diamond cutting wheels

I dont know that you would use a diamond wheel for that?
Is that what you actually want to do or are you just drawing an analogy?
I think a variable speed drill would be way to slow for a Dremel tool bit of any sort?
tell us what you are actually attempting to do and maybe someone can assist?

Diamond blades come in many different types. For optimum use they must be run with a coolant and at the correct speed and preferably with a constant slow advance. Depends on the material but you could destroy the blade very quickly. A blade made for use with ceramics would last no time at all if used on hardened steel. For the use that you describe you want a very thin (yes they do make them) carborundom disc but you must make sure you do not get any of the abrasive particles into any of the inner workings !

I would only try cutting the bolt head off as a last resort. Have you tried welding another bolt/nut onto the top of damaged one ?

Gday,
Dont do it! Diamond blades are meant for masonary, not steel.
Just use a normal metal cutting blade, They now come in 1mm thick blades and are really handy.
Cheers

Earwig.
What I would do in this situation is find a piece of steel tube with a bore that is the same size as the screw head, cut of a piece about 30 mm long and then drive it over the screw head. then get a die grinder with a stone the same size as the bore and grind down on the broken allen key until it is gone, you may have to replace the tube during the process if it gets ground away. When the key is gone you can either drill out the screw or keep grinding. Eric.

the abovementioned 1mm mini discs for cutting steel are only about 100 mm diam. perhaps you could just use them elsewhere till they wear down to the 40 mm diam you need