I'm trying to remove the metal box that holds a bathroom exhaust fan into the ceiling. I've got all the wood screws out but there are 2 sheet metal screws that appear to connect the box to the duct to the outside. The problem is the screw heads are inside the duct and I can't access them. All I can see are the threads. Is there a way to remove the screws from the threaded end? I've tried cutting them off with a hacksaw and a cold chisel. I also tried pounding them out with a hammer.
If there's enough sticking out that you can grab on to, a pair of locking pliers would likely help turn it but it will be hard to fully unthread since there will come a point where the pliers don't have enough to grip on to. If you're lucky, you might get enough clearance to cut it or turn it from another part of the screw.
Another option is to simply drill it out and then tap new, slightly larger threads to use with a larger screw when re-installing.
this would probably have been easier if you hadn't hack-sawed off the exposed threaded ends….:D
Now that that's done, I expect your best bet is to try to 'tap' them w/ a screw-removal bit. They're reverse-threaded drill bits made for drilling into the shaft of broken-off screws. The challenge will being able to push hard enough against the screw to get it to bite.
Alternatively, get a set of metal sheers and snip the sheet metal of the fan.
I finally figured it out. I used a Fein MultiMaster to cut the screws down to the sheet metal. One of them helpfully fell out after that. For the other I tried a nail punch but it was stuck too well. So I used a crowbar between the outside of the box and the vent it was attached to. That did it. Thanks for all the answers.