Surfacing and flattening toolpaths
A surfacing toolpath sweeps a wide flat bit across an area in overlapping passes to make it level with the machine's own plane of motion. Use it on the spoilboard so your work sits true, and on slabs to flatten them.
What surfacing actually achieves
It does not make a surface flat in an absolute sense. It makes it parallel to the plane the machine moves in, which is far more useful. After surfacing, a Z depth of 0.125 inches means the same thing everywhere on the bed. Everything downstream, cut depth, V-carve depth, through cuts, gets more accurate at once.
Setting it up
| Setting | Guidance | Why |
|---|---|---|
| Bit | A surfacing bit, 1 to 1.5 inches for most hobby machines | Wide flat cut, few passes |
| Depth per pass | Very light, often 0.01 to 0.03 inches | You are skimming. Deep cuts with a wide bit stall routers. |
| Stepover | 40 to 70 percent | More overlap hides tramming error and leaves a better surface |
| RPM | Lower than you would use for a small bit | The edge of a wide bit is already moving fast. High RPM burns. |
| Feed | Brisk | Slow feeds on a wide light cut means rubbing and burn marks |
| Direction | Consistent, and consider a final pass across the first direction | Crossing passes evens out any directional pattern |
Reading the result
| What you see | What it means |
|---|---|
| Even sheen, no pattern | Trammed and flat. Done. |
| Regular ridges between passes | Spindle tilt. Tram it, do not just reduce the stepover. |
| One side of each pass deeper than the other | Tilt in a specific axis, and it tells you which way to adjust |
| Untouched patches | The surface was lower there. Take another pass a little deeper. |
| Burn marks or a glazed look | Feed too slow or RPM too high for the bit diameter |
| Chatter marks | Depth too heavy, or the material is not held flat underneath |
Surfacing the spoilboard
Take the least material that gets you a clean surface everywhere. Every pass is thickness you will never get back, and a spoilboard has a finite number of resurfacings in it before it is too thin and has to be replaced. See spoilboards.
Surfacing a slab or a workpiece
Different problem: the piece is not flat and you want it flat, and the machine can only reference its own plane. The key is that it must not rock or flex. Shim it, wedge it, or hot-glue it so the high points cannot move, flatten the top, flip it onto the now-flat face, and flatten the other side. See slab flattening for the whole process.