Once again, the problem of obtaining high quality materials here in China made for a little more work in creating the valves. To obtain the necessary material, I used heat-treated stainless bolts sourced from McMaster-Carr in the US as my base material.
The bolts, p/n 97646A165, were 5/16″x1.5″ 18-8 stainless flange head bolts. These were heat-treated to 100,000 psi and around HRB100 in hardness. I like working with pre-heattreated material – not only is it higher strength, but machining it with carbide tools results in very nice surface finishes.
With a 1mm radius insert, I’m profiling the neck area as well as turning the shaft to the finish O.D. At this point, the shaft is still about 1/4″ long – the excess, and the center, will be trimmed off later.
There is almost 5/16″ of excess material to remove from the face end of the valve. With the valve neck now at 0.140″, I was concerned about bend or twisting the valve if I tried to remove this amount of material by just clamping on the valve stem. Therefore, I decided to grip the valve by as much of the face as possible.
To do this, I pressed on a short piece of brass to my 5C stop rod. The stop was then adjusted to sit 0.080″ shy of the face of the collet. With the 5C collet stop reassembled and attached to my 12mm collet, I then drilled and reamed a 0.1865″ hole in the end of the brass. I did this after the stop was assembled to the collet to insure concentricity.
I made split collet out of brass to hold the valves while machining the faces. Again, I could have used a 3/16″ collet, but would risk scratching the polished valve shaft with the hardened collet.
For this whole process, I’ve rotated my tool a few degrees from square so it’s cutting with the 0.4mm corner radius instead of the cutter face.
The next step was to turn the O.D. from 12mm down to the 0.437″ finished dimension. I first plunged most of the excess off with the face of the tool, and then fed across allowing the back corner to cut the final dimension (remember the tool is rotated a few degrees).
For drilling the 0.046″ retainer pin hole, I made a quick drill jig out of brass. I didn’t have a correctly sized drill bushing, so I made one out of a small piece of 1/8″ hardened dowel pin drilling through with a #56 carbide circuit board drill.
To drill the holes in the mill, I’ve set up a vise stop and a single parallel along the fixed jaw, and indicated in the hole position. The valve is loaded into the jig, and the jig clamped into the vise. The act of clamping holds not only the jig in the vise, but the valve in the jig. I’ve used a small scrap of paper to keep from marring the face of the valve.
Since these valves were pre-hardened and they also tend to work harden, it was very important to use a high-quality cobalt drill, a firm feed rate, and lots of cutting fluid to make the holes. I don’t suggest carbide here as a drill this size is very brittle and likely to snap when exiting the far side of the valve.
The final step will be to lap these valves into the heads.
The plans call for a simple rigid valve stem chuck. I was concerned that this could induce side loads during the lapping process and given that there is 0.002″ of clearance between the stem and the valve towers might cause some sealing problems.
To counter this, I made a lapping chuck with a flexible section. The flexible section was a piece of 6mm O.D. x 1mm wall urethane tubing. This was attached to a 0.250″ O.D. chuck that fit over the valve and was drilled for the valve retainer pin.
Over this was a 5/16″ O.D. aluminum collar that slid down to cover the retention pin. On the other end of the flexible tubing was a 5/16″ rod to fit in the portable drill chuck.
At this point, all I need is a sparkplug and some non-embedding garnet lapping compound. A good source for this compound is your local school band instrument supplier. It is used to lap the valves on the Brass instruments. I ordered mine online from MusicMedic.com
- Add some photos here showing the actual lapping process
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All material, including the CAD drawings, relating to the construction of the Hodgson Radial presented on this site is free to use any way you see fit. However, no guarantees are made regarding the accuracy or correctness of the material presented here.