Stainless Steel Machining Processes
What milling, turning, threading, EDM and finishing each do to a stainless part, which grade suits which process and what to write on the drawing so the quotes you get back are comparable.
What stainless does at the tool
Three behaviours decide how a stainless part is made and what it costs. None of them needs an exotic machine. They need a shop that cuts stainless every week. Machinability figures on this page are rated against B1112 at 100, published typical values for the alloy.
Work hardening
Austenitic grades such as 304L and 316L harden where a tool rubs instead of cuts, so the next pass meets harder metal. Work hardening is why stainless is cut with a real feed and a sharp edge, never slowly and gently.
Heat stays in the cut
316L conducts heat at around 16 W/m K, roughly a tenth of 6061 aluminium. The edge runs hot and wears faster, and the part grows while it is cut, which matters on tight features.
Long chips and built-up edge
Austenitic and duplex grades make tough, stringy chips and weld to the edge. 303 and 416 have sulphur added so the chip breaks, which is why they cut fastest and why they corrode first.
Process against part feature
Which process makes which feature, the tolerance shops typically quote and what stainless changes. Typical values, never a promise. The shop that quotes confirms what it can hold on your geometry.
| Process | Typical parts | Typical tolerance | What stainless changes |
|---|---|---|---|
| CNC milling, 3 axis | Manifolds, brackets, housings, plates | ±0.05 mm routine, ±0.02 mm with a finishing pass | Austenitic grades work harden under a dwelling or rubbing tool. Constant engagement toolpaths avoid it. |
| CNC milling, 4 and 5 axis | Valve bodies, pump housings, instrument parts | ±0.02 to ±0.05 mm on positions | Fewer setups, fewer re-clamps and fewer chances to drift on a part that holds heat. |
| CNC turning | Shafts, fittings, bushings, valve stems | ±0.02 mm on diameters, ±0.01 mm on 303 and 416 | 304L and 316L make long stringy chips. Chip breaking and coolant pressure control them. |
| Swiss type turning | Small pins, instrument shafts, connector bodies | ±0.01 mm common on small diameters | The guide bushing supports slender parts. 303 and 416 run fastest, 316L is common for medical. |
| Drilling and deep hole drilling | Fluid passages, cross holes, bolt holes | Reamed holes typically H7 | A drill that rubs at the bottom of a hole work hardens it for the next tool. Cross hole burrs need a plan. |
| Tapping and thread milling | Ports, fastener threads, fittings | 6H and 6g classes typical | Taps snap in 316L and the part is often scrapped. Thread milling recovers. Galling on assembly is a grade question. |
| Wire and sinker EDM | Thin slots, sharp corners, hardened parts | ±0.005 to ±0.01 mm typical on wire | No cutting force, works on hardened 17-4 PH and 440C. Leaves a recast layer. |
| Grinding and honing | Bearing journals, seal faces, bores below ±0.01 mm | Below ±0.01 mm | Often a second operation or a second supplier. Expect it to show as its own line. |
Grade against process
The grade that machines easily is often the one you cannot use. Full composition, PREN and weldability for 18 grades are in the grade reference.
| Grade | UNS / EN | Machinability | Work hardening | What it means in the shop |
|---|---|---|---|---|
| 303 | S30300 / 1.4305 | 78 | Low | Fastest to cut and thread. Sulphide stringers start pits, so not for food contact, medical or marine. |
| 304L | S30403 / 1.4307 | 45 | High | The default austenitic. Welds well. Keep the tool engaged and sharp. |
| 316L | S31603 / 1.4404 | 36 | High | Molybdenum for chloride resistance. Slowest of the common three. Medical, food and pharma ask for it by name. |
| 17-4 PH | S17400 / 1.4542 | 50 | Moderate | Machined in Condition A, aged afterwards. Size moves slightly on ageing. |
| 416 | S41600 / 1.4005 | 85 | Low to moderate | Free-machining martensitic. Cheapest to cut, weakest against corrosion. |
| 2205 | S32205 / 1.4462 | 30 | Very high | Duplex strength and chloride resistance. Hard on tools, needs rigid setups. |
Heat treatment and welding in the sequence
Where heat goes in the route decides which dimensions a shop can promise.
- 17-4 PHMachined in Condition A, then aged. H900 is the hardest and least corrosion resistant, H1150 the toughest and most resistant. Ageing contracts the part slightly and predictably. See 17-4 PH machining conditions.
- 410, 420, 440CRough machined annealed, hardened, then ground or hard finished on the critical features. Hardening distorts, so leave grinding stock.
