cnc milling design rules
CNC Milling Design Rules
Milling removes material from flat faces, and every rule is about the cutter reaching the feature, staying rigid while it does, and leaving a surface the next operation can reference. A milled part is cheapest when it is designed to be made from one side.
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- Pocket depth
- ≤ 4 × cutter diameter
- Corner fillet
- ≥ cutter radius (3 mm for a 6 mm tool)
- Slot width
- ≥ 1 × cutter diameter, 1.5× for depth
- Faces per setup
- Each additional face is a setup
Milling is a reach problem before it is a tolerance problem. The design rules are all about giving the cutter room to reach the feature without becoming a long, thin, vibrating tool.
Specifications
| Pocket depth | ≤ 4 × cutter diameter |
|---|---|
| Corner radius | ≥ cutter radius; ≥ 1/3 pocket depth |
| Slot width | ≥ 1.2 × cutter diameter |
| Hole position | ±0.05 mm standard · ±0.01 mm bored |
| Faces | One per setup on 3-axis; five-axis removes setups |
| Surface finish | Ra 1.6 µm typical on milled faces |
Frequently asked questions
How deep can you mill a pocket?
About four times the cutter diameter. Deeper than that the shank rubs the wall, the finish deteriorates and the tool is likely to break. A narrow deep pocket needs a small long tool, which is the worst combination.
Why do milled corners have a radius?
Because the cutter is round. A 6 mm end mill leaves a 3 mm radius at an internal corner. Anything sharper needs a smaller cutter with less reach, or EDM.
What slot width should I design?
At least 1.2 times the cutter diameter, so the tool cuts on one side with chip clearance rather than rubbing on both. A slot exactly the cutter width is the slowest cut on the machine.
What position tolerance can milling hold?
About ±0.05 mm on a hole pattern, and ±0.01 mm on a bored hole. Tighter than that across multiple faces means more setups and higher inspection cost.
Can five-axis machining reduce cost?
Yes, when it removes setups. A part that would need three clampings on a three-axis machine can be made in one on a five-axis, which usually beats the higher hourly rate.
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