Prototype Services: 7 Checks Before You Order CNC Parts
How to pick a supplier for first-article and low-volume machined parts. Written for design engineers and sourcing teams who need real tolerances, real lead times and a quote they can compare. Read this and you can judge a shop in one call.
Key takeaways
What to compare across prototype services
Use the same line items for every supplier, or the prices will not be comparable.
| Check | Weak answer | Strong answer |
|---|---|---|
| Tolerance | "We hold ±0.05 mm" | ±0.005 mm on stated features, with inspection report |
| Quote detail | One total price | Material, machine time, finish, inspection listed |
| Lead time | "About a month" | Production start inside 24 hours, parts in 3–5 days |
| Minimum order | 500 pieces minimum | No MOQ, one piece accepted |
| Certifications | ISO 9001 only | ISO 9001, IATF 16949, ISO 13485, ISO 27001 |
| Confidentiality | Verbal promise | NDA signed before files are opened |
| Machine list | "CNC machines" | 16 simultaneous 5-axis, 127 machines total |
Pick on evidence, not on the price line
The cheapest quote is rarely the cheapest part. Compare tolerance scope, inspection reports, MOQ, lead time and certifications line by line, then choose the shop that answers all five.
What prototype services actually cover
A prototype order is not just cutting metal. It starts with a 3D model and a drawing, then moves through DFM feedback, material selection, setup, first-article inspection and finish. If a supplier only quotes the cut, you will absorb the rest of the work yourself. Ask what is included before you compare prices.
The first deliverable is usually a DFM report. A good shop flags thin walls, deep pockets, sharp internal corners and features that need a second setup. We return quotation and free DFM analysis within 12 hours, so the feedback arrives while the design can still change cheaply.
Volume changes the process. One housing might be cut from a billet on a 3-axis mill with simple soft jaws. Fifty units justify a fixture and a 4-axis or 5-axis setup, which lowers the per-part cost and tightens repeatability. Say the expected build quantity in your request.
Finish is often the last thing specified and the first thing that causes a delay. Anodizing, bead blasting, electroless nickel and laser marking all add time and each has process limits. Laser marking needs a minimum character height of 1.5 mm, for example.
Tolerance and inspection: where the cost sits
±0.005 mm is the tight end of routine CNC work, and it is not free. It needs the right machine, a stable setup, temperature control and a CMM check. Put that callout only on features that touch something else: bearing bores, seal grooves, dowel holes, mating faces.
General dimensions can sit at ±0.1 mm without hurting function. Marking every dimension tight raises the price and slows the schedule, and it makes the drawing harder to read. A tolerance stack review during DFM usually removes half of them.
Surface finish follows the same logic. Ra 0.2–0.8 μm needs fine passes and often a second operation. Ra 0.8–1.6 μm covers most sealing and sliding surfaces. As-machined Ra 1.6–3.2 μm is fine for brackets, covers and internal structure.
Ask for the inspection plan, not just the numbers. Raw material check, in-process monitoring and final inspection before shipment are the baseline. Reports on request. Dimensions only mean something if someone measured them.
Materials, machines and part size limits
Aluminum 6061-T6 is the default for prototypes: easy to cut, stable, cheap to anodize. Use 7075 when you need strength, 2024 where fatigue matters, and ADC12 only when the part will later move to die casting.
Stainless 303 machines cleanly and suits most fixtures. 316L is for medical and marine exposure. 17-4PH gives high strength after heat treatment. Titanium TC4 (Ti-6Al-4V) and Inconel are cut here too, but expect longer cycle times and higher tool wear.
Plastics behave differently. POM and PA are stable and machine well. PEEK holds up at temperature but costs more than most metals. ABS and PC are usually chosen to mimic an injection-molded part before the tool is cut.
Size decides which machine you get. Our largest travel is 4,000 × 400 × 150 mm. Medium work runs on 750 × 1,150 × 550 mm or 600 × 600 × 600 mm tables. Compact parts use 500 × 500 × 450 mm or 500 × 310 × 200 mm, with a Ø400 mm rotary table for round features.
Lead time, minimum order and paperwork
Lead time claims are easy to make and hard to verify. Ask for two numbers: when production starts, and when parts ship. Here production can start within 24 hours of an approved drawing, and parts ship in 3–5 days. Historical late-delivery probability is below 2%.
Minimum order quantity is a hidden cost driver. A shop with a 500-piece minimum will either refuse a one-off or quote it at a bad price. We work with no minimum order quantity, from one prototype to 10,000+ part runs, on the same equipment.
Certifications tell you which markets the supplier can serve. ISO 9001:2015 is the base quality system. IATF 16949:2016 is what automotive programs expect. ISO 13485:2016 covers medical devices. ISO 27001:2022 matters if you send sensitive CAD data.
Confidentiality should be settled before files move. Uploads are kept secure and confidential, and an NDA is available on request. If your IP is the product, sign first and quote second.
Seven steps from drawing to first article
Each step lists the parameter range or the mistake to avoid.
- 1Send STEP plus a 2D drawingSTEP defines geometry, the drawing defines tolerance and finish. Missing datum callouts are the most common cause of a wrong first article.
- 2Read the DFM report before approvingCheck flagged thin walls, deep pockets and sharp internal corners. Corner radii should be at least one third of pocket depth where possible.
- 3Freeze material and temper6061-T6 and 6061-O machine and anodize differently. Write the temper on the purchase order, not just the alloy.
- 4State the build quantity honestlyOne piece, 20 pieces and 500 pieces get different setups. The quantity changes the fixture and the price.
- 5Confirm finish and maskingAnodizing adds 5–25 μm per surface. Mask threaded holes and bearing bores, or the fit will close up.
- 6Approve the first-article reportCompare measured values against the drawing, not against the model. Ask which features were checked and with what instrument.
- 7Plan the bridge to productionIf the part will be molded or cast later, keep the machined version within the draft and wall rules of that process.
Questions engineers ask before ordering
How fast can a machined prototype ship?
Quotation and free DFM analysis come back within 12 hours. Once the drawing is approved, production can start within 24 hours and parts ship in 3–5 days.
That timeline assumes the material is in stock and the drawing does not change after setup. Late design changes reset the clock.
Is there a minimum order quantity?
No. We run from one prototype to 10,000+ part runs. A single part is quoted on the same machines as a small batch.
Per-part cost drops once a fixture is justified, usually somewhere between 20 and 100 units depending on geometry.
How tight a tolerance should I call out?
±0.005 mm is available, but reserve it for mating bores, seal grooves and dowel holes. General dimensions at ±0.1 mm are enough for most brackets and covers.
Over-tolerancing raises cost and can force slower cutting and extra inspection.
Which materials are available?
Aluminum 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12; stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH; steel 1018, 1045, 4130, 4140, 4340, A36 and tool steel.
Also copper and brass grades, titanium TA1, TA2 and TC4, Inconel, magnesium AZ31B and AZ91D, plus ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre.
Can you sign an NDA before I share files?
Yes. Uploads are kept secure and confidential, and a non-disclosure agreement is available on request.
We hold ISO 27001:2022 for information security, which covers how design data is stored and accessed.
What decides whether I get a 3-axis or 5-axis part?
Feature access. If every face can be reached from three directions, a 3-axis machine is cheaper and faster. Undercuts, deep angled pockets and contoured surfaces usually push the job to 4-axis or 5-axis.
We have 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, so the routing follows the geometry rather than the other way round.
Send your drawing, get a quote in 12 hours
Upload a STEP file and a 2D drawing. We return pricing plus free DFM feedback before you commit to anything.
12-hour quoteNo MOQ100% inspectionNDA on request
More from the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
