It turned out to be a little on the light side for me. Being the poor grad student that I am, I started thinking about how to make it heavier, rather than just buying a heavier hammer. First, I duct-taped a 2.5 lb plate to the top. That works pretty well. Since I'm not actually striking anything with the hammer, I think it will stay put. As an added bonus, now the top of the hammer has a nice flat surface to rest on.
But then I thought, it would be nice if there were a convenient way to adjust the weight on-demand. As it turns out, the handle gets thicker near the head of the hammer, and I can slide an olympic plate down the handle, and clamp it securely with a standard spring collar. Presto!
I'm a little concerned about whether the handle can manage the extra weight though, depending how much I add. I wouldn't want the head to go flying off when I'm swinging it in my apartment. That would be bad.
I have a few ideas for an improved design. One way is to get a ~3.5 ft (sledge hammer length), 2" diameter (olympic plate hole diameter) metal (iron?) pipe. I'll put a permanent fixture on the end, which the plates will rest on. I can then use a standard collar to keep the plates from sliding down the handle. The diameter is a bit thicker than a sledge handle- I think it will make it more interesting by challenging the grip more.
You could go with a smaller diameter though, and maybe use some kind of fitting to get 2" diameter on the end. Actually, this is how a normal olympic bar is constructed. The part you grip is narrower (1"?), and there's an adapter to make the ends 2" thick. You might even be able to get a short bar, and then just put a standard adapter on one end of the bar. I think this route might be a little trickier, and is probably more expensive.
I'm probably overthinking the whole thing, just thought I'd share
