crockett said:
It looks like a screw tip. What size hole and tap for the threads? How did you drill and work the tip? Turn first on a lathe and then drill the hole?
It is Philly collared screw tip as described in Art DeCamp's Pennsylvania Horns of the Trade book.
The tip and collar are a complicated process - I suggest taking Art's courses for the best source of all of these skills coordinated together. He is the master for good reason.
But I will answer your question on the tip as best I can. I have used several methods to make tips of various types.
This tip can be drilled partly through and tapped so that the tap can be used as a spindle in the lathe. Make a center dimple on hte far side so that this can be lined up and drilled through while in the lathe after the exterior shaping on one side is complete. We did a few this way while learning, but I do not do it that way now - had some run-out on one and went to another method.
It can be drilled through initially and gripped by a pen turning mandrel. I do not have this tool, but have heard of it being used by others.
It can be drilled through initially after a shoulder is turned. Then turn it around and lock it into a chuck against the shoulder to continue the fine turning.
All have advantages and limits.
As to the drill sizes and taps, we change these for each project. We simply match them to an original; read descriptions in books (like Art's) or "guestimate" using analysis of photos against known measurements. My buddy keeps a variety of drillbit and tap sets in a rack and we choose them as needed without a lot of consideration of the actual sizes. (I do not see up close well either, so I give it even less attention...)
We are currently working on chasing threads and studying how to make our own taps, but there is a learning curve here...
Hope some of this helps.
CS