Stainless steel CNC machining

Custom stainless steel parts, milled and turned

Stainless steel is chosen for corrosion resistance, strength, hygiene and heat resistance: shafts, fittings, valve bodies, food and lab equipment, marine hardware, medical-style instruments and fasteners. It is harder to machine than aluminum or brass, so the grade you pick has a direct effect on cycle time, tool wear and price.

We machine stainless on 3- to 5-axis mills and on lathes and mill-turn centers (most stainless parts are round, so CNC turning is common). Standard tolerance is ±0.03 mm; tighter tolerances on critical features are confirmed in the quote. Mill certificates are available on request, and passivation is offered as a finish.

303 vs 304 vs 316 vs 17-4 PH

Typical properties for annealed bar (303, 304, 316) and 17-4PH in common aged conditions. These are textbook typicals; check the governing ASTM standard (for example A276 or A564) where minimums matter.

303: free-machining stainless

303 is 304 with added sulfur. The sulfur forms manganese sulfide inclusions that break chips and reduce tool wear, so 303 machines noticeably faster than 304 or 316. The cost: lower corrosion resistance (the sulfides are initiation sites for pitting), poor weldability and reduced ductility. Use it for turned parts, fittings and screws that live indoors or in mild environments.

304: the general-purpose grade

304 (18% chromium, 8% nickel) is the most common stainless. It resists atmospheric corrosion and most foods and cleaning agents, welds well and is easy to source in every bar and plate size. 304L, the low-carbon version, avoids sensitization when welded.

316: for chlorides and salt water

316 adds 2–3% molybdenum, which greatly improves resistance to pitting in chlorides: seawater, de-icing salts, many chemicals. It is the usual choice for marine hardware and wetted parts. It machines a little worse than 304 and the stock costs more.

17-4 PH machining

17-4PH is hardened by a low-temperature aging treatment rather than a quench, so distortion is small. It is magnetic and roughly four to five times stronger in yield than 304 in the H900 condition. Parts are normally machined in Condition A (solution treated) or in a softer aged condition like H1150, then aged if the drawing calls for a harder condition. State the required condition, for example H900 or H1025, on your drawing. Aging causes a small, predictable shrinkage, so very tight features may need finishing after heat treatment.

What drives the cost of stainless steel CNC machining

• Work hardening. Austenitic grades (304, 316) harden at the cutting edge when the tool rubs instead of cutting. Machinists use sharp tools, constant feed and no dwell, which means lower speeds than aluminum and more careful programming.
• Tool wear. Stainless is tough and conducts heat poorly, so heat stays in the tool. Tools wear faster, especially on small end mills, deep holes and fine threads.
• Grade. 303 is the fastest to machine. 316 and aged 17-4PH are the slowest. If corrosion requirements allow 303, the part will usually cost less.
• Small, deep features. Small taps (M3 and below) and deep holes over about 6× diameter are a risk in stainless because broken tools are hard to remove. Use larger threads or thread milling where possible.
• Heat treatment and finishing. Aging 17-4PH, passivation and polishing add steps. They are worth it when needed, but do not specify them by default.

Our CNC machining cost guide covers the general drivers, and our tolerance guide explains how to avoid paying for precision you do not need.

Passivation and other finishes

Machining can leave free iron from tools and fixtures on the surface. Passivation is a chemical treatment, typically nitric or citric acid per ASTM A967 or AMS 2700, that removes this iron and helps the chromium oxide layer re-form. It does not change dimensions or appearance noticeably. It is a sensible default for parts in food, medical-style and wet environments.