Very little is simple and without complications when you are making replacement parts for an 1881, Baldwin built, 2-6-2 Prairie type steam locomotive and I would not have it any other way. The fabrication of four new brass oil cups for the “EMMA NEVADA” is a case to illustrate the point.
The Baldwin oil cup design is simple enough: a hollow brass casting which needs only threading and polishing to produce the finished oil cup. Not so for the new oil cups. Because of the handsome number of Dollars asked for castings, it was decided to make the new oil cups in two pieces and silver solder them together. In order to make sure of sufficient wall thickness in the finished oil cup, a 10x scale layout of the two pieces was used to design the separate pieces. The machining and assembly of the “in-the-rough” oil cups was without incident, if one ignores the minor melting of one assembly during the soldering process. The time had come to face-up to the “what-in-hell-thread-is-it” question that had nagged us from the start.
Using the “wisdom” this eighty one year old machinist has accumulated, I started with the assumption that I had no hard understanding of the thread’s O.D. or pitch, or even if it was a sixty degree profile! The root cause of this softness was the pitiful condition of the threads in the locomotive frame and of the old oil cup we had as a sample part. The drawing available to us informed us it was a 7/8 ” O.D. x 14 t.p.i. thread. After much fiddling with my old and beautiful Lufkin thread gage and my equally old and beautifully worn Starrett one inch micrometer, I decided to go with a 3/4″ x 14 thread. I could not rule out a 55 deg. profile but 60 deg. would be easier on me. Finally, the “hard understanding” was at hand. Or so I thought.
After consulting our plentiful library of hand books and after rooting through our equally plentiful supply of historical (aka old and dull) taps and dies, I realized I was stuck with a bastard thread! Oh well.
I threaded one oil cup using our faithful 16 inch lathe, hewing closely to the proper dimensions including the flats on the root and crest. I then walked over to Grizzly Flats where folks there persist in the mistaken belief that 3 ft. track gauge is normal. It should be noted there is a similar problem with the folk in Barn One involving 3 1/2 ft. gauge. Upon arrival in 3 ft. land, and with the confidence that stems only from the certain knowledge that your assumptions are correct, I screwed the oil cup into the frame. Or,to be truthful about it, I tried to screw the oil cup into the frame. In vain, because the oil cup went about two turns before becoming hand tight. I Returned to the lathe with my head held high in despite my dismal two turn engagement failure;this mainly because nobody else was there. I removed “a little” from the now suspect thread, returned to 3 ft. land and got about three, or so, turns to hand tight. After several more of these cycles of failure, I stopped because the thread was approaching a sharp Vee profile. It took me several weeks to recognize the now obvious fact that a sharp Vee thread is what was wanted because that is the way threads were in 1881! A litle older and wiser.
Sharp Vee threads, and a good cleaning of the frame threads, did the trick.