CNC machining services for precision parts

• 1
  Key takeaways
• 2
  What to check before you send a PO
• 3
  CNC machining services for precision parts: what tolerance really means
• 4
  Lead time, capacity and the size your part needs
• 5
  Material choice and finishing drive the quote
• 6
  Certifications, inspection and documentation
• 7
  How to qualify a supplier in six steps

A tolerance is a range, not a target. When a shop says ±0.005 mm, ask on which feature. A 20 mm bore in 6061-T6 held in a collet is routine. The same tolerance on a 300 mm thin-wall housing is a different job. Thermal drift, tool wear and fixture stiffness all sit inside that number.

The machine matters more than the claim. Simultaneous five-axis centers hold position on angled features because the part is not re-fixtured between operations. On a three-axis machine, the same angled hole needs a second setup, and the stack-up eats your tolerance. Ask for the machine list, not a generic precision statement.

Surface finish and tolerance trade against each other. Ra 0.8–1.6 μm is a normal machined finish. Pushing to Ra 0.2–0.8 μm takes slower passes, smaller stepovers and more time. If your drawing calls out a fine finish on a non-functional face, you are paying for it without a reason.

Material behavior decides the rest. Titanium and Inconel spring back and work-harden. Plastics like POM and PEEK move after cutting as they release stress. For those parts, rough, stress-relieve, then finish. If a supplier quotes one pass and promises ±0.005 mm on a PEEK housing, the quote is optimistic.

Send a drawing with datum callouts and a critical-to-quality list. It tells the machinist which features actually matter. Without that list, he protects the tightest number on the print and may ignore the one your assembly needs.

• 1
  Tolerance applies per featureA single number on the title block does not cover every surface.
• 2
  Five-axis removes setup stack-upAngled and contoured features hold better in one setup.
• 3
  Finish costs timeReserve fine finishes for sealing and sliding faces.

Lead time is a scheduling question. A shop with 127 high-precision machines and open capacity can move fast. GreatLight starts production within 24 hours of a released order and ships parts in 3–5 days. Those numbers only hold when the drawing is final and the material is in stock.

Size changes everything. Compact travels of 500 × 500 × 450 mm cover most brackets, housings and manifolds. Large parts need a machine that can take them: 4,000 × 400 × 150 mm travel for long rails and beams, or a Ø400 mm rotary table for round work. If your part is 2 m long and the quote came from a shop with 600 mm travels, ask how.

Prototype and production runs use different setups. One part often runs on a five-axis center with soft jaws. A 10,000-part run wants fixtures, tool-life tracking and in-process gauging. The same shop can do both, but the quote should say which path it assumes.

Watch for the hidden queue. A low price sometimes means your job sits behind a bigger customer. Ask for the production start date in writing and hold the supplier to it. A shop that tracks its own on-time rate can answer that question in a minute.

Material is usually the largest line item. Aluminum 6061-T6 machines fast and is the default for fixtures and housings. 7075 gives higher strength for aerospace brackets but cuts slower. Stainless 304 and 316L are common for medical and food-contact parts; 17-4PH adds strength after heat treatment.

Titanium and Inconel are slow. TC4 (Ti-6Al-4V) needs sharp tools, low cutting speeds and flood coolant. Inconel work-hardens if the tool rubs, so the feed must stay up. These parts cost more for a reason, and a quote far below the others usually means the shop has not run the material.

Finishing is a separate operation with its own lead time and fixtures. Anodizing, hardcoat, electroless nickel, zinc plating, powder coating and black oxide all add days. Laser marking needs a minimum character height of 1.5 mm, so plan your part numbers and traceability marks around that limit.

Decide what the surface must do before you specify it. A sealing face needs a controlled Ra. A cosmetic cover needs color and texture. A hidden bracket needs corrosion protection and nothing else. Specifying finish by habit is the fastest way to add cost without adding function.

Certificates tell you what system the shop runs, not how your part was made. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive standard. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when your drawings are confidential.

Check the scope. A certificate issued to a trading company does not cover a subcontractor's machines. Ask which site will cut the part and whether that site is on the certificate. For regulated work, the answer needs to be a specific factory address.

Inspection is where precision is proven. Raw material check, in-process monitoring and final inspection before shipment are the baseline. 100% inspection is stronger than sampling. Ask for dimensional reports, material certificates and first article inspection on the features you marked as critical.

Confidentiality is part of the package. Uploads should be secure, and a non-disclosure agreement should be available before you send CAD. If a supplier hesitates on an NDA for a prototype, that tells you how they treat production files later.