A few weeks back I looked at our production log. We'd machined just over 23,000 parts that quarter. About 4,000 of them were stainless steel. The rest were mostly aluminum, some plastics, a handful of titanium jobs.
Here's the thing about stainless: it fights back. Aluminum doesn't. Plastic definitely doesn't. Stainless steel pushes against your tool with every pass, generates more heat than your coolant can pull away, and will happily destroy a $60 carbide end mill if you get the speeds wrong.
But when you need it, there's no substitute.
This is what I've figured out about machining stainless — the grades, the tricks, the mistakes engineers make when they spec it, and how to get clean stainless parts without blowing your budget.
Stainless Steel Grades We Machine Every Week
Not all stainless steels machine the same way. Not even close. The difference between 303 and 304 is night and day on the shop floor. And if someone sends me a drawing calling for 440C with a 3mm internal radius in a deep pocket, I'm going to pick up the phone before I program the job.
Here are the grades we run most often, with real numbers on how they behave:
| Grade | Relative Machinability | Tool Wear Rate | Typical Tolerance | Cost Index | Surface Finish (Ra) |
|---|---|---|---|---|---|
| 303 | 78% (best) | Low | ±0.005 mm | 1.0x (baseline) | 0.4 µm |
| 304 | 45% | Medium-high | ±0.01 mm | 1.3x | 0.6 µm |
| 316L | 40% | High | ±0.01 mm | 1.5x | 0.6 µm |
| 17-4 PH (annealed) | 48% | Medium | ±0.01 mm | 1.6x | 0.5 µm |
| 17-4 PH (hardened) | 15% | Very high | ±0.02 mm | 2.5x+ | 0.3 µm |
| 420 (annealed) | 50% | Medium | ±0.01 mm | 1.4x | 0.5 µm |
| 420 (hardened) | 10% | Extreme | ±0.03 mm | 3.0x+ | 0.2 µm |
The machinability percentages are relative to 1212 carbon steel at 100%. 303 at 78% means you can run it at 78% of the speed you'd run mild steel. 304 at 45% means less than half the cutting speed. When you're programming a job, that number decides whether the part takes 20 minutes or 50 minutes — and whether you need one end mill or three to finish it.
304 vs 316L: The Question I Get Every Other Day
An engineer emails me: "I need this bracket machined from stainless. What grade should I use?"
Nine times out of ten, the answer is 304.
304 has more than enough corrosion resistance for most applications. Food equipment, indoor structural parts, general industrial hardware — 304 handles all of it. The cost difference between 304 and 316L bar stock is about 25-40%, and the machining time is roughly the same. But 316L costs more in both material and tooling because it work-hardens faster and puts more heat into the tool.
316L exists for one reason: chlorides. Salt water, pharmaceutical cleaning chemicals, anywhere with aggressive corrosion. The molybdenum in 316L (2-3%) is what makes the difference. Without molybdenum, chlorine ions pit 304 in hours. With it, 316L holds up for years.
If your part lives indoors and never sees salt or strong acids, buy 304. Put the money you saved toward tighter tolerances or a better surface finish.
If your part is going on a boat, into a surgical suite, or anywhere near hot chlorinated cleaning solutions, the 316L premium is non-negotiable.
One more thing: if the drawing says 304 or 316 (not 304L or 316L) and there's welding involved, switch to the L grade. The low-carbon versions won't precipitate chromium carbides during welding, so you keep your corrosion resistance at the weld joint. I've seen welded 304 parts rust at the seam within a month because nobody caught this.
303: The Stainless That Machines Like Butter
303 exists because someone got tired of fighting 304 on jobs with lots of small threaded features.
The sulfur addition (0.15-0.35%) makes chips break instead of stringing. That matters enormously when you're running a Swiss lathe and chip evacuation determines whether you scrap one part or fifty.
The trade-off: 303 has slightly lower corrosion resistance than 304, and it can't be welded as easily. But for fasteners, bushings, shafts, and fittings — parts where machinability drives cost more than corrosion performance does — 303 is almost always the right call.
We ran a job last month: 500 stainless couplings, each with internal M6 threads, from 303 instead of 304. The client saved 28% on the unit price, and the parts exceeded their corrosion test requirements by 30%. That's the kind of win you get by picking the right grade for the actual requirement, not the default one.
17-4 PH and 420: When the Part Needs to Be Hard
17-4 PH
You machine 17-4 PH in its annealed state (Condition A), when it's roughly as machinable as 304. Then heat treat it. After precipitation hardening at 480-620°C, the tensile strength jumps from 1,000 MPa to as high as 1,300 MPa. The hardness goes from about HRC 33 to HRC 44.
We machine all the critical features — holes, threads, bearing seats — before heat treatment. After hardening, we do a light finishing pass on the tightest tolerances. There's dimensional change during heat treat (roughly 0.1-0.2%), so you need to account for it in the pre-heat-treat geometry.
Airbus and Boeing both use 17-4 PH extensively for structural components. If your part is going into an aircraft, this is probably the grade you want.
420
420 is a martensitic stainless that hardens to HRC 50+. In its annealed state it machines fine. Hardened, it chews through carbide like it's personal.
