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[What follows is specifically about the MTD mulching lawnmower model cited in the title of this post. Much of it is applicable to any rotary mower.]
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Here's a view of the blade's fastening underneath.
That bolt head takes a 5/8" wrench. A shallow 1/2" square drive socket wrench on a breaker bar is ideal for loosening the bolt. Hold the blade end steady with a rag to save wear-and-tear on your hand. (The bolt will be quite tight, but I've never encountered a seized one.) Unscrew the bolt and the big washer and the blade will be free to come off.
Here's a view of the blade fastening hardware off the mower.
The bolt is SAE grade 8; 3/8" - 24 x 2 1/2", with an oversize (5/8" A/F) head. (The standard hex head size for 3/8" bolts is 9/16" A/F.) At reassembly,[1] it doesn't hurt to apply some oil or grease to the bolt's threads as an anti-seize measure.
This old blade is in rough shape. It looks like it was never sharpened while it was in service, and it's been outside rusting on an idle mower for a few years. Here's a view of one of the blade's ends.
That's a grass flogger, not a grass cutter. Here's a blade end after a lot of grinding with a belt grinder.[2]
It's still not perfect and never will be, but it will cut grass.
That upward-curved portion of the blade complicated the grinding job a bit. I take it that that's the part of the blade that does the mulching.
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Notes:
[1] Pay close attention to getting the blade back on the right way up. Inadvertently flipping the blade is an easy mistake to make. I once picked up a perfectly good mower from the side of the road that turned out to have its blade on upside down. I suppose the owner got fed up with the poor cutting action, and discarded the mower.
[2] A belt grinder with a 1" wide belt is the best tool I know of for this sort of sharpening chore -- far superior to a wheel grinder. See this post for more information on a suitable belt grinder.
A lot of sharpening instruction I've seen makes much of checking and correcting blade balance. You'd have to do a pretty uneven grinding job to affect blade balance to any significant degree. Blade balance is not something I fret about; I just try to do a reasonably even grinding job.
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