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SUS304 Machining: A Guide for Buyers Working From JIS Drawings

Materials / 21 Sept 2026

If your drawing says SUS304 and you are buying the part outside Japan, two things decide whether you get a usable quote: whether the shop can read the rest of the title block, and whether it has cut austenitic stainless often enough to price the cycle time honestly. The material itself is not exotic. The way it behaves under a tool, and the way a JIS drawing specifies it, is where quotes go wrong.

This guide covers what SUS304 is, why it costs what it costs to machine, and what a shop needs from you before it can commit to a price.

What SUS304 is, and what a certificate will say

SUS304 is the JIS designation for the standard 18-8 austenitic stainless: roughly 18 % chromium, 8 % nickel, carbon held at or below 0.08 %. It appears in JIS G 4303 for bar and JIS G 4305 for cold rolled sheet and plate.

The same alloy is AISI 304 in the American system, EN 1.4301 in Europe, and it also appears as UNS S30400. The composition ranges overlap closely enough that a shop buying EN 1.4301 bar is buying the material your drawing intends.

The documents are a different matter. A European stockist will certify bar to EN 10204 3.1 against EN 10088. A Japanese mill sheet is written against the JIS standard. Both describe material that meets your drawing. Only one of them will satisfy an inspector who was told the certificate must name JIS G 4303. Decide which you need and put it in the notes, because it changes where the shop can buy the bar and therefore what the part costs.

Why SUS304 costs more to cut than the bar price suggests

The published machinability index for annealed SUS304 is about 45, where 100 is AISI B1112 free-machining steel. SUS303 sits at about 78 and SUS316 at about 36. Those numbers are handbook comparisons, not cycle times, but the ratios hold up in practice: the same part in SUS303 and SUS316L can be two jobs of very different lengths.

Three properties drive it.

It work hardens while you cut it. Austenite transforms to martensite under strain. A tool that rubs instead of cutting, a feed that pauses, or a second pass that is too light all leave a hardened skin behind. Annealed SUS304 is around 200 HV. A work hardened surface can be double that. The next pass then has to cut through the mess the last one made.

It holds heat in the cutting edge. Thermal conductivity is roughly 16 W per metre kelvin, about a third of plain carbon steel. Heat that would normally leave in the chip stays at the tool nose instead, and tool life pays for it.

It builds up on the edge. Austenitic stainless is gummy. Long stringy chips weld to the rake face, break away and take coating with them, which spoils the surface finish and eventually the tool.

None of this is a problem for a shop that runs stainless every week. It is a real problem for one that mostly cuts aluminium and quoted your part off a feeds and speeds table. That difference is invisible on a website and obvious in the quote.

What the JIS title block tells a shop

The single most common reason a quote comes back wrong is that nobody read the general tolerance call-out.

JIS B 0405 covers linear and angular dimensions that carry no individual tolerance, in four classes: f for fine, m for medium, c for coarse and v for very coarse. It is the JIS equivalent of ISO 2768-1. The class is not a single number. For a dimension between 6 mm and 30 mm, class f allows plus or minus 0.1 mm, class m allows plus or minus 0.2 mm and class c allows plus or minus 0.5 mm. On a 30 mm to 120 mm dimension those become 0.15 mm, 0.3 mm and 0.8 mm.

JIS B 0419 adds the geometrical general tolerances, in classes H, K and L, covering straightness, flatness, perpendicularity, symmetry and circular run-out. A drawing marked JIS B 0419-mK is carrying a medium dimensional class and a K geometrical class, and the K run-out limit of 0.2 mm may well be tighter than anything explicitly drawn on the part.

A shop that quotes your part without reading those two lines is quoting a looser part than the one you asked for. You will find out at first article inspection.

Two more conventions catch shops outside Japan. JIS B 0001 uses third angle projection, the same as ASME, so a shop used to European first angle drawings can read a feature on the wrong side of the part. And dimensioning follows JIS Z 8317, which is close to ISO but not identical in how tolerances are stacked.

Surface finish: Ra, Rz and the triangle marks

Current JIS drawings specify surface texture to JIS B 0601, in Ra or Rz, using the same tick symbol as ISO. That part is unambiguous.

Older drawings, and there are a great many still in circulation, use the triangle marks that were withdrawn in the 1990s. The mapping people generally use is:

Symbol	Typical Ra
Wavy line	No machining, as supplied
One triangle	Ra 25
Two triangles	Ra 6.3
Three triangles	Ra 1.6
Four triangles	Ra 0.2

Treat that table as a reading aid, not a specification. Before anyone prices a finish off a legacy drawing, confirm the intended Ra with whoever owns the drawing. The difference between three and four triangles is the difference between a finish turn and a grinding or polishing operation, and it is not a small line in the quote.

