High-quality black-and-white photographs of large old machines and tools

[Frank S]

Making large diameter threads is not much of a task. I've made both internal and external 18" diameter threads 4 TPI on a 16" gap bed lathe using a homemade 20 inch diameter face plate instead of a chuck to take advantage of more of the gap Obviously not hex stock. One of the strangest threads for me at least was when I made a 3 start 8 TPI thread full length on a 4inch diameter bar and the nuts to go with it.
For those who don't know doing a 3 start *TPI thread you only make the cuts as deep as if you were doing a 24TPI but you must be precisely 120° between starts

[Isambard]

"For those who don't know doing a 3 start *TPI thread you only make the cuts as deep as if you were doing a 24TPI but you must be precisely 120° between starts"

Memory fade Frank?
Thread depth is as normal. However the lathe must be capable of thread cutting at 1/3 of the tpi. i.e 3 start 12 TPI would cut at 4 TPI.

[Toolmaker51]

Making large diameter threads is not much of a task. I've made both internal and external 18" diameter threads 4 TPI on a 16" gap bed lathe using a homemade 20 inch diameter face plate instead of a chuck to take advantage of more of the gap Obviously not hex stock. One of the strangest threads for me at least was when I made a 3 start 8 TPI thread full length on a 4inch diameter bar and the nuts to go with it.
For those who don't know doing a 3 start *TPI thread you only make the cuts as deep as if you were doing a 24TPI but you must be precisely 120° between starts

All the more reason to have a degree wheel, or degree tape prepared in advance; it'd be utterly correct to have a vernier or some means to locate quadrants accurately. Something tells me a Pi tape (easily determines .001 -.0005 accurately) and a fine tipped pointer is just the ticket. Not much reason beyond 2-3 and 4's.