.

Someday down the road I may be able to afford to add a spindle encoder and purchase the software unlock for rigid tapping; but this would be a good alternative until then.

[Dave_C]

Short answer is that it is not likely to work well but if you used a floating tap holder, then maybe. Tormach did this on their mills prior to the newer models that have spindle encoders. Now they can ridged tap, re-enter a hole and so on.

I played with this idea when I was still using Mach 3 and I made a floating tap holder for the lathe tail stock. I made some air threads with it and it looked like it was going to work, then I trashed the whole setup and bought an Acorn.

Life has been a whole lot easier since!

Dave C.

[CRM]

The entire quill is a floating holder in my scenario. I am not trying to slave an axis drive motion to spindle rotation. The W axis (quill) is not powered. It is simply a DRO. All I really want the control to do is watch the depth that the tap is pulling the quill to, then reverse the spindle at the appropriate time. Machinists have been tapping like this on Bridgeports for millennia. Throw the tap in a drill chuck, shift to back gear, turn the spindle on, lower the quill to the part and let the tap bite in and start pulling. Bang reverse at just the right time, guesstimating the coast amount it takes to reverse the spindle.
And I already have the mill converted over to ALLIN1DC; Acorn would be a step backwards.

[Dave_C]

Ok, I've done that as well in the past. It works pretty well on through holes but not so good on blind ones.

Dave C.

[CRM]

Which is why letting the CNC's cat-like reflexes hit reverse lightning quick and repeatably is an idea worth pondering.

[mikebramel]

Sounds like power tapping on a bridgeport.... instead of using the bolt circle on the dro you're using the servos to move to position?

Tap in a drill chuck will let it slip when it hits bottom. Youd need manual control of the spindle, the quill, and have to do a fake tool change or something after every hole so it didnt advance too quickly?

[CRM]

But that's the point, Mike. Centroid has control over positioning and spindle speed/direction. I ran a Bridgeport E-Z Trak 2 axis CNC knee mill that did NOT have spindle or Z axis control (only Z axis DRO on the quill). We would run a conversational program for a hole pattern or bolt circle, once to spot drill, once to drill, and once to tap. Because it was a 2 axis machine, the parser would look for every z axis position change and halt X &Y movement until the cycle start button was pressed again to acknowledge the operator had positioned the z axis to the new position.
Since the machine I have the Centroid control on is a bed mill with a Bridgeport style head mounted on the Z-axis ram, it should be possible to write a PLC program to utilize the manual quill with a glass scale readout (W axis) to watch that readout and reverse the spindle at the appropriate depth during a G84 cycle. My problem is that I don't understand how to write the PLC logic.
Do you really trust "slipping" the tap in a drill chuck (never mind that it definitely doesn't work with keyless chucks)?
All the data to do what I want is contained in the canned cycle parameters. Spindle speed would already have been set and rotation initiated with the Sxxxx and M3 words. The servo driven Z axis (the ram) would position to the R-plane after X & Y position to the hole location. At that point, the PLC parser needs to halt program execution and start monitoring the W axis (the quill) for movement by the operator (this would be movement in the negative direction). Just like when you power tap manually on a Bridgeport, the tap is going to engage the workpiece and draw the quill down. The PLC will be watching this movement, and when the proper depth (defined in the canned cycle) is reached, it will send a reverse signal to the VFD and the tap will back out of the hole, at which point the operator (who will be holding light upward pressure on the quill handle) will feel the tap disengage the part and draw the quill back up to the top of stroke (quill home position). The PLC will see that and issue the spindle forward command again and position to the next XY coordinates where it will again wait for the operator to lower the quill into the part, and repeat the sequence until the G84 is canceled.
I can't see a logical reason this would not be doable with the right PLC programming. If any of the forum gurus can explain why it's not possible, I'd appreciate it.

[cncsnw]

You would need to make it a CNC macro, called with G65. You cannot make it automatically happen through G84. Therefore you would need to customize your postprocessor, in whatever CAD/CAM system you are using, to write out an appropriate G65 call.

Other than that, yes, you should be able to do what you describe with a suitable combination of CNC macro programming and PLC programming.

[frijoli]

.... to write out an appropriate G65 call.

Other than that, yes, you should be able to do what you describe with a suitable combination of CNC macro programming and PLC programming.

Interesting. Now I have something else to take up my time.

[CRM]

You would need to make it a CNC macro, called with G65. You cannot make it automatically happen through G84. Therefore you would need to customize your postprocessor, in whatever CAD/CAM system you are using, to write out an appropriate G65 call.

Other than that, yes, you should be able to do what you describe with a suitable combination of CNC macro programming and PLC programming.

How does the Centroid control interpret Z axis moves when it is installed on a 2 axis mill with a Z axis DRO? Curious as to how it differs from the 2 axis EZ-Traks I ran years ago.