CNC Machine Tool Services: How to Vet a Supplier

• 1
  Five checks that decide the outcome of CNC machine tool services
• 2
  Which supplier type fits your part
• 3
  Machine envelope and tolerance claims in CNC machine tool services
• 4
  Inspection and the paperwork behind CNC machine tool services
• 5
  Order quantity, materials and finishing under one roof
• 6
  Quotation, confidentiality and what the price covers
• 7
  How to run a first order, step by step

Start with the largest dimension and the heaviest feature. A shop with 16 simultaneous 5-axis centers can cut complex geometry in one setup, but the part still has to fit the rotary table and the spindle travel. At GreatLight the largest machine handles 4,000 × 400 × 150 mm, the medium class covers 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, and the compact class covers 500 × 500 × 450 mm and 500 × 310 × 200 mm. If your part sits near the edge of a travel limit, ask how the fixture will be oriented. A small rotation can push a feature out of reach.

Tolerance is the second filter. A quotation that lists ±0.005 mm as a general capability is making a machine-level claim. Ask which feature the number applies to. On a long aluminum rail, ±0.005 mm over 1,000 mm is a different problem from ±0.005 mm on a 30 mm bore. Even a well-kept machine moves with thermal growth during a long cut. For most brackets and housings, ±0.05 mm is enough and costs less.

Surface finish usually follows tolerance. Ra 1.6–3.2 μm comes off the cutter as machined. Ra 0.8–1.6 μm needs a finishing pass with a sharp tool and light depth of cut. Ra 0.2–0.8 μm is a grinding or lapping operation, not a milling pass. If the drawing calls for a fine finish on one face only, say so. Specifying it everywhere raises the price for no reason.

• 1
  Send the 3D model and the 2D drawing
• 2
  Mark which features are critical
• 3
  State the material and temper
• 4
  Say how the part will be measured

Any shop can write ±0.005 mm on a quote. Fewer can show how they measured it. Ask what instrument covers the critical feature and whether the report travels with the parts. At GreatLight, inspection covers raw material check, in-process monitoring and final inspection, with 100% inspection before shipment and reports on request. In-house CMM and laser scanning cover the dimensional callouts. If a supplier subcontracts inspection, the feedback loop gets longer and the first article arrives later.

The paperwork should match the industry. ISO 9001:2015 is the baseline for general machining. IATF 16949:2016 matters for automotive and EV parts, where process control and traceability are audited. ISO 13485:2016 covers medical devices and their records. ISO 27001:2022 is about information security, which matters when your CAD files are the product. A shop that holds all four is telling you it can run different jobs under different rules.

One practical test: ask for a first article inspection report on a feature you care about, not a generic sample. If the response is a certificate with no numbers, the shop is selling paper. If the response includes the gauge used, the datum and the measured value, the process is under control. This question costs you nothing and filters suppliers fast.

Minimum order quantity is a cost structure, not a policy. A shop with no MOQ can run one prototype because the setup time is small relative to a flexible schedule. The same shop can run 10,000 pieces when the volume justifies a fixture. Ask what changes between the two: tooling, inspection frequency, and whether the parts run on a dedicated cell. At GreatLight, runs go from one prototype to 10,000+ parts with no minimum order quantity.

Material availability decides the schedule more often than machine time. Aluminum 6061, 2024, 5052, 6063, 6082, 7075 and ADC12 are common. Stainless 303, 304, 316L, 17-4PH and 440C are stocked in bar and plate. Titanium TA1, TA2 and TC4 (Ti-6Al-4V) plus Inconel and magnesium AZ31B and AZ91D need a check before quoting. Engineering plastics such as POM, PEEK, PA and carbon fibre cut differently and may need sharp, uncoated tooling.

Finishing is where schedules slip. Anodizing, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing are separate operations. Ask whether they happen in-house or at a partner. In-house finishing keeps the parts in one quality system. If a step is outsourced, add days and confirm who owns the rework if the finish fails.

Laser marking and engraving are worth planning early. Minimum character height is 1.5 mm, so a serial number on a 10 mm boss may not fit. Move the mark to a flat face before the drawing is released.

• 1
  No MOQ means one piece is a valid order
• 2
  Confirm the material temper before quoting
• 3
  Keep finishing in the same quality system
• 4
  Plan the laser mark location

A useful quotation shows the process plan, not just a number. It should list the machine, the number of setups, the material, the finish and the inspection method. A price with no process detail is hard to compare against another shop, because you cannot tell whether the difference is labor, tooling or margin. At GreatLight, quotation and free DFM analysis come back within 12 hours, and production can start within 24 hours after approval.

Confidentiality belongs in the first conversation, not the last. Uploads are secure and confidential, and an NDA is available on request. If your part is a new product, ask who inside the shop sees the files and whether the data is covered by an information security system. ISO 27001:2022 is the standard that answers this.

Read the price for what is excluded. Deburring, marking, packing and inspection reports are sometimes separate lines. Ask about them before you compare two quotes. A lower number that omits inspection is not cheaper, it just moves the cost to your incoming quality team.

Delivery history is a fair question. Ask how often parts ship late. A shop that tracks the number can tell you. At GreatLight the historical late-delivery probability is below 2%.