Purchasing CNC Machining Prototype Service: 5 Proven Checks Before You Send the PO

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  Key takeaways
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  What to check and what a real answer looks like
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  Match tolerance and finish to the feature that matters
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  Read the quote line by line, not the total
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  Confirm the machine actually fits the part
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  Check certifications against your industry
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  Test the DFM feedback before you commit
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  Four traps that show up after the parts arrive
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  Step by step: how to place a prototype order

A prototype drawing usually has one or two features that decide whether the part works: a bearing bore, a sealing face, a mating hole pattern. Those features carry the tight numbers. The rest of the part does not need them, and paying for ±0.005 mm everywhere raises the price without improving function.

When you send an RFQ, mark the critical dimensions. Tell the supplier which ones will be measured on a CMM and which ones are reference. A shop that asks this question before quoting is reading your drawing. A shop that ignores it will machine to whatever the machine happens to hold.

Surface finish follows the same logic. Ra 0.2–0.8 μm is a fine finish that needs a separate operation on most parts. Ra 0.8–1.6 μm is a high finish you can often reach in the same setup. As-machined at Ra 1.6–3.2 μm is fine for brackets and internal parts that nobody sees.

For prototypes, a finish callout on a non-functional face is wasted money. Keep it where a seal, a bearing, or a sliding surface needs it.

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  Mark critical dimensionsBox them on the drawing or list them in the RFQ note.
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  Separate finish calloutsFine finish only on functional faces, standard elsewhere.
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  Ask for the measurement planWhich features get CMM time, which get calipers.

Two quotes for the same prototype part can differ by 40% and both be honest. The gap is usually scope, not price. One quote covers material, machining, deburring and dimensional inspection. The other adds anodizing, laser marking, an inspection report and express shipping as extras.

Ask for the quote broken into five lines: material, machining, surface finishing, inspection reporting and shipping. That structure lets you compare suppliers directly and lets you drop a line if the prototype does not need it.

Watch for the material substitution line. Aluminum 6061-T6 and 7075 look similar on paper, but 7075 machines differently and costs more. If the quote says "aluminum" with no grade, the shop will pick whatever is on the shelf. For a prototype that will be tested, that is a problem.

Shipping language also matters. Parts ship in 3–5 days is a production statement. It is not a delivery guarantee, and no honest shop will give you one before the drawing review is done.

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  Five-line quoteMaterial, machining, finishing, inspection, shipping.
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  Named material grade6061-T6, 7075, 316L, not just "aluminum" or "steel".
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  Inspection as a line itemIf reports cost extra, you know what you are buying.

A prototype shop can hold a tight tolerance on a small part and struggle on a large one. The machine envelope decides what is possible. A 4,000 mm maximum processing size covers long rails and housings. A 750 × 1,150 × 550 mm travel covers most enclosures and plates. Compact travels of 500 × 500 × 450 mm suit small, high-detail parts.

The number of axes matters for geometry, not for size. A part with undercuts, angled holes or sculpted surfaces needs 4-axis or simultaneous 5-axis work. A part with holes on five faces can often be done on a 3-axis machine with two setups, which is cheaper and just as accurate.

Ask the shop how they plan to hold the part. Thin walls deflect under clamping pressure. Deep pockets need long tools that chatter. A supplier who names the workholding approach before quoting has already done the hard thinking.

For prototypes with a rotary or round feature, a Ø400 mm rotary table covers most work. Beyond that, you are into mill-turn territory.

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  Match envelope to part sizeFrom 500 mm compact travels up to 4,000 mm.
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  Match axes to geometry3-axis with two setups often beats 5-axis on simple parts.
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  Ask about workholdingThin walls and deep pockets are clamping problems first.

Certificates are not a scorecard. Each one answers a different question. ISO 9001:2015 covers a general quality management system and applies to most industrial work. IATF 16949:2016 is the automotive standard and matters if the prototype feeds a production program with PPAP requirements.

ISO 13485:2016 is the medical device standard. If your prototype is a surgical instrument or a device component, this is the first certificate to check. ISO 27001:2022 covers information security, which matters when you are sending CAD files for an unreleased product.

Ask for the certificate scope, not just the logo. A certificate that covers machining in one plant does not automatically cover a second plant or a subcontracted finishing line.

For a prototype order, the practical question is whether the shop can produce the inspection paperwork your quality team will accept. That is a document question, not a marketing one.

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  ISO 9001:2015Baseline for general industrial prototypes.
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  IATF 16949:2016Required when the prototype leads to automotive production.
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  ISO 13485:2016Medical device components and instruments.
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  ISO 27001:2022Information security for confidential CAD data.

The cheapest way to find a design problem is to have someone read the model before any metal is cut. A useful DFM review flags a wall that is too thin for the material, a pocket deeper than the tool can reach, a tolerance that no process can hold at that size, or a corner radius smaller than the cutter.

This feedback costs nothing to request and it tells you a lot. A shop that returns the model with a markup in a day is engaged. A shop that returns only a price has not looked at it closely.

DFM also protects the production run. A prototype made with a feature that cannot be molded or cast will pass the test and fail the transfer. Fixing it at the prototype stage costs a drawing revision. Fixing it later costs tooling.

Ask for the review in writing, even if it is a short list of three points. Written feedback is the record you use when the next revision comes back.

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  Request DFM with the quoteFree analysis and quotation within 12 hours at GreatLight.
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  Expect named issuesThin walls, deep pockets, small radii, unholdable tolerances.
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  Keep it in writingA short list is enough to drive the next revision.

The first trap is the silent material swap. The drawing says 6061-T6, the shop uses a generic 6061 and skips the heat treat. The part machines fine and measures fine, then deforms or wears faster in test. Named grades in the quote prevent this.

The second trap is an inspection report with no numbers. A certificate of conformance says the parts meet the drawing. It does not say which dimensions were measured or what the readings were. If you need data, ask for it in the RFQ.

The third trap is finishing done outside the shop with no visibility. Anodizing and plating are wet processes with their own tolerances. A hardcoat adds thickness, which changes a press fit. Confirm who does the finishing and whether it is dimensionally controlled.

The fourth trap is a prototype that cannot scale. A design optimized for one machined part may not transfer to casting or molding without changes. Ask about the intended production process at the prototype stage, and let the DFM review account for it.

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  Silent material swapSpecify the grade and the temper, not just the alloy family.
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  Empty inspection reportAsk for dimensional data, not just a conformance statement.
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  Finishing thicknessHardcoat and plating change fits; confirm the control.
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  Prototype that cannot scaleName the production process before the design is frozen.