ステンレス鋼 CNC 加工：2026 ガイド (303, 304, 316L, 17-4PH)

Why and when to choose stainless steel

Stainless steel earns its name from chromium (≥10.5%) forming a self-healing chromium-oxide passive layer that resists rust. Beyond corrosion resistance, the family offers:

• Strength — most grades yield at 200–500 MPa, with PH grades reaching 1100+ MPa.
• Hygiene — non-porous, easy to clean, autoclavable. Standard for food, medical, pharma.
• Heat tolerance — useful service to 400–800°C depending on grade.
• Aesthetic durability — keeps appearance for decades without coating.
• Recyclable — 100% recyclable, scrap value supports lifecycle costs.

Grade comparison — pick the right family

Machinability differences (this is where stainless gets tricky)

Unlike aluminium, stainless steel work-hardens during cutting — meaning a dull tool leaves the surface harder than the bulk material, making the next cut even harder. This drives several rules:

• Sharp tools always. Worn tools cause work hardening and surface damage. Replace at half the wear interval used for steel.
• Constant chip load. Stop-start motion (intermittent cutting) work-hardens the surface. Use trochoidal toolpaths for pockets.
• Slower than aluminium, faster than titanium. Surface speed: 60–90 m/min vs aluminium 300+ m/min vs titanium 30 m/min.
• Aggressive coolant. Heat builds fast in the cutting zone. Flood coolant minimum, high-pressure ideal.
• Carbide tooling preferred. HSS works for low-volume, but carbide lasts longer and cuts cleaner.
• 303 is dramatically easier than 304. The added sulphur in 303 acts as a chip-breaker and lubricant — but at the cost of corrosion resistance.

Achievable tolerances on stainless

For ±0.005 mm or tighter on stainless — typically aerospace pumps, medical implants — JLYPT uses dedicated 5-axis cells with thermal compensation and CMM verification. See our tolerances and GD&T guide.

Passivation and finishes (don't skip passivation)

CNC machining exposes free iron from the underlying material. This iron will rust in saline or moist environments, even on "stainless" parts. Passivation removes this iron and restores the chromium-oxide passive layer.

1. 1

   Degrease

   Remove cutting fluid, oils and machining residues with appropriate cleaning chemistry.

2. 2

   Citric or nitric acid bath

   Citric acid (gentler, environmentally friendly) or nitric acid (aggressive, faster) — typically 30–60 minutes per ASTM A967 or AMS 2700.

3. 3

   Rinse and neutralise

   Multiple deionised-water rinses to remove all acid residue.

4. 4

   Dry and inspect

   Visual inspection + optional copper sulphate test to confirm passive layer is present.

5. 5

   Document

   Issue passivation certificate referencing ASTM/AMS standard for traceability.

Industry applications by grade

• 304 — kitchen equipment, brewery tanks, automotive trim, indoor industrial. The default "good stainless".
• 316L — surgical instruments, marine fittings, chemical processing, swimming pool components, off-shore. The default "premium stainless".
• 303 — non-critical machined parts: shafts, custom fasteners, valve internals where slight corrosion compromise is OK.
• 17-4PH — aerospace pump shafts, downhole oil tools, high-strength fasteners. See oil & gas components.
• 440C — bearings, ball valves, knife blades, hardened wear surfaces.
• 2205 duplex — chemical processing, offshore platforms, desalination — anywhere stainless commonly fails to chloride pitting.
• 321 — exhaust manifolds, heat exchangers, jet engine hot-section parts.

Real cost ranges

Indicative pricing for a typical 50×50×25 mm stainless bracket, passivated, batch quantities:

Stainless parts are typically 2–3× the cost of equivalent aluminium parts because: material is more expensive (×3), machining is slower (×2), and inspection takes longer (×1.5). For 100+ unit batches the gap narrows but stainless never matches aluminium on price.

Design tips for stainless parts

1. Use 303 when corrosion isn't critical. Saves 20–30% in machining time over 304.
2. Don't mix grades in one assembly. Galvanic corrosion at the interface; use the same grade throughout, or insulate.
3. Specify passivation on the drawing. "Passivate per ASTM A967" — tells the shop to include it and certify.
4. Avoid sharp internal corners. 1 mm minimum radius reduces stress concentration and tool breakage.
5. Allow extra material for stress relief. For PH grades, plan rough → heat treat → finish; aging shifts dimensions slightly.
6. Use larger thread engagements. Stainless galls easily; spec full thread depth + anti-seize for assembly.
7. Specify surface finish where it matters. Default Ra 1.6 µm is fine for most. Demand Ra 0.4 only on sealing/sliding surfaces.
8. Mark food/medical parts for traceability — alloy + lot number etched per regulations.