All the repairs to the Mogul are being done in Dad's shop. The problem is that I only get there about three days a month. He wants to be able to use the machinery during the in-between times. I had to devise techniques that avoid major set-up times between visits. The eccentrics on the valve gear are a case in point.
Step one is to cut off a blank. In this case, the outside diametre of the eccentric is 2 1/2", the strap diametre is 2", and the axle is 1".
Centre up using a dial test indicator. Neither end is square at this point, so you cannot depend on the three-jaw alone.
Face it up square, turn around and face the other side.
Clamp it to the drill press table, drill one bolthole #20 and tap 10-32, then put in a cap screw. Drill the other boltholes, tapping and bolting each one in turn.
Photo 1 is for illustration only - it shows how the fourth bolthole aligns on the jig. We are looking at the opposing side. Photos 2 and 3 show mating surfaces. Note how the centred part of the jig requires four bolts while in use. Parting puts some nasty strains on this operation. The eccentric turning is simple drilling and facing - much less stress.
Slow the lathe down and cut a part 1/8" from the end of the blank. The depth of the part . . .
. . . is measured with the depth finder on the end of a dial vernier, or using calipers. You want the part diametre to be several thousandths less than the finished diametre of the eccentric strap. In this case, we indicate 0.258" deep, giving us a finish diametre of 1.984".
Move the parting bit over to give you a total slot width of .050" wider than the strap, and part again.
Change to a narrow round-nose bit, turn down the running surface to diametre, then take a strip of emerycloth and polish it up to a mirror surface. You want the lathe humming on this operation - I used 1260 rpm, because that is as fast as she'll go.
Turn the eccentric around in the jig. Put in the three bolts, and tighten. You will notice that both sets of boltholes in the eccentric jig are countersunk, so that the capscrews are always below the surface. Drill the axle hole out. Note that the fourth mounting bolt hole is there - do not drill through it, because the drill will wander,
but instead, use the boring bar to get past this point.
Turn down the boss on the side of the eccentric. If you take note of the micrometer collar reading when your first cut bites, and again when the cut goes all the way around, you will have an indication of the total throw of the eccentric. In my case, it is 0.662", which is an improvement over the 0.5" of the original cams. This number will be used in your calculations to make the valves.
Put a vee-block on the drill press, and a 1/8" bit in the chuck. Drill two holes through the boss at 90 degrees, going into the axle hole. Go to a #29 drill, and put in a third hole. Tap this one 8-32. This is for a grub screw, to hold the eccentric while setting the valves. The first hole will be enlarged to #29 extended 3/16" into the axle when the eccentric is finally set, then tapped to hold the valve setting. The second 1/8" hole will be to reset the cam if there are any changes to be made - a "hedged bet," so to speak. It is easier to do it now on the drill press than it will be later, between the frames...
There is the final product. It would have taken less time to machine the eccentrics individually, but, by using the jig, the four cams are within a thou of one another in throw and concentric. When you get the four of them together between the frames, and the valves set . . .
. . . I tell you it is quite a feeling to put the air to it and see the wheels spin, smooth as silk, and to hear the exhaust bark through the open pipe. Ideal valve setting accelerates the wheels as you notch up, and this one did just that.
There have been a lot of learning experiences with this engine. Like that counterbalance on the weighshaft, top-centre of the photo above. Step one - make sure your propane bottle weighs more than the chunk of metal you want to silver-solder...
I have redone the blast nozzle and smokebox for higher efficiency, according to The Ultimate Steam Pages.
Link to hear the story of this experience, on CBC's "Out Front" programme.
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