Why is my bit digging into my material during the roughing pass on a 3D carve?

First, find out who is doing the digging

Reset the preview, then preview the roughing toolpath on its own.

• The preview digs in too. The toolpath is cutting exactly what it was told to. The cause is a setting: see Software causes.
• The preview looks right, but the real cut digs in. The file is fine and the cause is at the machine: see Machine causes.

This one check saves you from changing toolpath settings to fix a slipping bit, or tightening collets to fix a setting.

Software causes (the preview shows it)

1. Z zero does not match the job. The material thickness and the Z Zero Position (material surface or machine bed) in Material Setup must match how you zeroed the machine and how thick the board really is. If they disagree, every pass lands deeper than planned. See the entry on Z-zero mismatch.
2. It digs around the outside of the model. With a Material Boundary or a large Boundary Offset, the tool clears the field around the model down to the bottom of the model's height range. Use Model Boundary or a selected vector with a modest offset, or add a Zero Plane component as a floor. See the entry on stopping a 3D toolpath cutting too deep around the model.
3. The model sits deeper than you think. If you see "Model thickness exceeds material thickness", or a component reaches below the zero plane after tilting or changing its base height, the toolpath follows the model down. See the entry on that message.
4. No Machining Allowance. The official documentation describes the allowance as a virtual thickness added to the model so roughing leaves a skin of extra material. Set to zero, the large roughing bit cuts right to the finished surface, where aggressive cuts are more likely to chip it.
5. Pass depth too big for the bit. Z Level roughing steps down by the Pass Depth set for that tool in the Tool Database. A pass the bit or machine cannot handle loads it up and pulls it down into the work. Lower the Pass Depth for that tool and recalculate.
6. Straight plunges. Turn on Ramp Plunge Moves in the roughing form. The official documentation says ramping in reduces heat that damages the cutter and the load on the spindle and Z axis bearings.
7. The toolpath uses a different bit than the one in the spindle. The software assumes the bit in the collet matches the tool in the toolpath, at the diameter in the Tool Database. Check the toolpath's tool before you run it.

Machine causes (the preview is right)

1. The bit is slipping in the collet. Dust in the collet or an under-tightened nut lets the bit creep down under load, so the cut gets deeper as the job runs. Dark marks or grooves on the shank usually mean slippage and a worn collet. Clean the collet, nut and spindle taper, seat the shank about 80% of the collet's depth, and replace a worn collet. See the entry on collet depth and collet care.
2. The Z axis is losing steps or dropping. Depth drifts deeper partway through the job. Check the Z coupler screws and make sure the lead screw cannot slide up and down. As a test, cut the feed rate and pass depth by about 30% and rerun. If the problem stops, the settings were too aggressive for the machine. See the entries on lost steps and on telling software from machine faults.
3. The board is not flat or not held down. A cupped or uneven board cuts deeper where its surface is low, and a board that lifts or shifts gets cut in the wrong place.
4. Z was not re-zeroed after a tool change. If the roughing and finishing bits are different, Z zero has to be set again for each bit. See the entry on wrong depth after a tool change.

How to confirm it is fixed

• Preview the roughing toolpath alone after a reset. It should stay above the model by your Machining Allowance and stay inside your boundary.
• If you are unsure, run an air cut. The official documentation suggests jogging Z up to a point where the toolpath's maximum depth will not reach the material, setting Z zero there, and running the toolpath to watch the moves.

Pro tips

• Roughing and finishing work as a pair. For which strategy to use and why a finish pass can bite too deep after Z Level roughing, see the entry on Z-Level versus 3D Raster roughing.
• For bit sizes, stepover and feeds on a first 3D carve, see the walkthrough entry on your first 3D carving.