CNC Factory Guide: How to Pick a Shop That Hits Your Print

• 1
  Key takeaways
• 2
  Which machine class fits your part
• 3
  What a CNC factory guide should tell you about tolerance
• 4
  Lead time, MOQ and how the shop is actually tooled
• 5
  Certifications, inspection and paperwork
• 6
  Reading a quote: what separates a real number from a guess
• 7
  Red flags in a CNC factory guide checklist
• 8
  Step by step: qualifying a CNC supplier

Tolerance is the first number buyers compare and the one most often misread. A shop quoting ±0.005 mm is describing its best case on a rigid setup with a sharp tool and stable temperature. Your part may not offer that setup. A thin wall, a long reach or a deep pocket will move under cutting force no matter how good the machine is. The honest question is not what the shop can hold, but what it can hold on this drawing.

Ask for the tolerance split by feature. A bolt circle, a bearing bore and a cosmetic outer profile rarely need the same callout. Tightening everything to ±0.005 mm adds inspection time and scrap risk without adding function. Most parts we quote run at ±0.01 mm on general features, with ±0.005 mm reserved for the two or three surfaces that mate with something else.

Surface finish follows the same logic. Ra 0.8–1.6 μm covers most functional faces and comes off the machine with a light pass. Ra 0.2–0.8 μm needs a finer stepover, a fresh tool and often a second operation. On a 200 mm aluminum housing that difference can add hours. Specify finish only where a seal, a sliding contact or an optical surface demands it.

Watch for drawings that stack tolerances across a chain of dimensions. Three features each held at ±0.01 mm can drift 0.03 mm apart at the end of the chain. Call out the critical dimension from a single datum instead. That change alone often lets a shop move the job to a faster machine class and cut cost.

Lead time quotes are only meaningful when you know what starts the clock. Some shops count from PO receipt. Others count from approved DFM, from material arrival, or from a deposit clearing. Ask which. A 12-hour quote and a 24-hour production start mean nothing if the material sits on a dock for a week. On standard aluminum and stainless, most shops in Dongguan can start within a day.

Typical delivery for machined parts runs 3–5 days after production start, but that figure assumes no finishing. Anodizing, plating and powder coating each add a queue. Hardcoat anodizing is slower than clear. If your schedule has no slack, split the order: ship the machined parts now and the finished parts later, rather than holding everything for the coating line.

MOQ is a signal about the factory, not just a purchasing rule. Shops built around high-volume automotive work have fixtures and tooling optimized for long runs, and their setup cost is spread across thousands of parts. A single prototype from that shop gets priced at full setup. Shops that run one-off work are the opposite: fast on prototypes, less efficient past a few thousand pieces.

The practical test is to ask for a quote at three quantities: one piece, one hundred, and ten thousand. If the per-piece price barely moves between them, the shop is either not quoting realistically or is hiding setup in the unit rate. A healthy curve drops sharply from one to one hundred, then flattens.

Certification matters when your part carries a regulatory burden. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive standard and comes with traceability and PPAP expectations. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters more than most buyers expect when drawings and CAD files leave your network.

For a simple bracket, ISO 9001 is enough. For a part that goes into a vehicle or a patient-contact device, the higher standards are not optional, and a supplier that holds them has already built the documentation habit. Ask for the certificate scope, not just the logo. A certificate can cover a different plant or a narrower product range than the one quoting your job.

Inspection is where quotes quietly differ. A shop that inspects 100% before shipment and keeps raw material, in-process and final records will cost more than one that ships on a visual check. For tight-tolerance work the difference shows up as fewer surprises at incoming inspection. Reports are usually available on request, and you should request them on the first order to see the format.

Confidentiality belongs in the same conversation. Uploads should be treated as confidential, and a signed NDA should be available when your drawings are sensitive. If a supplier hesitates to sign a standard NDA, treat that as a signal about how the rest of the relationship will go.

A quote that arrives as a single line item tells you almost nothing. Ask for the breakdown: material, setup, cycle time, finishing, inspection and packaging. With that structure you can see whether the shop understood the part. A quote that omits setup entirely usually means the estimator did not look at the drawing closely.

Material pricing is the easiest line to verify. Aluminum 6061 and 7075 move on different indexes, and 17-4PH stainless costs several times what 303 does. If the quote for a 17-4PH part matches the price of the same part in 303, the material line is wrong and the rest of the quote is suspect. Titanium and Inconel carry the same warning, larger.

Cycle time is where experience shows. An estimator who has actually run your geometry will quote a number close to the machine time. A wildly low cycle time usually means the shop plans to run it slower and bill you for the overrun, or that it never noticed the deep pocket. Ask how many setups the quote assumes. Two setups is normal for a part with features on both ends.

Finally, confirm what the price includes. Deburring, chamfering, thread checking and basic packaging should be listed. Laser marking, special packaging and full dimensional reports are usually extras. Knowing which is which before the PO goes out prevents the awkward change order later.

A quote in minutes with no questions about the drawing is the first flag. Machining a part involves tool access, fixturing, stock size and tolerances that a real estimator checks. Fast is good. Instant, with no follow-up, is not.

The second is a tolerance promise with no method. A shop that says it holds ±0.005 mm should be able to name the machine, the workholding and the metrology that gets it there. If the answer is vague, the number is aspirational.

The third is a refusal to show process. Photos of a similar part, a sample center, or a short walkthrough of the setup are normal requests. A shop that cannot show any of that may be brokering the work to a third party you have never vetted.

The fourth is a lead time that does not move when the job changes. A one-piece prototype and a ten-thousand-piece run cannot share the same delivery promise. If both quote three days with no discussion of material and finishing queues, the schedule was never built from the work.