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How to Choose the Correct CNC Material

Picture of David Pechter
by David Pechter
Jan 10, 2019

CNC is a computer controlled fabrication technique and about 30 years older than 3D printing. Fast, accurate, and versatile, it belongs in the advanced maker's toolkit.

Quick Introduction to CNC -- I Came, I Sawed, I Conquered

The basics of milling.
The basics of milling. Source: APC Asian Production & Components

CNC is the “subtractive 3D printer”. Just imagine three-axes of motion, software, and a spindle with a cutter in the place of an extruder, and you have a CNC router. CNC stands for computer numerical control, a term that goes back to when a computer was the size of a refrigerator. CNC’s range from desktop hobby machines to four-foot furniture-making machines and metal working behemoths.

CNC starts with a block or plate of clamped material. Then, using a g-code motion recipe (like a 3D printer), CNC moves the cutter in paths to cut pockets, outlines, and through-holes as well as face off surfaces.

CNC offers these advantages over 3D printing:

  • works with wood, plastic, aluminum, and steel (with an industrial kit)
  • can be extremely accurate and deliver very smooth surfaces
  • large work envelope — a 24-inch square is typical but it can go up to 4′ x 8′
  • much faster than 3D printing

CNC has these downsides:

  • parts are solid under the surface (no analog to infill in CNC)
  • color choice is very limited
  • the starting material is the bounding box of the final part… and a lot of it gets turned into chips
  • the machines can be expensive
  • the cutting process is NOISY!

But the next question is, assuming you have a machine and earplugs, what kind of material should you feed into it? Let’s take a tour of the basic CNC material choices for the hobbyist maker.

Short Tour of the Hobbyist CNC Maker's Go-To Materials

CNC milling acetal.
CNC milling acetal. Source: cruddCNC / YouTube

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.

CNC Material #1: Baltic Birch Plywood

You can almost smell it.
You can almost smell it. Source: Woodworkers Source

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

CNC Material #2: Polycarbonate

PC is even bullet resistant!
PC is even bullet resistant! Source: http://protectivestructures.com

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

CNC Material #3: Aluminum

Aluminum
Aluminum Source: http://www.industrialsupplierbd.com/wp-content/uploads/2018/02/aluminum-raw-material.jpg

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 

CNC Material #4: UHMW

If you need slippery, go with UHMW.
If you need slippery, go with UHMW. Source: insidethegames.biz

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)

CNC Material #5: Acetal

Great for functional objects.
Great for functional objects. Source: Craftech Industries

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

Other Options

Milled MDF.
Milled MDF. Source: ShopSabre CNC / YouTube

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.

License: The text of "How to Choose the Correct CNC Material" by All3DP is licensed under a Creative Commons Attribution 4.0 International License.

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