Occasionally, I have trouble separating the anamorphic attachment from the front of the prime lens for movie projectors.
Either some idiot (me) cross-threads them or screws them on too tight. Sometimes repeated heating and cooling cycles make the parts stick together. (Yes, movie projectors get hot! Hot enough for the focus to drift out of whack as the movie plays.)
I have two different ways to un-stick stuck parts like that.
1) I wrap a leather strap around the offending parts.
2) I use differential heating and cooling.
If you wrap a leather strap around an object it will progressively tighten as you try to turn them. Carefully done, the pressure will be kept even around the part so as not to throw it out of round. Alternatively, you can get a strap wrench from Lowes, Home Depot, etc.
Heating and cooling can often help. As parts expand and contract you might be able to get them to loosen easier. Don't use a torch! Use a heat lamp or a blow dryer. Just be careful! If your lens elements are cemented together with balsam, too much heat will F-them up, right quick!
Regardless of the method, remember: "Slow and steady wins the race!"
Work steadily and methodically. Don't get in a rush and try to keep your head. If you get in a hurry or get peeved off you're more likely to make a mistake and risk destroying something.
Don't ask me how I know that...