"MiamiCuse" writes:
I bought a 4-1/2" angle grinder from Harbor Freight, I don't have much
grinding to do so I just wanted to get a cheap one and use it for this
project which requires grinding off a few door hinges that has been set into
metal door jambs that extended into the slab. The grinder came with metal
disc but I also need to make some cuts in concrete so I went and got a
4-1/2" diamond blade from the big box store only to find that those discs do
not fit...apparently the metal grinding disc that came with it has a
depressed center so that the locking nut can be secured from the outside.
If I put in a flat disc there is not enough room for the lock to catch -
well even if there was I am not sure it's a good idea as the lock may break
the thin disc.

I have never used this type of grinder before does it mean I need a special
depressed center diamond cutting blade or do I need a special fitting to use
standard blades?

What happens when you tighten the grinder's shaft nut? Does the diamond
wheel still rotate? Try flipping over the nut - does it snug down on
the diamond wheel now?

On many grinders, the washer that sits against the gear case has a small
raised boss that is designed to keep the grinding wheel properly
centred. The nut has a similar raised boss, and it's supposed to face
the washer when using thick discs. But with thin disks, the two raised
areas contact each other before the disc is clamped. The other side of
the nut has a recess in the same area, so flipping over the nut allows
it to clamp even very thin discs.

All of this is independent of whether the wheel has a depressed centre
or not. The thick wheels intended for grindng on the face of the wheel
always seem to have the depressed centre, presumably so you can get the
wheel face down almost parallel to the work without striking the nut.
But abrasive cutoff wheels, where the edge does the cutting instead of
the face, are available with depressed centre or flat. And the edge-cutting
diamond wheels always seem to be flat.

Dave