1) Take a wild ass guess as to how, how much and how long 2) Lose my ass at first 3) Re-adjust just about everything about the job (how to do it, how much, how long) 4) Hit it again with some experience under my belt.
And I thought no one else used my secret method!
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We used tons of AR400 at work, with a little AR500 thrown in. Neither are pleasant, both are somewhat machineable.
AR500 hardness in Brinell: 450 minimum / 500 nominal.
AR400 hardness: 360 minimum / 400 nominal.
Both are quenched & tempered products. If your material is 1" or less in thickness, it will be hard from top to bottom. If 2" or thicker, you'll have 1/2" of hard plate on the top & bottom, the middle will be easily to machine.
Drilling with HSS bits isn't bad. Use strong flood coolant, speed around 10-12 SFM, as much feed as your machine & drill point can stand - .025 IPR is a typical number. The drill point should never squeal until it starts to dull. Grind the cutting edge for the least relief possible to avoid chipping. Thin the web to reduce drilling pressure.
Tapping with HSS taps, even hi-tech coated taps, is nearly impossible unless you spot anneal with a small rosebud.
Milling, like drilling, can be done at reduced speeds. Take your normal carbide speed & reduce by 1/3 to 1/2.
Pretty much sums it up.
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Barry Milton