rigid tapping fail advice and "discriminant" err

I just tried the ngcgui subroutine for rigid tapping on my lathe. It was a first attempt so I didn't really expect success and indeed I wasn't disappointed

I was using a M8 1.25 tap. I drilled a 17/64th hole first. I entered these parameters in the dialog:

1 (mm)
1.25 (pitch)
2 (start point)
-10 (end of thread - wanted 10mm deep)
1000 (rpm)
4 (tool)
9 (no coolant)

It was fairly fast (is 1000 rpm perhaps too fast?), it seemed like it struggled a bit as it went in near the end, but that could be a perception and not reality. What did happen though is that it didn't come all the way out and the turret moved to machine 0 which broke the tap violently. I noticed that the machine zero position in Z on the machine was screwed up too. Also when it was done I had this error on the screen: "Disriminant -0.012892 <0 in velocity calculation!"

I then tried a bunch of cutting in air and I often, but not always, get this error (with a different first number each time). I have tried air cutting at 275 rpm (slowest my spindle will go) and 2500 rpm and I get this error quite often.

Any advice on rigid tapping and what this error means?
Thanks,
-Tom

BigJohnT wrote:

Not sure about the error but 1000 RPM is pretty fast for rigid tapping. Your machine would need some high performance servos to keep up with that I'd assume. I generally tap at 200 to 300 RPM. You need to tell us more about the machine like spindle encoder count and stepper or servo axes etc.

JT

The machine has 5-phase stepper motors on the axes. The 3HP spindle can run from 275rpm to 3400 rpm using a Automation Direct GS2 VFD. The encoder on the spindle is a disk with 150ppr plus an index.
-Tom

It uses a Mesa 6i25 for step generation.
-Tom

BigJohnT wrote:

Hardware step generation should be able to keep up fine. Are you using a 7i76 with the 6i25?

And the most important question I forgot is what version of LinuxCNC are you using?

JT

I have 7i84 and a 7i85S daughter cards. I am using Linuxcnc version 2.7.0~pre6.

Could you give us a short gcode example that shows the discrim error? (and your ini file)

If you are energetic - you could try this branch and let us know if it fixes your issue. (it is not perfect yet)

github.com/robEllenberg/linuxcnc-mirror/...gent-improvement-2.7


sam

skunkworks wrote:

Could you give us a short gcode example that shows the discrim error? (and your ini file)

If you are energetic - you could try this branch and let us know if it fixes your issue. (it is not perfect yet)

github.com/robEllenberg/linuxcnc-mirror/...gent-improvement-2.7


sam

Sam,
Attached are the ngc file I run and an image of the error it generated. Again, this doesn't happen every time, but does happen quite often.
-Tom