How to Choose a Rapid Prototype Company for CNC Machining

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  Key takeaways
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  What to check before you send an RFQ
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  Does the rapid prototype company actually machine your geometry?
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  Tolerance, surface finish, and what they cost
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  Lead time: what the timeline really includes
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  MOQ, unit cost, and prototype-to-production continuity
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  Certifications, materials, and finishing in one place
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  Inspection, data handling, and the questions that expose a weak supplier
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  Step by step: how to vet a rapid prototype company

Most prototype parts are not simple blocks. They have pockets with deep corners, angled faces, thin ribs, or a bore that has to line up with a boss on the other side. Ask which machine would run your part and why. A shop with 16 simultaneous 5-axis machining centers can cut five faces in one setup, which holds position between features far better than moving a part across three fixtures.

Travel limits decide what is possible. A 4,000 × 400 × 150 mm envelope covers long rails and frame sections. Medium platforms at 750 × 1,150 × 550 mm and 600 × 600 × 600 mm handle most enclosure and manifold work. Compact machines at 500 × 500 × 450 mm and 500 × 310 × 200 mm are the right call for small, high-detail parts where rigidity matters more than size.

If a supplier answers with a machine count and nothing else, push back. The useful answer names the machine, the number of setups, and the axis count. That tells you whether your datum scheme survives the process.

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  5-axisComplex geometry, undercuts, five-sided features, one setup
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  4-axisCylindrical parts with cross holes or flats
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  3-axisPrismatic parts with features on one or two faces

±0.005 mm (±0.0002 in) is the tight end of normal CNC work, and it is not free. Reaching it means a stable setup, sharp tooling, temperature control, and slower passes. Apply it to the features that mate with something else, and leave the rest at general machining tolerance.

Surface finish works the same way. Ra 1.6–3.2 μm is as-machined and usually fine for a bracket or a housing. Ra 0.8–1.6 μm needs a finishing pass. Ra 0.2–0.8 μm means a separate operation, sometimes hand polishing, and the price reflects it. If your drawing calls for Ra 0.4 μm on a face that gets covered by a gasket, you are paying for nothing.

A good rapid prototype company will read the drawing and tell you which callouts drive cost. A weak one quotes the whole thing at the tightest value it sees. That difference shows up as a 30–50% gap between two quotes for the same part.

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  As-machinedRa 1.6–3.2 μm — covers most non-sealing faces
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  Fine finishRa 0.8–1.6 μm — sealing faces, sliding surfaces
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  PolishedRa 0.2–0.8 μm — optical, medical, or bearing fits

Quoting speed is the first signal. A shop that returns a quotation and free DFM analysis within 12 hours is set up to process incoming work, not to batch it once a week. From there, production can start within 24 hours of PO and material release, and parts ship in 3–5 days for standard prototype quantities.

That 3–5 day window assumes the material is in stock and the drawing is frozen. If your part needs 7075 aluminium or Ti-6Al-4V and the shop has to order it, add the mill lead time. If you are still changing the model, every revision restarts the setup. Freeze the geometry before you release the PO and the quoted date holds.

Ask what happens when a part fails final inspection. A shop that reworks in-house keeps the schedule. One that outsources finishing or heat treatment loses days to shipping both ways. Historical late-delivery probability below 2% is a reasonable benchmark to request, but only if the supplier can back it with their own records.

Prototypes exist to be iterated. If a supplier sets a minimum order quantity, every design change costs you a full batch. No minimum order quantity, from one prototype to 10,000+ part runs, is the right model at this stage. It also tells you the shop is not treating prototype work as an unwanted interruption to production.

Unit price on a single piece is not a useful number on its own. Ask for a price at 1, 25, 100, and 1,000 pieces. The curve shows where setup cost stops dominating and where material and cycle time take over. If the drop from 1 to 100 is small, the shop is probably charging mostly for material. If it is steep, you are paying for setup and programming.

The other question is whether the same shop can take the part into production. Moving from a machined prototype to a die-cast or vacuum-cast part means new tooling and a fresh tolerance stack. A supplier that offers machining, die casting, vacuum casting, and sheet metal under one roof can plan that transition instead of handing you a drawing and wishing you luck.

Certifications are a filter, not a badge. ISO 9001:2015 covers general quality systems. IATF 16949:2016 is what automotive and EV programs expect. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when your uploads are unreleased designs. Match the certificate to your industry before you compare price.

Material range is the next filter. Aluminium 6061, 7075, and 6082 are common. Stainless 303, 304, 316L, and 17-4PH cover most corrosion and strength needs. Titanium TC4 (Ti-6Al-4V), Inconel, and magnesium AZ31B are available but slow to cut and priced accordingly. On the plastic side, POM and ABS machine easily, PEEK and carbon fibre do not.

Finishing is where schedules slip. Anodizing, electroless nickel, powder coating, bead blasting, and laser marking all add a step. Laser marking has a minimum character height of 1.5 mm, so a 0.8 mm part number will not reproduce. Confirm the finish sequence and who owns the outside process before you approve the quote.

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  AnodizingClear, colour, hardcoat, conductive
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  PlatingElectroless nickel, zinc, silver, gold
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  MechanicalBead blasting, tumbling, brushing, polishing

100% inspection before shipment, covering raw material check, in-process monitoring, and final inspection, is the baseline. Inspection reports on request should be normal, not an upsell. If a supplier only inspects the first and last part of a run, dimensional drift in the middle goes out the door with your parts.

Confidentiality is a real commercial risk at prototype stage. Uploads should be secure and confidential, and a non-disclosure agreement should be available on request without a negotiation. If a shop hesitates on an NDA for a prototype job, that is a signal about how they handle everything else.

Finally, ask two questions. What would you change in this design to make it cheaper? And what feature here is most likely to fail inspection? A shop that answers both with specifics has actually read your drawing. A shop that answers with generalities has read your email.