I’m trying to repair an office chair. One of the casters won’t stay on because the threaded hole into which it screws (part of a metal starfish into which all five casters screw) is stripped. Is there any way to repair the thread?
Drill and tap a new hole 1/2’’ or so away.
The most satisfactory solution will probably turn out to be, sob, buying a new chair.
If it’s a machine screw thread you may be able to use a Helicoil to repair it. The repair kit has a drill to cut away the old damaged threads and a tap to make larger, fresh threads. A spiral of diamond profile wire is threaded into those threads and the original machine screw is threaded into that. Look at an auto parts store and bring the original screw to check the thread size.
As an alternative, there’s JB Weld. Not certain it’s suitable for your application, though.
How thick is the material that is stripped? If it is thin (say less than 3/8") a heli-coil ain’t gonna cut it.
A nut-sert will work on sheet metal or other thin material, but the installation equipment probably costs more than a new chair.
There are some chemical products for repairing a stripped hole (scroll down to find Permatex® Stripped Thread Repair) but I have no experience with these items
Best single bet is to tap out the hole to the next larger size and replace the screw with a larger one. You will have to buy a tap (and maybe a drill bit) and a new screw.
Good luck
JB Weld would be the easiest & cheapest solution. It’s a 2-part epoxy which, after it dries, is so hard that it can be machined or drill’n’tapped.
I assume the caster will never need to be removed from the chair in the future.
Unless you have some experience w/ this type of repair, it would probably be best to take it to a small machine shop. If the value of the chair justifys it, I would remove the base and take it, w/ the wheel, to such a shop. I’d guess they could fix it for under $50.00. A few phone calls could determine if a shop is willing to do the job.
My problem is usually finding a stripped screw hole! 
If it doesn’t ever have to come off again, you might want to just weld it on. My friend and I are software developers by trade, but together we own a light electric welder that we bought for fun. Perhaps if you ask someone you know will turn out to own a welder.
I hird the JB Weld idea if, as Rick said, the metal is thick enough. If thin, you may consider JB along with a lead wall anchor (like for conc. block). Drill the hole a bit larger, JB the anchor in place and reset the castor with a self-tapping metal screw.
Please let us know what you end up doing. So often we get no closure with these things. 
Correct me if I’m wrong, but the caster stays in place as long as you don’t lift the chair off the floor? You may be able to get away with using JB Weld to just glue the caster in. If you want to get fancy, you can fill the hole with JB Weld, then drill and tap it, but that doesn’t sound necessary to me. It doesn’t seem to be a really critical repair.
I’m going to make a few assumptions about your chair here, you can confirm if any of them are correct.
1.) The caster has a threaded stem, probably a 5/16" or 3/8" diameter.
2.) The chair base is a relatively thin hollow metal, possibly aluminum, assembly with matching screw holes.
3.) The threaded hole is stripped (OK, that’s a given).
Since the caster is only weight bearing from above then nothing is trying to pull it out except gravity when you pick up the chair. So the repair doesn’t have to be super strong, but you need a good fit to keep the caster from buckling. THe best method would be to thread a nut and washer on the caster stem from inside the chair base, but that’s likely impossible without damaging the chair base. Second best would be to epoxy, weld, or other wise permanently affix the caster. Third would be to buy a new chair.
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Sorry to post and dash. I had to drive from Columbus to Detroit (which I expected). When I got there, relatives were waiting (which I did not expect).
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The chair is at my ex’s place (in Columbus), so I am going on memory now.
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The caster pretty much stays in place unless the chair is lifted. But it won’t screw in enough to balance with the other casters. So the chair is unstable, which means that it gets lifted a lot.
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The caster never needs to come out again. In factm, I wish it wouldn’t.
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The metal chair bottom seems to be around 3/8". The threaded part appears to be a separate assembly that is welded into the bottom.
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The caster indeed has a threaded stem.
I’d clean off the the mating parts with IPA (isopropyl alcohol-not India Pale Ale) or lacquer thinner to remove grease, dirt, bond breakers, and then use JB Weld. Let the base sit until the mix has cured. Maybe drink an IPA while you wait.
Thanks!!