Diverse CNC Machining Service: How to Judge One Before You Order

• 1
  Key takeaways
• 2
  What to compare across diverse CNC machining suppliers
• 3
  Axis count and travel decide which parts a diverse CNC machining service can take
• 4
  Tolerance and finish: what to specify and what you will pay for
• 5
  Material range and finishing options that keep you on one supplier
• 6
  MOQ, lead time and certifications: the commercial filters
• 7
  Step by step: how to qualify a supplier in one week

The first question is geometric, not commercial. A part with pockets on four sides, a cross-drilled hole and a contoured top can be made on a 3-axis mill, but it needs three or four setups. Every setup adds a datum error and a queue slot. A 5-axis machine reaches the same features from two directions in one setup, so the position tolerance between faces comes from the machine, not from the fixture.

Look at the actual numbers. A shop with 16 simultaneous 5-axis machining centers can hold tight angular relationships across a part in one pass. Twelve four-axis mills handle cylindrical work, slots along a rotating axis and parts that are long relative to their diameter. Twenty-seven three-axis machines are not wasted capacity: for flat plates, brackets and simple housings, they are faster and cheaper per part.

Size is the second limit. Travel ranges differ per machine, so ask which machine will run your part rather than what the largest machine on site can do. On the large side, 4,000 × 400 × 150 mm covers long extrusion-like profiles and frame rails. Medium frames around 750 × 1,150 × 550 mm and 600 × 600 × 600 mm suit most enclosures and manifolds. Compact cells at 500 × 500 × 450 mm and 500 × 310 × 200 mm are where small, high-mix work belongs.

Mill-turn centers matter more than buyers expect. A shaft with milled flats, a cross hole and a turned OD usually travels between a lathe and a mill, which means two fixtures and two tolerance stacks. A mill-turn center produces it in one program. If your part family looks like that, ask how many mill-turn centers the shop runs and what bar or chuck size they accept.

• 1
  One setup beats tight fixturesFewer setups mean fewer datum shifts, and the tolerance stack stays short.
• 2
  Match machine to batchSimple plates belong on 3-axis machines; complex contoured parts justify 5-axis time.
• 3
  Confirm the specific machineAsk for travel and spindle hours of the machine assigned to your job.

Tolerance should be written per feature, not as a blanket note on the title block. A ±0.005 mm callout is achievable on a bearing bore or a mating face, and it is checked with a coordinate measuring machine. Applying the same value to a clearance hole that no one measures only adds cost. Mark the features that touch another part, and leave the rest at a general tolerance.

Surface finish follows the same logic. As-machined surfaces sit around Ra 1.6–3.2 μm, which is fine for brackets, covers and internal structure. Sealing faces, sliding surfaces and cosmetic panels usually need Ra 0.8–1.6 μm. Optical and fluid-handling parts sometimes need Ra 0.2–0.8 μm, which means slower feeds and a finishing pass, so budget the cycle time.

Material choice changes all of this. Aluminium 6061-T6 and 7075 cut cleanly and hold thin walls well. Stainless 304 and 316 work-harden if the tool rubs, so the shop needs sharp tooling and steady feed. Titanium TC4 (Ti-6Al-4V) and Inconel generate heat at the cutting edge and need lower surface speed and more coolant. A supplier that lists these alloys as routine stock is more likely to quote them sensibly.

Do not accept a tolerance promise without a measurement plan. Ask what instrument checks your critical feature: micrometer, bore gauge, height gauge, or CMM. Ask whether the report ships with the parts. At GreatLight, parts get raw material checks, in-process monitoring and a final inspection before shipment, and inspection reports are available on request.

• 1
  Tolerance per featureSpend the tight tolerance on mating surfaces only.
• 2
  Finish follows functionRa 1.6–3.2 μm as machined; Ra 0.8–1.6 μm for seals and sliding faces.
• 3
  Hard alloys need different feedsTitanium and Inconel cut slower, so cycle time and price rise.

A diverse CNC machining service is judged by how many process steps stay in house. Sending a machined aluminium housing out for anodizing adds freight, a second schedule and a second party who can scratch the part. When machining, plating and coating sit under one quality system, the paper trail is shorter and a rework loop is possible.

On the metal side, the common range covers aluminium 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12; stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH (SUS630); steel 1018, 1045, 4130, 4140, 4340, A36 and tool steel; copper and brass C101, C103, C110, beryllium copper, C27400, C28000 and C36000. Titanium TA1, TA2 and TC4, Inconel, and magnesium AZ31B and AZ91D round out the specialty list.

Plastics behave differently. ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre all machine, but they move. PEEK and carbon fibre are abrasive, so tool wear shows up in the last parts of a run. POM and PA hold dimension better after a stress-relief pause. If your part is a plastic prototype, say so early; the feeds and the inspection timing change.

Finishing decides how the part looks and how it survives. Anodizing comes in clear, colour, hardcoat and conductive types. Electroless nickel, zinc, silver and gold plating cover conductivity and corrosion needs. Powder coating and black oxide handle larger frames and wear surfaces. Bead blasting, tumbling, brushing and polishing set the cosmetic baseline. Laser marking works down to a minimum character height of 1.5 mm, which matters when you need a serial number on a small boss.

• 1
  Keep steps in houseMachining plus finishing under one quality system shortens the route.
• 2
  Name the grade6061 and 7075 behave differently; the quote must state the alloy and temper.
• 3
  Plan marking earlyLaser marking needs at least 1.5 mm character height and a flat area.

Minimum order quantity tells you who the shop is built for. Some factories optimize for long runs and treat a five-piece order as noise. Others run from a single prototype to 10,000+ part runs without a minimum order quantity. If your program starts with one functional prototype and grows, the second type is the only one that will quote you fairly at both ends.

Lead time has two parts: how fast you get the price, and how fast you get parts. A quotation and free DFM analysis within 12 hours lets you close a design review the same day. Production can start within 24 hours of approval, and parts typically ship in 3–5 days. Treat those as planning inputs, not guarantees, and ask what happens when a feature needs a second operation.

Certifications are a scope question, not a logo question. ISO 9001:2015 covers general quality management. IATF 16949:2016 is what automotive and EV programs look for. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when your CAD files and drawings are the customer's intellectual property. Ask which sites and which processes the certificate covers.

Confidentiality is part of the same conversation. Uploads should be secure and confidential by default, and an NDA should be available on request before you send the first STEP file. If a supplier hesitates on that, the rest of the commercial terms do not matter much.

• 1
  No MOQ means one part is a normal orderUseful when the prototype is the decision, not the volume.
• 2
  Quote speed affects your schedule12-hour quoting keeps a design review on track.
• 3
  Match certificate to industryIATF 16949, ISO 13485 and ISO 27001 each answer a different risk.