CNC Machined Stainless Steel Parts

Which stainless steel grade should the part be?

Stainless steel is steel with at least 10.5% chromium. The chromium forms a thin passive film that keeps corrosion from spreading into the metal and repairs itself when it is scratched (source 1). The grades differ in what else goes in. The table covers 303, 304, 316 and 17-4 PH, the grades with pages of their own here. For a part that sits in salt water, the British Stainless Steel Association (BSSA) advises that 303 and 304 should not be considered for seawater service (source 5).

Each grade has more on its own page: 303 and 304 share one, and 316 and 316L covers the chloride and weld detail. The 17-4 PH page covers that grade in depth, including which material condition to name on the print.

What stainless parts get quoted?

Round stainless parts are shafts, pins, sleeves, bushings, hex standoffs and fittings, turned from bar. Shafts, dowel pins, and mold leader and guide pins to a print, including pins in 420 or 440C stainless, are on custom shafts and pins. A plain sleeve bearing is a bushing, and a hex or round spacer cut to a length the catalog skips is on spacers and standoffs. A shaft sleeve or wear ring for a pump is one of the pump parts, and a threaded adapter is one of the custom fittings.

Milled stainless parts are brackets, plates, manifold blocks, housings and flanges. A flat part cut from stainless sheet, with holes tapped or bends formed, goes to laser-cut stainless steel parts, and one order can hold machined and laser-cut parts together. The 304-or-316 choice for sheet has its own page, laser-cut stainless 304 vs 316.

Why does a stainless part cost more than the same part in aluminum?

Stainless is tougher and gummier than carbon steel and work hardens quickly as it is cut (source 8). Published starting ranges put stainless at 100-350 surface feet per minute (source 9), against 800-1,500 for wrought aluminum (source 10). Slower cutting means more machine time, and machine time is most of what a custom part costs, so the same part usually quotes higher in stainless than in aluminum.

Within stainless, the grade moves the price too: BSSA rates free-machining 303 about 60% easier to machine than 304 (source 4), so it is worth naming wherever the table allows it. If the part lives somewhere dry and a finish can hold off rust, carbon or alloy steel is often the cheaper part.

How is passivation called out on a stainless part?

A finish called out on the print is quoted with the part. Parts come back finished and inspected. Passivation removes iron contamination from the surface and restores the passive oxide layer that keeps the part from rusting, and ASTM A967 and AMS 2700 are the most common specifications for it (source 11). A967 covers nitric acid, citric acid and electrochemical treatments (source 12), so “passivate per ASTM A967” on the print is enough to quote from; add citric or nitric if your customer names one.

What goes on the print for a stainless part?

Give the grade and the finish callout. On a welded part, put the L on the grade: 304L and 316L hold carbon to 0.030% maximum instead of 0.08%, which keeps the metal beside the weld from corroding along its grain boundaries, at some cost in strength (source 6). Mark the dimensions that fit another part with their tolerances. Without a drawing, a STEP file is quoted to a ±0.005 in general tolerance or the one your notes name; what file to send has the full rule. If the grade is still open, choose “Not sure - please advise” for material on the quote form, say where the part will live (salt, washdown, a weld, a load), and the quote recommends one. Other alloys and plastics quoted on request. Name the grade on the print. Ask for more than one quantity, such as 1, 25 and 100, and the quote shows each.

What ships with the parts, and how long does it take?

No C of C or material certifications ship with parts by default. If your PO needs them, say so before the quote. Inspection reports on critical dimensions aren’t included by default. If you need one, ask when you place the order.

Rush 1-2 weeks applies to machined parts with no secondary operations. Heat treat, grinding, EDM and finishing add time, and the quote gives the real date. Lead time starts at order approval. Put your need date in the part description and the quote answers it.

Sources

All accessed 27 September 2026. Where a figure here differs from your print, the print governs.

1. Wikipedia, "Stainless steel": en.wikipedia.org (chromium content of 10.5% or more; a passive film that blocks corrosion and repairs itself; austenitic grades usually non-magnetic, slightly magnetic after work hardening; martensitic grades magnetic).

2. Wikipedia, "SAE 304 stainless steel": en.wikipedia.org (18.0-20.0% chromium and 8.0-11.0% nickel; food handling and processing equipment).

3. Wikipedia, "SAE 316L stainless steel": en.wikipedia.org (2-3% molybdenum; greater resistance than 304 to localized attack by chlorides; chemical, food processing, pharmaceutical, medical and marine uses).

4. British Stainless Steel Association, "Stainless Steels to Behold: 303": bssa.org.uk/stainless-steels-to-behold-303-%E2%9C%A8/ (0.15% minimum sulfur; machinability about 60% better than 304; pitting resistance significantly lower than 304; poor weldability).

5. British Stainless Steel Association, "Selection of 316, 304 and 303 types of stainless steels for seawater applications": bssa.org.uk/bssa_articles/selection-of-316-304-and-303-types-of-stainless-steels-for-seawater-applications/ (304, and more especially 303, not to be considered for seawater service).

6. British Stainless Steel Association, "Comparison of 304 or 316 and 304L or 316L type compositions and effect on corrosion resistance": bssa.org.uk/bssa_articles/2-comparison-of-304-or-316-and-304l-or-316l-type-compositions-and-effect-on-corrosion-resistance/ (carbon 0.08% maximum for 304 and 316, 0.030% for the L types; below 0.030% the grain-boundary corrosion after weld temperatures does not occur; lower strength of the low-carbon type).

7. Wikipedia, "17-4 stainless steel": en.wikipedia.org (martensitic precipitation-hardened grade, about 17% chromium and 4% nickel; magnetic; about 190,000 psi (1,300 MPa) ultimate tensile strength at about 44 HRC; corrosion resistance comparable to 304).

8. Wikipedia, "Machinability": en.wikipedia.org (stainless steels machine poorly compared with regular carbon steel because they are tougher, gummier and work harden very rapidly; 303 and 416 are easier to machine because of added sulfur and phosphorus).

9. Harvey Performance Company, "Slaying Stainless Steel: Machining Guide", In The Loupe, 19 June 2018: harveyperformance.com/in-the-loupe/stainless-steel-machining/ (stainless steels at 100-350 SFM).

10. Harvey Performance Company, "Attacking Aluminum: a Machining Guide", In The Loupe, 24 April 2018: harveyperformance.com/in-the-loupe/aluminum-machining-guide/ (wrought 2024, 6061 and 7075 at 800-1,500 SFM).

11. Wikipedia, "Passivation (chemistry)": en.wikipedia.org (stainless passivation removes exogenous iron and restores the passive oxide layer; ASTM A967 and AMS 2700 the most prevalent standards).

12. ASTM International, "A967/A967M-25: Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts": store.astm.org (nitric acid, citric acid and electrochemical treatments).

13. Peter Zelinski, "Understanding Swiss-Type Machining", Modern Machine Shop, 26 June 2013, updated 14 July 2022: mmsonline.com/articles/understanding-swiss-type-machining (the stock feeds through a guide bushing, so the tool cuts near the point of support; suited to long and slender turned parts).