Surfacing bits and slab flattening bits for CNC
A surfacing bit is a wide flat cutter, usually one to two inches across, for flattening large areas in few passes. Diameter is the main decision: bigger covers ground faster but demands more power and a well trammed spindle, because any spindle tilt shows up as ridges across a wide cut.
What they are
A wide cutter with a flat bottom, designed to remove a shallow layer over a large area. The two common forms:
| Insert type | Brazed carbide tipped | |
|---|---|---|
| Construction | Steel body with replaceable carbide inserts | Carbide tips brazed to a steel body, one piece, nothing to misassemble |
| When dull | Rotate or replace the insert, cheap | Replace the whole bit, or resharpen |
| Typical sizes | 1 to 2.5 inches | Usually 1 to 1.5 inches |
| Finish | Very good if the inserts are level with each other | Very good |
| Cost to own | Higher upfront, cheaper long term | Lower upfront |
| Note | Inserts must be seated clean. One high insert does all the cutting. | Nothing to misassemble |
Choosing a diameter
Bigger covers more ground per pass, which means fewer passes and less time. It also means:
- More power needed. A 2 inch cutter in hardwood asks a lot of a trim router.
- Lower RPM required. The outer edge of a wide bit is already moving very fast. High RPM on a big surfacing bit burns.
- Tramming errors get magnified. A spindle tilted by a tiny angle leaves a visible arc across a 2 inch cut, and the ridges between passes get taller with diameter.
Rule of thumb For most hobby machines with a trim router, a one inch to one and a quarter inch cutter is the practical sweet spot. Larger cutters suit spindles with real low-speed torque.
Settings
Starting point Surfacing is a light cut over a large area, so depth is small and the limits are heat and tramming rather than load.
Why your surfaced finish has ripples
Almost always tramming, not the bit. A perfectly trammed spindle cuts a flat circle. A tilted one cuts a shallow dish, and the overlapping dishes leave ridges. If your spoilboard shows a regular corduroy pattern after surfacing, go and tram the spindle before you do anything else. Reducing the stepover just makes finer ripples.
The finishing pass that makes it look machined
Surface in two stages: a normal pass to get flat, then a final pass removing a very small amount at a slower feed with a generous overlap. The last pass is not removing material so much as leaving a surface, and it is what produces the even sheen people associate with a properly surfaced slab.