Cutting hardwoods on a CNC: maple, walnut, cherry, oak, ash, poplar
Maple and cherry burn easily and need a real chipload. Walnut machines beautifully and is forgiving. Red oak tears out at the pores and fuzzes. White oak is harder and cleaner than red. Poplar is soft, cheap and fuzzy, and it is the right thing to make mistakes in.
Species by species
Hard maple
Hard maple is dense, closed grain and pale, which means every scorch mark shows. It is the material that teaches you about chipload, because a chipload that gets away with pine will leave brown streaks in maple every time. Keep the bit sharp, keep the feed up, and consider lowering the RPM rather than slowing down. Soft maple is noticeably easier and looks similar in many applications.
Walnut
Walnut cuts cleanly, holds detail, resists burning better than maple, and it is dark enough that small scorches disappear. It is the default choice for CNC relief work for good reason. Dust is a known irritant for some people, so run collection.
Cherry
Machines well and holds detail nearly as well as maple. It burns readily, and cherry burns are especially visible because the wood is pale when fresh. It darkens with light exposure over months, which is worth mentioning to customers so they are not surprised. Sharp tools and a real chipload are the whole story.
Red oak
Open, coarse pores that run through the board like straws. Consequences:
- Fuzzy edges where the cut crosses the pores. Sanding is part of the plan, not a failure.
- Fine detail does not hold. A crisp V-carved serif can land on a pore and simply crumble.
- Finish soaks in unevenly unless you use a grain filler or a conditioner.
- Tearout at the pores is common, especially on cross-grain cuts.
Red oak is inexpensive and widely available, which makes it tempting. For detailed carving it is a poor choice. For bold signage and rustic work it is fine and the grain is part of the look.
White oak
Harder and denser than red oak, with pores that are closed off by tyloses, so it machines noticeably cleaner and holds better detail. It is also harder on tooling and more expensive. Quartersawn white oak with ray fleck is beautiful and worth the extra care.
Ash
Similar structure to oak, open pored, but generally lighter in colour and slightly easier to machine. Good for bold work, poor for fine detail. It has become inexpensive in many regions because of emerald ash borer salvage.
Poplar
Poplar is the material to make your mistakes in. It is cheap, it is stable, it cuts easily, and nothing about ruining a piece of it will hurt. It fuzzes rather than tearing out, and a sharp downcut bit and a light finishing pass go a long way.
Birch
Solid birch machines much like maple: dense, pale, prone to burning, good at holding detail. Not to be confused with Baltic birch plywood, which behaves completely differently.
General hardwood strategy
| Concern | Approach |
|---|---|
| Burning | Sharp bit, real chipload, lower RPM before slower feed. See burning. |
| Tearout | Mind cut direction relative to grain, take a light finishing pass. See tearout. |
| Fuzzy edges in open grain | Downcut for top surfaces, sharp tooling, plan to sand |
| Wood movement | Let boards acclimate in the shop before machining. See wood movement. |
| Hidden defects | Look for knots and check reclaimed wood for metal before it goes on the machine |
Exotic species, briefly
Many exotic hardwoods are dense, oily or abrasive, and several are respiratory sensitisers. Purpleheart and ipe are extremely hard and burn readily. Cocobolo and other rosewoods are oily, which affects both machining and glue-up, and are known irritants. Take dust seriously with any species you have not cut before, and test on a small piece first.