Hi. I ran into a strange problem recently. I was trying to make a back plate for a chuck on my recently acquired South Bend 9 lathe. It required a 1 1/2-8 thread for the spindle nose. Unfortunately, my Craftsman 109 does not have the correct change gears for this thread. I tried an old blacksmith's trick for making a thread. It doesn't have to be very good. It just has to be good enough to get the 4 jaw chuck on the lathe so I can use it to thread a real back plate. This is done by cutting slots in a piece of plate that have rise to run of 1/8" over the length of the plate which is the circumference of the thread. Then, the assembly is rolled up and pushed into a pipe and forge brazed, or in this case, tig welded. Of course, the thread doesn't fit, and it is no good to force it. The little Craftsman 109 can chew out little pieces of the thread without gears, but unfortunately the plate, being from a scrap bin, was not mild steel, and it got too hard in some of the heat affected spots. I tried to use blue to find them, but this is very hard. According to posts on the Internet, the non-drying hi spot blue paste is best for this. This was not the case, in my experience. The trial female thread would be uniformly blue, but after a tight trial fit, none would get rubbed off. Dykem layout fluid was considered inappropriate by the Internet posts, but it seemed to work better. It still was not good enough. There were still tight spots, and neither bluing could find them.
Here's what worked. I took an old nail and forged it into a short scroll which fit exactly into a thread. This is a female thread, so thread wires won't work. I then cut it off from the nail and used calipers going around the thread to find high spots, just like using thread wires. This worked. After knocking down two or three high spots with a dremel cutoff wheel, the thread went right on! Attached are some pics. This is really a sad lowly looking tool, but blacksmiths like stuff that works.
Here's what worked. I took an old nail and forged it into a short scroll which fit exactly into a thread. This is a female thread, so thread wires won't work. I then cut it off from the nail and used calipers going around the thread to find high spots, just like using thread wires. This worked. After knocking down two or three high spots with a dremel cutoff wheel, the thread went right on! Attached are some pics. This is really a sad lowly looking tool, but blacksmiths like stuff that works.