- WeldmentsWeld, stress relieve if specified, then machine the datum and fitted features. 304L and 316L keep carbon low so the weld zone does not sensitise. See machining stainless weldments.
- After machiningDeburr, clean, then passivate or electropolish, then inspect. Marking and packing come last so nothing contaminates the passive surface.
Finishing, passivation and surface roughness
Machining leaves free iron and a disturbed surface. Passivation removes the iron and lets the chromium oxide layer reform. It does not change the finish. More in passivation against electropolishing and stainless surface finish.
| Finish | Typical result | Where it fits |
|---|---|---|
| As machined | Ra 1.6 to 3.2 µm | General surfaces. The common default is Ra 1.6. |
| Careful finishing pass or polish | Ra 0.8 µm | Food contact to 3-A, seal faces, sliding fits. |
| Grinding, fine turning, polishing | Ra 0.4 µm | Pharma contact, high pressure seals. |
| Electropolishing to ASTM B912 | Lowers Ra, removes metal | Medical, pharma and vacuum surfaces. Allow for removal in the tolerance. |
| Passivation to ASTM A967 or AMS 2700 | Unchanged | Removes free iron after machining. Citric or nitric, named on the drawing. |
| Bead blast | Even matte | Appearance. Glass or ceramic media only, then passivate, because steel media embeds iron. |
What a tolerance costs
Each step down adds a pass, tooling, settling time or inspection on every piece. Put the tight numbers on the two or three features that fit something. Detail in stainless tolerances and what they cost.
| Tolerance | What it takes | Relative cost |
|---|---|---|
| ±0.1 mm | Standard practice, no special effort | Baseline |
| ±0.05 mm | Routine, a little care on setup | Slightly above |
| ±0.02 mm | Dedicated finishing pass, fresh tooling, controlled setup | Noticeably above |
| ±0.01 mm | Process control, thermal settling, dedicated inspection | Substantially above |
| Below ±0.01 mm | Grinding, honing or lapping, often a second supplier | Different conversation |
What to put on the drawing
Each line removes a decision a shop would otherwise make for you. The full list is in what to put in a stainless RFQ, and the industries that bring their own paperwork are on the industries page.
- Grade and standardFor example 316L to ASTM A276 or A479, or 17-4 PH with the condition. A grade without a standard is a guess about the certificate.
- Condition of dimensionsFor 17-4 PH and martensitic grades, say whether dimensions apply before or after heat treatment.
- Thread methodState the thread class and whether thread milling is acceptable. It removes the scrapped part risk on 316L.
- Finish per surfaceRa only on the faces that need it. A blanket Ra 0.8 multiplies the finishing time on every face.
- Finishing chainPassivation standard and method, electropolish if needed, and the certificate you want back.
- Mating partsFlag stainless on stainless sliding or threaded joints. Galling is fixed by grade or pairing, not by machining.
- Welded assembliesSay whether the part is machined after welding, and use 304L or 316L where the weld sees corrosion.
Keep reading
- Process
Work hardening in stainless parts
Why small features harden fastest and what a shop does about it.
- Process
Threading and tapping stainless
Taps against thread mills, and why a broken tap costs the part.
- Materials
303 against 304 against 316L
The machinability and corrosion trade in the three common grades.
- Process
Citric against nitric passivation
What each method does and how to name it on the drawing.
Questions about stainless processes
Something else? Email .
Why does 316L cost more to machine than 303?
303 has sulphur added so the chip breaks, and it rates around 78 on the B1112 machinability scale. 316L rates around 36. It work hardens, makes long chips and wears tools faster, so the same geometry holds the machine longer. The sulphur that helps 303 is also why it is kept off food, medical and marine parts.
Should 17-4 PH be machined before or after heat treatment?
Usually before. 17-4 PH is machined in Condition A, then aged to the condition on the drawing, for example H900 or H1150. Ageing shrinks the part slightly and predictably, so tight features are either sized for it or finished after ageing. Say which condition the drawing dimensions apply to.
Is passivation the same as electropolishing?
No. Passivation is a chemical clean, citric or nitric, that removes free iron and lets the chromium oxide layer reform. It does not change the surface finish. Electropolishing removes a thin layer of metal, lowers Ra and rounds micro burrs. Medical and pharma parts sometimes get both, in that order or as the method the standard names.
Can stainless be wire EDM cut?
Yes. Every stainless grade conducts, so wire EDM cuts thin slots, sharp inside corners and hardened 17-4 PH or 440C without cutting force. It leaves a thin recast layer and is slower than milling on open features, so it is chosen for the feature, not for the whole part.
Have a stainless drawing ready?
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