Micro CNC Machine Service: How to Pick a Supplier Who Holds Tolerance

• 1
  Key takeaways
• 2
  What to weigh before you choose a micro cnc machine service
• 3
  Process choice for small features
• 4
  What micro cnc machine service really means on the shop floor
• 5
  Which micro parts belong on a CNC, and which do not
• 6
  Tolerance, inspection, and the paperwork behind the part
• 7
  Cost drivers buyers usually miss
• 8
  Lead time, lot size, and what to put in writing
• 9
  Step by step: qualifying a micro cnc machine service

Micro machining is not a smaller version of standard milling. The physics change. A 0.5 mm end mill has almost no stiffness, so deflection, not the machine's positioning accuracy, becomes the limiting factor. Spindle runout of 2 μm is tolerable on a 12 mm cutter and fatal on a 0.5 mm one. Tool life drops fast, and a cutter that survived 40 minutes in aluminum may fail in 6 minutes in 316L stainless.

For buyers, that means a micro cnc machine service should be judged on tooling and process control, not just on the machine list. A shop with 127 high-precision CNC machines still needs the right collets, the right coolant delivery, and an operator who can hear a cutter starting to chatter.

The practical tolerance band at GreatLight is ±0.005 mm (±0.0002 in) on qualified features. That is the number to plan around. It is achievable on small bores, slots, and faces when the geometry gives the tool room to cut, and it is not achievable on a 0.4 mm wide, 6 mm deep pocket in titanium without changing the process.

• 1
  Feature size drives tool choiceInternal corners are limited by cutter radius, not by the machine.
• 2
  Depth-to-diameter ratio is the real limitAbove roughly 5:1, expect a reduced step-down or a second operation.
• 3
  Material temper matters6061-T6 machines cleanly; soft 6061 gumms up small flutes.

Micro CNC is the right call when you need metal, tight tolerance, and a geometry that cannot be molded or stamped. Typical fits are machined housings for sensor packages, implant-grade fixtures, connector bodies, fuel and fluid manifolds, and prototype parts where the design is still moving. One-off and 10,000-piece runs use the same program, which keeps the transition from prototype to production short.

It is the wrong call when the part is a thin, flat, high-volume item with no tight features. A 0.2 mm thick shim at 500,000 pieces belongs on a stamping die. A plastic clip with a simple shape belongs in injection molding. CNC wins on complexity and tolerance, not on unit cost at volume.

There is a middle zone worth thinking about. Micro parts with one critical bore and loose everything else can sometimes be machined on the critical feature and left looser elsewhere. Marking that on the drawing is the single easiest way to cut cost, because it lets the shop skip a finishing pass on surfaces nobody measures.

Surface finish is the other side of the trade. As-machined finishes land around Ra 1.6–3.2 μm. A fine finish of Ra 0.2–0.8 μm is available on request but adds a separate operation and its own inspection step. If your drawing calls for Ra 0.4 μm across the whole part, expect to pay for it on the whole part.

• 1
  Good fitSmall metal parts with tight bores, slots, or sealing faces.
• 2
  Poor fitThin flat parts at high volume, or simple plastic shapes.
• 3
  Cost leverTolerance the critical feature only, not the whole drawing.

A tolerance on a drawing is a contract. The question is how the shop proves it. For micro work we inspect 100% of parts before shipment, with raw material checks, in-process monitoring, and a final inspection pass. Reports are available on request, and for a first article they are worth asking for by default.

The instrument matters as much as the number. A ±0.005 mm bore cannot be verified with calipers. It needs an internal micrometer, a bore gauge, or a CMM with a small enough stylus. If a supplier cannot say what they measure with, the tolerance claim is unverified.

Material certification is the second layer. Medical and automotive buyers usually need mill certificates traced to the heat number, plus a certificate of conformance. GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022. Each one changes how records, traceability, and information handling are managed, and they are not interchangeable.

For proprietary designs, confidentiality is part of the technical review. Uploads are handled as confidential, and an NDA is available on request. If your part is a competitive advantage, settle that before you send files, not after.

• 1
  100% inspection before shipmentNot sampling, on tight-tolerance micro features.
• 2
  Ask for the first article reportIt shows which features were measured and with what.
• 3
  Match the certificate to the industryISO 13485 for medical, IATF 16949 for automotive.

Quotes for micro parts often differ by 3× between suppliers for the same drawing. The gap is rarely the hourly rate. It is setup count, tooling, and how much inspection the shop plans to do. A part that can be cut in two setups on a 5-axis center may need five setups on a 3-axis machine, and each setup adds fixture time, re-datum risk, and scrap risk.

Tool cost is the second driver. Micro end mills are consumables that break. A job with a deep 0.6 mm slot may burn through several cutters per batch, and that cost sits in the quote whether or not the supplier itemizes it.

Deburring is the third and most underestimated. On a part with a 0.3 mm edge, a 0.02 mm burr is not cosmetic, it is a functional defect that can jam an assembly or shed particles in a cleanroom device. Manual deburring under magnification takes time. Tumbling is faster but rounds edges you may need sharp. The drawing should say which edges matter.

Finishing adds a fourth layer. Anodizing, plating, and passivation are separate operations with their own handling and inspection. Keeping them under one roof reduces shipping risk and the finger-pointing that follows a finish defect.

• 1
  Setup countFewer setups means less re-datum error and lower unit cost.
• 2
  Tool breakageDeep small slots consume cutters; expect it in the price.
• 3
  Edge specificationState which edges need a controlled break and which stay sharp.

Micro parts are usually needed fast, because they sit in a build that is already late. Quotation and DFM come back within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. Those are the working numbers, and they assume the drawing is released and material is in stock.

Lot size flexibility matters more than most buyers expect. There is no minimum order quantity, so the same program runs one prototype or a 10,000-piece batch. That removes the usual second supplier for low-volume bridge builds, which is where tolerance drift often starts.

Put three things in writing before the PO. First, the datum scheme, so both sides measure the same way. Second, the critical feature list with its tolerance. Third, the finish and edge specification. Most disputes on micro parts trace back to one of these three being left to interpretation.

Schedule risk is worth a direct question. Historical late-delivery probability at GreatLight is below 2%. Ask any supplier how they measure their own on-time rate and over what period. A shop that tracks it will answer quickly.

• 1
  Quote and DFM in 12 hoursEnough time to re-check your own design before committing.
• 2
  One piece to 10,000+Same program, same datum, no supplier switch mid-build.
• 3
  Write the datum downIt prevents a passing part being rejected on a different setup.