rigid tapping fail advice and "discriminant" err

More
25 Jul 2015 06:08 - 25 Jul 2015 06:10 #60899 by tome
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
Last edit: 25 Jul 2015 06:10 by tome.

Please Log in or Create an account to join the conversation.

More
25 Jul 2015 21:41 #60908 by BigJohnT
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

Please Log in or Create an account to join the conversation.

More
25 Jul 2015 23:22 #60913 by tome

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

Please Log in or Create an account to join the conversation.

More
25 Jul 2015 23:36 #60917 by BigJohnT

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


A 150ppr should be ok, 1000 RPM on a stepper driven machine I would not think possible. Are you use software step generation (parallel port) or hardware step generation (Mesa or Pico card)?

I run a GS2 on my Anilam 3 axis servo converted BP knee mill and rigid tap on it all the time at 250 RPM by running the motor at 900 RPM and setting the speed dial to 500. IIRC the encoder is 1000 or 1200 ppr.

JT

Please Log in or Create an account to join the conversation.

More
25 Jul 2015 23:46 #60918 by tome
It uses a Mesa 6i25 for step generation.
-Tom

Please Log in or Create an account to join the conversation.

More
26 Jul 2015 01:14 - 26 Jul 2015 01:14 #60923 by BigJohnT
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
Last edit: 26 Jul 2015 01:14 by BigJohnT.

Please Log in or Create an account to join the conversation.

More
26 Jul 2015 02:11 #60927 by tome

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.

Please Log in or Create an account to join the conversation.

More
27 Jul 2015 17:46 #60953 by BigJohnT
I got in touch with the developers as I was stumped and got this reply.

we are in the process of fixing that problem. A sample of gcode that shows the problem would help.
rob has a branch that takes care of the issue - but causes a few violations he hopes to look at this week.


JT

Please Log in or Create an account to join the conversation.

More
27 Jul 2015 19:58 #60957 by skunkworks
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

Please Log in or Create an account to join the conversation.

More
28 Jul 2015 01:43 - 28 Jul 2015 01:47 #60965 by tome

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
Attachments:
Last edit: 28 Jul 2015 01:47 by tome. Reason: adding .ini file

Please Log in or Create an account to join the conversation.

Time to create page: 0.294 seconds
Powered by Kunena Forum