Machining 303, 304, and 316 stainless steel is where feed discipline and tooling choice separate a clean, cost-controlled part from a scrapped one. All three are austenitic stainless grades, and all three work harden if you let the tool dwell or rub. The good news is that the failure mode is predictable and avoidable. This guide covers how each grade behaves, how to beat work hardening, achievable tolerances and finishes, and what actually drives the cost of a machined stainless part.
The three common austenitic stainless grades
Grade selection controls machinability, corrosion resistance, and price. Picking the right one for the application prevents overpaying for corrosion resistance you do not need.
303: the free-machining grade
- Machinability: the easiest of the three, rated around 78 percent, thanks to added sulfur that breaks chips.
- Trade-off: the sulfur reduces corrosion resistance and weldability. Use it for fittings, shafts, and fasteners that do not see harsh environments.
- Best fit: high-volume turned parts where cycle time and chip control matter most.
304 and 304L: the general-purpose standard
- Machinability: around 45 percent, notably tougher than 303. It work hardens readily.
- Corrosion resistance: excellent for most environments. 304L has lower carbon for better weldability.
- Best fit: food-grade parts, enclosures, brackets, and general corrosion-resistant hardware.
316 and 316L: the marine and chemical grade
- Machinability: similar to 304, around 45 percent, and equally prone to work hardening.
- Corrosion resistance: the best of the three, with added molybdenum for chloride and marine resistance. 316L is standard for medical and pharmaceutical work.
- Best fit: marine hardware, medical components, chemical processing, and anything exposed to salt or acids.
Beating work hardening: the core of stainless machining
Work hardening happens when the tool rubs instead of cutting, or dwells in the cut. The surface layer hardens, the next pass rubs even more, and the part turns into a heat-glazed problem. The fixes are consistent across all three grades:
- Constant, positive feed: never let the tool dwell. Keep the cutter engaged and moving. Feed hesitation is the single biggest cause of hardened surfaces.
- Sharp, positive-rake tooling: coated carbide (TiAlN or AlTiN) with a sharp edge shears cleanly instead of pushing.
- Moderate speed, firm feed: typical 300 to 500 SFM for 304 and 316, higher for 303. Slower speeds with a firm chip load beat high speed with a light one.
- Depth below the hardened layer: each pass should cut beneath the previously work-hardened skin, not skim it.
- Flood or high-pressure coolant: heat control is critical because austenitic stainless conducts heat poorly.
- Rigid setup: deflection causes rubbing, so minimize overhang and stick-out.
Small diameter and long parts
For small, slender, high-precision turned stainless parts, Swiss machining supports the part right at the cutting zone with a guide bushing, which controls deflection and lets you hold tight tolerances on long, thin geometry. Larger diameter shafts and hubs move to conventional CNC turning. L:D ratios above 8:1 are where guide-bushing support earns its keep.
Tolerances and surface finish in stainless
- General tolerance: +/-.005 in without special call-out.
- Precision features: +/-.001 in is routine on turned diameters and milled features.
- Critical diameters: +/-.0005 in on Swiss and precision turning with finishing passes and temperature-stabilized inspection.
- Surface finish: Ra 63 uin as-machined; Ra 32 uin with finishing passes; Ra 16 uin achievable on turned diameters, which matters for seal surfaces and medical parts.
Stainless takes an excellent finish when the tool is sharp and the feed is steady. A glazed or torn finish is almost always a work-hardening symptom, not a limitation of the material.
What drives the cost of a machined stainless part
Stainless costs more than aluminum on nearly every line: slower cutting speeds, more tool wear, and pricier raw material. The ranges below are typical illustrative estimates for a small to mid-size machined stainless part, not a binding quote.
| Cost driver | What it covers | Typical range |
|---|---|---|
| Setup / programming | CAM, fixturing, first-part proveout (amortized over the lot) | $175 to $650 per setup |
| Material | 303 is cheapest; 304 mid; 316/316L highest due to molybdenum | $12 to $90 per part |
| Machine time | Slower speeds and firmer feeds mean longer cycles than aluminum | $70 to $140 per hour |
| Tooling wear | Higher insert consumption than aluminum, carried into the rate | Built into machine rate |
| Passivation / finishing | Citric or nitric passivation, bead blast, or electropolish | $4 to $30 per part |
| Inspection | In-process checks, CMM report, first article | $60 to $275 per lot |
To control cost: pick 303 when corrosion resistance is not critical, keep tolerances loose where the function allows, and quote in volumes that spread setup across more parts. Passivation is often specified on 304 and 316 to restore the chromium oxide layer after machining.
Inspection, passivation, and certs
Stainless parts frequently carry medical, marine, or defense requirements, so documentation matters. In-house Brown & Sharpe CMM inspection supports first article inspection and full dimensional layouts, and every lot can ship with a material certification traceable to the mill heat. Passivation per ASTM A967 (citric or nitric) is available to restore corrosion resistance after machining. Our inspection and quality process is ISO 9001-aligned, with DFARS-compliant material sourcing on request.
Choosing your stainless grade
- High volume, easy chip control, mild environment? Choose 303.
- General corrosion resistance, food-grade, or weldable? Choose 304 or 304L.
- Marine, chemical, or medical exposure? Choose 316 or 316L.
Send the model and print, and we will confirm grade, feed strategy, finish, and passivation before machining begins.