Why CNC cost is mostly a design problem.
Engineers often assume CNC cost is set by the shop — labor rate, overhead, how aggressive the quote model is. In our experience quoting 14,000+ parts a year, the shop accounts for maybe 30% of the variance between a cheap part and an expensive one. The other 70% is baked into the CAD before anyone quotes it.
Below are the ten design choices we see most often that inflate cost without adding functional value. Each includes the approximate cost multiplier based on comparable jobs we quoted in 2025, and a suggested fix.
§01 — Sharp internal corners.
Internal corners on milled parts must be cut with a round tool. A corner with R0.5 requires a Ø1 mm end mill — which has low rigidity, slow feed, and short tool life. Zero-radius corners can't be milled at all; they require EDM or sinker broaching.
Cost delta: An aluminum bracket with R0.5 internal corners throughout quoted at $42/part (qty 50). The same part with R3 corners quoted at $18. A 2.3× increase for corners that had no functional purpose.
§02 — Deep pockets and thin walls.
Deep pockets (depth > 3× tool diameter) require either long reach tooling (fragile, slow) or plunge machining (very slow). Thin walls below 1 mm chatter, work-harden, and often require spark-out passes.
Cost delta: A 6061 housing with 0.8 mm walls and 40 mm deep pockets quoted at $180/part. Same housing with 1.5 mm walls and 30 mm pockets: $72. That's a 2.5× premium for thinner walls that weren't load-critical.
§03 — Tight tolerances on every dimension.
This is the single biggest cost inflator we see. Drawings with ±0.02 mm on every dimension, when only two or three dimensions actually matter for fit. The shop has to inspect and prove every feature — CMM time, scrap rate, and setup rigidity all go up.
Cost delta: A titanium medical bracket had 14 dimensions at ±0.01 mm. We requoted with only 3 critical fits at ±0.01 mm and the rest at ISO 2768-f. Price dropped from $412 to $147 per part (qty 25). Same finished geometry, same function.
ISO 2768-mK in the title block. Add tight tolerances only to features that must mate with specific parts or must meet specific assembly loads. Read our full tolerance chart for guidance on which tier to pick.
§04 — Exotic threads, non-standard hole sizes.
Custom-pitch threads, imperial threads on metric parts, odd hole sizes (e.g. Ø9.37 mm when Ø10 would work) all require special tooling or boring. Non-standard threads especially kill cost — every custom tap is a tool order and a setup.
Cost delta: A stainless manifold specified M8×1.0 fine-pitch in a place where M8×1.25 standard would have worked. Setup and tooling added $14/part; part quantity was 200. Total avoidable cost: $2,800.
§05 — Ra 0.4 surface finish on non-functional faces.
A blanket "Ra 0.4 all over" note on the drawing forces finish passes on every face — even the ones nobody ever sees. Each finish pass adds cycle time; a face-milled aluminum part at Ra 0.4 cuts 3–4× slower than the same face at Ra 1.6.
Cost delta: Same 6061 top plate, two versions: Ra 0.4 all over = $58/part. Ra 0.4 only on sealing face, Ra 1.6 elsewhere = $31/part. 1.9× premium for a finish nobody needed.
§06 — Specifying 7075 instead of 6061.
7075 is great if you need the strength. But many designs specify 7075 "because stronger is better" when 6061 would have worked fine. 7075 costs 2.5–3× more in raw material, and isn't as easy to anodize to a uniform color.
Cost delta: Drone gimbal frame in 7075: $86/part. Same frame in 6061-T6: $39/part. Strength margin against worst-case load was 8× in both — i.e. neither material was stress-limited. Pure material overspec.
§07 — Designs that need 4+ setups.
Every setup on a 3-axis machine adds 10–20 minutes of re-fixturing, re-zeroing, and re-probing. A part that needs 5 different orientations may spend more time being repositioned than being cut. 5-axis avoids this — but not every shop has the capability, and 5-axis time is billed 50–80% higher per hour.
Cost delta: Stainless valve body designed with features on 5 faces: $94/part, 3-axis, 5 setups. Redesigned with 4 faces (combined two features that could be on the same plane): $54/part, 3-axis, 3 setups. 1.7× premium for the extra setup.
§08 — Arbitrary chamfer callouts.
"0.5 × 45° chamfer all edges" looks innocent. But on a part with 30 edges, that's 30 individual tool paths — often a separate toolpath for each edge because they're on different faces. And chamfers break into corners in ways that sometimes require a different tool to finish cleanly.
Cost delta: Simple aluminum housing, 24 edges chamfered 0.5×45°: 18 minutes added per part. At qty 100, that's 30 hours of CNC time.
§09 — Tight hole position tolerance from a rough face.
Datum selection matters. If you specify Ø0.05 true position on a hole, but the datum is a rough bandsaw-cut edge, the shop has to first mill that edge true — adding setup time — or measure from an imaginary feature that no tool can find.
Cost delta: Steel linkage plate with ±0.02 mm hole location datum'd to a rough sheared edge: $48/part. Same plate re-datum'd to a machined pad: $22/part.
§10 — "No burrs allowed" without defining "allowed."
Nothing sinks a part to 100% inspection faster than a subjective quality note. "No burrs" gets interpreted conservatively: the inspector will reject anything the fingernail catches. This means hand-deburring every edge, every part, which is expensive and still doesn't guarantee acceptance.
Cost delta: Brass fittings with "no burrs allowed, all edges smooth to touch": $19/part after 12% scrap. Same fittings with "burrs < 0.1 mm per ISO 13715": $11/part, 2% scrap.
The one-page checklist.
Before you release a CNC drawing, walk through these ten questions:
| Check | If yes, fix |
|---|---|
| 1. Internal corners < R1.5 mm? | Open up to ≥ R1.5 unless mating requires sharp |
| 2. Walls < 1.5 mm or pockets > 3× tool dia deep? | Thicken walls, reduce depth, or flag as thin-wall |
| 3. Tolerances tighter than ±0.05 on non-fit dimensions? | Default title block to ISO 2768-mK |
| 4. Non-standard threads or hole sizes? | Use M3–M12 coarse or UNC/UNF; H7 reamed holes |
| 5. Ra < 1.6 specified "all over"? | Specify per face; default Ra 1.6 or 3.2 |
| 6. 7075, titanium, or Inconel called out? | Verify strength margin; 6061/304 may work |
| 7. Part requires > 3 setups? | Consolidate features onto fewer faces |
| 8. Explicit chamfers on every edge? | Replace with "break sharp edges 0.2–0.5" note |
| 9. Datums on raw / unmachined surfaces? | Datum from machined faces only |
| 10. Subjective quality notes ("smooth," "no burrs")? | Reference ISO 13715 with numeric limits |
Work through this list before you send the RFQ. If you're not sure on any of them, send us the drawing — we'll DFM-review it and come back with markups within a business day. No obligation, no fee.