Going That Extra Inch

Some odd behavior from my machine, and I’d like to see if anyone else has experienced this and perhaps has an explanation. Using Aspire, I made a design that goes on a 6"x24" surface with about 1/4" clearance top, bottom and sides. I saved the toolpaths using the x-carve (inch) post-processor from Vectric, and used UGS for the project. I homed the spindle and then sent it to XY zero, which is the same every time. It found it correctly and I used my z-probe to set Z for the v-carve bit. When it started the cut, the machine added about 0.8" to the Y starting point, which pushed the top of the design off the material. I hit cancel and told it to return to Zero, but when it did, it was 0.8" off zero on the Y axis. For curiosity I repositioned it to the original XY zero, changed the bit for a profile toolpath, and ran the second toolpath. This time it started where it should have, which of course meant it was .08" off from the first cut.

Anybody have some insight into this?

I have the X-controller. Latest drivers, as far as I know. I installed them about three weeks ago.

Thanks, guys. The screws were tight but the belts were loose. Keeping my eye on the offset checkbox. Haven’t had the problem again through several runs.