DFM Checklist for CNC Turned Parts

By CHX Metal Engineering Team · Published 2026-10-08

Twelve design-for-manufacturing checks before you send a turned part for quotation - wall thickness, L/D, undercuts, threads, tolerance, finishes, datums. The factory covers 5,000 m2 and produces up to 5 million precision parts per month.

Most quotation delays and cost surprises on turned parts come from a handful of design details. Running these twelve checks before you send a drawing shortens the quote loop and usually removes cost before it is designed in.

Geometry checks

1. Wall thickness. Thin walls deflect and chatter. For turned metal parts, walls under about 0.5 mm become a special case; tell the shop early so the process and fixturing can be planned.
2. Length-to-diameter ratio. Above roughly 3:1, a Swiss-type machine with a guide bushing becomes the practical route. Above 10:1 it is usually the only repeatable one.
3. Undercuts and relief. An undercut that cannot be reached by a standard tool nose forces a special tool, a grind or a second operation. Add relief grooves where a shoulder meets a larger diameter.
4. Corner radii. Match internal radii to the tool nose radius the shop actually stocks. A drawing that calls for a sharp internal corner is asking for a broach, an EDM or a compromise.

Feature checks

5. Threads. State the thread standard, class and whether it is cut or rolled. Rolled threads are stronger but need the diameter called out correctly; cut threads are more flexible on short runs.
6. Cross-holes and milling features. These are cheap on a Swiss-type with live tooling and a sub-spindle, and expensive as a separate milling operation. Flag them so the route can be planned in one cycle.
7. Knurling and splines. These add tooling and setup; if they are decorative rather than functional, say so.
8. Marking. Laser marking is fast in-house, but confirm the character height and whether it must survive surface treatment.

Tolerance and finish checks

9. Tolerance allocation. Do not put a tight tolerance on every dimension. Standard tolerance is +/-0.005 mm; reserve +/-0.005 mm for the features that actually control function, and let the rest run loose.
10. Datums and GD&T. Choose datums that reflect how the part is held and measured. A concentricity or runout callout referenced to a datum that the fixture cannot access will drive cost and argument.
11. Surface finish. Specify Ra only where the function needs it. Blanket finishes across a drawing add polishing time without adding function.
12. Process sequence. Note whether heat treatment or finishing happens before or after machining. Hardening a part before a tight final cut changes the tooling and the order of operations.

One more habit that saves time

Send the 3D model as well as the 2D drawing, and state the drawing revision. A STEP file removes ambiguity about geometry, while the 2D drawing still governs tolerances and GD&T. When the two disagree, the revision number decides which is authoritative.

At CHX Metal every order is quoted by an engineer who works to your drawing revision, and DFM feedback is returned with the quotation - typically within 12 working hours. There is no strict MOQ, so a single prototype can be validated before a batch is committed.

Frequently asked questions

What is the most common DFM problem you see on turned parts?

Over-tolerancing. Tight tolerances applied to every dimension, including features that have no functional relationship to each other, add inspection time and cost without improving the part.

Should I send a 2D drawing, a 3D model, or both?

Both, if you have them. The STEP or IGES model removes geometry ambiguity and the 2D drawing governs tolerances and GD&T. State the revision that is authoritative.

Can you work from a physical sample with no drawing?

Yes. We can measure the sample, produce a drawing for your confirmation and quote from that, but an original drawing is always faster and removes interpretation risk.

Do you charge for a DFM review?

No. DFM feedback is returned with the quotation, and there is no obligation to order.

Shenzhen Changhexin Precision Hardware Co., Ltd. (CHX Metal) · Shajing Street, Bao'an District, Shenzhen, Guangdong, China · ISO 9001:2015 (NOA198428) · ISO 14001:2015 (NOA2100184)