There have been many “How-To” articles on how to make piston rings, and here is another one. I mostly followed the instructions in Joseph F. Nelson's book “So You Want to Build a Live Steam Locomotive”. During the process I discovered a problem or two, and decided to correct one during construction, and document the correction for the other. I used the dimension table from the book and turned the outside and inside diameters on a piece of cast iron held in the three jaw chuck on the lathe. I then faced the outside surface and parted off the ring leaving 0.015-0.020 extra width for the facing operation, repeating this for as many rings as I could get out of what was sticking out of the chuck. The parting operation leaves the newly parted ring with a rough, and not particularly parallel, surface. I marked the faced surface.
Step two is to make a fixture to hold the ring for facing. I followed the drawing in the book. Here is where I encountered the first problem. The drawing in the book has one slot all the way through and the other two slots only down to the first step in the part. When the chuck closes the fixture to hold the part, the single slot causes the fixture to collapse unevenly. If I were to do it over, I would use the following design with 3 equal depth slots:
Place the ring with the marked surface toward the inside and face it to the proper dimension.
Next I tried to slot the rings using various methods. I tried a hacksaw, and a slitting saw on the mill. Both methods produced more broken rings than good ones. Problem two. Gordon Carlson suggested slotting the rings with two, halfway through, adjacent slots. I made the following fixture:
After mounting the fixture in the mill, offset the tool the diameter of the tool (I used a 5/32 end mill). Mill a slot trough the cover enough past the ring inner diameter to assure a complete cut, and deep enough to cut into the bottom of the fixture (for aligment). Set the tool depth to 1/2 the thickness of the ring. Make the first cut on the ring. Flip the ring over and line the left edge of the slot just cut with the right edge of the slot in the fixture. I used a manigifier to assist in the aligment. Cut the second slot.
Finally, the outside diameter is turned while the ring is held in its compressed state. You guessed it, another fixture:
I made the depth 0.32, enough to hold 6 rings. I put the rings on the fixture and put the cap on (the same cap from the sloting fixture) but didn’t tighten it. Then I compressed the rings using a hose clamp close to the size of the rings. After compressing them, I tightened the cap and removed the clamp. The pressure of the cap holds the rings in place.
The rings on the fixture prior to turning the final diameter.
The rings after the final turning.
With the help of Matthew Kobel and Gordon Carlson (see his work on http://www.locogear.com/page15.html).