Circle problems

[mrmfwilson]

I have a brand new Legacy CNC.  I am trying to cut a 3" circle, .06 deep with a .125 bit.  I can run the cut as slow a 10ipm and still get the same results.  Has anyone seen something like this?  The machine uses a Smooth Stepper USB interface.  I have tried using arcs and also raster and get the same results.  Some folks say it is backlash.  The manufacturer says its probably not.  One opinion is that the Mach data is getting scrambled.  The movement is so smooth that it looks intentional.  Using the lock down version of Mach and the Smooth Stepper driver/plugin.

[Hood]

Looks like backlash to me on Y axis.
Hood

[Picengraver]

I vote with Hood.  I've personally experienced the same problem, even with new anti-backlash nuts.  Tracked it down to a combination of machine flex and sticky axis bushings on my machine, but it can be any of several mechanical problems.  It is not a Mach3 problem.  Disconnect the axis drive and see if it moves smoothly end to end by hand.

Please let us know what you find.
Regards,
John Champlain
www.picengrave.com 

[mrmfwilson]

You are correct.  .065 backlash in both directions.  Actually a bad stepper motor.

[zephyr9900]

You're not implying that the axis is using the stepper motor bearings as a mount for the leadscrew, are you?  That would be a serious design flaw.  Stepper motor bearings are only there to hold its shaft central in the windings, and usually have endfloat built in to protect against binding as the motor heats up in operation and thermally expands.  The leadscrew must be separately supported, both radially and axially.  The motor is only there to turn the leadscrew.

Randy

[zephyr9900]

Ahhh, strike that last comment.  I just saw that Legacy machines are rack-and-pinion drive.  I'm sorry for the leap of assumption. 

Randy