Wall Thickness Minimums
Thin walls vibrate during cutting. This causes chatter marks and poor finish. It can also break the tool or the part.
For aluminum, keep walls at least 0.5mm (0.020") thick. For steel, use at least 0.8mm (0.031"). For plastics, aim for 1.5mm (0.060") because they flex more.
Taller walls need to be thicker. A good rule: the wall height should not exceed 15 times the wall thickness. A 1mm thick wall should be no taller than 15mm.
If your part allows it, use 1mm walls for aluminum and 1.5mm for steel. Thicker walls cut faster. They also hold tighter tolerances. Your parts will cost less to make.
Pocket Depth Limits
Deep pockets need long tools. Long tools flex during cutting. This causes poor finish and loose tolerances.
Keep pocket depth to 4 times the pocket width. A 10mm wide pocket should be no deeper than 40mm. This keeps the tool stiff enough for a clean cut.
If you need deeper pockets, widen them. A wider pocket lets you use a bigger, stiffer tool. You can also step down the pocket walls to reduce tool load.
What Happens with Deep Pockets
- The tool deflects and the walls taper slightly
- Surface finish gets rougher near the bottom
- The machine runs slower to reduce deflection
- Cycle time and cost go up
Corner Radii
End mills are round. They cannot cut sharp inside corners. Every inside corner will have a radius.
The corner radius equals the tool radius. A 6mm end mill leaves a 3mm corner radius. A 3mm end mill leaves a 1.5mm radius.
Use a corner radius of at least 1/3 the pocket depth. For a 12mm deep pocket, use a 4mm or larger corner radius. Smaller radii need smaller tools, which cut slower.
Sharp inside corners require very small tools. Small tools break easily. They also cut slowly. If your mating part needs a sharp corner, add a relief cut (dog bone or T-bone) in the corner instead.
Outside Corners
Outside corners can be sharp. The tool naturally leaves a sharp edge on outside corners. You do not need to add a radius here.
Hole Depth Rules
Standard drill bits reach 4 times the hole diameter. A 6mm drill can cut 24mm deep. This is the cheapest and fastest option.
For deeper holes, use peck drilling. The drill goes in partway, backs out to clear chips, then goes deeper. Peck drilling reaches up to 10 times the diameter.
Holes deeper than 10 times the diameter need special tools. Gun drills or boring bars can go deeper. But they cost more and take longer.
Flat-Bottom vs. Pointed Holes
Standard drills leave a pointed bottom. The point angle is usually 118 or 135 degrees. If you need a flat bottom, the shop uses an end mill. This costs more.
Only call out a flat bottom when your design needs it. Let the shop use a standard drill when a pointed bottom works.
Threads, Fillets, and Undercuts
Thread Depth
Thread depth should be 1.5 to 3 times the diameter. A M6 thread needs 9mm to 18mm of depth. Going deeper adds cost with little benefit. Most of the holding force comes from the first few threads.
Fillets
Add fillets where walls meet floors. Fillets reduce stress and help the tool flow. A fillet radius of 1mm to 3mm works well for most parts.
Undercuts
Avoid undercuts when possible. Standard tools cannot reach under a ledge. Undercuts need special T-slot cutters or extra setups. Each extra setup adds cost and time.
Draft Angles
CNC parts do not need draft angles. Draft is for molded parts. CNC machines cut straight walls with no trouble. Adding draft to CNC walls just adds cost.
Quick Reference Table
| Feature | Recommended Limit | Why |
|---|---|---|
| Wall thickness (aluminum) | 0.5mm (0.020") min | Thinner walls vibrate and break |
| Wall thickness (steel) | 0.8mm (0.031") min | Steel is stiffer but harder to cut thin |
| Pocket depth | 4x pocket width max | Deeper pockets cause tool deflection |
| Inside corner radius | 1/3 of pocket depth min | Smaller radii need smaller, slower tools |
| Hole depth (standard) | 4x diameter | Standard drill reach limit |
| Hole depth (peck drilling) | 10x diameter | Requires peck cycle, costs more |
| Thread depth | 1.5x to 3x diameter | Deeper threads add cost, not strength |
| Fillet radius | 1mm to 3mm | Reduces stress, helps tool flow |
| Undercuts | Avoid if possible | Need special tools or extra setups |
| Draft angles | Not needed for CNC | Only needed for injection molding |
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Upload CAD for Instant QuoteFrequently Asked Questions
What is the minimum wall thickness for CNC parts?
For aluminum, keep walls at least 0.5mm (0.020") thick. For steel, use at least 0.8mm (0.031"). Thinner walls vibrate during cutting. This causes poor finish and broken tools. When possible, use 1mm or thicker walls for best results.
Why do CNC parts need corner radii?
End mills are round. They cannot cut sharp inside corners. Every inside corner will have a radius equal to the tool radius. Design your corners with a radius of at least 1/3 the pocket depth. This lets the tool cut smoothly without chatter.
How deep can I make pockets in CNC parts?
Keep pocket depth to 4 times the pocket width or less. Deeper pockets need longer tools. Long tools flex and chatter. If you need deeper pockets, widen them or accept a rougher finish on the walls.
Do CNC parts need draft angles?
No. Draft angles are for molded parts, not CNC parts. CNC machines cut straight walls easily. You do not need to add draft to vertical surfaces. Adding draft to a CNC part adds cost with no benefit.
What hole depth is standard for CNC drilling?
Standard drill bits reach 4 times the hole diameter. For example, a 6mm drill can cut 24mm deep. Deeper holes need peck drilling, which costs more. For holes deeper than 10 times the diameter, consider a different approach like boring.
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