If you’re off by 1/8" and using a 1/8" bit, the first thing I’d check is that you’re cutting on the correct side of the line (such as cutting outside the line instead of on the line). That should provide the largest potential source of error that isn’t due to lost steps, and is usually an error that you can figure out because it is a multiple of the bit diameter.
Smaller sources of error are incorrect bit diameters and uncalibrated stepper settings (steps/mm):
- You should measure your bits with a micrometer or calipers and use the measured size as your bit diameter. It’s very rare to find a 1/8" bit that is actually .125", and so on. Another way to measure the effective bit diameter is to cut a single line with the bit and measure the size of the line itself.
- I have made a video that shows one method of calibrating steppers that you may find useful, it’s in the Maintenance and Troubleshooting thread: X-Carve Maintenance/Troubleshooting Videos - Add Your Own!. There’s other ways to do it, but this was the way that worked best for me.
These should help, assuming you’re not losing steps. If you’re losing steps, it will typically (but not always) show up in only one axis. Typical sources for lost steps are loose pulley screws, tight V-wheels, tight/loose bolts, and incorrect pot settings. If it looks like your project has been scaled in only one direction, or if the solutions above don’t work for you, a good thread to check out is the Guide to Lost Steps: [Guide] Offset cutting, Lost steps, Positioning errors, etc.
Good luck, and let us know what you figure out!