304 stainless steel is the general-purpose austenitic grade: 505–620 MPa tensile strength, roughly 215 MPa yield, 8.0 g/cm³ density, non-magnetic in the annealed condition, and excellent corrosion resistance in most environments. SkillsProto machines 304 to ±0.005 mm on critical dimensions and ±0.05 mm on general dimensions, with 3–7 working day lead times and a 48-hour express option. Minimum order is one piece, and every order ships with a first-article inspection report.
304 Stainless Steel at a glance
304 is an 18-8 chromium-nickel austenitic stainless: about 18% chromium and 8% nickel. It cannot be hardened by heat treatment, only by cold work, and it stays tough at low temperatures. It is the default material for food equipment, chemical fittings, medical trays, marine hardware for non-immersed service, and any bracket that must survive weather without coating.
| Property | 304 | 316 | 303 | 17-4PH (H900) |
|---|---|---|---|---|
| Tensile strength | 505–620 MPa | 515–620 MPa | ~620 MPa | ~1070 MPa |
| Yield strength | ~215 MPa | ~205 MPa | ~240 MPa | ~1000 MPa |
| Density | 8.0 g/cm³ | 8.0 g/cm³ | 8.0 g/cm³ | 7.8 g/cm³ |
| Hardness | ~170 HB | ~170 HB | ~180 HB | ~38 HRC |
| Magnetic when annealed | No | No | Slight | Yes |
| Chloride and pitting resistance | Moderate | High (2–2.5% Mo) | Low | Moderate |
| Weldability | Excellent | Excellent | Poor | Good |
| Machinability (B1112 = 100%) | ~45% | ~45% | ~78% | ~45% |
| Relative stock cost | 1.0× | 1.2–1.4× | 1.0–1.1× | 1.8–2.5× |
The table explains most sourcing decisions. 316 costs 20–40% more per kilogram and buys chloride resistance through molybdenum. 303 is the machinist's grade for turned parts but welds poorly and corrodes sooner. 17-4PH is the step up when yield strength near 1000 MPa is required.
Tolerances SkillsProto holds on 304
304 machines cleanly if the tool stays in the cut at a constant feed. Because the alloy work hardens, tolerances depend more on toolpath discipline than on machine accuracy, which is why SkillsProto controls finishing allowance and tool engagement rather than simply slowing everything down.
| Feature type | General capability | Tight-tolerance capability |
|---|---|---|
| Linear dimensions | ±0.05 mm | ±0.005 mm |
| Hole diameter (reamed or bored) | ±0.03 mm | ±0.01 mm |
| Hole position (true position) | ±0.05 mm | ±0.015 mm |
| Flatness | 0.05 mm over 100 mm | 0.02 mm over 100 mm |
| Perpendicularity | 0.05 mm | 0.01 mm |
| Surface roughness, as-machined | Ra 1.6 µm | Ra 0.4 µm |
| Threads | 6H / 6g | 6H / 6g with gauge verification |
A first-article inspection report ships with every 304 order and CMM data is available on request. Material certificates trace the bar or plate lot back to the mill, which matters when the part goes into regulated equipment.
Cost drivers on 304 parts
Stainless costs more than aluminum before the first cut and again during every minute of cutting. Five variables set the price:
| Cost driver | Typical impact | How to control it |
|---|---|---|
| Raw stock | Material is often 25–40% of a small stainless part; 304 costs roughly 2–3× more per kg than 6061 and is about 3× denser | Buy near-net blanks, avoid oversized plate |
| Cutting speed and cycle time | Stainless runs at roughly 40–60% of aluminum speeds and feeds | Avoid geometries that force small tools and long reach |
| Tool wear | Carbide inserts wear faster on work-hardening alloys, and tooling cost lands in machine time | Avoid interrupted cuts and scale on hot-rolled stock |
| Tolerance and finish class | Going from ±0.05 mm to ±0.005 mm adds 10–25% on affected features; electropolishing adds 1–2 working days | Tighten only functional features |
| Batch size | Unit price often drops 20–40% from 1 to 10 pieces, and another 10–20% at 50 | Request quotes at 1, 10 and 50 |
SkillsProto returns quotes with DFM feedback within 24 hours on working days — wall thickness, corner radii a standard cutter can reach, and tolerances that can be relaxed without affecting function.
