Plumbing problems -- how to remove bathtub (cast iron) drain ( cross bars are already broken).

Get some strong helpers first lol or a good hand truck  or cart. Measure the tub and measure possible exits to figure out where you will go with it once it's out. Leave the plumbing to the end if you'll need to wash the area or yourselves. Where the walls meet the tub there is a lip that the walls are hiding. You'll have to remove the nails or screws that are through that lip and go into backing or and wall  studs . You'll have to remove  about 2 inches of the wall to  get to them. With the back, sides and also the front of the tub at the floor loose and the path you have measured and decided on clean up the mess then disconnect the drain. If you cant get under the tub to get to the drain pipe unscrew the drain flange inside the tub and unscrew the drain lever  cover screws if there is one and remove it. Remove the spout by unscrewing it.Just be careful not to tweak the pipes much or joints feet away could be damaged.If it was possible to do this step at first it's  your call.I like to take precautions to prevent leaks which like to turn up when the jobs all buttoned up. If you have room to be safe and  have a cutting torch cut the tub up. Thats the easy way. There could be connectons under the tub and if there are undo them. Remove tub.

short stroke saw-zall. cut interior of drain opening and then angle the saw to cut part of the flange. repeat cut about 3/8 " away. pry out cut section. the rest should unscrew relatively easy, if not, pry strainer away from tub shoe and just pop it out. be careful not to cut into the tub itself. a roto - zip also works nicely.

It's quite obvious the first gentleman missunderstood your question, but the second gentleman didn't. His answer would be the same as mine..You just have to be very careful with your cuts as not to damage the tub itself. A side grinder , if available, works nice for the cuts. Remove the flange part of the drain and removal of the rest of the body should be pretty easy.