I have a couple solid-wood-legged barstools that I want to temporarily (for a year or so) shorten by 6 inches. Is there any good way to cut the legs so they'll be more stable upon reattachment? Or any good way to reattach them solidly?
This sounds like a problem that's best avoided rather than solved.
I'm picturing a "classic" wooden bar stool where the legs are not vertical, but tapered out to be wider at the bottom than the top, e.g.:
This means that the legs experience a bending load when someone sits on the stool (imagine someone trying to pull the legs wider apart at the bottom).
The parts of the legs that help resist this bending load are the wood fibers around the outside (circumference) of the leg, the center of the leg doesn't do much (and doesn't need to).
If you cut the leg and re-attach with a hanger bolt or double-ended lag bolt, you've prevented the critical parts of the leg from doing their job--you've severed the outside fibers, and attached the leg only in the center. You can imagine that a leg put back together in this way wouldn't be hard to break over your knee. Therefore, it wouldn't be great in resisting the bending load that it needs to. It'd be fine as a plant stand, but I'd be sweating if Norm stopped by for a drink.
The only solution I can think of to retain the bending strength is some kind of a slip-fitting like you'd find on a patio umbrella or kayak paddle. By the time you're done buying and installing all the slip fittings, you'd probably be better off buying a second set of stools.
In theory you could cut off the lower end of each leg and later reattach using a double ended lag bolt (really a screw):
This type of reconnection should be a one time thing.
For repeated removal you could use a fastener called a "hanger bolt" that has a lag screw on one end and a machine screw on the other:
The screw end would be placed in the cut off section of the leg and the machine screw end could be threaded into a T-nut type fastener set into the chair side of the leg:
However, stools take a lot of weight and are subject to lateral pressure (especially if the legs are splayed). I would be wary of trying any reattachment unless the stools are hardwood, and the bolts and t-nuts would need to be long enough to give a very solid connection. Also the cuts should be very clean and flat so that the cut faces mesh well, and the heads of the t-nuts should be recessed to avoid a gap.
If the reattachment is to be permanent, gluing the faces of the cut ends, in addition to the lag screw or hanger bolt, would substantially improve the strength.
I bought pairs of bar stools at Menards for less than $40. Also from barstools.com for less than $20 each. I'd buy some shorter ones for the time you need them and store the taller ones. When you're done with the shorter ones, put them on a garage sale or give them away and bring the taller ones back out. That way your taller ones won't be messed up.