Surface finishes for 304
- As-machined — deburred with all sharp edges broken, typically Ra 1.6 µm; Ra 0.4 µm achievable on specified faces.
- Passivation — nitric or citric acid treatment that removes free iron from the surface with no measurable build-up; the standard finish for corrosion-critical parts.
- Electropolishing — removes 5–30 µm of material and brings surfaces down to about Ra 0.2 µm, improving cleanability in food and medical equipment.
- Bead blast — uniform matte texture around Ra 1.2–2.0 µm that hides tool marks on cosmetic faces.
- Powder coating — 60–120 µm, for outdoor hardware that needs colour and impact resistance.
- Electroplating, nickel — 5–25 µm, used on mating or sliding surfaces.
- Mirror polishing — for visible trim parts; adds cost that rises sharply with part complexity.
Design rules that cut cost on 304
- Keep walls at 1.0 mm or thicker. Below 0.8 mm, 304 deflects under cutting load and the resulting chatter work-hardens the surface.
- Use internal corner radii of at least one third of the pocket depth, with 0.5 mm as the floor. Sharp corners force tiny tools and slow finishing passes.
- Keep pocket depth-to-width at 4:1 or less; deeper cavities need long-reach tooling and produce the chatter that ruins surface finish on stainless.
- Limit drilled hole depth to about 5× diameter with standard tooling. Deeper holes need peck cycles and add cycle time quickly.
- Specify thread depth at 1.5× diameter, and for holes below M6 or any joint assembled repeatedly, use thread inserts — 304 galls against itself.
- Leave 0.2–0.3 mm finishing allowance. Cuts lighter than about 0.05 mm rub instead of cutting and harden the surface, which then destroys insert life on the next pass.
- Group tight tolerances on one datum face so the part finishes in a single setup.
When to choose a different material
304 is the right default for corrosion resistance at a reasonable price, but not for every part. Choose 316 or 316L for marine immersion, chloride exposure, chemical service or welded assemblies where weld decay matters. Choose 303 when the part is turned, high volume, and never welded — machinability near 78% against roughly 45% for 304 cuts cycle time significantly. Choose 17-4PH when the design needs yield strength near 1000 MPa or hardness around 38 HRC. Choose 6061-T6 when weight dominates: at 2.70 g/cm³ against 8.0 g/cm³, an aluminum part weighs about a third as much, and it is cheaper to machine. For enclosures, brackets and panels, sheet metal fabrication usually wins once quantities reach a few dozen; above roughly 500–1000 pieces, die casting or injection molding takes over on unit cost.
Frequently Asked Questions
Is 304 stainless steel magnetic?
In the annealed condition, no — 304 is austenitic and effectively non-magnetic. Cold work such as bending, deep drawing or heavy machining can induce slight magnetic response at the worked surfaces. SkillsProto notes this on the inspection report when magnetic permeability matters to the assembly.
What is the difference between 304 and 316 stainless steel?
316 adds 2–2.5% molybdenum, which raises resistance to chlorides and pitting. Tensile strength is nearly identical at 515–620 MPa, and yield is slightly lower at about 205 MPa. 316 stock costs 20–40% more per kilogram, so it is worth specifying only for marine, chemical or medical service.
Can 304 stainless steel be welded after machining?
Yes. 304 is one of the most weldable stainless grades by TIG, MIG and resistance methods, and it does not require pre-heat. Use 304L or 316L filler with low carbon content for joints that will not be annealed afterwards, and machine critical sealing faces after welding rather than before.
Why does 304 cost more to machine than aluminum?
Material, speed and tooling all move together: 304 stock costs roughly 2–3× more per kg than 6061 and is about three times denser, cutting speeds run at roughly 40–60% of aluminum, and carbide inserts wear faster on a work-hardening alloy. SkillsProto reduces the gap by controlling finishing allowance.
What tolerance can SkillsProto hold on 304 stainless steel?
±0.005 mm on critical dimensions and ±0.05 mm on general dimensions. Hole position holds to ±0.015 mm and flatness to 0.02 mm over 100 mm on tight-tolerance work. Every order includes a first-article inspection report, and CMM data is available on request.
How fast can SkillsProto ship 304 parts, and what is the minimum order?
Standard lead time is 3–7 working days from order confirmation, with a 48-hour express option; passivation, electropolishing and plating add 1–2 working days. DDP shipping to the UK, EU and US takes 3–5 days door to door. Minimum order quantity is one piece.
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