CNC Processing Bay Area: How to Pick a Supplier
Most hardware teams here cannot wait three weeks for a first article. This guide lists the checks we would run before sending a PO: tolerance fit, machine travel, certification scope, MOQ, and what a real quote should contain. Read it and you can tell a capable shop from a broker in under an hour.
In this article
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Key takeaways
What to check before you release a PO
Use this as a checklist. If a supplier cannot answer two or three rows with numbers, keep looking.
| Check | What good looks like | Why it matters |
|---|---|---|
| Tolerance | ±0.005 mm on critical features | Drives machine choice and inspection cost |
| Machine travel | 750 mm+ for large housings | Small machines cannot reach the feature |
| MOQ | No minimum, 1 to 10,000+ | Prototype and production fit one supplier |
| Certifications | ISO 9001, IATF 16949, ISO 13485 | Gate for auto, medical, and defense work |
| Quote turnaround | DFM feedback within 12 hours | Catches design errors before cutting metal |
| Lead time | Production start within 24 hours | Keeps your build schedule intact |
| Inspection | 100% before shipment, reports on request | Avoids surprises at incoming QC |
| Finishing | Anodize, plating, bead blast in-house | One handoff instead of three vendors |
The verdict
Pick the shop that answers your tolerance, travel, and certification questions with numbers. A fast quote with DFM notes beats a cheap quote with silence.
Start with tolerance, then work backward to the machine
Every quote starts with a tolerance callout, and that callout decides which machine runs the part. A ±0.005 mm bore on a 40 mm aluminum housing is a 5-axis job with a probe and a temperature-stable setup. The same housing at ±0.05 mm is a 3-axis job that ships faster and costs less. Engineers who mark everything ±0.01 mm 'to be safe' pay for precision they never measure.
Look at the drawing the way a machinist does. Which features actually mate? Which are datum? A sealing face, a bearing bore, and a dowel hole carry the function. Cosmetic bosses and cable reliefs usually do not. When you tolerance only the functional group, you give the shop room to fixture the part for rigidity instead of chasing tenths everywhere.
For parts under 100 mm, ±0.005 mm is routine on a 5-axis center with a Ø400 mm rotary table. Above 500 mm, thermal drift starts to matter. A shop that runs large parts in a temperature-controlled bay will say so; one that does not will quote the tolerance and hope. Ask where the part is cut and how the machine is probed.
- 1Functional features firstBores, faces, and dowel holes get the tight callouts.
- 2One datum schemeMixed datums force the shop to re-fixture and stack error.
- 3Surface finish follows functionRa 0.8–1.6 μm covers most sealing faces.
Match part size to machine travel
Travel is the number buyers forget to ask for. A shop with 127 machines may still have nothing that reaches a 900 mm chassis rail. The large travel envelope we run is 4,000 × 400 × 150 mm, with 750 × 1,150 × 550 mm and 600 × 600 × 600 mm for mid-size work, and 500 × 500 × 450 mm or 500 × 310 × 200 mm for compact parts. If your part falls between envelopes, say so early.
Size also changes fixturing. A long, thin rail deflects under cutting force, so the shop has to support it along its length or accept chatter. A tall part needs a tombstone or an angle plate, which eats travel in Z. When a supplier quotes a large part in one setup without asking about wall thickness, the quote is a guess.
For prototypes, the opposite problem shows up. A one-off bracket does not need a 5-axis center; it needs a shop willing to set up a 3-axis machine for a single piece. Many high-volume shops quietly pass on this work or pad the price. Check the MOQ policy before you send the file.
- 1Ask for the envelopeGet X, Y, and Z in millimeters, not a brochure claim.
- 2Flag thin wallsUnder 1.5 mm, deflection drives the setup.
- 3Confirm one-off willingnessNo MOQ is a real capability, not a slogan.
Read the certificate scope, not the logo
Four certificates show up on most supplier pages: ISO 9001:2015 for general quality systems, IATF 16949:2016 for automotive, ISO 13485:2016 for medical devices, and ISO 27001:2022 for information security. Each one covers a defined scope. An automotive certificate does not make a shop qualified for surgical instruments, and a quality certificate says nothing about how your CAD files are stored.
For medical work, the useful questions are about traceability and material certificates, not the logo. Can the shop supply a mill certificate for the heat lot? Does it keep inspection records tied to a serial number? ISO 13485:2016 sets the framework, but the records are what an auditor will ask for. Ask to see a sample inspection report before you commit.
Information security matters more than most buyers expect. ISO 27001:2022 covers how files, credentials, and networks are controlled. If your design is unreleased, pair that with a signed NDA. We keep uploads secure and confidential and sign an NDA on request. A supplier that hesitates on either point is telling you something.
- 1Match cert to industryIATF for auto, ISO 13485 for medical, ISO 9001 as a baseline.
- 2Ask for a sample reportInspection records reveal more than a certificate PDF.
- 3NDA before uploadSign first, then share the model.
Quote speed tells you how the shop is organized
A quote is not just a price. It is a DFM review, a process plan, and a schedule. When a shop returns a quotation and free DFM analysis within 12 hours, that means someone read the model and flagged the thin wall, the deep pocket, or the tolerance that needs a second setup. A number with no comments is a number you will pay for later.
