Backlash or something else??

[dphens]

Hi,

I have a new CNC machine and need some assistance. I am fairly new to the CNC world - been dabbling in it for about a year now and just upgraded to a 24x36 machine. I've been running some tests, making basic geometric shapes and verifying the size. Everything had been ok for the most part except circles. Larger circles appear fine, but smallers circles have flat spots on the left and right of the circle (along the X axis). I assumed I had some backlash, but when I use a dial indicator to try an measure it, I am getting results of .003 or less on the X axis. Is that enough to throw off a small circle (.5" or so)?

Are there any other issues that may also cause this?

If it matters, I am using Vcarve Pro to crate the gcode along with Mach3.

Any help is appreciated!

Dave

[RICH]

If you have backlash you will "egg" instead of a true cirlcle. I use backlash comp on the mill and it works well. Now i would suggest that you experiment some to find just what your machne will do. This way you can cut under or over and use finish cuts to get the od or  id very close to dimension.

Did you really mean " .5" off...........? 

RICH

[Hood]

I would think you would probably  notice it more in smaller circles as you change direction in a much shorter distance.
How much of a flat is there? Also worth checking for flex as their will likely be more pressure in small circles as opposed to large.
Hood

[ger21]

It could be flex in the machine. It won't show up when measuring backlash, but can act like backlash when cutting.

[dphens]

No, I was indicating that circles .5" or so in size is where I was seeing the problem.

I'll have to measure the flat spot. How can spot or measure flex?

I'll play around with the backlash comp too and run a few tests.

Thanks for the replies!

Dave

[RICH]

Sorry about that......I can't read but certainly can type

RICH

[dphens]

No problem Rich. I think that sentence was a little misleading. 

[dphens]

OK, I added .003 to the X axis for backlash comp and it's definitely helped. Still not perfectly round, but I can seem to find a number the eliminates it all. I've toyed with the idea of a DRO for the X axis so that I could see how far the gantry moves. Do you think this would help me determine more accurately how much backlash I have?

I was/am hoping to use the machine for small engravings, with letters less that .25" high. The backlash my make this unfeasible though.

[Hood]

Not really sure what you mean by a DRO for the X axis?
Hood

[dphens]

Sorry. I've seen digital readouts that you can add to tooling equipment. An external device that measures actual axis movement. Seems to be something more geared to retrofitting a manual mill, but thought it might be a neat project to try (especially if it gave me more accurate distance measurements).

Like these: http://www.hobbymetrix.com/scales/digitalscalemain.htm