CNC Milling

CNC machining combines the responsiveness of additive manufacturing processes with parts that are milled from real engineering-grade plastic and metal for improved material selection, part functionality and cosmetic appearance. Final parts are nearly indistinguishable from molded parts, making for high-quality prototypes often used as jigs, fixtures and one-offs.

During the machining process, a solid block of material is clamped into a CNC mill and cut into a finished part. We employ three-axis milling from up to six orthogonal sides of the part to machine as many features as possible. The range of material choices allows parts to be made with the desired material properties, and good tolerances yield parts suitable for form, fit and function testing.
1 to 200+ parts
Shipped in 1 to 3 days
Parts start at $65

Design Guidelines


  • Maximum part size: 10 in. by 7 in. (254mm by 178mm)
  • Minimum part size: 0.25 in. by 0.25 in. by 0.25 in. (6.4mm by 6.4mm by 6.4mm)
  • Maximum part depth: ranges from 0.75 in. to 3.75 in. (19mm to 95mm). The maximum depth that can milled from either side of the part is 2 in. (51mm). Please see our materials selection guide for more information.

Larger Sizes

Larger part sizes may be available in ABS, acetal, aluminum 6061 and 7075, and magnesium materials.
  • For these materials with depths up to 1.75 in. (44mm), maximum size increases to 10 in. by 14 in. (254mm by 356mm).
  • For these materials with depths up to 0.75 in. (19mm), maximum size increases to 22 in. by 14 in. (559mm by 356mm).
  • For parts in aluminum 6061, depths up to 1.25 in. (32mm) are possible with a maximum part size of 22 in. by 14 in. (559mm by 356mm).
Larger parts can be milled from top and bottom only — no side setups. Both standard and larger envelope sizes are nominal; parts that fit within these constraints occasionally cannot be manufactured due to fixturing or other considerations.
Firstcut size limits


Proto Labs' CNC machining process typically requires nominal part thickness to be larger than 0.040 in. (1mm). Parts may be as thin as 0.020 in. (0.5mm) in regions as long as the nominal part thickness stays above 0.040 in. (1mm). Features thinner than 0.020 in. (0.5mm) typically do not survive the machining process.


In general, tolerances of ±0.005 in. (0.13mm) are expected and achieved.


Sharp inside corners on a part will be radiused (rounded) as a natural result of the CNC milling process. Our automated quotes highlight areas where these radii will occur.

Text on parts

Recessed text should have a minimum stroke width of 0.020 in. (0.5mm). The spacing between characters on raised text should be 0.020 in. (0.5mm) or greater. If your design permits it, opt for raised text versus recessed.

Surface finishes

All machined plastic parts are left as-milled, which typically means they show visible tool marks. Some metal parts, on the other hand, allow more choice. Magnesium, like plastic, is always left as-milled. Soft metals like brass, copper and aluminum can be lightly bead blasted leaving a fine matte finish. Hard metals like steel and stainless steel can also be de-burred with a light bead blast which removes metal slivers and sharp or particularly rough areas.
Surface finishes
Firstcut plastic as-milled finish
Plastic as-milled
Firstcut metal as-milled finish
Metal as-milled
Firstcut bead-blasted finish
Firstcut de-burred finish
Firstcut plastic as-milled finish
Plastic as-milled
Firstcut metal as-milled finish
Metal as-milled
Firstcut bead-blasted finish
Firstcut de-burred finish

Threaded holes

Proto Labs provides the ability to easily add threaded holes to your CNC-machined parts. We currently support UNC and UNF threads from #2 up to 1/2 in., and metric threads from M2 to M12. Because of the lack of standards between CAD packages for specifying threads, our automated systems decide where to put threads by looking for holes that could be pilot holes that are oriented suitably. Don't model internal threads; it will confuse Proto Labs' pilot hole finder. The Proto Labs process uses three-axis milling, so holes can only be milled and threaded if they align with one of the three cardinal axes. More details.
Threaded holes
Firstcut threaded holes


There are more than 30 different engineering-grade plastic and metal materials in Proto Labs that can be used in prototyping. See full list of materials.
  • ABS
  • Acetal
  • CPVC
  • HDPE
  • LDPE
  • Nylon 6
  • Nylon 6/6
  • PEEK
  • PEI
  • PET
  • PMMA
  • Polycarbonate
  • Polypropylene
  • PPE/PS
  • PPSU
  • PS
  • PSU
  • PTFE
  • PVC
  • UHMW
  • Aluminum
  • Brass
  • Copper
  • Magnesium
  • Stainless Steel
  • Steel Alloy
  • Steel Mild Low Carbon