A clogged bathtub drain can be the easiest or the hardest drain to clear in the entire house. It's the easiest drain to unclog when all you must do is clear hair off of the cross bar, or pop-up plunger. It is the hardest when you have to try to get a drain cable to go through the bends of a 1 ½ inch cast iron P-trap.
There are lots of types of tub drain assemblies. I can't give detailed instructions on all of them, so I'll add some photos of the common ones when I get a chance.
Remember that when you see striations on parts where you'd normally see wrench facets, it means that you must hand-tighten and hand-loosen that part. If it won't hand-loosen, try to position your wrench or pliers so that the teeth are not directly on the brass. That can leave scars on the brass and create metal slivers.Even after all these years of being a plumbing contractor in Portland, if I even think about putting "teeth” on brass, I can feel my journeymans eyes watching my every move.
If it comes down to using the drain cable to clear the clogged bathtub drain, the overflow of the tub is the accepted clean-out site. Some overflows have fittings to maintain the grip of the overflow gasket on the back of the tub when the trim plate is removed. The gasket is outside the tub between the tub and the face of the overflow tube.
If you do not have a way to hold the top of the overflow tube in place you must be careful not to dislodge it. That could create more work stopping a leak as the gasket may tear or even fall into the wall.
If the drains are plastic you are not likely to be trying to cable them. If the drains are iron, the cable does not like to make that last turn from the J-bend of the trap into the trap arm. A lot of twisting and pushing with a drain tool at that point may move unsecured drain parts.
Psssst... Sometimes you can snake a clogged bathtub or bathroom sink from the roof. That's only if the drain in question is vented directly to the roof without any offsets. This is the case sometimes when the fixture is on the highest floor, and is set all alone on its side of the room. (It must be away from the toilet so that its vent doesn't join the toilet vent at mid wall.) If this is the case, it can make clearing the clogged bathtub drain very easy. But please be careful!
Plumbing Safety Warning!!
Anytime you get on a roof or use a ladder, you're making the job
much more dangerous. I'm not qualified to instruct you on roof and
ladder safety so I am not telling you to get on a ladder or a roof!