What RPM should I use for a 50 mm mild steel bar?

With a carbide insert at about 180 m/min: 1000 × 180 ÷ (3.1416 × 50) ≈ 1146 rpm. With HSS at about 30 m/min it is near 190 rpm.

Why is stainless steel run slower than mild steel?

Stainless steel work-hardens and conducts heat poorly, so the heat stays at the cutting edge. A lower speed with a steady feed keeps the tool alive.

Should I use the job diameter or the cutter diameter?

In turning, the diameter being cut (the job). In milling and drilling, the diameter of the cutter or drill.

Cutting speed and RPM calculator: turning, milling, drilling

OperationMaterialToolDiameter (mm)(1.97 inch)Turning: the job's diameter. Milling and drilling: the cutter or drill diameter.Cutting speed Vc (m/min)Leave empty to use a typical value for the material and tool.FeedTurning and drilling: mm per revolution. Milling: mm per tooth.

Spindle speed: 1,146 rpm
Cutting speed used: 180 m/min
Feed rate: 229 mm/min

Typical speeds for good rigidity and coolant. Reduce for long overhangs, interrupted cuts or a weak setup.

How it is worked out

Spindle speed N (rpm) = 1000 × Vc ÷ (π × D), with Vc in m/min and D in mm.
Turning and drilling: feed rate (mm/min) = feed per revolution × rpm.
Milling: feed rate (mm/min) = feed per tooth × number of teeth × rpm.

Questions

What RPM should I use for a 50 mm mild steel bar?: With a carbide insert at about 180 m/min: 1000 × 180 ÷ (3.1416 × 50) ≈ 1146 rpm. With HSS at about 30 m/min it is near 190 rpm.
Why is stainless steel run slower than mild steel?: Stainless steel work-hardens and conducts heat poorly, so the heat stays at the cutting edge. A lower speed with a steady feed keeps the tool alive.
Should I use the job diameter or the cutter diameter?: In turning, the diameter being cut (the job). In milling and drilling, the diameter of the cutter or drill.

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Results use standard formulas and typical values. Use them for estimating and planning; check critical figures against the drawing, the material certificate or the maker's data.