CNC quotation fee explained: what actually moves the number

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  CNC quotation fee explained as a time and setup sum
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  How part geometry changes the cutting time
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  Where tolerance and surface finish add real cost
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  Why quantity changes the unit price so sharply
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  Material choice, finishing, and hidden handling
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  Which driver dominates your part

A machine shop sells machine time, operator time and tool wear. That is the whole business. When a quote looks high, one of three things happened: the part needs many setups, it needs a long cut, or it needs a material and finish that eat tooling. Everything else is a detail of those three.

Setup is charged once per operation, not once per part. A simple plate faced on one side in a three-axis vise might take 20 minutes to set up. Flip it three more times for back-side features and you pay four setups. On a 10,000-part run that setup cost nearly disappears. On a single prototype it is most of the invoice.

Run time is the cutting itself. It scales almost linearly with part count, which is why unit price drops fast in the first few hundred pieces and then flattens. When a supplier quotes the same part at two very different numbers, ask how many setups they assumed and how many tools they plan to load.

Material is the third leg. A 6061-T6 bracket and a 17-4PH stainless bracket can share identical geometry and differ by a factor of three in price, mostly because stainless cuts slower and wears tools faster. The geometry did not change. The machining physics did.

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  Fixed costSetup, programming, fixture build. Paid once, spread over the batch.
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  Variable costCutting time, material, tool wear, inspection. Scales with quantity.
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  Risk costTight tolerance, thin walls, exotic alloys. Added as a buffer.

The cutter has to reach every feature. If a pocket is 4× deeper than its width, the tool has to be long and thin, so you must slow the feed and take lighter passes. A 30 mm deep pocket with a 6 mm end mill is normal. A 120 mm deep pocket with a 6 mm end mill is a problem the quote will reflect.

Sharp internal corners force a small tool. A 1 mm corner radius means a 2 mm cutter, which removes metal slowly. Open that radius to 3 mm and a 6 mm cutter can rough most of the cavity at several times the removal rate. This single dimension often changes a quote more than material choice.

Thin walls are the other trap. Below roughly 1 mm wall thickness on aluminium, the part deflects under cutting force and vibrates. The shop has to reduce depth of cut, add support, or run a semi-finish and finish pass. All of that is time. A 2 mm wall is routine. A 0.5 mm wall is a different job.

Conversely, a part that can be reached from one direction is cheap. If every feature faces the spindle in a single orientation, one setup covers it. Add a cross-hole or a back-side counterbore and you buy another setup, another fixture, and another chance for position error.

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  Keep depth-to-width under 4:1Deeper pockets need longer tools and slower feeds.
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  Corner radius ≥ 1/3 of pocket depthLets a larger cutter clear the cavity.
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  Wall thickness ≥ 1 mm on aluminiumBelow that, chatter control costs time.
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  Design for one setup when possible5-axis work can reach five faces without refixturing.

General machining holds ±0.05 mm without drama. Push to ±0.01 mm and the shop starts checking temperature, using sharper tools, and measuring more often. At ±0.005 mm, which is our standard floor, boring and grinding operations enter the plan, and the part may need to be measured in a temperature-stable room.

Only the features that need tight tolerance should carry it. A bolt clearance hole at ±0.005 mm is wasted money. A bearing seat at ±0.005 mm is essential. Mark the critical dimensions on the drawing and leave the rest general. Shops quote what the drawing says, not what the function needs.

Surface finish behaves the same way. As-machined at Ra 1.6–3.2 μm is the natural result of a normal finishing pass. Ra 0.8–1.6 μm needs a finer stepover and a sharp tool. Ra 0.2–0.8 μm usually means a separate finishing operation, sometimes hand polishing, and that is where cost climbs quickly on large surfaces.

Inspection follows the tolerance. A general-tolerance part gets a caliper check. A ±0.005 mm part gets a CMM report. We inspect 100% of parts before shipment and can supply reports on request, but the measurement effort is part of the quote, and pretending otherwise helps nobody.

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  ±0.05 mmStandard milling and turning, no special handling.
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  ±0.01 mmSharper tooling, more in-process checks.
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  ±0.005 mmFine boring or grinding plus CMM verification.

The first part pays for everything. Programming, fixture, first-article inspection. If that is 4 hours of engineering and the part runs 10 minutes, the first unit is 25 times the pure cutting time. That is not a markup. That is arithmetic.

By part 50, the fixed cost is spread thin. By part 500, the shop can justify a soft jaw or a dedicated fixture that cuts cycle time further. By part 5,000, bar feeders and pallet changers run unattended, and the quote reflects lights-out machining rather than attended machining.

This is why a single prototype and a 10,000-part production run should never be compared per piece. We quote both, with no minimum order quantity, but the numbers sit in different cost regimes. If you need a production price, send the drawing and the annual volume together so both can be planned.

The counterintuitive part: sometimes a higher quantity costs more per unit than expected, because a dedicated fixture is added. That fixture pays back only if the volume is real. If the program is uncertain, say so, and the shop will quote a flexible setup instead of a hard one.

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  1–10 partsSetup-dominated. Expect high unit cost.
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  50–500 partsRun-time-dominated. Fixtures start to pay.
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  1,000+ partsAutomation-dominated. Tightest unit price.

Aluminium 6061-T6 is the default for a reason: it cuts fast, holds tolerance, and costs little. Moving to 7075 buys strength and loses machinability. Moving to 17-4PH stainless or Inconel buys corrosion resistance or heat resistance and costs several times more in both stock and tool wear.

Titanium TC4 (Ti-6Al-4V) is a specific case. It conducts heat poorly, so the heat stays in the cutting edge. Tool life drops, feeds drop, and the quote rises. If the application does not need the strength-to-weight ratio, aluminium or stainless will do the job for less.

Finishing is a second operation with its own setup. Anodizing, plating, powder coating, bead blasting and laser marking all require the part to be handled again, sometimes racked, sometimes masked. Laser marking has a practical floor of about 1.5 mm character height; below that the mark becomes unreliable and rework climbs.

Every added finish adds a shipping step and a risk step. Not a reason to avoid finishing, but a reason to specify only what the part actually needs. A hidden internal surface that no one sees rarely needs decorative anodizing.

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  Aluminium 6061-T6Fastest to machine, lowest risk, most common.
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  Stainless 303 / 304 / 17-4PHBetter corrosion resistance, slower cutting.
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  Titanium TC4, InconelHigh strength at high temperature, highest cost.
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  Plastics POM, PEEK, PCLow cutting force, but heat and chip control matter.