When I was trying to design my 6″ gate valve I ran into the problem of – where the heck do I find something small enough to look like a valve stem for something that small. I found a chart for Kennedy AWWA Gate Valves where they show the stem dia for a 6″ valve is 1-1/4″. In 1:48 we are talking about a threaded rod that is only 0.026″ dia. Woah.
Very small dia threaded rod can be found. You can purchase #0000-160 (0.021″ dia) .. for about $50 a foot! I saw that and quickly decided .. uhhhh … no. (on an aside that is 160 threads per inch. If we reduce that to 1:48 that is something like 3.3 threads per inch)
I had fair success in taking a single-cut file and rolling it across a piece of 0.020″ solder. Holding the file at an angle (“Single-cut files have rows of teeth cut parallel to each other at an angle of about 65 degrees from the centerline.“) the teeth empressed grooves into the solder that looked very much like threads. The problem with this method is that you are restricted by the tooth-spacing of the file you are using – this of course relates directly to the ‘threads-per-inch’ of the pesudo-threaded rod we are trying to create. A ‘Smooth Cut’ has the greatest number of threads per inch – and a 6-inch file has more teeth per inch then a 12-inch file.
This running around in circles brought me finally to Acme threads. This is what would be used on a valve stem. The Acme thread was a development of the square thread. It has a 29° thread angle with a thread height half of the pitch; the apex and valley are flat. A 1, 1-1/4″ dia rod would have a Pitch of 1/5 (5 threads per inch or .200″).
Now .. at 1:48 – one inch is 0.021″ (rounded). Dividing that by 5 we find that the pitch would be 0.004” .. woziers!