Stainless Steel CNC Machining
303, 304, 316L and 17-4 PH — fittings, shafts, valves, and food- and medical-grade hardware, with passivation and electropolishing in the same order. Stainless steel CNC machining services from prototype to repeat production, with finishing in the same order.
Corrosion-Resistant Parts, Machined to Drawing
Stainless steel CNC machining is where cycle-time honesty matters: stainless cuts slower than aluminum, work-hardens if the tool rubs, and punishes weak setups. We machine it every day — fittings, shafts, valve components, and food- and medical-grade hardware — on rigid milling centers and live-tooling lathes, with passivation and electropolishing handled in the same order. If your last stainless quote came back late, over tolerance, or with work-hardened bores, the sections below explain how we avoid all three.
Stainless Grades We Machine
- 303 — the free-machining grade. Sulfur addition gives the best chip formation of the austenitic family and the lowest machining cost; slightly reduced corrosion resistance. Our default when the drawing allows it
- 304 / 304L — the general-purpose corrosion-resistant grade for food equipment, enclosures, and hardware. Gummier than 303; needs sharper tools and firm feeds
- 316 / 316L — molybdenum-alloyed for marine, chemical, and medical exposure. The standard for fittings and components in chloride environments
- 17-4 PH (630) — precipitation-hardening stainless, heat-treatable to H900/H1150 conditions for high-strength shafts, pins, and structural parts
- 416 — free-machining martensitic grade, hardenable, for shafts and fasteners where corrosion demands are moderate
Machining Stainless: What Actually Matters
Stainless work-hardens. Every parameter choice follows from that:
- Never let the tool rub — feeds stay above the minimum chip load; dwell and spring passes are programmed out, because a work-hardened surface destroys the next tool
- Lower surface speeds, firm feeds — carbide at roughly 250–450 SFM (75–140 m/min) depending on grade, with positive-rake, sharp-edged inserts
- Flood coolant, always — heat concentrates at the cutting edge; we run high-volume coolant and, on turning, high-pressure delivery to break chips
- Rigid setups — stainless's toughness amplifies any vibration into chatter and poor finish; short tool overhangs and solid workholding are standard practice
- 303 substitution — where a drawing specifies 304 but the application allows, we flag 303 as a cost-down option in the DFM review
Expect stainless cycle times roughly 2–3× aluminum for equivalent geometry — that's physics, and we quote it honestly. Where a design mixes materials (an aluminum housing with stainless inserts, for example), we machine both in the same order so the assembly ships together.
Tolerances and Surface Finishes
General features hold ±0.005 in (±0.13 mm); sealing diameters, bores, and threads to ±0.001 in (±0.025 mm) quoted against the drawing. As-machined finish Ra 3.2 µm (125 µin), to Ra 0.8 µm (32 µin) on sealing faces.
Finishes: passivation per ASTM A967 (standard on 300-series parts), electropolishing for micro-burr removal and maximum corrosion resistance, bead blast, and polishing. Stainless parts ship uncoated unless the drawing says otherwise — that's the point of the material. See surface finishes for spec references.
Typical Stainless Parts
Hydraulic and pneumatic fittings, valve bodies and stems, pump shafts, fasteners, food-processing hardware, and medical device components. Sealing faces and thread accuracy are the usual critical features — we gauge threads and inspect sealing surfaces on every fitting-class part. Most fittings and shafts run on CNC turning with cross-holes and flats completed in one setup. For medical applications, see medical CNC machining.
What to Specify on the Drawing
Call out the exact grade and condition (316L, not just “stainless”) — L-grades matter for welded assemblies, and 17-4 PH must carry its heat-treat condition. Note passivation explicitly if your quality system requires a cert, and mark sealing surfaces with an Ra value rather than a note like “smooth finish.” For NPT threads, state whether they’re gauged; for BSP threads, state the standard (ISO 228 vs ISO 7) because they are not interchangeable.
Stainless Machining — Capability Summary
| Capability | Specification |
|---|---|
| Grades machined | 303, 304/304L, 316/316L, 17-4 PH, 416 (others on request) |
| General tolerance | ±0.005 in (±0.13 mm); precision features to ±0.001 in (±0.025 mm) |
| Threads | NPT, BSPT/BSPP, metric — gauged to drawing |
| As-machined finish | Ra 3.2 µm (125 µin); sealing faces to Ra 0.8 µm (32 µin) |
| Finishes | Passivation (ASTM A967), electropolish, bead blast, polishing |
| Batch range | 1 pc prototype to 10,000+ pcs production |
| Typical lead time | Prototypes from 5 days; production typically 2–3 weeks |
Quote Your Stainless Parts
Upload STEP files and drawings for pricing and lead time within 1–2 business days. Include grade, passivation, and any thread-gauging requirements on the print so the first quote is the accurate one. Other alloys on the full materials page.
Quote Your Stainless Parts
Upload STEP files and drawings for pricing and lead time within 1–2 business days.
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