Design Guidelines: CNC Milling

Our basic guidelines for CNC milling include important design considerations to help improve part manufacturability, enhance cosmetic appearance, and reduce overall production time.



Maximum Dimensions

SIZE (3-axis milling) 10 in. x 7 in.
SIZE (5-axis milling) 2.6 in. x 2.9 in. x 3.9 in.
DEPTH (3-axis milling) 0.75 in. to 3.75 in.
Maximum depth that can be milled is 2 in. from either side of part
SIZE (3-axis milling) 254mm x 177.8mm
SIZE (5-axis milling) 66mm x 73mm x 99mm
DEPTH (3-axis milling) 19.05mm to 95.25mm
Maximum depth that can be milled is 50.8mm from either side of part

Minimum Dimensions

SIZE 0.25 in. x 0.25 in.

0.040 in.

SIZE 6.35mm x 6.35mm


Typically, Proto Labs can maintain a machining tolerance of +/- 0.005 in. (0.127mm). Parts may be as thin as 0.020 in. (0.508mm) in regions as long as nominal part thickness is above 0.040 in. (1.016mm).

With 3-axis milling, 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).
applications engineer working on CAD model

Design Feedback
For additional questions, please contact an applications engineer at or 877.479.3680.

5-Axis Milling Capabilities

We use 5-axis indexed milling—in addition 3-axis milling and turning—to machine parts. With 5-axis indexed milling (also referred to as 3+2 milling), two axes are stopped while the tool is engaged with the part. This allows for more complex geometries and non-orthogonal features like off-axis holes.

Better finishes are also possible on non-orthogonal surfaces and because 5-axis milling increases machining efficiencies, more cost-efficient parts can be had.

milling off-axis features and holes


  • ABS 
  • Acetal
  • CPVC
  • HDPE 
  • LDPE
  • Nylon
  • PEEK
  • PEI
  • PET 
  • PMMA
  • Polycarbonate
  • Polypropylene 
  • PPSU
  • PS
  • PSU
  • PTFE
  • PVC 
  • Aluminum
  • Brass
  • Copper 
  • Magnesium
  • Stainless Steel
  • Steel Alloy
  • Steel Mild Low Carbon


milling a block of plastic

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 deburred with a light bead blast which removes metal slivers and sharp or particularly rough areas.



As-machined, deburred, or deburred with a light bread blast
machined metal gear

Threaded Holes

We currently support UNC and UNF threads from #2 up to 0.5 in., and metric threads from M2 to M12. Avoid modeling internal threads on your part design.



threading guidelines for CNC machining


Sharp inside corners on a part will be radiused (rounded) as a natural result of the CNC machining process. Resulting radii will be identified before the part is milled.



machining a radiused corner


Recommended text considerations for CNC milling:

  • Recessed text should have a minimum stroke width of 0.020 in. (0.508mm).
  • The spacing between characters on raised text should be 0.020 in. (0.508mm) or greater.
  • If design permits it, opt for raised text versus recessed.
machining text into a part

team of applications engineers

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