When I made my first call 15 years ago, a short box, there wasn't much info on the Internet on how to make a box call. I did have Neil Cost's video on making box calls. Very helpful video and great to watch the master walk viewers step-by-step through how to make a box call, and I probably could not have made that first call without that video. Since that first box call, I've made long boxes, scratch boxes, hooters, crow calls, glass, slate and copper calls, wingbones, duck calls, goose calls and some others I'm forgetting. With two exceptions, call makers are usually pretty generous with advice. Duck call makers and trumpet call makers hold their secrets tightly. While there's a lot of good advice on making duck calls, duck call makers and trumpet call makers generally do not share specifics of how they make calls. Whenever someone asks questions on specifics, the responses generally are "you have to put in the work to find the answer" or "I'm not just going to give you what it took me years to learn and perfect." I understand those responses and am not criticizing call makers for not wanting to give up their dimensions, or tooling or secrets. In the spirit of the generosity of Neil Cost's video, I am offering some insights to demystify trumpet call making and to help others get started.
First, you have to be very proficient on a lathe to turn a trumpet. The tolerances and fitting needed to turn a good sounding trumpet are not for beginners.
Second, you need to acquire a trumpet and learn to run it well. It doesn't have to be an expensive, high-end collector grade call, just one that is well made.
Third, you need a good starting point for external and internal dimensions. For my calls, my mouthpieces are 2.75" OAL and have a 3/32" bore. Here's one my "secrets": At the end where the mouthpiece meets the barrel, I expand the bore to 1/8" for about 3/16" of an inch. This helps the 1/8" starting bore of the barrel line up with the mouthpiece bore. The barrels of my calls are 6", which is fairly standard. Shorter or longer barrels can effect the sound, pitch, volume and playability of the call, and that is something you DO have to experiment with and decide for yourself. I chuck my barrel blank in a four jaw chuck and drill on the lathe using a Jacobs chuck in the tailstock. Drill the blank through with a 1/8" bore. I start with a shorty 1/8" bit, then move to a standard 1/8" bit and finally to a 1/8" aircraft bit. Using these three different length bits helps ensure your bore will be straight with little or no runout. Drill slowly, removing and clearing the bit often since 1/8" bits load quickly and will cause the bit to drift. Next you begin step drilling the barrel to increase the bore size. Here's a "secret" that it took me a long time to figure out and what I believe makes the sound in a trumpet: Leave at least half the total barrel length (from the mouthpiece end) with a 1/8" bore. The flaring or enlargement of the bore should begin at approximately 1/2 the total barrel length. I step drill beginning with a 3/16" bit, which I measure and use masking tape to mark at 3" and then run that 3/16" bit into the bore 3 inches. Next, a 1/4" bit is run 2.25" into the bore, then a 3/8 bit is run 1.25" inch into the bore and finally, a 1/2" bit is run .75" into the bore. To remove the steps, I use a 3/8" Fuller Taper Point bit from Jamestown Distributors
https://www.jamestowndistributors.com/userportal/show_product.do?pid=473 I tape mark and run this bit into the bore 2.75" Finally, I run a 1/2" stepless drill bit into the bore the full cutting length
https://www.harborfreight.com/3-piece-stepless-drill-bits-66463.html Whatever reamer or set of bits you use to build your internals, the critical rule is to create a GRADUAL taper, or flaring of the inside the barrel. After I've bored the barrel, I turn the bell end to shape while the blank is still in the four jaw chuck. I also shape the barrel all the way to where the blank is gripped by the chuck. I then remove the blank from the chuck and tighten a homemade wood mandrel in the chuck. This mandrel is a four inch section of a 3/4" oak dowel, turned to a cone/taper for about 2" I insert this taper into the bell end of the barrel and use a live center in the 1/8" bore hole on the mouthpiece end. I tighten the tailstock down to create enough hold to turn the mouthpiece end of the barrel to shape and sand the call.
This is just one of many, many methods of turning a trumpet, and my internals and tooling are what work for me. I mainly wanted to give folks a starting point for making trumpets that I know works and produces a good sounding call. Hope this helps.
