Design for machining: 9 changes that cut part cost
Machined parts get cheaper when the design respects how a CNC cuts: open tight tolerances only where function needs them, give internal corners a radius a standard end mill can reach, keep pockets shallow relative to tool diameter, avoid unnecessary thread depth, design for fewer setups, start from standard stock sizes and specify surface finish only where it matters.
Guilherme Rodrigues ItinoseUpdated 2 min read
Key takeaways
- Tight tolerances everywhere are the most expensive line on a drawing.
- Internal corners need a radius; sharp internal corners need EDM or extra operations.
- Every extra setup adds fixturing time, cost and error risk.
- Standard stock sizes and common tools save material and programming time.
- Ask the shop for a DFM review before releasing the drawing.
Two drawings of the same part can produce quotes that differ by a factor of two. The difference is rarely the material; it is the time the shop spends setting up, cutting and inspecting features that did not need to be that hard.
Nine changes that reduce cost
- Open tolerances that do not matter. Use a general tolerance for non-functional dimensions and tighten only fits and interfaces.
- Radius internal corners. Choose a radius slightly larger than a standard end mill, for example 3 mm instead of 2.9 mm.
- Keep pockets shallow. Depth beyond three to four times the tool diameter slows everything down.
- Limit thread depth. Thread engagement beyond about 1.5 times the diameter rarely adds strength.
- Design for fewer setups. Features that can be reached from one or two sides save fixturing.
- Use standard holes and threads. Standard drill and tap sizes avoid special tools.
- Start from stock sizes. A part 1 mm larger than a standard bar may force the next size up.
- Specify finish only where needed. A blanket Ra 0.8 on every face multiplies cutting time.
- Avoid thin walls. They vibrate, deflect and need light cuts.
Tolerance has a price
| Tolerance on a dimension | Typical consequence |
|---|---|
| General (e.g. ISO 2768-m) | Standard cutting, no special inspection |
| ±0.05 mm | Careful cutting, more inspection |
| ±0.01 mm | Fine finishing, controlled temperature, CMM |
| Fits like H7/g6 | Reaming or boring and gauge inspection |
Ask for a DFM review
Before releasing the drawing, ask the machinist which features drive the price. A short conversation often removes a setup or a special tool.
We design and review parts with machining in mind, from the model to the drawing. See CAD engineering and our page for machine shops.
When to outsource this
- Your machining quotes are higher than expected and you do not know why
- You are preparing a new product for series production
When not to
- One-off prototypes where speed matters more than cost per part
FAQ
What tolerance should I use by default?
Use a general tolerance standard such as ISO 2768-m for non-critical dimensions and apply tighter tolerances only on fits, sealing faces and features that affect function.
Why are sharp internal corners expensive?
Rotating tools leave a radius. A truly sharp internal corner needs EDM, broaching or another operation, which adds cost and lead time.
How deep can a pocket be?
As a rule of thumb, pockets deeper than about three to four times the tool diameter get slower and less accurate. Deep, narrow pockets need long tools and lighter cuts.
Does surface finish change the price?
Yes. Fine finishes need extra passes or grinding. Specify roughness only on faces that slide, seal or are visible.
Send us one drawing
We return a review with issues found, suggested fixes and a fixed-price scope.
