Circular pockets not cutting round - Help!

[watsonstudios]

Here is a diagram of what is happening. I cut some 3/8 circular pockets in a practice part and they're not coming out perfectly round. The flat areas are in the Y direction - does this mean I need to adjust for some Y backlash? I'm using Mach 3. Becausr the flat areas are in the Y direction - it's like the Y is not moving far enough out on each end of the circle. The circle in the X direction is accurate. Is this a issue of backlash?

[watsonstudios]

Problem Fixed!

I found out that my Y axis linear bearings were way too tight. Once I disconnected the Y motor, I could barely move it along the rails. The linear bearings have small set screws that will tighten the bearing around the rail - so I loosened them up so I had free movement along Y and did another circular pocket test and now all is well. -It's a good thing I fixed it because the backlash config in Mach was a little screwy - once activated, the job did not run properly.

[Buzz9075]

I tried using the back lash in Mach3... NO GO it did a really bad job on stuff, all kinds of funny results depending on what I was cutting. If you are looking at backlash being your problem correct the mechanical problem.

[alexccmeister]

Me too. I used a dial gauge to read my backlash. It worked out about 0.8mm but when I put this value into mach3, the backlash seems smaller than what I put in. The circle isn't very circle. I am using the stock leadscrew. Got to change to ballscrew.

[Buzz9075]

I converted to backlash device from www.dumpstercnc.com device and it appears to be doing a good so far.

[wcarrothers1]

Just as a note.. Backlash support in MACH is NOT for cutting procedures. It is for spot drilling and alignment of drilling type procedures..

Basicly it alows mach to go at a point from both driections before setteling on the actual spot (ie auto cancel of the backlash)

Try to Imagin what it is trying to do when it's trying to resolve comming from both points of the backlash while trying to cut a line or curved line. Really can't be done and isn't what it was designed for.

If you use it for what it is intended you will notice more accurate hole drilling locaitons anyway.

b./

[NC Cams]

This subject was discussed extensively in an EzTrak thread i brought up quite some time ago.

Circular milling requires very careful tuning of the servo's/steppers and much care/attention to friction and clearance/preload of the ball screw thrust bearings. Lost motion due to clearance or binding will cause hang ups, especially at direction reversal

Do an "ExTrak" and "ball screw" search via the search engine. You might also try "circular milling" search.

NOTE: backlash comp WON'T fix slop or stick-shun of tables when you have direction changes with CNC equipment.