[tshumay]

I'm experimenting with my new ShopBot and trying to solve a bit of a mystery - I suspect the answer to this may be pretty obvious, so I thank you for your patience in advance.

Here is the piece I am making - cut from scrap 2x4 (excuse the two screw holes):

Attachment 253726 Basically, it's two cylinders - one added to a rectangle shape, and the other subtracted. They are perfectly identical and round in Rhino.

I am doing two passes - one roughing pass with a 0.25" end mill, and a finishing pass with a 0.25" ball end. Everything works well. I preview the passes in RhinoCAM and they look good. Export and preview in the Shopbot software and everything looks good.

However, when I cut, the two cylinders are not perfectly round. If you look at the profile of the shape, you can see that the left side of the cutout cylinder looks fairly round, but the right is stretched:

Attachment 253728

Because it looks fine in both previews in RhinoCAM and the ShopBot preview, I am assuming the fault is something mechanical or to do with my tool selection?? Is it that my selection of a 0.25" ball mill is too large for this radius of curve, or am I missing something else?

Thanks!

[IN-WondeR]

I would have said a mechanical problem. Either a grub screw loose or something like that. But when all else looks fine, it must be a programming fault.

It looks to me like that the two curved surfaces, both the top part, going round over, and the bottom part where it is routed inside, is stretched in the same direction, or should the area in the top of your piece look like that??

[tshumay]

That is correct, they are both stretched in the same direction. The parts should be perfectly round - I modelled them from cylinders, but for some reason, when I cut, I get the stretch as you noted. In preview/render, it looks round as well. The problem occurs only when I run the program on the CNC

[ger21]

Looks like some type of backlash in the Z axis maybe.