Precison CNC Machining Services: What to Check Before You Order

• 1
  Key takeaways
• 2
  Which supplier profile fits your part
• 3
  Tolerance: which machine actually holds it
• 4
  Surface finish and the cost it hides
• 5
  Order size, setup cost and where MOQ hurts
• 6
  Certifications and the paperwork behind them
• 7
  Lead time, quote speed and DFM feedback
• 8
  Quotingscope and the questions that expose gaps
• 9
  Step by step: qualifying a supplier in one week

A drawing that says ±0.005 mm is a request, not a capability. The answer depends on the machine, the material and the thermal state of the shop floor. A 3-axis mill in an uncontrolled room can drift 0.02 mm over a long cycle as the spindle warms up. The same part on a 5-axis center with a temperature-controlled room and in-process probing lands far closer.

Ask which machine will run your part, then ask for its travels and spindle hours. GreatLight runs 127 high-precision CNC machines across three wholly-owned plants, including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Largest processing size reaches 4,000 mm, and the rotary table on the large 5-axis cells is Ø400 mm.

Material moves too. Aluminum 6061 cuts cool and holds size well. Titanium TC4 and Inconel push heat into the tool and the part. A shop that quotes the same tolerance for both without changing the process is guessing. For stainless 17-4PH or 316L, expect a stress-relief step before the finishing cut if the part is long and thin.

One more thing. Tolerance on the drawing is never uniform across the part. Datum faces, bore diameters and bolt patterns each carry their own callout. Send the full GD&T and ask the shop to confirm the critical ones in writing. That single reply tells you more than any capability page.

Finish is where quotes diverge most. An as-machined surface of Ra 1.6–3.2 μm comes straight off the cutter with a normal stepover. A high-finish surface of Ra 0.8–1.6 μm needs a lighter finishing pass, sharper tooling and often a different coolant. Fine finish at Ra 0.2–0.8 μm usually means a separate operation: lapping, polishing or a slow finishing path with small stepover.

Look at the function before you tighten the number. A bearing bore or a sealing face earns a fine finish. A bracket that bolts to a frame does not. Engineers who put Ra 0.4 μm on every face pay for polishing on surfaces nobody touches.

Finishing also changes handling. A polished aluminum face marks easily, so the shop needs protection film, foam trays and gloves on the line. If your drawing calls for anodizing after polishing, check the sequence. Hardcoat anodizing adds 20–50 μm of build-up and can round a sharp edge. Specify the finish before or after plating, not both ways.

GreatLight keeps finishing in-house: anodizing, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing. Keeping those steps under one roof removes a freight leg and one round of re-inspection.

A minimum order quantity is a signal about how the shop schedules work. A shop with no minimum runs one prototype and a 10,000-part lot on the same floor. That flexibility costs the shop in setup time, and it shows up in the unit price of small runs. It also means the shop knows how to hold a first-article part and a production part to the same drawing.

For one-off parts, the setup is most of the price. Fixture design, tool selection and first-article inspection can take longer than the cut. Ask the shop to separate setup from run time in the quote. If the quote is a single number with no breakdown, you cannot tell whether a small change will cost you 5 percent or 50 percent.

Volume changes the process, not just the price. At a few hundred parts, a 3-axis mill with soft jaws is fine. At tens of thousands, a dedicated fixture, bar feeder or mill-turn cell pays for itself. A shop that quotes the same route for both ends of that range is not planning your job.

One prototype or a 10,000+ part run, the same drawing applies. The difference is how much of the process the shop builds around your geometry. Say the annual quantity in the RFQ even if the first order is small. It changes the fixture and the tooling choice.

Certificates are a filter, not a ranking. ISO 9001:2015 covers a documented quality system and works for most industrial parts. IATF 16949:2016 adds automotive-specific control plans, traceability and PPAP expectations. ISO 13485:2016 covers medical device manufacturing and the records that go with it. ISO 27001:2022 covers information security, which matters when your files are the product.

A certificate on a wall does not tell you who inspects. Ask for the inspection plan: raw material check on arrival, in-process monitoring, final inspection before shipment. GreatLight inspects 100 percent of parts before shipment and shares reports on request. For a first order, ask for the dimensional report with the parts, not after you find a problem.

Traceability is the part buyers forget. A medical or automotive customer may need to know the heat number of the metal and which machine ran the part. If the shop cannot pull that from a job number, the certificate will not save the audit.

Confidentiality runs on the same track. Uploads should be secure by default, and an NDA should be available on request before you send STEP files. If a shop asks for drawings over a personal email account, that is your answer.

Lead time claims are easy to make. Ask instead what happens between the PO and the first chip. A shop that can start production within 24 hours has material on the shelf and a machine slot open. A shop that needs a week to source aluminum will push your date.

Quote speed is a proxy for engineering attention. A quotation and free DFM analysis within 12 hours means someone opened your model, looked at the tool reach and flagged the thin wall before pricing. A quote that arrives in three days with no comments usually means the number came from a spreadsheet.

Standard turnaround for machined parts sits in the 3–5 day range once production starts. That figure assumes the drawing is final and the material is standard. Change the revision mid-run and the clock resets. Lock the model before you release the PO.

Ask about late delivery history in plain terms. A shop with a late-delivery probability below 2 percent has a schedule it can hold. That number is only useful if you also ask how it is measured, and on how many jobs.

A useful quote lists material, machining, finishing, inspection and freight as separate lines. It names the material grade, the tolerance it will hold, and the finish range. If two suppliers quote the same part and the prices differ by 40 percent, the line items explain why. One of them is not doing something the other is.

Read the exclusions. Does the price include first-article inspection, a material certificate, plating, or laser marking? Laser marking has a minimum character height of 1.5 mm, so a 0.8 mm part number will not reproduce and the shop should say so at quote stage.

Ask what the shop will not do. A supplier that names its limits, such as no deep-hole drilling beyond a certain ratio or no mirror polish on internal corners, is easier to work with than one that accepts everything.

Finally, compare the same scope. Send the same drawing, the same material and the same finish to each shop. Then compare the tolerance commitment and the inspection report, not just the total. That is the only comparison that predicts what arrives in the crate.