Designing for CNC machining

A CNC mill makes a part by removing material with a spinning cutter. That single fact explains the design rules, and it is the opposite of 3D printing, where you add material layer by layer. A shape that is trivial to print can be impossible to machine, and the reverse is true too.

Think like the cutter

The cutter is round, it has length, and it reaches the part from one direction at a time. Three questions cover most of the rules.

1. Can the tool get there? (Tool access)

A standard 3-axis mill approaches from above. Anything it cannot see from above has to be machined in a different setup.

• A deep, narrow slot is hard to reach because the cutter would need to be long and thin, which makes it flex and chatter.
• An undercut (a groove inside a hole, a lip facing downward) cannot be reached by a straight cutter at all.
• A pocket's depth should generally stay within about 3 to 4 times its width. Deeper is possible, but costs more.

2. What shape is the corner? (Inside radii)

A round cutter cannot make a sharp inside corner. The corner of a pocket will always have at least the cutter's radius.

• A 6 mm end mill leaves an inside corner radius of at least 3 mm.
• Design inside corners with a radius slightly larger than the cutter radius (for example 3.2 mm for a 6 mm tool), so the tool does not stop abruptly in the corner.
• Use the same radius everywhere you can, so one tool does the job.
• Outside corners are different. You can have them sharp or add a chamfer easily.

If a mating part truly needs a square corner, put a relief (a small round notch) in the corner of the pocket, or round the corner of the mating part instead.

Give every inside corner of the pocket a 3.2 mm radius so a 6 mm end mill can cut it.

3. How many times must it be flipped? (Setups)

Each time the machinist flips or re-clamps the part is a setup. Every setup adds time, cost and a small alignment error between the features cut in different setups.

• Features that can all be cut from one side cost less.
• If features on opposite faces must line up precisely, say so on the drawing so the shop plans its setups to hold that relationship.
• A design that needs six faces machined can quickly cost several times more than one needing two.

Other rules of thumb

Setting it up in Solid Vibes

1. In Make, choose CNC Mill as the process.
2. Pick a material such as AL 6061-T6, Steel 1018, Stainless 304 or Brass.
3. Run the check. Look for tool access, sharp internal corner, undercut and small feature issues.
4. Fix them. Some fixes are automatic, such as adding a corner radius, and others go through the agent.
5. Create a drawing that tells the shop what you care about. See Drawings for a CNC shop.

Design intent helps here

State the rules once, and they hold:

Always keep inside corner radii at 3.2 mm or larger. Never let a wall go below 1.5 mm. The overall length must stay under 150 mm so it fits our stock.

3D printing vs CNC at a glance

A common pattern: prototype in plastic with the printer, then convert to a machined design once the shape is settled. Solid Vibes helps with that. Branch the Thing, ask the agent to adapt the design for machining, and compare.

Try it

On the L-bracket template:

Adapt this bracket for CNC machining from 6 mm aluminum plate. Make all inside corners 3.2 mm radius, keep hole depths through, and tell me how many setups it needs.

Read the answer, then check the result with the CNC Mill process in Make.