Making a Bronze Diaphone Piston


This fall, I took a machine shop class at South Shore Vocational Technical School in Hanover, MA. During the 10 week course, I learned the basics of metal lathe and milling machine work by producing a bronze piston for my Gamewell Type B Diaphone horn. Upgrading the horn from a 4 oz aluminum piston to a 15+ oz bronze piston lowers the pitch and also increases the power of the horn, as more air pressure is required to drive it.

	The Beginning The first thing that I did was take some measurements from original Gamewell pistons. I had a very nice sample of a brass billet piston for the critical measurements, as well as an aluminum piston with the "ring of 40 holes" baffle plate which I also wished to reproduce. The brass billet piston is shown at the left.
	The Drawing From careful measurements of the original pistons, I came up with a shop drawing (actually several). The overview drawing is shown at the left. Please note that while I normally believe in sharing, I have chosen to leave out the dimensions on this drawing. It was a lot of work to record and fine tune all the dimensions, and I hope to make some pistons for profit, to offset the cost of my lathe. For this reason, I do not wish to give away the fruits of my research to the entire world for free just yet.
	The Raw Material This is the starting point, a 3+" diameter rod of leaded tin bronze cut to my starting size on a Makita metal chop saw.
	Stage 1 - Rough Cuts Over the first couple of classes, I basically learned how to use the machines in the shop. When I finally got up and running on the lathe, what I got done first was learning how to get the stock running true in a 4-jaw chuck, and the rough cuts to the diameters you can see at the left.
	Stage 2 At this point, I have ground several form tools and done all the grooving on the outside of the part. I have also taken the final finishing passes on the two critical diameters, checking fit in my cylinder as I go. I still have a big hunk of un-turned material for the chuck to hold onto when I do the next step--boring.
	Stage 3 Here is the piston after boring. I drilled out a bunch of material and then finished with a boring bar.
	Stage 4a - Part-off I've now parted off the raw material hunk.
	Stage 4b - The Recess Here you can see that I turned the piston around into a soft-jaw chuck and bored out the recess in the back.
	Stage 4c - The Blank I now have a piston "blank". That is to say, all the lathe work is done and what remains goes to the milling machine.
	Stage 1 Here is the setup on the CNC mill's MILLPWR 2-axis controller for doing the ring of 40 holes in the baffle. Each hole was started with a long center drill and finished with a jobber drill to final size, then chamfered to give the holes a nice look and to deburr them.
	Stage 1 Here is the result--the holes are spot-on.
	Stage 1 This is the final product, after cutting the 36 voice slots in the mill using a jeweler's saw. You might notice that the original aluminum piston in this photo has 6 internal support ribs, and mine does not. Interestingly, these ribs are not actually needed for support when you cut the voice slots with a saw. The ribs were part of the original casting pattern so that the voice slots could be cut on the lathe, which is indeed how the earliest pistons were done. Gamewell switched to using a saw for milling the slots, but there was no need to remove the ribs from the castings as they still added extra--though now unnecessary--support. On Gamewell's pistons made from brass billet, the ribs are absent (see the first photo on this page).