The only time we machine hardened 420 is for finishing operations. Roughing always happens in the annealed state. The sequence is: rough machine in annealed condition → heat treat to target hardness → finish machine critical surfaces → grind if needed.
Surgical instruments, pump shafts, and mold components use 420 regularly. If you need both stainless corrosion resistance and steel-like hardness, 420 is your answer.
What Actually Drives Stainless Machining Cost
Everyone asks about the material price. Here's the reality: for stainless steel CNC parts, material is maybe 20% of your cost. The rest is machining time and tooling.
Stainless steel machining costs more than aluminum for three reasons:
- Cutting speed. You run stainless at roughly half the SFM of aluminum. Same spindle, same tool — the stainless part takes twice as long.
- Tool consumption. A carbide end mill that cuts 200 aluminum parts might cut 40 stainless parts before it's done. Stainless work-hardens at the cut interface, which accelerates flank wear.
- Setup rigidity. Stainless pushes back. If your workholding isn't rigid, the part vibrates, surface finish degrades, and tolerances drift. We use heavy vises, short tool holders, and sometimes custom soft jaws just to keep the part still.
Typical cost multipliers vs 6061 aluminum for the same geometry:
| Material | Cost Multiplier | Main Driver |
|---|---|---|
| 6061-T6 Aluminum | 1.0x | Baseline |
| 303 Stainless | 1.8x | Slower speeds + tool wear |
| 304 Stainless | 2.2x | Work hardening + tooling |
| 316L Stainless | 2.6x | Premium material + aggressive work hardening |
| 17-4 PH | 3.2x | Material cost + post-machining heat treatment |
| Titanium Grade 5 | 4.5x | For context — very different machining challenge |
These are ballpark numbers from our shop. Your part geometry changes everything. A simple turned pin in 316L might only be 1.5x aluminum. A complex multi-axis part with deep pockets and tight tolerances could be 4x.
Design Rules That Keep Stainless Parts Affordable
After machining thousands of stainless parts, here's what separates designs that come in on budget from ones that don't:
Keep internal radii above 3mm if possible. Sharp internal corners in stainless concentrate cutting forces and kill tools. A 3mm radius lets us use a 6mm ball end mill with reasonable tool life. Drop to 1.5mm and we're running a 3mm tool that will break or burn through inserts.
Avoid deep, narrow pockets. Stainless doesn't clear chips well. Deep pockets trap chips against the tool, causing recutting and accelerated wear. If you must have a deep pocket, make it wide enough that the chip has somewhere to go — diameter at least 3x the depth.
Threads in stainless are expensive. Tapping 304 will break taps. Thread milling is more reliable but slower. If your part has 20 threaded holes in 304, those threads might be 40% of your machining cost. Consider whether every one of those holes actually needs to be threaded.
Don't spec tight tolerances everywhere. If you put ±0.01mm on every dimension of a stainless part, you're asking for trouble. Stainless moves during machining as internal stresses release. Pick the 3-5 features that actually matter for function and loosen everything else.
Consider post-machining processes early. Passivation? Bead blast? Electropolishing? Each adds cost and lead time. Specify them on the drawing, not in an email three days after you approve the quote.
Surface Finishing: Passivation Is Not Optional
Raw machined stainless steel has microscopic free iron particles embedded in the surface from the cutting tools. Those particles will rust. Passivation dissolves them with nitric or citric acid and leaves a chemically clean chromium oxide surface that actually resists corrosion.
Every stainless part we ship gets passivated unless the customer explicitly declines it. It costs about $3-5 per batch and adds one day to lead time.
Beyond passivation:
- Electropolishing removes surface material (5-40µm) and produces Ra 0.1-0.4µm. It's what you want for pharmaceutical equipment or anything with hygiene requirements. Cost: $30-80 per batch depending on part size.
- Bead blasting gives a uniform matte finish. Good for cosmetic parts, less effective than passivation for corrosion resistance. We usually blast first, then passivate.
- PVD coating (TiN, CrN, AlTiN) puts down a thin, hard decorative or functional layer. Common on consumer products and medical tools.
When Stainless Is the Wrong Choice
I will say no to a job when I think the material choice is bad. Here's when stainless doesn't make sense:
- The part never sees moisture. If it's an indoor bracket in a climate-controlled environment, 6061 aluminum with clear anodize costs half as much, weighs a third as much, and works fine.
- The primary requirement is thermal conductivity. Copper machines at 4x the thermal conductivity of 304. If you're making a heat sink, stainless is the wrong material.
- Weight matters more than corrosion resistance. If the part flies, titanium Grade 5 gives you equivalent strength at 40% less weight than 17-4 PH.
- You just need it to be metal and cheap. Carbon steel with zinc plating costs less, machines faster, and for many indoor applications, it's perfectly adequate.
Get a Quote on Your Stainless Part
We machine stainless every day. 304, 316L, 303, 17-4 PH, 420. From prototypes to production runs. CMM inspection on every order, passivation included unless you tell us otherwise.
Send your drawing for a free DFM review. I'll look at it myself, give you a quote within 24 hours, and tell you honestly if your grade choice makes sense — or if there's a better option that saves you money.