How to Reduce CNC Machining Cost — 8 Design Changes and One Worked Example

CNC machining cost falls fastest when a drawing stops paying for precision it does not need. Three changes do most of the work: hold ±0.005 mm only on true functional interfaces and leave the rest at ±0.05 mm, finish the part in one or two setups instead of five, and use stock-friendly hole sizes and corner radii. SkillsProto runs all eight checks in the free DFM review returned with every quote within 24 hours, typically removing 15–40% from a first-quote price.

This page is about the changes that cut the number. For the breakdown of where a quote actually comes from, see the companion guide: What Drives CNC Machining Cost? 9 Factors That Move Your Quote.

Where the money actually goes

A prototype-quantity CNC quote is not a material price with markup added — it is machine time, and everything else is either a multiplier on machine time or an hourly charge beside it. The ranges below are typical for low-volume work; the split shifts with quantity, because setup and programming amortise across the batch while machining time does not.

Read the table as an attack list: machine time is the largest line, but setup count and inspection are the two a designer controls on the drawing — and the two most drawings inflate by default.

8 design changes that cut cost

1. Loosen the tolerances that do nothing

General dimensions at ±0.05 mm instead of a blanket ±0.005 mm callout. Holding ±0.005 mm means slower finishing passes, controlled measurement and a higher scrap allowance. Across a 12-dimension drawing, this one change typically removes 10–25% of the quote.

2. Use one internal corner radius everywhere

Pick one radius — R3 or R6 for an aluminium bracket — and use it on every internal corner. One cutter diameter means one tool change instead of four, and a larger, stiffer cutter can run at higher feed: typically 10–20% less cycle time on pocket-heavy parts.

3. Design for one or two setups

Every time a part leaves the machine to be flipped or re-chucked, the job pays setup labour plus the risk of losing datum repeatability. A part finished in 1–2 setups routinely costs 30–50% less than the same geometry needing five. Put clamping faces and datums on the drawing.

4. Keep cavities shallow

Depth beyond roughly 3–4× the cutter diameter forces a smaller, less rigid tool and slower feeds, or a 5-axis approach. If a pocket must be deep, open the radius and add draft so a bigger tool reaches in.

5. Hold wall thickness at 1 mm or more

Walls below about 1 mm in metal deflect under the cutter, chatter and drive scrap. Uniform thickness also reduces distortion and removes stress-relieving passes. Where a rib is needed, make it 0.6–0.8× the wall rather than thickening everything.

6. Limit thread depth to 1.5× diameter

A threaded hole deeper than roughly 1.5–2× its diameter adds tapping time, chip-clearing pauses and broken-tap risk for no extra joint strength, because the first few engaged turns carry the load. Blind holes should leave at least one diameter of clearance below the thread.

7. Specify standard hole diameters

Holes on the standard metric drill and reamer series run with tools already in the carousel. A 7.3 mm or 11.4 mm hole usually means a special tool or a boring pass. Rounding three hole sizes to the standard series is a five-minute drawing change.

8. Consolidate parts

Three machined plates bolted together become one part with pockets, removing two sets of setups, two FAI reports and the fastener stack. On small assemblies this is often the largest saving available — and it improves stiffness and alignment too.

Tolerance: the most expensive line on the drawing

Tolerance is the line engineers add by habit and suppliers price with care. Moving a dimension from ±0.05 mm to ±0.005 mm typically adds 10–25% to the affected features, and not linearly: it buys slower feeds, extra finishing passes, controlled measurement and a scrap allowance.

The discipline fits on one line of the title block: general tolerance applies unless a feature is marked. Critical dimensions — bearing bores, press fits, sealing faces and locating interfaces — get the tight callout plus CMM-verified FAI; everything else gets the general note. SkillsProto holds ±0.005 mm on critical features and ±0.05 mm generally, and the DFM review flags every dimension tighter than the function requires.

Quantity breaks that matter

Unit cost does not fall smoothly with quantity — it falls in steps, and each step has a different cause.

The index is illustrative, not a price list: step size depends on how setup-heavy the part is. Quote two or three quantities on the same drawing before fixing the order — the penalty for ordering 5 instead of 10 is often larger than expected.

Material and finish choices that save money

• Match the envelope to standard stock. A part that fits a standard bar or plate section avoids buying an oversized block and machining away half of it — often worth 5–15% across the material and machine-time lines.

• Specify the alloy the function needs, not the alloy the last project used. 6061 aluminium machines roughly two to three times faster than 303 stainless on equivalent geometry; where a harder alloy's strength is not doing work, the cycle-time saving is immediate.

• Leave invisible surfaces as-machined. As-machined is around Ra 3.2 µm. Requesting Ra 0.8 µm adds finishing passes on every surface called out; Ra 0.1 µm means manual polishing. Call out finish on cosmetic and sealing faces only.

• Group finishing into one batch. Anodising 25 identical parts costs far less per part than anodising five singles across five orders.

A worked example

A motorsport sensor housing in 6061 aluminium, 25 pieces, released with a blanket ±0.005 mm note, five setups and a full Ra 0.8 µm finish callout. SkillsProto's DFM review returned eight flagged items within 24 hours; the table shows the five that moved the price.

Nothing about the housing's function changed: two bores still locate the sensor and the sealing face still seals. The part ships on the same 3–7 working day cycle, or in 48 hours on express, and reaches a UK address in 3–5 days by DDP air freight.