How do I engrave slate coasters or a slate sign on a CNC router?

Why slate is different

Slate tiles are split, not machined, so the faces are neither flat nor parallel. Any pointed bit cuts a wider line where the surface is high and a thinner one where it is low. The project is mostly about reducing that variation.

Steps

1. Blanks. 4 by 4 inch slate tiles sold for backsplashes work. Sort them and keep the flattest.
2. Level each tile. Slide thin shims under the low corners until the tile does not rock, then hold it in a simple pocketed jig or against stops so every coaster sits in the same place.
3. Bit. Makers who started with a 90 degree V-bit reported uneven line widths and moved to a tapered solid carbide engraving bit. A steeper tip changes width less for the same height error.
4. Artwork. Import and trace the image, or type text. Bold shapes read better than hairlines on a rough surface.
5. Toolpath. Quick Engrave suits this job. Outline runs the tip along the vectors; Fill hatches inside them, and its stepover is set on the form, not taken from the Tool Database.
6. Depth. A reported working recipe is 0.005 inch per pass at 25 inches per minute, to a total of 0.020 inch. The extra total depth is there so the low areas of the tile still get marked.
7. Zero Z on the highest point of the tile, or on an average spot once you have shimmed it level.

Alternatives

• A spring-loaded diamond drag bit follows an uneven surface by itself. In Quick Engrave it uses a Pressure setting instead of a depth.
• The Use Nose Cone option exists for the same reason: material that is not flat.

Pro tips

• Slate dust is abrasive. Vacuum it; do not blow it into the machine's rails.
• Stick adhesive-backed cork on the underside so the finished coaster does not scratch furniture.
• See the entry on the Quick Engrave toolpath for the fill pattern options.