Updates CNC Machining: How to Judge a Supplier

• 1
  What decides a good supplier
• 2
  What to compare between suppliers
• 3
  Start with the tolerance the quote actually covers
• 4
  Match the geometry to the machine list
• 5
  Read the lead time as three separate numbers
• 6
  Check material stock and traceability
• 7
  Certifications only count when they match your industry
• 8
  How to evaluate a supplier in seven steps

Two suppliers can quote the same part at very different prices because they are not quoting the same process. A shop that holds ±0.005 mm needs more setup time, more in-process checks and a temperature-stable room. A shop at ±0.05 mm can run the part faster and skip some of that work. Before you compare numbers, ask each supplier to write the tolerance on the quote.

Look at your drawing and mark which dimensions are functional. A bearing bore, a dowel-pin hole or a sealing face usually drives the whole part. If only three features need ±0.005 mm, say so. The rest of the part can sit at a looser band and the price will drop without hurting the assembly.

Surface finish is the second half of this. A sealing face at Ra 0.8–1.6 μm machines differently from a bracket at Ra 3.2 μm. Call out the finish per feature, not per part. Otherwise you pay for polished surfaces on faces that never touch anything.

One warning sign: a quote that lists no tolerance and no finish at all. That is not a cheaper offer, it is an undefined one. You will find out what you bought when the parts arrive.

Ask what machines will run your part, then think about setup count. A part with features on five sides costs more on a 3-axis mill because each face needs a new fixture and a new datum. On a simultaneous 5-axis center, those features come off in one or two setups, and the datums stay consistent.

Mill-turn centers handle parts that mix turned and milled features, such as a shaft with cross-holes or a fitting with a milled flat. If you split that work between a lathe and a mill, you add a second setup and a second chance for position error.

Size matters too. A supplier with 4,000 mm travel can take a long extrusion or a frame in one piece. If the largest machine is 500 mm, that same part becomes a bolted assembly with more joints and more tolerance stack-up.

GreatLight runs 127 high-precision CNC machines across three plants, including 16 simultaneous 5-axis centers, 16 mill-turn centers and a Ø400 mm rotary table. That range covers small medical components and long industrial housings from the same shop.

A single delivery figure hides where the time goes. Break it into quote time, production start and shipping. GreatLight returns a quotation and a free DFM analysis within 12 hours, can start production within 24 hours, and ships parts in 3–5 days once cutting begins.

The DFM analysis is the part buyers skip and later regret. It flags thin walls, deep pockets that need long tools, and holes too close to a wall to drill cleanly. Fixing those in the model costs minutes. Fixing them after the first article costs a week.

Ask about the late-delivery record. A shop that tracks it can tell you a number. Ours is below 2% historically. If a supplier cannot answer the question at all, treat the promised date as an estimate, not a commitment.

Rush work has a limit. A 5-axis job with a complex fixture cannot be pulled forward without adding a second machine or overtime. Say which parts are urgent so the shop can sequence them first.

Material choice drives tool wear, finish and price more than most buyers expect. Aluminum 6061 and 7075 cut fast and hold tight tolerances well. Stainless 316L and 17-4PH work-harden, so feeds and speeds need care. Inconel and titanium TC4 (Ti-6Al-4V) cut slowly and wear tools quickly, which shows up in the price.

Ask whether the shop stocks the grade or buys it per job. A stocked grade means a faster start and a consistent heat lot. A per-job buy can add days and, on some alloys, a minimum purchase you end up paying for.

Traceability matters in aerospace, medical and automotive work. You want the mill certificate tied to the parts. GreatLight performs a raw material check before cutting, so the certificate links to your order, not to a generic batch.

Plastics are their own case. POM and PEEK hold dimensions well; ABS and PP move with temperature and clamp pressure. If your part is plastic, ask for the finish and the annealing step in writing.

A wall of certificates means little if none of them covers your product. Match the standard to the market you sell into. ISO 9001:2015 is the baseline. IATF 16949:2016 is what automotive and EV programs expect. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers how your drawings and data are handled.

Then ask how inspection is done, not just that it happens. A useful answer names the stages: raw material check, in-process monitoring, and final inspection before shipment, with reports on request. A vague answer tells you the process is not written down.

First-article inspection is worth requesting on any new part with tight features. It confirms the process before the shop runs the full batch. On a 10,000-part order, that one check protects the whole run.

GreatLight holds a 99.99% qualification rate and inspects 100% of parts before shipment. Those numbers come from written procedures, not from operator confidence.