Cutting External Threads on a Screw tip Horn
(credit to Art DeCamp)
During the Conner Prairie Horn class, Art DeCamp had a simple but ingenious way of solving the problem of ragged, crumbly external threads on a screw tip horn. In emails since the end of the class we discussed the problem and Art made further suggestions to me, knowing that I wanted to set up my lathe for making more. He also suggested that I could share his method on ALR and sent me photos that he has used for instruction.
The methods offered below are modern threading techniques. Art wished me to stress "that this method is not the authentic "old time" way of chasing threads. It is a modern expediency that allows 21st century horners to make good threads with tools that are generally available today."
It is my assumption that you are somewhat familiar with lathes. You should know how to adjust the tool rest and choose the best tool for the cut you are making. Without this knowledge and also a SLOW turning lathe, you will have difficulty.
The process of preparing to thread a horn neck begins by sawing off the tip. We will need .600” for the threaded section (5/8 x11). We shot for 3/4” Next we drilled the neck for either ¼” or 5/16” tap. From here on we will deal with 5/16. A drill bit #F (.257") is used for 5/16 X18. (17/64" will work.) Photo #1 shows the horn tip cut off with the tap. Turn the tap into the horn and leave the tap in, for the next step.
Photo #1
In this step the 5/16” tap is chucked in the lathe. The lathe speed needs to be SLOW. Art’s lathe is variable speed, and he thinks the chuck is turning 80-100 rpm. (My lathe’s slowest speed is about 800 rpm. I am working on a reduction in speed to approximate Art’s turning speed.)
Using a variable speed, we set the control to zero, turned the lathe on, and then adjusted the speed until we were comfortable. Too fast and you might snap the tap. This is why Art likes 5/16 x18 over ¼ x 20. Be very careful not to bump the horn as it spins. There is a lot flying around. A bump could mean a broken tap - big trouble to remove .
The first cutting is to face off the end of the horn tip with a parting tool. This will give you a clean flat end to the horn. You have to stop short of the tap. Once you have faced off the tip at least part way, you can finish this by hand after the tap is removed.
Next turn the part of the horn that will be threaded. For a 5/8 x 11 thread we turned to a diameter of .600”. This needs to extend ½ “. The shoulder behind the threaded part was turned to ¾” This is somewhat arbitrary based on the horn.
Photo #2 shows three things to look for:
1. the faced-off that stops short of the tap
2. the .600” step for the threads
3. the ¾” shoulder beyond the threads.
The area beyond the 3/4" shoulder will be cleaned and shaped. Art starts with a draw knife and works from the tip. It works fast and very well.
Photo #2
Photo #3 shows the tools used in the threading step. Notice the threading guide- that looks like a stepped bolt. The large diameter end is a 5/8 x 11 bolt., matching the die you’re about to use. The small end is threaded to match the tap you used in threading the tip, probably 5/16. Screw the 5/16” end into the horn tip.
Photo #3
Photo #4 shows the horn ready to tap. Thread the die unto the threading guide. Spin the die up to the horn. You’ll note that the die spins easily and is oriented to smoothly begin the threading of the horn. A little lube will help. At Conner we used oil. At home I use lanoline or Crisco.
Photo #4
After lubing, turn the die unto the horn. If the die is sharp, it should cut like butter. When you reach the shoulder, spin the die off, reverse it and spin the die up to the shoulder with the back side of the die leading. This step will cut the threads up closer to the shoulder.
Photo #5
That should about do it. Using the threading guide insures a straight shot at the horn and should leave you with clean, crisp, well formed threads. Threading guides are fairly easy to make. I made one for 1/4 to 5/8". I plan to make one using 5/16 to 5/8" to have a larger, stronger tap.
Threading and fitting the tip is not covered here. There will be some hand work when doing the final fit.
My thanks to Art for his instruction during the class and his advice in emails since the class was over. I claim no credit for this threading advice. I’m simply the messenger. Thanks for everything, Art.
Regards.
Larry Pletcher
