Prototype CNC Machine Shop Service: 5 Essential Checks Before You Order

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  Key takeaways
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  What to weigh when comparing prototype CNC suppliers
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  Tolerance on a prototype cnc machine shop service: read the drawing, not the slogan
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  DFM feedback: the fastest thing a prototype cnc machine shop service can give you
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  Machine capability: does the shop's list fit your geometry?
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  MOQ, materials and certifications on a prototype cnc machine shop service
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  Lead time, inspection and the late-delivery question
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  Step by step: qualifying a prototype cnc machine shop service

Every shop advertises a number. The useful question is which features carry that number. A housing might need ±0.005 mm on two bore centers and nothing tighter than ±0.1 mm anywhere else. If the whole part is quoted at the tight figure, you pay for accuracy you cannot use.

General machining holds ±0.005 mm (±0.0002 in) on a capable 5-axis cell with the right fixturing. That is a realistic figure for bores, spigots and bearing seats on aluminum and stainless. It is not automatic on a 4,000 mm long part, where thermal drift and fixture stiffness start to dominate.

Surface finish moves with tolerance. A bearing seat usually lands at Ra 0.8–1.6 μm. A sealing face or a sliding bore may need Ra 0.2–0.8 μm, which means a separate finishing pass, sometimes a different tool, and a longer cycle. Ask for the finish callout early, because it changes the quote more than the tolerance does.

One practical test: send a drawing with one deliberately tight callout and one loose one. A shop that prices them differently understands your part. A shop that returns a single blended cost either did not read it or is padding.

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  Name the mating featuresMark the two or three dimensions that must be right, and let the rest run standard.
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  Ask how they will measure itA CMM, a micrometer and a pin gage answer different questions.
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  Watch for tolerance stackingTwo ±0.02 mm features can still fail when assembled.

A prototype exists to answer a question. If the shop can answer part of that question before cutting metal, you save a cycle. Good DFM feedback is short and specific: this wall is 0.6 mm and will chatter, this pocket has a 2 mm internal corner that needs a 2 mm cutter with a long reach, this hole breaks into a slot and will push the drill off center.

At GreatLight, quotation and free DFM analysis come back within 12 hours. That window matters more than it sounds. A design review that arrives on day one can be folded into a revised model before the first setup. One that arrives after the first parts ship becomes a second order.

Watch for feedback that is really a sales script. Phrases like "we recommend upgrading to 5-axis" without naming a feature are noise. Useful notes point at a specific face, a specific dimension or a specific tool.

If your part is simple, good DFM feedback may be one line: nothing to flag. That is a fine answer. Silence with no explanation is not.

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  Send the native CAD fileSTEP or Parasolid plus a PDF drawing beats a PDF alone.
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  State the functionA bracket that holds a cable and one that holds a load need different setups.
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  Ask what changedIf they revise the model, get a marked-up copy back.

Prototype work is where odd geometry shows up. A shop with one machine type will quietly route everything through it. Ask what will run your part and listen for travel numbers and axis count.

For deep pockets, undercuts and compound angles, simultaneous 5-axis is the difference between one setup and four. GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That mix lets a shop pick the setup that matches the part instead of forcing it.

Size decides the rest. The largest travel is 4,000 × 400 × 150 mm for long extrusions and frame rails. Mid-size cells cover 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, which handles most enclosures and manifolds. Compact cells at 500 × 500 × 450 mm and 500 × 310 × 200 mm suit small precision parts where rigidity matters more than envelope. A Ø400 mm rotary table gives you 4th-axis indexing on round and prismatic parts without a second op.

If your part is 900 mm long and needs a true position callout at both ends, a compact 3-axis machine cannot get there with one datum. That is the kind of mismatch to catch before you pay.

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  Match axis count to featuresOrganic surfaces and undercuts want simultaneous 5-axis.
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  Check the envelope against the blankStock size, not finished size, drives the machine choice.
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  Ask about fixturingA soft jaw or a vacuum plate can cost more than the cutting time.

Prototyping does not want a minimum order quantity. GreatLight has none, and runs from one prototype to 10,000+ part runs on the same process. If a shop quotes a single part at five times the unit price and calls that a setup fee, ask what the fee covers. Setup, programming and a first-article check are real costs. A tool that only makes sense at volume is not.

Material availability drives schedule as much as machining does. Common grades in aluminum, stainless, steel, copper alloys, titanium and engineering plastics are stocked or quickly sourced. 6061-T6, 304 and 316L, 17-4PH, Ti-6Al-4V and POM cover most prototype needs. Exotics like Inconel or magnesium AZ31B are workable but expect a longer material lead time and a different cutting strategy.

Certifications tell you which quality system governs the floor. ISO 9001:2015 is the baseline. IATF 16949:2016 matters when the prototype feeds an automotive program. ISO 13485:2016 matters for medical devices. ISO 27001:2022 covers information security, which is relevant if your drawings are the product. Ask which certificate covers the specific plant doing the work.

Confidentiality belongs in this list. Uploads should be treated as confidential, and an NDA should be available before files are opened, not after.

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  Ask for the certificate scopeA group certificate may not cover the plant running your job.
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  Confirm the material grade6061 and 7075 machine differently and cost differently.
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  Settle NDA timingSign first, upload second.

Lead time in prototyping is a chain: quote, DFM, material, machining, finish, inspection. A shop can only compress the parts it controls. Ask which link is longest for your part and get an answer in days, not weeks.

Typical timing on a straightforward machined prototype: quotation and free DFM analysis within 12 hours, production start within 24 hours, parts ship in 3–5 days. Finishing adds time. Anodizing, plating, powder coating, bead blasting or laser marking each need a pass, and laser marking has a minimum character height of 1.5 mm, so tiny text will not reproduce.

Inspection is where a prototype order either closes cleanly or opens a dispute. Look for 100% inspection before shipment, with a raw material check, in-process monitoring and a final inspection. Reports should be available on request. If you need dimensional data on three features, ask for it at the quote stage, not after the parts arrive.

Late delivery is the quiet cost. A historical late-delivery probability below 2% is a useful benchmark to ask about. Any shop should be able to tell you how often it misses a promised ship date, and what it does when it does.

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  Get dates per stageOne end date hides where the risk sits.
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  Separate machining and finishingFinishing is a second supply chain.
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  Request the report up frontInspection scope agreed before cutting avoids arguments later.