What is hard milling?
Hard milling involves machining pre-hardened stock (up to 60+ on the Rockwell hardness scale!) as opposed to machining the “soft” material and hardening the finished part.
Why is this useful?
In many circumstances, hard milling allows for faster lead time on parts – you can go right from machining to shipping out the part to your customer. Machining the part before it is hardened requires a shop to either heat treat and harden the part themselves, or send it off to a third party to be anodized before sending it to the customer.