DFM Guide · CNC Turning

Spinning chuck.
Stationary tool.
Design for it.

CNC turning rotates the workpiece against a stationary cutting tool. Specific design rules apply — different from milling. Cost-optimal turned parts follow these guidelines.

Free DFM Review CNC turning service

01 · Geometry

Turning-friendly geometry.

Symmetry

Round about axis

Turned parts are inherently axially symmetric. Features must be on or about rotation axis. Off-axis features need secondary milling operation.

Length-to-diameter ratio

Keep L/D < 4

Long thin shafts deflect during cutting. L/D > 4: requires steady rest support. L/D > 8: Swiss machine territory.

Undercuts

Tool clearance

Undercuts (grooves narrower at top) require specialty tooling. Add chamfer or relief instead when possible.

Threading

Standard preferred

Standard metric (M3-M50) and inch (UNC, UNF) threads cheapest. Custom thread pitches require specialty tools.

02 · Diameter steps

Stepped shaft design.

0.5-1mm

Step transition radius

Generous fillet at diameter changes. Reduces stress concentration.

≥ 5×

Tool relief width

Allow 5× tool width for tool to enter relief without gouging.

Finish per zone

Different Ra OK

Different surface finish on different zones is fine — bearing journals smooth, OD areas standard.

Concentricity

±0.01 mm typical

Concentricity between turned features achievable to ±0.01 mm without specialty processes.

Knurling

Specify pattern

Diamond, straight, or angle knurl available. Common widths 5-25 mm.

Center drilled

For grinding

Center drill at ends if subsequent grinding planned.

03 · Threading rules

Threading on lathe.

Single-point threading: precision and any custom pitch. Slower than die threading. Ideal for precision threads on shafts > 25mm diameter.

Die threading (chasers): faster, repeatable. Limited to standard pitches. Most production threads cut this way.

Thread relief: always specify thread relief at thread end. Allows tool runout, ensures full thread depth.

Class 2A/2B: default for most production. Class 3A/3B for precision applications (aerospace, instruments).

04 · Live tooling

Combined turning + milling.

Live tooling enables

• Cross holes in turned parts
• Flats and slots on cylindrical features
• Hex flats for wrench engagement
• Multiple operations in single setup
• Faster than turning + separate milling

Pure turning when

• Simple round geometry
• Higher production volumes
• Better economy without secondary features
• Standard turret-only equipment

FAQ

Length-to-diameter limits?

L/D up to 4 standard turning. L/D 4-8 with steady rest. L/D > 8 requires Swiss-type lathe with sliding headstock and guide bushing.

Threading vs tapping?

Threading on lathe (single point or die): for external threads or large internal threads. Tapping: for internal threads up to about M16 in production. Choose based on size and thread quality.

Surface finish on turned parts?

Standard Ra 0.8-1.6 µm. Fine turning Ra 0.4 µm. Polished finish Ra 0.1 µm with subsequent operation. Specify per function.

Roundness achievable?

Roundness ±0.005 mm achievable on quality lathes with proper tooling. Sub-µm requires grinding or precision lapping.

Live tooling cost?

Live tooling adds 10-25% to standard turning operation cost. Saves significantly vs separate milling setup. Net win for parts needing both turning and milling features.

Bar feed automation?

Yes — bar feed enables unattended production runs. Standard for production volumes 200+. Adds setup time but dramatically reduces per-part cost.