Most CNC machining cost is decided at the CAD stage. Small design changes, such as a larger corner radius, a standard hole size or a shallower pocket, can cut machining time significantly without affecting function. These 12 rules cover the most common issues we see on incoming drawings.
1. Add radii to internal corners
End mills are round, so internal vertical corners always have a radius. Design internal radii at least one-third of the pocket depth, and slightly larger than the tool radius so the cutter doesn't stall in the corner. If you need a sharp internal corner for a mating part, use a dog-bone or T-bone relief instead.
2. Keep pockets shallow
Pocket depth up to about 4× the tool diameter machines efficiently. Beyond that, longer tools deflect and must cut slowly. If a deep pocket is unavoidable, widen it so a larger tool can be used.
3. Avoid very thin walls
As a guideline, keep metal walls above about 0.8 mm and plastic walls above about 1.5 mm. Thin walls vibrate and deflect, which hurts tolerance and finish.
4. Use standard hole sizes
Standard drill diameters are fast and cheap. Non-standard diameters require interpolation with an end mill or a special reamer. For precision fits, use standard reamer sizes and tolerance classes such as H7.
5. Limit hole depth
Drilled holes up to about 4–6× diameter are routine. Deeper holes need peck drilling or gun drilling. Where possible, drill through rather than blind, and if blind, allow for the drill point at the bottom.
6. Don't over-specify thread length
Thread strength comes mostly from the first few engaged threads. A thread length of around 1.5× the nominal diameter is usually sufficient. Long threads add time and risk tap breakage.
7. Tolerance only what matters
Use a general tolerance note and reserve tight tolerances for functional features. This single change often has the biggest effect on price. See CNC machining tolerances explained.
8. Minimise setups
Every new orientation is a new setup. Keep features on as few faces as possible, or design for 5-axis access if the geometry demands many faces.
9. Avoid unnecessary undercuts
Undercuts need special T-slot or lollipop cutters. If you need one, use standard cutter dimensions and leave access for the tool shank.
10. Don't specify fine finishes everywhere
Standard machined finish is around Ra 3.2 µm. Specify Ra 0.8 µm or better only on sealing surfaces, bearing seats and sliding faces.
11. Design text and logos to be engraved, not embossed
Engraved (recessed) text is quick to machine. Raised text requires removing all the material around it. Use a simple sans-serif font at a reasonable size.
12. Choose a machinable material
If the application allows, choose free-machining grades such as 303 stainless, 6061 aluminium or leaded brass. Harder alloys like titanium and Inconel are excellent materials but take much longer to machine. See machining titanium and machining Inconel.
Get a DFM review
At Aeroedge International, our engineers review incoming drawings in SolidWorks before quoting and suggest changes that reduce cost without compromising function. Send us your design and ask for DFM feedback with the quote.
