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I have done a mistake by adding metal epoxy to a machine screw.
I added that because that screw did not fit. but later it was turned out that it is a wrong screw that I used. Now I cannot unscrew it. I tried adding alcohol, WD40. But it didn't work. Do you have any suggestion to remove this epoxy and unscrew it.
What is the metal you're screwing into part of? Depending on that and the screw material, there's a high probability heating it up sufficiently with a torch (As little as 200F/93C may soften it) will allow you to get the offending bolt out and expose the surface of the epoxy so that you can try to use a powerful adhesive remover to dissolve it. I'm assuming the epoxy has cured, but if it hasn't, you can look up other weaker solvents to use. If you find you can't get enough cleared out with solvents, order the correct closed end hole tap and use either solvents or heat to soften the epoxy while you clear out the threads.
As a backup you can drill the center of the offending bolt with increasing sized bits to get rid of it(a huge pain, but I've done it) and use a pick and or tap and or heat/solvent to clear the bolt threads from the hole threads and then a tap and heat/solvent to prime the threads before using the correct bolt.
Depending on the size of the bolt you used and how concerned you are about ripping the head off it, you may wish to take an equal sized bolt and drive it into something else and intentionally rip the head off it to see what it takes. Automotive impact drivers and torque bars are particularly effective for ripping bolt heads off without destroying your socket. If you don't have an automotive impact driver a wrench can withstand more force than a socket. Knowing how much force you can use to get it moving will help you know if it's time to try more heat. On the other hand, heating the bolt up may allow you to heat the epoxy without distributing too much heat to the surrounding material, but will reduce the force required to tear the head off the bolt.
Depending on whether the metal is tempered or could be ruined by uneven heating, you might be stuck with mechanical removal and/or solvents, but a last ditch option is to drill and thread the hole to a larger size and/or use a thread insert to reduce it back to the size you want.
User @Jason recommends MEK as a solvent from experience if you need to go the solvent route. Whatever solvent you use, you may want to double check it's OK on the metal.
