DFM
Most CNC cost is designed in before the quote. These are the changes that remove setups, tools and inspection without changing what the part does.
Key takeaways
- Tolerance on non-functional faces is the most common cost driver.
- Corner radii, wall thickness and stock size often decide the route.
- Cosmetic faces should be marked before the fixture is built.
- A 3D model plus a 2D drawing removes a round of questions.
- Read the DFM note before you release the batch.
1. Put tolerances only where they matter
Why it costs: a global ±0.01 mm callout forces slower cuts, more inspection and sometimes a second operation. What to change: keep ±0.002 mm for the features that locate or seal; use ISO 2768-mK for the rest. Mark the critical few.
2. Increase corner radii in deep pockets
Why it costs: a square internal corner needs EDM or a special tool. What to change: a radius larger than the cutter allows the pocket to be milled in one setup. If the corner must be square, say so and expect the EDM line.
3. Keep walls thick enough to machine
Why it costs: below 1 mm in aluminum, the wall deflects, chatters and distorts. What to change: add a rib, reduce the pocket depth or choose a different grain direction. A design change is often cheaper than a slower cut.
4. Design around standard stock
Why it costs: a plate much thicker than the finished part means more material removal, longer cycle time and more distortion. What to change: pick a stock thickness close to the finished size; the quote can show the difference.
5. Use standard threads and avoid deep small holes
Why it costs: a deep hole with a small diameter needs a long, fragile tool and a slow peck cycle. What to change: use metric or UNC/UNF threads; keep hole depth within 4–5× diameter where possible.
6. Mark the cosmetic faces
Why it costs: a clamp mark on a visible face is a defect; on a hidden face it is not. What to change: mark cosmetic faces on the drawing before the fixture is built. We plan workholding around them.
7. Avoid undercuts that need a special tool
Why it costs: a feature that cannot be reached from any setup becomes a partner operation. What to change: add a relief, split the part or accept a two-piece assembly.
8. Specify the finish by function
Why it costs: Ra 0.8 on a seal face is a different route from Ra 0.8 on a cosmetic panel. What to change: say what the surface has to do — seal, slide, reflect, look good — and mark only the area that needs it.
9. Split a complex part when it is cheaper
Why it costs: a deep pocket, an undercut or a difficult internal feature can force extra setups. What to change: two simple parts bolted together can be cheaper and easier to replace. Ask for both routes.
10. Keep one datum scheme
Why it costs: changing datums between operations forces the shop to re-align and re-check. What to change: use one datum scheme through the drawing and tell us which features locate the part.
11. Send a 3D model and a 2D drawing
Why it costs: a model without tolerances forces assumptions; a drawing without a model forces the shop to rebuild geometry. What to change: send both — STEP for geometry, PDF/DWG for tolerance, threads and finish.
12. Ask for the DFM note before you release the batch
Why it costs: the two or three changes that save the most are visible before the first chip. What to change: read the note, decide what the part must do, and release the batch with the assumptions written down.
Cost impact at a glance
| Change | Typical effect | Risk |
|---|---|---|
| Relax non-functional tolerances | Lower inspection and cycle time | None if the function is unaffected |
| Increase pocket corner radius | Removes EDM or a second operation | Check the mating part |
| Use stock close to finished size | Less material removal and distortion | May need a different supplier stock size |
| Mark cosmetic faces | Avoids rework and polishing | Needs a clear drawing |
| Split a complex part | Fewer setups, easier replacement | Assembly and tolerance stack |
DFM checklist before you send
- Critical tolerances marked, not global.
- Corner radii in deep pockets.
- Wall thickness and cosmetic faces marked.
- Threads and hole depths standard.
- One datum scheme.
- STEP + 2D drawing + quantity + finish.
- Ask for the DFM note with the quote.
Free DFM review · Custom CNC parts · Cost calculator
FAQ
What is DFM?
Design for manufacturing: changing the drawing so the part is easier, cheaper and more consistent to machine without losing function.
What is the most common DFM mistake?
Tight tolerances on non-functional faces. They add inspection and slower cuts without making the part work better.
Can you review my drawing before I quote?
Yes. The free DFM review marks the features that drive cost and the changes that remove a setup or an EDM operation.
Does DFM delay the project?
It usually saves time. One review before the run is cheaper than reworking a batch.
Will you change my design?
No. We mark the cost drivers and suggest options; you decide what the part must do.