cnc turning design rules
CNC Turning Design Rules
Turning is the cheapest way to make a round part, and the rules are all about one thing: how far the workpiece sticks out of the chuck. A part that is 2× its diameter long turns easily at any tolerance; the same part at 8× needs a centre, a steady or a second operation, and the price reflects the difference.
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- Rigid length
- ≤ 3 × diameter unsupported
- Needs support
- > 3 × diameter (centre or steady)
- Thread relief
- Undercut required at a shoulder
- Standard tolerance
- ±0.01 mm on diameter
Turning is the cheapest route to a round part, and the constraint is always the same: how much of the workpiece is hanging in the air.
Specifications
| Rigid work | ≤ 3 × diameter unsupported |
|---|---|
| Slender work | > 6 × diameter — light passes, support required |
| Diameter tolerance | ±0.01 mm standard |
| Length tolerance | ±0.1 mm standard |
| Thread | Class callout required (6g / 2A) |
| Cross features | Live tooling or a second milling operation |
Frequently asked questions
What length-to-diameter ratio can a lathe turn?
Up to about 3× diameter without support, 3–6× with a tailstock or steady rest, and beyond 6× the part is slender: deflection dominates and the process is slower and more controlled.
Why does my part need an undercut at the shoulder?
Because the turning tool has a nose radius and cannot cut a square internal corner. If a mating part must sit flush against the shoulder, the corner needs an undercut groove, and the drawing has to show it.
Do turned threads need a relief groove?
At a shoulder, yes. Without a relief the last thread is incomplete and a nut cannot seat. The groove width and diameter belong on the drawing.
Can a lathe cut a cross hole?
Only with live tooling. Otherwise it is a second milling operation, which means the part is handled twice and the price reflects it.
What tolerance is normal on a turned length?
About ±0.1 mm unless a shoulder is the datum. Diameters are easier — ±0.01 mm is routine — because the tool is in continuous contact with the surface.
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