5 Checks on CNC Machining Inc Quality Before You Send a PO

• 1
  What matters most when you judge quality
• 2
  Five checks and what a weak answer looks like
• 3
  Tolerance is only the first line of cnc machining inc quality
• 4
  Certifications and inspection planning
• 5
  DFM feedback, order size and the quote itself
• 6
  Lead time, capacity and the machines behind the promise
• 7
  Confidentiality and the questions you should ask twice
• 8
  How to run the five checks on a new supplier

Most quotes lead with a tolerance figure. ±0.005 mm sounds decisive, and it is a real capability, but it says nothing about which features carry it. A bore, a bolt pattern and a cosmetic outer wall do not need the same limit, and a shop that applies one number to the whole drawing is usually quoting from habit rather than reading the part.

Ask which features hold the tight number and how they are verified. A CMM report on request is a reasonable answer. So is a first-article inspection on the critical dimensions. What you want to avoid is a supplier who cannot name the datum scheme or the fixture that holds the part during measurement.

The second half of tolerance is surface finish, because it changes how the part behaves in service. Ra 0.8–1.6 μm covers most sealing and bearing surfaces. Ra 0.2–0.8 μm is for optical, medical and low-friction sliding faces, and it costs more time because it needs a separate finishing pass or a finer tool path.

Material choice sets the floor here. Aluminium 6061-T6 and 7075 hold tight limits more easily than 316L stainless, which moves as cutting heat builds. Titanium TC4 and Inconel move more again. If your drawing calls for ±0.005 mm in Inconel across a long thin wall, expect the shop to raise the fixture and the pass strategy, not just the spindle speed.

A certificate tells you the management system exists. It does not tell you the system is used on your job. The useful question is which certificate matches your industry and what it forces the shop to do on the floor. ISO 9001:2015 is the general baseline. IATF 16949:2016 adds automotive traceability, PPAP-style documentation and change control. ISO 13485:2016 adds medical device process validation. ISO 27001:2022 covers information security, which matters if you are sending CAD for an unreleased product.

Then ask how inspection is scheduled. GreatLight runs raw material checks on incoming stock, in-process monitoring while parts are still on the machine, and a final inspection before shipment, with 100% of parts inspected and reports available on request. That sequence matters more than the label on the wall, because a fault caught at the machine costs a re-cut, while the same fault caught at final inspection costs a whole batch.

For a run of 10,000 parts, the inspection plan should say how often a feature is sampled and what triggers a full re-check. For a single prototype, the plan is closer to a full dimensional report. A supplier who gives the same answer to both questions has not thought about it.

Documentation depth should scale with your risk. Aerospace and medical buyers usually want material certs, heat-lot traceability and inspection records tied to a serial number. Industrial machinery buyers often need only a dimensional report and a finish check. Say which one you are at the start, and the quote will be accurate rather than padded.

A quote without engineering feedback is a price, not a review. The useful output is a short DFM note: which corner radii will chatter, which wall is too thin for the chosen material, where a tolerance can be opened without affecting function, and whether a 5-axis setup removes a second operation. GreatLight returns quotation and a free DFM analysis within 12 hours, and production can start within 24 hours once the design is settled.

Order size is the next filter. Some shops quote a low unit price and then add setup charges that only make sense at volume. Others refuse small runs. GreatLight sets no minimum order quantity, so one prototype and a 10,000+ part run go through the same quoting process. That is a useful signal: a shop set up for both usually has its fixturing and tooling under control.

Read the quote line by line. Material grade, finish spec, inspection level and packaging should each appear. Be cautious when a quote lists a finish as "as machined" without a Ra value, or when anodizing is priced without stating the type. Clear, hardcoat and conductive anodizing behave differently and are not interchangeable.

Also check that the finish matches the drawing callout. Laser marking needs a minimum character height of 1.5 mm to stay legible after anodizing. Powder coating adds thickness that can close a tight bore. Black oxide and bead blasting change surface texture in ways that affect fit. Each of these is a small trap that a careful supplier will flag before cutting metal.

Delivery promises are easy to make and hard to verify. Ask for a number instead. Historical late-delivery probability below 2% is a figure you can hold a supplier to, and it tells you whether the schedule is built from data or optimism. A shop that cannot quote its own late rate is usually guessing at capacity.

Capacity should be specific enough to match your part. GreatLight runs 127 high-precision CNC machines across three wholly-owned plants covering 7,600 m², including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That mix matters because a mill-turn center finishes a shaft in one setup, while a three-axis machine needs two or three, and each extra setup adds a datum error.

Size limits decide whether a shop can even quote. Maximum processing size reaches 4,000 mm, with travels such as 4,000 × 400 × 150 mm for long parts and 750 × 1,150 × 550 mm or 600 × 600 × 600 mm for mid-size work. Compact travels of 500 × 500 × 450 mm and 500 × 310 × 200 mm suit small housings. A Ø400 mm rotary table supports parts that need indexed features around a bore.

Standard delivery for machined parts runs 3–5 days after production starts. For a complex housing with five faces and a tight bore, ask which machine is planned and how many setups it needs. The answer predicts the schedule better than any promise, and it also tells you whether the shop read the drawing.

If your CAD describes an unreleased product, confidentiality is part of quality. Secure upload handling and an NDA on request are the baseline. ISO 27001:2022 certification is a stronger signal because it covers how files, access and devices are controlled, not only how parts are made. Ask who inside the shop can open your files, and whether the data leaves the building.

Some answers should be checked twice because they are easy to give and hard to prove. "We inspect everything" is one. "We hold ±0.005 mm on all features" is another. "No MOQ" but a setup fee that only works at 500 pieces is a third. Each one is worth a follow-up question, and a good supplier will not mind it.

Trust the physical details. A shop that asks about your datum scheme, your assembly stack-up and your finish callout before quoting is reading the part. One that returns a price in an hour with no questions is working from a template. The first conversation is a small sample of the whole project.

Finally, ask what happens when a dimension comes out of spec. A written path from detection to disposition, with a report to you before shipment, is what keeps a project on schedule. That process is the real meaning of cnc machining inc quality, and it shows up in the paperwork long before it shows up in the parts.