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Author Topic: Circles not round, rectangles out of proportion (slightly)  (Read 3168 times)

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Circles not round, rectangles out of proportion (slightly)
« on: April 03, 2012, 10:07:36 PM »
I just completed a new machine (CNCRouterparts 48x48) and am loving it over my old machine.  Ran some test cuts and calibrations tonight and have a question.

First some background:  All files created with Vectric VcarvePro, and I checked them for errors... they're good.  I calibrated the machine by having it drill a 4x4 square of plunge holes and measured with pins, plus used the Mach set-up for axis steps/inch.  It's good and square and the steps/inch are quite good.

The problem project was 9 copies of a 1.25 x 3 rectangle with a .26 hole in the middle and a slight pocketed hole around the through hole (for a washer to be pressed in).  Problem:  the pocketed holes are out of round, slightly off-center (could be a function of out of round) and are undersize by .020 or so.  Each one is off, and they're all slightly different. The rectangles are dead on 1.25 (all 9 of them)  but the 3" dimension is 3.010-3, all of them, exactly.   :(   Cut with a .25 downcut spiral.  The over-size dimension is a single axis- X.  No issues with rounded corners, they're sharp like they're supposed to be.

Other things worked great- a 4" square test of surfacing (bigger bit, slower travel) was dead on 4" both ways, and the test pin set-up is really good.

Is this a CV setting issue?  Suggestions?  I've double checked the drawings and they're correct.  But odd that not everything is doing it.

Re: Circles not round, rectangles out of proportion (slightly)
« Reply #1 on: April 04, 2012, 12:36:51 PM »
It's been a while but the last time I experienced a similar problem, I found that it was mill related and not my software. It was either something sticking or slipping, I don't recall exactly. My best guess is that it was at the motor - leadscrew coupler. But fortunately it was an easy fix once I located the problem.
David Morrow
Re: Circles not round, rectangles out of proportion (slightly)
« Reply #2 on: April 04, 2012, 01:04:02 PM »
Thanks David-

In all of the set-up business I never checked for backlash, and I have it.   ::)  It's a CNCRP rack and pinon set up on the primary axes.  I need to adjust the tension UP a bunch and try this all over again.    My bad for getting excited without checking all the simple-to-check items off the list first!

Not sure why the backlash appeared on the one dimension of the rectangle (and long!?) and not the other, but it certainly could explain the issues with the slightly out of round, not quite centered circles.