CNC Milling — design guidelines
Almost everything in milling comes back to one question: can the tool reach the feature while staying short enough to be rigid? Get the answer right at the CAD stage and the part is cheap; get it wrong and no amount of programming will rescue the price.
Constraints specific to this process
These are the limits that come from this process in particular. The general rules on wall thickness, corner radii and hole depth apply on top of them.
| Parameter | Design to thisNo surcharge, no discussion needed | We can reachAchievable, but affects price or lead time | Where the limit comes from |
|---|---|---|---|
| Clamping allowance on the blank | 10 mm on one face | 5 mm with vacuum/glue | The vice needs something to hold. If the finished geometry runs to every edge we have to add a sacrificial tab and a second operation to remove it. |
| Flatness on a large face | 0.05 mm / 100 mm | 0.01 mm / 100 mm | Stock relieves internal stress as material comes off and the plate bows. Tighter than 0.02 mm needs rough-machining, a stress relief cycle, then finishing — three steps instead of one. |
| Boss / island height | ≤ 4 × width | ≤ 8 × width | A tall narrow boss is cut from all sides, so it flexes away from whichever side the tool is on and ends up out of tolerance in both directions. |
| Datum scheme | 3 mutually perpendicular faces | Any, but state it | We set the part zero from your datums. If the drawing has no datum scheme we pick one, and on a multi-setup part that choice decides where the error stacks up. |
What we see go wrong most often
Recurring issues on the drawings that reach us. None of them are hard to avoid once you know to look for them.
- A pocket whose corner radius is smaller than the floor radius. That needs two cutters and a tool change on every pocket — matching them costs nothing.
- Tolerances tightened on the clamping face. That face is held in the vice, so it is the one surface we cannot machine in the same setup as everything else.
General machining rules
The numbers that apply across every cutting process — wall thickness, radii, holes, threads, tolerances and cost drivers.
- Wall thickness
- Internal corner radii
- Holes and threads
- Pockets, cavities and undercuts
- Tolerances, text and surface callouts
- What actually drives the price
Every quotation includes a manufacturability review against these rules. If something in your model falls outside them, we tell you before you commit, not after the first article fails.
Not sure whether your part clears these limits?
Send the STEP file. A manufacturing engineer marks up anything that will be difficult and returns it with the quotation — no charge, no obligation.