Production can start within 24 hours once the drawing and material are confirmed. Parts ship in 3–5 days for most jobs. That rhythm holds because the shop runs its own finishing, so anodizing or bead blasting does not add a week of outside transit. Historical late-delivery probability is below 2%, which matters when your assembly line is scheduled.
Be skeptical of a quote that is 30% below everyone else with no explanation. Either the shop missed a setup, or it plans to cut corners on inspection. Ask what is included: material certs, first article report, finishing, and packaging. The cheapest quote often becomes the most expensive when a batch arrives out of tolerance.
- 1Demand DFM notesA quote without comments skipped the review.
- 2Ask what is excludedFinishing and inspection are common omissions.
- 3Confirm the ship dateGet it in writing against your build schedule.
Material and finish choices that change the price
Aluminum 6061-T6 is the default for housings and brackets because it machines fast and takes anodizing well. Switch to 7075 when you need strength, and expect more tool wear and a slower cycle. Stainless 303 and 316L behave differently: 303 is free-cutting, while 316L work-hardens and needs lighter passes. Titanium TC4 (Ti-6Al-4V) and Inconel are for high-temperature or high-load parts, and they cut slowly.
Plastics follow the same logic. POM and ABS are easy. PEEK holds tight tolerances at temperature but costs more per kilogram than most metals. Carbon fibre composites need diamond tooling and dust control. When a supplier quotes a material without asking about the application, the number is generic and will move.
Finishing is where hidden cost lives. Anodizing can be clear, colored, hardcoat, or conductive, and each changes masking and racking. Electroless nickel, zinc, silver, and gold plating all have different thickness tolerances. Laser marking needs a minimum character height of 1.5 mm to stay legible. Decide the finish before the first cut, not after.
- 16061-T6 firstMove to 7075 only when strength demands it.
- 2Watch work-hardening316L and titanium punish heavy passes.
- 3Pick the finish earlyAnodize type changes masking and lead time.
Step by step: how to vet a supplier
Run these in order. Each step takes minutes and filters out most mismatches.
- 1Send the model with a tolerance calloutInclude the STEP file, a 2D drawing for critical features, material, and finish. Mark datums. A model with no tolerance scheme forces the shop to guess.
- 2Ask for the machine envelopeGet X, Y, and Z in millimeters and the number of setups planned. If your part exceeds 750 mm, confirm a large travel machine is available.
- 3Request DFM feedback with the quoteA useful response arrives within 12 hours and names at least one risk: thin wall, deep pocket, or a tolerance that needs a second op.
- 4Confirm certification scopeAsk which certificate applies to your industry and request a sample inspection report. For medical, ask for heat-lot traceability.
- 5Check MOQ and prototype policyIf you need one piece, confirm the shop will run it. No minimum order quantity means the setup cost is already in the unit price.
- 6Agree on inspection and reports100% inspection before shipment is the baseline. Ask for dimensional reports on the first article and on request for later lots.
- 7Lock the schedule in writingProduction start within 24 hours and shipping in 3–5 days are realistic targets. Put the dates in the PO so both sides measure the same thing.
Questions buyers ask
What tolerance can a typical shop hold on a 100 mm aluminum part?
±0.005 mm is achievable on critical features when the part is cut on a 5-axis center with probing and a stable setup. General dimensions are usually held at ±0.05 mm.
Tighter than ±0.005 mm is possible on small features, but it needs a specific inspection plan and often a temperature-controlled room. Ask what the shop measures and how.
Is there a minimum order quantity for a prototype?
Not with us. We run from one prototype to 10,000+ part runs, and the setup cost is built into the unit price rather than charged as a separate minimum.
If a supplier quotes a high minimum for a single part, they are signaling that prototype work is not their business. Expect long lead times and little DFM input.
Which certifications matter for automotive and medical parts?
IATF 16949:2016 is the automotive standard and covers production part approval and traceability. ISO 13485:2016 is the medical device quality standard.
ISO 9001:2015 is the general baseline, and ISO 27001:2022 covers information security. Match the certificate to your industry, then ask for records that prove it is in use.
How fast can parts ship after I approve the quote?
Production can start within 24 hours once material and drawings are confirmed. Most parts ship in 3–5 days, including finishing when it is done in-house.
Historical late-delivery probability is below 2%. Complex parts with multiple setups or outside processes will take longer, so ask for a realistic date rather than the shortest one.
What should I include in the RFQ to get an accurate price?
Send a STEP file, a 2D drawing with tolerances and datums, the material grade, the finish, and the quantity. Note any cosmetic requirements and the assembly the part fits into.
If you have a target budget or deadline, say so. It helps the shop propose a process that meets the real requirement instead of quoting the most expensive route.
How is confidentiality handled for unreleased designs?
Uploads are secure and confidential, and we sign an NDA on request before files are shared. File access is limited to the engineers and machinists on the job.
ISO 27001:2022 covers how systems and access are controlled. If your program is sensitive, sign the NDA first and share the model second.
Send your model, get DFM feedback in 12 hours
Upload a STEP file and drawing. We return a quotation and free DFM analysis, then start production within 24 hours of approval.
12-hour quote100% inspectionNo MOQNDA on request
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