For reference, Ra 1.6 is a normal machined finish and costs nothing extra. Below Ra 0.8 usually means an added operation. Below Ra 0.4 on stainless usually means grinding, lapping or electropolishing.

Threads, certificates and treatment

Three notes that decide the price and are regularly left off:

Thread class. M6 on its own is not a specification for a part that will be gauge checked. Give the pitch to JIS B 0205 and the tolerance class to JIS B 0209, so M6 x 1 - 6H for an internal thread. Say whether a go and no-go gauge result has to be recorded.

Certificate level. Mill certificate, EN 10204 3.1, first article report, full CMM dimensional report. Each one is real hours. Ask for what you need and nothing more.

Passivation. Even on a JIS drawing, passivation is normally called out to ASTM A967 or AMS 2700, since JIS does not have an equivalent in common use. If you want electropolishing, give the target Ra as well, otherwise every shop will assume a different one and you will be comparing quotes for different parts.

One more, specific to austenitic grades: cold work raises magnetic permeability. If the part sits near a sensor or inside a vacuum chamber, put the limit on the drawing, something like mu below 1.05, rather than mentioning it in an email after the quote.

When SUS304 is the wrong grade

SUS304 is the default, which means it is sometimes on the drawing because nobody revisited it.

Turned parts in volume, not welded, not in chlorides. SUS303 cuts roughly twice as fast. On a long run of shafts or fittings that is the cheapest change you can make.
Salt water, pool chemicals, chlorides of any kind. SUS316 or SUS316L, for the molybdenum. It machines slower than SUS304, and it is still cheaper than replacing pitted parts.
Welded then used in a corrosive service. SUS304L, to avoid carbide precipitation at the weld.
Indoor, non-corrosive, price sensitive. SUS430 is ferritic, magnetic, cheaper and closer to carbon steel under a tool.
Strength you cannot get from an austenitic grade. SUS630, the 17-4 PH, machined in condition A and aged afterwards.

The grade reference table on the home page pairs each JIS designation with its AISI and EN equivalents and its relative machinability, which is usually enough to have the conversation with your engineer.

A realistic sequence for a SUS304 part

For a typical turned and milled part in modest quantity, a shop that knows the material will plan something like this:

Buy bar with a mill certificate that names the standard your drawing asks for.
Rough with a heavy, constant feed. Depth of cut below the previous pass’s hardened layer, never a light spring pass.
Leave stock for finishing, typically 0.3 mm to 0.5 mm on a diameter, so the finish tool cuts fresh material rather than work hardened skin.
Finish with sharp, uncoated or sharply ground coated inserts, high pressure coolant where the machine has it.
Deburr, then passivate if the drawing calls for it.
Inspect, with a CMM report only if you asked for one.

If a quote comes back with a cycle time that looks like mild steel, that is usually the sign to ask a second shop.

What to send with the drawing

The fastest route to a firm quote:

A dimensioned PDF with the title block, so the tolerance class travels with the part.
A STEP or IGES model. The PDF governs, the model saves programming time.
Grade, and whether it is fixed or open to a suggestion.
Real quantity, and whether it repeats.
Certificate level and any surface treatment.
The date you need parts in your hand, not the date you would like a quote.

A photograph of a part with no drawing is not enough to price. If that is all you have, say so and a shop can tell you what it needs measured.

When the drawing is ready, get a quote. An engineer reads the tolerance class and the finish call-outs and prices the part within 24 business hours, and if you order we place it with a shop that holds what the drawing asks for, inspect it before packing and ship it on one PO and one invoice.

Questions

Is SUS304 the same as AISI 304?

For most purposes yes. SUS304 is the JIS designation, AISI 304 is the American one and EN 1.4301 is the European one, and the chemistry ranges overlap closely. The certificates are not interchangeable, so if your part needs a mill certificate, name the standard it has to be written against on the drawing.

Why is SUS304 slower to machine than mild steel?

Two reasons. It work hardens as it is cut, so a dull tool or a pause in the feed leaves a hard skin the next pass has to get through. And it conducts heat at roughly a third of the rate of carbon steel, so the heat stays in the cutting edge instead of leaving in the chip.

Should I switch to SUS303 to save money?

Only if the part is turned, is not welded, and does not live in salt or chlorides. SUS303 has sulphur added for chip breaking and cuts roughly twice as fast, but that sulphur is also where corrosion starts and it makes the grade unsuitable for welding.

What tolerance can a shop hold on SUS304?

Normal production work sits around plus or minus 0.02 mm on a turned diameter and h7 on a fitted one. Tighter is possible but it moves the part into grinding, temperature controlled inspection, or both, and the price moves with it.

Have a drawing for a stainless part?

Send it and an engineer reads the title block, quotes it within 24 business hours, and if you order we place, manage, inspect and ship the parts on one PO and one invoice.

Get a quote