I really really need some assistance with this. I have been playing at this for many months and spent a ton on birch ply. 95 percent of my cuts shadow cut and end up being garbage. I have tried to slow down the feed rate and cut depth to have exactly the same outcome. It doesn’t matter the size or type of the mill bit either. Right now I am extremely disappointed and can use a little bit of expertise. I attached a photo below for an example. I was just trying to make a gift for my sister in law. The mill makes a pass all the way around and then it deviates off the path…
It is hard to tell from your picture exactly what is going on, but it sure looks like you are losing steps badly. There are 3 main machine reasons this can happen.
your belt tension is not correct. Be sure all belts are “Guitar string tight”
Your Pulleys are slipping on the motor shaft. Be sure the set screws are tight
The voltage setting for the motors on the GRBL board are not correct. Search the forum for instructions on how to set them.
You can also loose steps if you are trying to cut too deep or move too fast. Keep your depth of cut to about half the diameter of your bit and keep the feed rate around 40 ipm (at least till you figure out what is going wrong)