How do I flatten an epoxy pour or river table on the CNC without melting or chipping the resin?

Why epoxy is different

Cured epoxy is a plastic. Laguna's guide puts it plainly: the goal is to cut it, not melt it, and the sign you have it right is chips and shavings coming off the cutter, not dust. Dust and a gummy bit mean heat, which comes from a feed that is too slow or a dull cutter.

Published setup (Laguna Tools)

1. Bit: a large flat bottomed surfacing bit. The guide uses a 2 inch spoilboard bit and notes that nearly any flat bottomed bit works.
2. Toolpath: a Pocket toolpath over a rectangle that covers the whole piece.
3. Speed and feed: 12,000 RPM, with 60 inches per minute as a starting feed for a four flute bit.
4. Depth per pass: 0.03 inch.
5. Entry: turn on Ramp Plunge Moves. The guide uses a 4 inch ramp so the wide bit never drops straight into the resin.
6. Z zero: zero on the highest point of the uneven surface.
7. Repeat: after a pass, zero Z on the new surface and run the same toolpath again until the whole face is cut.

Hold-down

The guide traps the piece between scrap blocks at each end and drives a wedge with a 5 degree taper to lock it. Nothing sits above the surface for the bit to hit. Shim any rock out of the slab first so it cannot shift mid cut. See the entry on flattening a warped board or slab.

After the CNC

A surfacing pass leaves tool marks. The guide removes them with a drum sander or a random orbit sander before finish sanding.

Pro tips

• Let the resin reach full cure first. Cure time depends on the product and the pour depth, so use the time on the resin maker's data sheet. No single number is given here because none was confirmed for all resins.
• Light passes at a brisk feed beat one heavy pass. If the bit starts to load up, stop and clean it before the built-up resin scars the surface.
• Draw the pocket rectangle larger than the piece so the bit runs off the edges instead of turning around on the work.
• For filling carved letters with resin, see the entry on epoxy inlay fills.