CNC Milling Costs: Typical Part Examples and What Drives Them

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  Key takeaways
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  Five typical parts and their main CNC milling costs
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  Plates and brackets: where CNC milling costs start
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  Enclosure housings: pocket depth and wall finish drive price
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  Shafts and pins: turning time plus milled features
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  Cavities and mold inserts: surfacing and polish dominate
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  Judging a CNC milling quote before you commit
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  Step by step: comparing CNC milling costs across suppliers

A flat mounting plate is the cheapest geometry in milling. One or two setups, a face mill pass, a profile cut and a hole pattern. On 6061-T6 at 100 pieces, most of the price is the plate itself, not the spindle time. If your plate is 200 × 150 × 12 mm with 20 drilled holes, the quote usually tracks material weight closely.

Brackets change that picture. The moment a part has two perpendicular faces that both need machined features, you need a second setup or a tombstone fixture. That second setup adds load, unload, re-clamp and a datum check. It is common for a bracket to cost more than a plate of twice the volume, because the plate never moves.

A practical test: count the directions from which a tool must approach the part. One direction is cheap. Two is normal. Three or more pushes you toward 4-axis or 5-axis work, and the hourly rate rises with the machine.

Watch for cosmetic callouts on brackets. A bead-blasted finish on a non-critical web can add a handling and masking step that is pure cost with no functional gain.

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  Cheapest caseSingle-face plate, standard hole tolerances, as-machined finish.
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  Cost jumpTwo machined faces at 90°, requiring a second setup and re-datum.
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  Ask the supplierCan the bracket be redesigned so all critical features sit on one face?

Housings are where CNC milling costs become non-linear. A shallow pocket 10 mm deep in a 100 mm wide block is routine. Push that pocket to 60 mm deep with a 12 mm corner radius and you need long-reach tooling, lighter depths of cut, and more passes. Cycle time can double without any change to the outer envelope.

Wall thickness matters just as much. A 2 mm wall next to a deep pocket will chatter. The machinist has to slow down, take smaller radial engagements and sometimes add support wax or a temporary rib. All of that is real time in the quote.

Finish callouts on the inside of a housing are often over-specified. An internal surface at Ra 0.8–1.6 μm is easy to reach with a finishing pass. Demanding Ra 0.2–0.8 μm inside a deep pocket usually means hand work, and hand work is the least predictable line in any quote.

If the housing is a prototype, ask whether the internal finish can stay as-machined at Ra 1.6–3.2 μm. On most electronics and automation enclosures it makes no functional difference.

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  Depth-to-width ratioAbove 4:1, expect longer reach tools and slower feeds.
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  Thin wallsBelow 2.5 mm on aluminium, chatter risk rises sharply.
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  Internal polishReserve fine finishes for sealing faces and sliding surfaces only.

A plain shaft is a turning job, and turning is fast. CNC milling costs appear when the drawing adds flats, keyways, cross holes or a hex. Each of those needs either a second operation on a mill or a mill-turn center that can do it in one chucking.

For a shaft with a single flat, a two-operation route is usually cheaper than a mill-turn machine. For a shaft with flats on three sides, cross holes at two angles and a thread, mill-turn becomes competitive because you avoid stacking concentricity errors across setups.

Concentricity is the hidden cost. If the drawing calls 0.02 mm runout between a bearing journal and a spigot, the shop must control the datum carefully, often by turning the critical diameters in one continuous operation and checking with a dial indicator before removing the part.

Material choice shifts the balance too. 17-4PH stainless at 40 HRC machines slower than 303 and will raise the turning portion of the quote noticeably, even for the same geometry.

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  Cheapest routeOne flat, no cross features, two setups on a lathe and a mill.
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  Mill-turn winsThree or more radial features with tight angular position.
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  Inspection pointRunout between bearing seats checked on every part or by sampling?

A cavity insert starts as a block and ends as a negative of your part. The roughing is straightforward. The cost sits in the 3D finishing passes and in the hand polish that follows. A contoured surface with a 0.5 mm stepover can run for hours on a 5-axis machine.

Surface finish requirements decide the last stretch. A tool-finished cavity at Ra 0.8–1.6 μm may be acceptable for some parts. A mirror finish at Ra 0.2–0.8 μm means manual polishing with progressively finer stones and diamond paste, and that time is difficult to estimate before the first cut.

Draft angle is another lever. Insufficient draft forces the polisher to work right into corners, which is slow and risks washing out the edge. Adding 1° of draft in the CAD model can cut polishing hours with no change to the molded part.

For low-volume work, consider whether a machined aluminium cavity is enough. It costs less than hardened tool steel, machines faster, and lets you test the geometry before committing to a production tool.

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  RoughingPredictable and fast; rarely the problem.
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  FinishingStepover and surface tolerance set the cycle time.
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  PolishThe least predictable line item; specify a finish grade, not a number.

Price alone tells you little. Ask for a quote that separates material, machining time, finishing and inspection. A supplier who cannot split those lines is guessing, and their guess will drift when the part changes.

Check how the shop handles the first article. At GreatLight, we run 100% inspection before shipment with raw material checks, in-process monitoring and a final report available on request. That matters on parts where a single out-of-tolerance bore scraps an assembly.

Look at lead time claims in context. We quote and return a free DFM analysis within 12 hours, start production within 24 hours, and ship parts in 3–5 days on typical work. Historical late-delivery probability sits below 2%. Those numbers only mean something if the shop states them up front.

Finally, confirm the certifications that apply to your industry. ISO 9001:2015 covers general quality systems, IATF 16949:2016 applies to automotive, ISO 13485:2016 to medical devices and ISO 27001:2022 to information security. A supplier holding all four is set up for regulated programs, not just general machining.

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  Quote structureFour separate lines: material, machining, finish, inspection.
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  DFM feedbackA real shop flags thin walls and deep pockets before you order.
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  CertificationsMatch the certificate to your industry, not to the shop's brochure.
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  Order sizeNo minimum order quantity matters when you are still validating a design.