Stainless steel is chosen for corrosion resistance, strength or hygiene, and every one of those properties makes it harder to machine. It work hardens under a rubbing tool, it holds heat at the cutting edge, and it is gummy in the grades that resist corrosion best. Expect to pay more per unit of material removed than for any common aluminium.
Why it is slower to cut
Stainless work hardens rapidly, so a tool that rubs rather than cuts creates a hardened layer that the next pass has to get through. It also has poor thermal conductivity, so heat stays at the edge rather than leaving in the chip. The practical consequences are lower speeds, positive rake tooling, rigid setups, consistent feed and never letting a tool dwell in the cut.
Grades and how they machine
Free machining grades with added sulphur cut far more easily and are used wherever their slightly reduced corrosion resistance is acceptable. The common austenitic grades machine moderately and work harden readily. Duplex and precipitation hardening grades are tougher again. Martensitic grades can be machined soft and hardened afterwards, which is often the cheapest route to a hard part.
What it does to the quote
Longer cycle times, more tooling consumed, more rigid fixturing and often a slower finishing pass. On a part with a lot of material removal the difference against aluminium is large. Where the corrosion or strength requirement is real that is simply the price; where it is habitual, the material choice is worth revisiting before the part is quoted.
Specifying it sensibly
Name the grade and the condition, because the difference between grades is bigger than the difference between suppliers. Say if a free machining grade is acceptable, since it can cut cost substantially. Keep tight tolerances to the features that need them, because stainless moves with heat during cutting. And say whether the part will be passivated, since that is a separate operation with its own requirements.
Questions people ask about cnc stainless steel
Why is stainless more expensive to machine?
It work hardens, holds heat at the cutting edge and wears tools faster, so cycle times are longer and tooling costs more per part.
Which stainless grades machine best?
Free machining grades with added sulphur, by a wide margin, where their slightly reduced corrosion resistance is acceptable for the application.
Does stainless need passivation after machining?
Often yes, to remove free iron picked up during machining and restore the passive layer. It is a separate specified operation, not automatic.
Can I substitute aluminium?
Only if the corrosion, strength or hygiene requirement genuinely allows it. Where it does, the machining cost difference on a heavily cut part is substantial.