I have been cutting lithophanes on my machine, and until recently all has been going well.

Now I am getting a doubling along the edges.   Using artcam to generate the code, reviewing the
toolpath in mach it looks ok, but doesn't come out ok.

Oddly I cut a smaller one (about 2.5" x 2") and it appears to not have the issue, which kinda confuses what
it might be in my mind...

Using a Gecko 540 as controller, and the room has been very hot at times recently, I thought that might be part
of the problem, but ran one while it was cool, and got the same issue.  Running a raster along the X (which is the
longer side of the image)..

You can see the doubling along the edges of the flower petals in the attached image.Any ideas?   

Very nice litho.
I cut a lot of lithos and have never had this problem.

I would be inclined to look for a machanical issue in the Z axis (also the X axis and spindle motor while you are doing it) like backlash, slack in the motor coupling, slack in the thrust bearing, loose parts etc. etc.

Hope this helps.

Tweakie.

I couldn't understand how a mechanical issue could cause it.  But you might be right.   The X axis is dual R&P, and I pulled both pinions down and reset them in the rack, and am re-cutting the rose litho now (with a 1/8 bit instead of 1/16 since I broke my last 1/16), and it appears to be working...  will let it run for a while and take another look and see how it is going.  Thanks for the input.

I will keep my fingers crossed 

You must post some pictures of your lithophanes perhaps under the 'What you have made with your machines' section we all love pictures.

Tweakie.

It looks exactly like backlash in the X axis. I bet dollars to doughnuts if you looked under a magnifying glass, you would see that every other pass is offset. I would also presume that if you cut in the Y direction, the problem would disappear, or at least be not as pronounced. Be sure to check the motor connection to the shaft as well. Something has definitely worked its way loose. Great looking rose, by the way.

Well, I cut most of the rose out with an 1/8 bit...  it appears the issue is still there.

The X is rack and pinion driven, and when the steppers are on, I can not move the gantry in the x position.   I did notice that there is a very slight amount of play in the Y... perhaps wear on the plastic anti-backlash nut, will order a new one to replace it...  but I think it's been like that for a while, so not sure that is what is causing it.   I suppose I will have to tear down the machine and do a good cleaning either way.

And recheck that everything is good and tight so there is no slipping.     

Tweakie, I'll go put up some pictures of lithos I have done...

I went over the machine... seems all is sturdy, can't twist or move anything.  So I started another with the 1/8 bit... but I got down close to watch.

One pass (going -x) it is all fine, then it moves over slightly on the y and goes the other way (+x)...   this is where the trouble happens.  I took a video and it
is very obvious.  The little bit of play in the Y is far smaller than how much you see the bit slip over... so I am not sure what is going on...   

To test, I manually ran the bit -x to cut, then over a little on the y then back up +x real slow.. does the same thing... actually, it jumps over within the first 1/4 inch when only
moving about 20ipm.    But I kept running it out that way, it obviously cut nothing, then without changing the y, ran back the other direction on X, and it moved back into place
and cut away the edge as it should have when moving the other direction...

Here is the video:

http://vimeo.com/14307039

Did one cutting on the Y, you are correct Sam, works fine that way.

Wow, that is indeed allot of slack.

Ok, think I got it fixed.  I was pushing and pulling and twisting everything I could, and the gantry could be twisted across the x rails.  I loosened up the carriages and move them back into place with clamps and tightened, and it appears to of taken care of the problem.   Thanks to everyone for the input.    Now I just need my new 1/16 ball nose mills to show up since I broke 3 fiddling with the machine trying to figure out the problem (all brain farts).