I have in mind some engine building with my new lathe. Actually that was the original reason for building it (uh if I remember that 5 years ago). I am thinking now about a long cherished project requiring boring a cylinder of about 4" dia by 7" long. For anything smaller I could just have made a boring bar to run between centers, but for a 7" long cylinder, I need maybe 16" of carriage travel on what is a 12" lathe. That's 12" between centers, with the tailstock in place.
If I remove the tailstock, I'll have sufficient travel, so I'm thinking about bolting a vertical arm into the end of the lathe shears. Those shears are two solid 3/4" x 4" vertical steel members. This arm would carry the boring bar thrust bearing.
For the arm, I'm thinking probably a 3/4" thick Zamak casting. I could probably do it with a 3/4" rectangular steel plate....... but, nah, it would be boxy looking and I think I could do a nicer shape with a casting. I don't have a cnc plasma cutter for fancy shapes.......well, nor any 3/4" steel plate. I do have the zinc, and wood for pattern making and a foundry furnace ready for a pour. Guess that settles that.
My lathe's center height is just a little more than 4-1/2" so the arm wouldn't need to be be very tall. Probably I'd slot it for mounting the bearing to allow vertical adjustment. I'll probably indicate the boring bar in before use. I'm expecting infrequent need for this rig, so adjusting the initial setup won't be a big pain. I could even make a gauge for that.
Not quite sure what to use for a bearing -- maybe just a plain sleeve or oilite bearing, I guess, since again, there will be infrequent use, and the attachment will probably kick around in a drawer for long periods. Fancier bearings might suffer there. I don't imagine there's going to be a lot of plain bearing wear.
I dunno, that's the idea, anyway.