Learning surface grinding
Hi Nick:
It's a bit difficult to give you decent advice without knowing a bit more about just where you're at on the learning curve.
Have you seen a knowledgeable operator using one before?
Have you used one before or been trained on one at all?
Have you read any literature on how to operate a surface grinder?
Grinding is an art like many other subspecialties of the metalworking trades...it's not too difficult to throw a block onto the magnet and make sparks, but there's a lot more to it and a really experienced hand can make a job that's a frustrating pain in the ass for you, look totally easy.
Your best bet by far is to get some one-on-one training by a toolmaker, preferably a specialist grinder hand...you'll learn more in a day with a guy like than than you will in years of farting about on your own.
Try to find a retired guy to help you out and don't forget to pay him!
Regarding your specific questions, you can choose any good tool steel, or even have mild steel case hardened; machinistrrt's suggestions will work fine.
My personal preference for making measuring and gaging equipment is A-2 toolsteel; it's air hardening and one of the most stable toolsteels especially if it's cryo treated after tempering.
It's capable of very high hardness, has good abrasion resistance (but is still reasonable to grind) and is tough enough to tolerate being knocked about a bit.
17-4 PH is a poor choice; it doesn't get nearly hard enough for a reference block.
I typically mill my blocks 0.005" per side oversize and almost never grind before heat treat.
I glass bead the blocks after I get them back to take off the firescale (keeps the wheel from glazing over).
Rough cuts are typically 0.001" downfeed and 0.025" or so crossfeed per stroke until I'm within 0.001" of target dimension.
I dress frequently and run coolant where ever possible.
Finish passes are typically very light...0.0001" if I need good precision and a perfect surface.
I dress the wheel with a nice sharp diamond at a fast traverse rate aind a 0.0005" DOC so my wheel is nice and open and will cut cool and free.
Wheel selection is a whole 'nuther topic for a future thread.
Making parts rectilinear is most commonly done by processing them against a known master...a grinding vise, an angle plate, a mag square.
There are other ways...if you know how to measure a rectangular block for squareness, you can compensate for deviations by grinding a step in your part that will tilt the part in the direction you wish to compensate, then flipping the block so the step side is against the magnet, grinding the second side, then flipping the block again to eliminate the step.
It's all very tedious which is why toolrooms spend thousands of dollars for good quality master square devices.
To get an idea, Google for Hermann Schmidt; they make very very nice stuff specifically for this purpose, but it's also very very pricey.
Trying to get a pair of surfaces orthogonal to within 0.0001" in 6" is not easy, and the price reflects that reality.
On a last note: grinders are among the most dangerous machines you'll ever encounter in a shop.
They look so innocent but they are utterly intolerant to fuckups.
Be extraordinarily careful, get into a routine that has you turn the mag chuck on and then immediately check the block to see if you can shift it on the chuck.
Do it every time...it's got to become a habit.
Make damn sure you don't rack the wheel into the job...the friction goes up orders of magnitude with only a very small increase in DOC, and will rip the job off the chuck and usually explode the wheel too.
Never stand a tall skinny block up on the chuck and expect to kiss the top of it...it's got to be packed properly or held in a vise or clamped (securely) to an angle plate or it'll take off like a rocket.
Grinding wheel burns are really, really painful and slow to heal...don't stick your fingers where they don't belong.
Lastly wear your safety shades...always...no exceptions.
Cheers and good luck
Marcus
www.implant-mechanix.com