China's Stainless Steel CNC Processing Guide

Stainless steel is not one material. The austenitic family (303, 304, 316, 316L) is tough, gummy, and work hardens fast. The martensitic and precipitation-hardening grades (420, 440C, 17-4PH) machine differently again, and 17-4PH changes hardness depending on heat treatment condition. A drawing that says only "stainless steel" forces the shop to guess, and the guess may cost you a rework loop.

Start from the service environment. Chloride exposure or medical cleaning cycles usually push you to 316 or 316L. General brackets, shafts and housings are fine in 304. High-volume parts that need heavy chip removal often run better in 303, which contains sulfur and breaks chips cleanly. Note the trade-off: 303 sacrifices some corrosion resistance and weldability for machinability.

Hardness targets drive the choice too. Shafts and wear pins that need 40 HRC or above usually land on 420, 440C or heat-treated 17-4PH. Those grades cut well in the annealed or solution-treated state and become difficult after aging or hardening, so the process sequence matters as much as the grade.

GreatLight stocks 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH (SUS630). When a print is ambiguous, the free DFM analysis returned with the quote will flag the grade question instead of silently picking one.

Austenitic stainless work hardens. Cut too lightly and the surface hardens under the tool, so the next pass cuts through a skin harder than the base metal. Tool wear accelerates, surface finish drops, and dimensions drift. The fix is counterintuitive: heavier chip load, lower surface speed, and no dwelling in the cut.

Heat is the second problem. Stainless conducts heat poorly, so the cutting edge keeps most of it. Carbide grades with a tough coating and generous coolant flow keep the edge alive. Peck drilling or interrupted cuts on thin walls make this worse, which is why deep holes and thin ribs need slower feed and more rigid setups.

Chatter shows up on thin-walled parts long before the tool breaks. A 2 mm wall on a 316L housing may need support from soft jaws, a tailstock, or a fixture that fills the bore. On a 4,000 mm part the same rule applies at a larger scale; deflection grows with length, so support points get added along the way.

Grade 303 is the exception that proves the rule. Its sulfur content makes it the easiest stainless to machine, which is why it dominates screw machine work. For a part that lives indoors and sees no chlorides, 303 can cut cycle time noticeably.

Stainless rewards simple geometry. Deep pockets, sharp internal corners, and threads that bottom out against a shoulder all raise cost and risk. A corner radius at least one third of the cutter diameter lets the tool run at a proper feed instead of rubbing. Bottoming taps in 316L break more often than any other feature we see.

Tolerance should match the function. GreatLight holds ±0.005 mm (±0.0002 in) when a feature needs it, but applying that number to every dimension inflates cost without adding value. Put tight tolerance on the mating surfaces, bore diameters and any datum that other features reference, then let everything else run general.

Datums deserve a second look. A 316L part with three loosely defined datums will be set up three times, and each setup adds stack-up error. Clean datums reduce setups, which reduces cost and improves concentricity at the same time.

Surface finish follows the same logic. Ra 1.6–3.2 μm is normal as-machined output. Ra 0.8–1.6 μm needs a finishing pass or a different insert. Ra 0.2–0.8 μm is a deliberate operation with its own time and inspection plan.

Prismatic parts with features on five faces belong on a simultaneous 5-axis center. One setup means one datum, and hole-to-hole position stays tight without a fixture sequence. GreatLight runs 16 simultaneous 5-axis machining centers, with travels of 750 × 1,150 × 550 mm and 600 × 600 × 600 mm on the medium platforms.

Shafts, bushings and threaded bodies go to turning or mill-turn. A Ø400 mm rotary table and 16 mill-turn centers cover round parts that also carry cross holes or milled flats. Doing both operations on one machine removes a re-chuck step, which is where concentricity usually dies.

Long parts up to 4,000 mm run on the large platform with 4,000 × 400 × 150 mm travel. At that length, thermal growth and clamping stress matter as much as the cutting parameters, so roughing and finishing are often split across shifts.

Small, high-count parts run on the 27 three-axis machines or the 12 four-axis mills. A four-axis mill with a trunnion is often the cheaper answer for a part that only needs work on four sides. Five-axis is not automatically better; it is better when setup count or feature access demands it.

Stainless takes most standard finishes, but a few behave differently. Bead blasting and tumbling soften tool marks and are safe on thin walls. Polishing gives the mirror look medical and food equipment buyers expect, and it removes a small amount of material, so polished bores should be toleranced with that in mind.

Electroless nickel and hardcoat anodizing do not apply to stainless the way they do to aluminum; plating choices for stainless focus on electroless nickel, silver and gold for conductivity or wear. Laser marking works well, with a minimum character height of 1.5 mm for legibility.

Inspection is where stainless projects quietly fail. Material certificates, in-process checks and a final dimensional report should be requested in writing. GreatLight inspects 100% of parts before shipment and will supply reports on request. For medical and automotive work, ISO 13485:2016 and IATF 16949:2016 shape the inspection plan.

Ask for the first article report on any new stainless part. It costs little and catches the setup error that would otherwise repeat across a full run.