CNC machining 303 stainless is the practical way to get stainless steel parts at the lowest machining cost. 303 is an austenitic 18-8 stainless with added sulfur, which forms manganese sulfide inclusions that break chips and lubricate the cut. Wexmar runs a steady volume of 303 on our lathes and Swiss machines in Canastota, New York, for fittings, shafts, pins and instrument hardware where corrosion resistance matters but welding and severe environments do not.
303 Stainless Machinability
303 has an approximate machinability rating of about 78 on the AISI 1212 = 100 scale. That is well ahead of 304 at about 45 and 316 at about 36, and it is the main reason to choose it. The sulfur additions make chips break into short segments, reduce tool pressure and extend tool life. It is still stainless steel, though, and it still work hardens if the tool rubs instead of cutting.
Tolerances and Surface Finish
For precision 303 turned parts, typical production tolerances are plus or minus 0.0005 in. on Swiss-turned diameters and plus or minus 0.001 in. on conventional lathe work, with ground diameters possible using our in-house centerless grinder. Typical finishes are 32 Ra on turned surfaces, 16 Ra on Swiss-finished diameters, and 63 Ra on milled features. These are typical results, not guarantees, and depend on diameter, length and feature layout.
303 Shop Knowledge: Work Hardening, Tooling and Finish
- Keep the tool cutting. Dwelling, rubbing or very light finishing passes harden the surface and dull the next tool. We hold a positive feed and take finishing passes deep enough to get under any hardened layer.
- Tool geometry. Sharp, positive-rake carbide with a light hone and a PVD coating performs well. Chipbreakers designed for stainless keep chips short even on long runs.
- Coolant. Flood or high-pressure coolant controls heat and flushes chips. On the Swiss machines, oil coolant improves finish and tool life on small diameters.
- Surface pitting. The sulfide inclusions that help machining can leave tiny pits on highly polished surfaces. Parts that need a mirror finish or high-purity service are usually better in 304 or 316.
- Burrs. 303 burrs are smaller than 304 burrs, but cross-drilled holes still get a deburr step in the program.
Finishing and Heat Treat Options
303 cannot be hardened by heat treatment; it is used in the annealed condition. Common finishes are handled through qualified outside processors:
- Passivation, using processes suited to free-machining grades to avoid flash attack on the sulfides
- Electropolishing for cleaner, smoother surfaces
- Black oxide for low-glare hardware
- Annealing or stress relief where a drawing requires it
303 is generally not recommended for welding because the sulfur promotes hot cracking. If a part will be welded, we suggest 304 or 304L.
Common 303 Parts and the Right Machine
- Fittings, adapters and nipples
- Shafts, pins and dowels
- Valve stems, seats and spools
- Fasteners, screws and threaded standoffs
- Instrument and connector hardware
303 Swiss machined parts up to 32 mm diameter run on our four Maier MLK-32 machines with FMB bar feeders, and repeat jobs can run lights-out; see Swiss machining. Larger diameters and parts with heavy cross features go to the Mazak and Doosan lathes with live tooling and Y-axis, described on our CNC turning page.
Cost Index: 303 and Other Stainless Grades
| Material | Approx. machinability (1212 = 100) | Cost index vs 6061 | Notes |
|---|---|---|---|
| 303 stainless | ~78 | 1.8 | Free-machining; not for welding |
| 304 stainless | ~45 | 2.2 | Weldable, work hardens |
| 316 stainless | ~36 | 2.5 | Chloride and marine service |
| 410 stainless (annealed) | ~55 | 2.0 | Martensitic, hardenable |
| 17-4 PH stainless (Cond A) | ~40 | 2.8 | High strength after aging |
| 6061-T6 aluminum | ~180 | 1.0 | Baseline |
| 1018 steel (cold drawn) | ~78 | 1.1 | Needs plating for corrosion |
These figures are typical and illustrative for a comparable part; actual quotes depend on geometry, quantity, tolerances and mill pricing.
Applications
Free machining stainless machining services at Wexmar support industrial OEMs building pumps, fluid power and instrumentation equipment, electrical and connector manufacturers, medical hardware assemblies and fastener programs. When a part needs welding or better corrosion resistance, 304 stainless and 316 stainless are the usual next steps.
Getting a quote
For 303 parts, upload a drawing for an instant estimate as a PDF or STEP file, or call (315) 400-2654. A human estimator confirms every quote and will note passivation, material cert and inspection requirements before release.