CNC Turning — design guidelines
Turning is governed by how far the part sticks out of the chuck. A short stubby part is almost free; the same diameter three times longer needs a tailstock, a steady rest, or both.
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 |
|---|---|---|---|
| Length-to-diameter, unsupported | ≤ 3 : 1 | ≤ 10 : 1 with tailstock | Beyond 3:1 the bar deflects under the tool and the diameter tapers along its length. A tailstock fixes that but blocks the end face until a second operation. |
| Bored hole depth-to-diameter | ≤ 3 : 1 | ≤ 6 : 1 | The boring bar is a cantilever inside the hole. Past 3:1 it chatters and leaves a spiral witness mark; past 6:1 the bore goes out of round. |
| Undercut / relief groove width | 2.0 mm or DIN 509 | 1.0 mm | Groove width sets the parting tool. A DIN 509 relief uses a stock insert; an arbitrary width needs a ground tool with its own lead time. |
| Concentricity between features | 0.02 mm same setup | 0.05 mm across setups | Features cut in one chucking share the spindle axis. Anything requiring the part to be turned around re-introduces run-out from the second grip. |
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 thread running right up to a shoulder with no relief groove. The tap or threading tool cannot produce a full profile at the end, so the last turn is always incomplete.
- Specifying Ra 0.4 on a turned diameter without allowing for grinding. Turning tops out around Ra 0.8; below that the part needs a grinding operation.
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.