AAA CNC Processing Service Guide

AAA is a marketing label, not a standard. No ISO or ASME document defines it. When a supplier places the phrase AAA CNC processing service next to their name, they usually mean the top tier of their own capability: tight tolerances, complex geometry, full inspection, and material range. It is a self-declared grade.

So the useful question is not what AAA means. The useful question is what the shop can hold on your specific part. Ask for the tolerance band they guarantee in production, not in a brochure. At GreatLight that number is ±0.005 mm (±0.0002 in) on qualified features. Anything looser than ±0.05 mm is easy. The last 0.02 mm is where cost and risk appear.

A real service also includes the work around the cut. Quotation with DFM feedback, material certificates, in-process checks, finishing, and packing. If a supplier quotes a price with no DFM questions, they probably have not read the drawing.

Three-axis machining moves the table in X, Y, and Z. The tool always points down. This is fine for prismatic parts with features on one or two faces. It is cheap, fast, and easy to fixture. If your part fits in a vise and has no undercuts, do not pay for more axes.

Four-axis adds rotation around one axis, usually A. The workpiece turns while the tool cuts. This suits cylindrical parts, cross-drilled shafts, and features on four faces of a block. One setup replaces three. Drill and mill operations can run in the same cycle, which protects position tolerance between them.

Five-axis adds a second rotation, typically B or C. The tool can approach from almost any direction. Undercuts, deep pockets with compound angles, and impeller blades become machinable in one setup. The gain is not speed. The gain is position accuracy between features and fewer fixtures. Every re-fixture adds stack-up error. A five-axis cycle removes two or three of them.

• 1
  3-axis is right forFlat plates, housings with one open face, simple brackets
• 2
  4-axis is right forShafts, bushings, parts with features on four sides
• 3
  5-axis is right forCompound angles, undercuts, deep cavities, thin walls
• 4
  5-axis is wasteful forOne-face plates and loose-tolerance frames

Aluminum is the default for prototypes and low-volume parts. Grades 6061 and 7075 machine cleanly and hold ±0.005 mm without drama. 7075 is stronger and costs more. It also moves more after stress relief, so thin walls need care. If the part is structural and light, start with 6061-T6.

Stainless 303 and 304 are common, but they work-harden. A dull tool or a stopped feed rubs the surface and hardens it, then the next pass breaks the edge. 17-4PH (SUS630) holds strength after heat treatment and is used for shafts and valve parts. 316L is for corrosive and medical environments. Expect slower cutting and shorter tool life than aluminum.

Titanium TC4 (Ti-6Al-4V) and Inconel sit at the hard end. They are chosen for temperature and strength, not for machinability. Cycle times can run five to ten times longer than the same part in aluminum. Copper and brass cut fast and hold fine detail, which makes them good for electrodes and RF housings. Plastics such as POM, PEEK, and PC need sharp tools and controlled chipload to avoid melting.

A tolerance claim means nothing without a measurement method. Ask which instrument checks the critical dimension and how often. Calipers are fine for ±0.1 mm. Below ±0.02 mm you need a micrometer, bore gauge, or CMM. Below ±0.01 mm you need temperature control and a stable setup.

GreatLight inspects 100% of parts before shipment. The flow is raw material check, in-process monitoring, and final inspection. Reports are available on request. For medical and automotive work this matters because the certification chain has to survive an audit. We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022.

Surface finish is measured separately from tolerance. As-machined parts land around Ra 1.6–3.2 μm. A good finish pass reaches Ra 0.8–1.6 μm. Fine finishing gets to Ra 0.2–0.8 μm, usually on sealing faces or bearing bores. Do not specify a fine finish across the whole part unless the function needs it. It adds cycle time.

Machining leaves tool marks, burrs, and sharp edges. Deburring is not optional on parts that will be handled or sealed. Bead blasting gives a uniform matte surface and hides light tool marks. Tumbling suits small parts in volume. Brushing and polishing give a directional or mirror look, mostly for visible surfaces.

Coatings change dimensions. Anodizing builds a layer on the surface, and hardcoat builds more. If a bore has a tight tolerance, mask it or allow for the coating thickness in the drawing. The same applies to electroless nickel, zinc, silver, and gold plating. Tell the shop which surfaces are functional and which are cosmetic.

Laser marking is common for part numbers, date codes, and logos. Minimum character height is 1.5 mm. Smaller text becomes unreadable after anodizing or powder coating. Mark on a flat surface, not on a curved or blasted one, if legibility matters.