Machining hardened steel after heat treatment: what actually changes
Most tool steel parts are still cut soft, hardened, then ground. Hard milling moves that sequence: the part is heat treated first, then finished on a machining centre at full hardness. It removes a grinding setup and it removes the distortion argument, but it is not the same job with a slower feed.
The stock allowance decides the job
Hard milling is a finishing operation. Leaving 2 mm of stock on a 58 HRC block is how inserts get destroyed. Realistic allowance after heat treat is 0.15-0.30 mm on a side for milling, tighter on small features. That means the soft-machining step before heat treat has to be planned around the hard-machining step — including growth from the quench.
If your drawing goes to a shop that only quotes soft machining, this allowance is the first thing they get wrong, and you find out six weeks later.
Tooling: CBN and coated carbide, not HSS
Above roughly 50 HRC, high-speed steel is out. The working set is:
- Solid carbide with AlTiN or AlCrN coating — the workhorse to about 60 HRC, ball nose for 3D surfaces.
- CBN inserts — above 60 HRC, and for hard turning of bores and shafts.
- Negative rake, heavy edge prep — a sharp positive edge chips instantly in hardened steel.
Depth of cut goes down, surface speed goes up, and the machine has to hold that without chatter. A 20-year-old mill with worn ways will not produce a Ra 0.4 face at 60 HRC no matter what tool is in the spindle.
Rigidity matters more than horsepower
The failure mode in hard milling is vibration, not lack of power. Cutting forces are lower than roughing mild steel; what kills the finish is deflection. That pushes the job toward shops with:
- thermally stable, well-maintained machines,
- short tool overhangs and shrink-fit or hydraulic holders,
- fixtures that clamp on solid material rather than on thin walls.
When a shop quotes hard milling from a photo of a machine list, this is the part you cannot see.
Air blast, not flood coolant
Flood coolant on an interrupted hard milling cut thermally shocks the CBN edge and cracks it. The usual setup is dry cutting with compressed air or a minimum-quantity lubrication mist. If a quote mentions flood coolant on a CBN finishing pass, ask why.
What this means for your RFQ
Three lines on the drawing change the price more than the geometry does:
- Hardness at the time of machining — “58-60 HRC, machined after heat treat” is a different job from “machine soft, we harden in house”.
- Where the tolerance actually lives — one bore at ±0.005 mm is fine; every dimension at ±0.005 mm doubles the inspection time.
- Surface finish on functional faces only — a blanket Ra 0.4 on all surfaces is the most common source of inflated quotes.
Send the drawing with those three answered and you get a comparable quote from every shop instead of three numbers that describe three different jobs.