Two parts that look nearly identical can differ in price by 5x. The reason is that CNC part cost is driven by a handful of specific engineering choices, and most of them are set the moment the drawing is released. If you know what drives CNC part cost up, you can control the number before you ever request a quote. Here are the biggest levers, ranked by impact, with real values.
1. Tolerances are the single largest cost driver
Tolerance is where money hides. Loosening a callout by half a thousandth can change whether a part needs one pass or three, plus extra measurement. Each tighter band adds cycle time, tool wear, scrap risk, and inspection.
| Tolerance band | Relative cost multiplier | Typical method |
|---|---|---|
| +/-.010 in | 1.0x (baseline) | Standard machining, no special care |
| +/-.005 in | 1.1x to 1.2x | Normal shop practice |
| +/-.001 in | 1.3x to 1.6x | Finish pass, temperature awareness |
| +/-.0005 in | 1.7x to 2.5x | Fine boring, CMM verification, sorting |
| +/-.0002 in | 2.5x to 4x+ | Grinding, climate control, 100% inspection |
The rule that saves the most money
Only tighten the features that must be tight. A shaft may need +/-.0005 in on the bearing journal but tolerate +/-.010 in everywhere else. Blanket tight tolerances across a whole print are the most common way buyers overpay. For tiny high-precision parts, Swiss machining holds +/-.0005 in efficiently on the features that need it.
2. Material choice and machinability
Material affects CNC part cost two ways: raw stock price and how hard it is to cut. Machinability is often the bigger surprise.
- 6061 aluminum: baseline, cuts fast, cheap stock. Machinability index near 100.
- 303 stainless: free-machining, roughly 1.3x to 1.6x aluminum cost.
- 316 stainless: gummier, slower feeds, roughly 1.6x to 2x.
- 17-4 PH stainless: tough, tool-hungry, 2x to 2.5x.
- Titanium 6Al-4V: slow cutting, heat control, 3x to 5x aluminum.
- Inconel 718: the extreme, aggressive tool wear, 4x to 7x.
A titanium part is not just expensive because titanium bar is costly. It is expensive because feeds and speeds drop, cycle time climbs, and cutting tools wear out faster and get charged into the job. DFARS-compliant sourcing and mill certs add a small procurement premium on regulated jobs.
3. Number of setups and workholding
Every time a part is unclamped and re-fixtured, you pay for a new setup and you risk stacking tolerance error. A part machinable in one setup is far cheaper than one needing five faces touched.
- 1 setup: baseline, fastest, best repeatability.
- 2 setups: flip operation, add $100 to $300 setup and a locating scheme.
- 3+ setups: custom fixtures or soft jaws, each adding cost and lead time.
- 5-axis in one setup: higher hourly rate but can beat multiple 3-axis setups on complex geometry.
Design features on as few faces as possible. Avoid geometry that forces a re-clamp just to reach one hole. Our CNC milling team will flag setup-reduction opportunities during quoting.
4. Surface finish requirements
Surface finish is a cost driver people forget. As-machined finish is roughly Ra 63 to 125 uin and costs nothing extra. Tighter finishes require slower finishing passes or secondary operations.
| Surface finish | Cost impact | How it is achieved |
|---|---|---|
| Ra 125 uin | Baseline | Standard as-machined |
| Ra 63 uin | Slight | Careful finish pass |
| Ra 32 uin | +10% to 25% | Dedicated finish tooling and speeds |
| Ra 16 uin | +25% to 60% | Fine finishing, sometimes polishing |
| Ra 8 uin or below | +60%+ | Grinding, lapping, or polishing |
5. Geometry that fights the tool
Certain shapes are simply slow to cut and drive CNC part cost up regardless of material or tolerance.
- Thin walls under 0.040 in: chatter, multiple light passes, possible fixturing aids.
- Deep pockets with small radii: long reach tools run slow to avoid deflection.
- High L:D bores: ratios above 4:1 need special tooling and slower feeds.
- Sharp internal corners: a true sharp corner is impossible to mill, so add a radius or pay for EDM.
- Tiny fillets forced by CAD defaults: a 0.015 in radius forces a small, slow tool across a big pocket.
6. Inspection and documentation demands
Regulated work carries paperwork cost. A print calling for first article inspection, full CMM ballooned reports, and material certs adds real hours. First article inspection typically runs $150 to $600 per part number and CMM time is billed by the hour. This is legitimate cost, and our in-house CMM inspection keeps it lower than outsourcing measurement to a third-party lab. We run an ISO 9001-aligned system and supply mill test reports on request.
Put the levers to work
The fastest way to cut a quote is to review tolerances, material, and setup count before release. All figures here are typical illustrative multipliers, not binding quotes. Send your model and print through our instant quote and we will point out which cost drivers are moving your number the most.