Cutting plywood and Baltic birch on a CNC
Plywood is layers glued with alternating grain, so the top and bottom veneers want to splinter as the cutter exits them. A compression bit solves both faces in one pass. Baltic birch has many thin void-free plies, which is why its edges look good enough to leave exposed.
What plywood is, and why it misbehaves
Thin layers of wood glued with each layer's grain running perpendicular to its neighbours. That cross-banding is what makes plywood stable and strong, and it is also the source of every problem it causes:
- The face veneers are thin and unsupported at a cut edge. A cutter exiting the top surface lifts them; exiting the bottom surface pushes them out.
- Glue lines are abrasive. Plywood dulls bits faster than solid wood of the same species.
- Voids inside cheap plywood appear as holes in a cut edge and there is nothing you can do about them after the fact.
- Sheets are rarely flat and rarely exactly their nominal thickness, which matters when you cut all the way through.
Grades, and what they actually get you
| Type | Plies | Edge quality | Good for |
|---|---|---|---|
| Baltic birch | Many thin, void free | Excellent, often left exposed | Boxes, puzzles, layered art, anything with a visible edge |
| Hardwood plywood (cabinet grade) | Fewer, may have a core of softer material | Good face, variable edge | Cabinet parts where edges get banded |
| Construction plywood | Few, thick, voids likely | Poor | Jigs, fixtures, spoilboard backing, shop furniture |
| MDF core plywood | Veneer over MDF | Very smooth face, MDF edge | Flat panels where the face matters |
Getting clean edges
| Approach | Top face | Bottom face | Notes |
|---|---|---|---|
| Upcut bit | Splinters | Clean | Good chip clearing, wrong for a visible top |
| Downcut bit | Clean | Splinters on breakthrough | Chips pack in a deep cut |
| Compression bit | Clean | Clean | The right tool. Must cut deeper than its upcut section. |
| Onion skin, then finish | Clean | Clean | Leave 0.01 in and cut or sand it off. Works with any bit. |
| Score pass | Clean | Depends | A shallow first pass with a downcut severs the top veneer before the main cut |
For production cutting of parts from sheets, the compression bit is the answer and nothing else comes close. For occasional work with the bits you already own, the score pass and onion skin approaches get you most of the way.
Cutting parts from a full sheet
- Hold down the whole sheet, not just the corners. The middle of a 4 by 8 sheet will lift and chatter. Screws into the spoilboard in the waste areas are the simplest reliable method.
- Use tabs so parts do not come loose and get thrown, and place them on edges that are easy to clean up.
- Cut inner features first, outer profiles last. Once the outline is cut, the part is no longer held by the sheet.
- Watch for the sheet not being flat. A cupped sheet means the cut depth varies. Screws pull it down; vacuum pulls it down harder.
- Expect the last few parts to move as the sheet loses rigidity. Plan the cut order so the biggest parts come out first.
Bit wear
The glue in plywood is abrasive and it dulls carbide faster than solid wood. If you cut a lot of sheet goods, expect shorter bit life and budget for it. It is also the reason a bit that cuts plywood beautifully on Monday can be burning by Friday, and why the first suspect for a plywood cut that has got worse is the bit.