Okay, you’re making a very deep, slotting cut.
First, check the machine mechanically. Per the machine operating checklist: Machine operating checklist , the basic points of adjustment for a machine are:
- Pulley set screws: http://docs.carbide3d.com/shapeoko-faq/shapeoko-3-how-to-check-the-pulley-set-screws/ — be sure to check all axes/pulleys including Z.
- V wheels / eccentric nuts: Tightening Eccentric Nuts - Carbide 3D
- Belt tension (see the relevant step in your instruction manual, e.g., Getting Started with Carbide Machines) Note that the X-axis motor is held in place on standoffs and if those bolts are loose this can cause belt tension issues. Also, belt tension for the Y-axis stepper motors needs to be even/equivalent on each side — a significant difference can cause skipping on one side eventually resulting in lost steps on both. Measuring belt tension, squaring and calibration
and then try off-setting to the outside to add geometry so as to cut as a pocket — where possible avoid slotting and add geometry and cut as a pocket
and/or
and consider leaving a roughing clearance and taking a finishing pass.