Hello Guest it is May 06, 2021, 12:57:07 PM

Author Topic: Egg shaped circles  (Read 4242 times)

0 Members and 1 Guest are viewing this topic.

Egg shaped circles
« on: June 11, 2012, 03:54:48 PM »
Hello everyone,

Well, I’ve got my NC router back up and running again.  It was down for a short period after Initial testing to work out a few issues, to finish construction and for painting.

Everything works good except for one small problem… I have a small amount of backlash in the ‘Y’ axis that I can’t seem to get rid of.  With the machine ‘powered up’ there’s about a ¼ inch free travel in the ‘Y’ axis.  The ‘X’ and the ‘Z’ axes are working fine.  If you cut a circle this backlash causes a small flat spot in the ‘X’ at the top and bottom of the circle (See attached pdf).

It looks like to me that the problem might be in the motor.  I thought, at first that the backlash was caused by a Lovejoy spider coupling between the motor and the ‘Y’ axis shaft.  I have since changed out the spider coupling with a spiral cut anti-backlash coupling and the problem still exists.  If you watch the mechanism while moving the carriage by hand with the machine ‘powered up’ you can trace the movement all the way back to the motor shaft.  I have a 400 oz in stepper motor operating the ‘Y’ axis.  From what I understand that motor is supposed to hold tight 400 oz in of force… I’m I right??  While moving the carriage by hand, you can feel a definite hard stop on either side of the free travel.  If you keep applying more force, you can feel it jump over the hard stop to the next one.  Also, while the motor is ramping up during jogging, you can detect a slight ‘jerking’ in the ‘Y’ direction.

I think that the problem could be caused by one of several things.  One is, the chopper board for this motor set for 1/8 step.  Should I try changing it to ¼ step or 1/16 step?  Could it be a bad motor or a problem with the motor wiring?  Could it be a motor tuning problem?

What do you think?  Does anyone have any ideas how to solve this problem that I could try?  Please help!!

Thanks,

Offline budman68

*
  • *
  •  2,354 2,354
    • View Profile
Re: Egg shaped circles
« Reply #1 on: June 11, 2012, 05:46:40 PM »
You could try swapping the X and the Y motors and see if it then repeats on the X like it did the Y axis.

Dave
----------------------------------------------------------------------
Just because I'm a Global Moderator, don't assume that I know anything !

Dave->    ;)
Re: Egg shaped circles
« Reply #2 on: June 11, 2012, 10:09:44 PM »
Thanks Dave.  I'll try it.  Hum... I wonder if I could swap the wires at the chopper board?

Dale

Offline Graham Waterworth

*
  • *
  •  2,322 2,322
  • Yorkshire Dales, England
    • View Profile
Re: Egg shaped circles
« Reply #3 on: June 12, 2012, 06:04:14 AM »
You need to remove the backlash or you will never get it sorted.

1/4" of backlash sounds like something is not bolted down properly.  Check the coupling is not slipping on the shaft and the ball screw nut is secure in the mounting.

Graham
« Last Edit: June 12, 2012, 06:09:33 AM by Graham Waterworth »
Without engineers the world stops

Offline RICH

*
  • *
  •  7,419 7,419
    • View Profile
Re: Egg shaped circles
« Reply #4 on: June 12, 2012, 06:16:50 AM »
Post some pictures of the y axis and we'll have a look. Need to see how the complete axis is put together.
The motor will have some play as they use bellvue washers in them to keep the rotor inplace but there is only a few thou of play
so that is not you problem. When the stepper is energized it will hold to the rating provided power is adequate.The problem is mechanical and not some setting.

Echo what Graham replied,
RICH

« Last Edit: June 12, 2012, 06:20:35 AM by RICH »
Re: Egg shaped circles
« Reply #5 on: June 12, 2012, 02:08:04 PM »
Rich and Graham,

I think I already determined that the problem wasn't mechanical...  I attached a small piece of tape on the shaft on the back of the motor and you can see the tape move with the translation.  I'll send pictures later...
The 'Y' axis is driven by a timing belt system coupled to the motor using a clamp type spiral cut anti-backlash coupling.
Re: Egg shaped circles
« Reply #6 on: June 12, 2012, 04:22:43 PM »
Hello everyone,

The tech support guy for Probotix says it could be caused by 'missing steps' in the motor, which is a motor tuning issue.  Anyone have any advice??

Thanks,
Dale

Offline RICH

*
  • *
  •  7,419 7,419
    • View Profile
Re: Egg shaped circles
« Reply #7 on: June 12, 2012, 05:05:53 PM »
Quote
I think I already determined that the problem wasn't mechanical
Ok, then no need to post a pic's of the axis.
Quote
Probotix says it could be caused by 'missing steps' in the motor, which is a motor tuning issue
What did they say to do?

RICH
Re: Egg shaped circles
« Reply #8 on: June 12, 2012, 09:46:51 PM »
Rich,

I'm going to talk to tech support tomorrow on the phone.  I'll let you know...

Thanks,
Dale

Offline rcaffin

*
  •  1,035 1,035
    • View Profile
Re: Egg shaped circles
« Reply #9 on: June 17, 2012, 06:04:19 AM »
Quote
The tech support guy for Probotix says it could be caused by 'missing steps' in the motor, which is a motor tuning issue.  Anyone have any advice??
Yeah - hang up on him. He hasn't a clue.

Either you have a huge amount of backlash compensation programmed into your system, OR you have a huge amount of mechanical backlash in your system
And the first option is not likely because of the direction it is going.

Cheers