First stop: baltic birch plywood. This is a great prototyping material for many CNC projects. It’s low-cost, high-quality, and paintable. Just be careful you don’t need any of the following:

• waterproof
• clear
• impact resistant
• slippery, bearing-like surface
• abrasion resistant

Next up is polycarbonate. If your part can be clear and doesn’t need to serve as a bearing or slippery surface, PC is often the way to go. If you need more strength or rigidity, consider an assembly, and remember that polycarbonate can be cold-bent on a bending brake, an excellent way to add structure and rigidity after machining. 

Now we come to a fork in the road:

• For more strength, hardness, or temperature resistance, move to aluminum.
• For thicker parts, with a slippery surface, impact strength and abrasion resistance, try UHMW.

Last stop on our short tour is acetal, for when you need a precise, slippery material available in black or white.

Getting confused between the different plastic options? Check out this chart comparing plastic properties.

For more detail, we take a more in-depth look at the five recommended materials for CNC.

Baltic birch plywood is great for CNC: It’s consistent, void-free, stable cabinet-grade plywood. It also has an attractive exposed edge. For more, see all about Baltic birch plywood.

• available thickness of 1/10″ to 1″
• about $3 for a 1/4″-thick, 12″ x 12″ piece
• about $6 for a 3/4″-thick, 12″ x 12″ piece
• easy to cut
• easy to join
• ready to finish with paint or stain

Polycarbonate, also known as Lexan or Makrolon, is a strong, impact-resistant engineering-grade plastic that mates very well with CNC. Thicker material does get very pricey. If you can work with 1/4″ clear sheet, this is a versatile material that is a pleasure to cut.

• available in thickness of 1/10″ to 3/4″
• about $15 for a 1/4″-thick, 12″ x 12″ piece
• about $200 for a 3/4″-thick, 12″ x 12″ piece
• easy to cut
• high impact strength
• cold bendable
• generally available only as clear or tinted

Aluminum is what they used to make airplanes of (until carbon fiber came along). Aluminum has an excellent strength-to-weight ratio, and is much stiffer than plastic. Working with aluminum has its challenges but opens the door to strong, high-tech parts coming right off your CNC machine.

• available in thickness of 1/32″ to multiple inches
• about $35 for a 1/4″-thick, 12″ x 12″ piece
• about $122 for a 3/4″-thick, 12″ x 12″ piece
• can be challenging to cut — requires lubrication and or cooling
• about 4x stronger and 30x stiffer than polycarbonate
• the basic engineering material — strong, light, and non-corroding
• post-CNC: choose 5052-H32 for bending and 6061-T6 for welding
• can be hard coated for abrasion resistance 

Ultra-high-molecular-weight polyethylene (UHMW) is like a poor man’s Teflon. Apart from having the same ultra-low coefficient of friction, UHMW is TOUGH. UHMW has a good bit of flex, so it isn’t a precision material. But if you need a bulky, slick, indestructible part with no moisture absorption, this is a good material to try.

• available in thicknesses of 1/8″ to 5″
• about $12 for a 1/4″-thick, 12″ x 12″ piece
• about $26 for a 3/4″-thick, 12″ x 12″ piece
• easy to cut
• slippery
• excellent abrasion, wear, and impact resistances
• generally available only in black or white
• better as virgin material (avoid reprocessed)

Alternatively known as POM or Delrin, acetal is a terrific engineering-grade material with very similar properties to nylon. It absorbs no water and cuts like butter.

• available in thicknesses of 1/32″ to 4″
• about $22 for a 1/4″-thick, 12″ x 12″ piece
• about $58 for a 3/4″-thick, 12″ x 12″ piece
• very easy and accurate to cut
• precise
• low friction
• low water absorption 
• generally available only in black or white

The following are some other popular options for CNC milling:

• MDF is a cost-effective wood-like composite, but can create nasty dust.
• Nylon is a bit more expensive than acetal, absorbs water, and can swell. It is, however, an excellent bearing material.
• Acrylic (plexiglas), when compared to polycarbonate, is clearer and more scratch resistant. But with a much lower impact strength, it can be difficult to cut. Note that it cannot be cold bent.
• G-10/FR4 is the stuff that circuit boards are made of. It’s an epoxy-grade industrial laminate and phenolic with high strength and low moisture absorption. A glass-filled material, it’s both stiffer and stronger than polycarbonate.