Rethreading opening after damaged by stripped knob

I have a chair that has a knob with a screw in type extension (about 1/4 inch in diameter). The threads on the screw in portion of the knob became stripped, and I obtained a new knob. However, the opening where the screw in extension belongs has become damaged and it needs to be rethreaded. I am pretty sure there are tools that will rethread the opening, and I am wondering if they are adjustable or if I have to buy a specific tool for the specific sized opening that I am dealing with.
Also, if it is simpler to just take the chair to a hardware store and get this small opening rethreaded I would be happy to do that. I just don't know where to start, and am looking for good tips.
Thanks,
JD
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Decij wrote:

The tool you need is called a "tap". The sizes of taps are specified by diameter and threads per inch...1/4 x 20 for example. You also need a tap wrench to hold the tap.
The socket in your chair may or may not be able to be rethreaded depending on how much metal there is. You will have to enlarge it by drilling to the next size bolt thread. For example, if the old thread was for a 1/4 bolt you will have to drill it out to use a 5/16 bolt. The drill you would need to use is *smaller* than 5/16 though so that when tapped there is sufficient metal in the hole for the tap to cut threads.
An easier solution for you may be to get an epoxy that sets hard, turn the chair upside down, pack the existing hole with epoxy, insert the new knob, wipe off excess epoxy and let the epoxy cure undisturbed for 24 hours.
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dadiOH,
Thanks a lot for your help. The chair I am dealing with only cost $25 at Sam's Club, but it has been a good chair for more than three years. The socket where the know extension goes seems to be damages only at the beginning (the extension did not reach very far into the extension). Is there some way I can just get it started without changing the thread size--buying several tools to fix the chair doesn't seem to be worth it. Also, the expoxy may not work because the knob does not go that far in.
Thanks again,
JD
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Decij wrote:

I'm taking a WAG that this knob may hold some essential part of the chair (like the backrest) in place, in which case you want the fix to be STRONG, or someone may get hurt, like if it fell off when they leaned back.
If that's it, you might want to take that knob down to the hardware store and see if you can find a bolt with the same sized thread, but somewhat longer threads, and a few washers to stack under the bolt's head if need be.
You can probably get the bolt you buy started in the mangled hole by filing its tip down slightly, and maybe even filing a couple of grooves at the tip to turn it into an erzats "self threading" screw.
If that works, you'll be tied down to using a wrench to tighten it, but if it's like most chairs, it won't have to get adjusted very often.
Better safe than sorry...
HTH
Jeff
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I would have thought that would have been quite an obvious & easy solution.

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Take the knob to the hardware store and get a tap the same size as it. Carefully screw the tap into the hole after putting a little oil on the cutting threads. Screw it in 1/2 to 3/4 turn, then back it out half that. Keep doing that untill the threads are clean. Chances are, that will do the job unless the threads are too messed up.
The next best step would bew to helicoil the hole. If you have to buy it, the helicoil set will cost more than the chair, but it would be a permanent fix.
I don't think the epoxy idea would last very long.
Bob
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