Standard CNC Machining Service: 5 Proven Checks Before You Order

• 1
  What decides a good fit
• 2
  Standard CNC machining service: five checks and what good looks like
• 3
  Tolerance and lead time: the two numbers that shape everything else
• 4
  Order quantity and the cost of a trial run
• 5
  Certifications: read the scope, not the logo
• 6
  Quote detail and the pitfalls that show up later
• 7
  How to run the five checks in order

Tolerance is not a promise, it is a capability statement. A shop quoting ±0.005 mm is telling you which machine class it will put your part on, how it will fixture it and how it will measure it. Ask for that chain. If the answer is only a number with no machine or inspection method attached, you are reading a sales line, not a process plan.

Aluminum and brass are the easy cases. 6061-T6, 2024, C36000 all cut cleanly and hold size well at ±0.005 mm on a 100 mm feature. The picture changes with thin walls, deep pockets and hardened tool steel. A 0.8 mm wall in 17-4PH will move after clamping release, and no machine can stop that. Design the wall thicker or accept a looser tolerance.

Lead time has two clocks, and buyers often confuse them. The first is the engineering clock: how fast does the shop return a quote and a DFM note? The second is the production clock: how fast do chips actually fly? A supplier that only quotes fast but starts slowly will still miss your build. Ask for both dates in writing.

At GreatLight, quotation and free DFM analysis come back within 12 hours, and production can start within 24 hours. Standard parts ship in 3–5 days. Those numbers assume the drawing is frozen. Every revision after the first article restarts part of the clock, so freeze the print before you release it.

• 1
  Ask for the measurement methodCMM, micrometer or optical comparator. Each has a different uncertainty.
• 2
  Name your datumA tolerance without a datum is open to interpretation at first article.
• 3
  Freeze the revisionLate drawing changes are the most common cause of missed dates.

A minimum order quantity is really a setup-cost policy. When a supplier demands 500 pieces before sampling, they are telling you that their setup is expensive and they will not absorb it. That is fine for a mature part. It is painful when you are still proving the design.

No MOQ changes the math on prototypes. You can order one part, measure it, adjust the model and order again. GreatLight runs from one prototype to 10,000+ part runs, so the first order carries no volume penalty. For engineers, that means the first article is a real data point, not a compromise.

Unit price falls with volume, but not evenly. The setup, programming and first-article inspection are fixed costs. Moving from 1 to 50 parts spreads those costs across more pieces. Moving from 5,000 to 10,000 mostly saves material and machine time, so the curve flattens.

Before you chase a lower piece price, check what leaves the quote. Dropping in-process monitoring or skipping the finish step can save 10–15% and cost you a whole batch. On tight-tolerance parts, inspection is not overhead. It is the product.

• 1
  Sample first, then scaleOne part proves the process; the batch proves the shop's consistency.
• 2
  Compare like for likeTwo quotes at different volumes are not comparable without the setup line.

Certificates are only useful when the scope matches your part. ISO 9001:2015 covers quality management, which is the baseline for any supplier. It says the shop has a documented process, calibrated equipment and traceable records. It does not say the shop knows your industry's rules.

If your part goes into a vehicle, IATF 16949:2016 is the relevant standard. It adds PPAP expectations, traceability and change control. For medical devices, ISO 13485:2016 governs process validation and record retention. If you handle customer drawings and personal data, ISO 27001:2022 covers information security. GreatLight holds all four.

Ask a simple question: which certificate applies to my part, and who audits it? A supplier that cannot answer that clearly is holding certificates for marketing. The useful answer names the standard, the scope and the last audit date.

Certification does not replace your own inspection plan. It raises the floor. You still need to state which dimensions you will check on the incoming inspection, especially on first articles and on any feature that is safety-related.

• 1
  Match the standard to the industryAutomotive, medical and information security each have their own.
• 2
  Ask for the audit dateAn expired certificate is a document, not a control.

A good quote reads like a process plan in miniature. It names the material and stock form, the machining operations, the finish, the inspection level and the packaging. When those lines are missing, the supplier is leaving room to change the price or the part after you order.

Watch for finish wording. Ra 1.6–3.2 μm is as-machined and fine for most brackets and housings. Ra 0.8–1.6 μm is the common standard finish for mating surfaces and sealing faces. Ra 0.2–0.8 μm needs a secondary operation and should be quoted separately. If a quote says "smooth finish" with no Ra value, you will not know what arrives.

Material substitution is another quiet risk. 6061 and 6061-T6 are not the same temper. 304 and 316L differ in corrosion behavior. 17-4PH in condition H900 and condition A are different materials for strength purposes. Put the grade and temper on the drawing and repeat it in the purchase order.

Finally, check the part size against the machine envelope. GreatLight machines up to 4,000 mm, with travels of 4,000 × 400 × 150 mm, 750 × 1,150 × 550 mm, 600 × 600 × 600 mm and smaller, plus a Ø400 mm rotary table. If your part is outside the supplier's envelope, no amount of quoting will fix the fixture problem.

• 1
  Name the finish in RaA number removes the argument at goods-in.
• 2
  Write grade and temper6061-T6 and 6061 behave differently under load.
• 3
  Check the machine envelopeOversize parts need a different supplier, not a longer schedule.