< Driving Wheel Assembly 2 >
The cranked axle for the second driving wheel is made of cast iron plates and silver steel shafts. The axle shaft already had been finished, so I tackled with the castings. First, back face and axle bore are finished in four-jaw.
As the photo shows, I prepared a jig on the face plate, so as to finish the crank hole in true distance from the axle hole. The stack of gears is balance weight for turning.
The shafts are glued into the castings with Loctite. Both ends of the crank shaft are thinned in diameter, in order to determine distance between the castings. Also a small pin on the axle positions the castings truly midpoint of the axle.
In case of Gresley derived motion, three crank axles have to be quartered exactly. Because the inner valve motion is a composition of two outer motions. I employed following procedure.
Pair of V-blocks is bolted down on the milling stage parallel. The cranked axle is mounted on it and the crank angle is set by a brass post of true height. The axle is clamped firmly and the key groove is cut by end mill. In the first photo, the post is hidden behind the clamping blank. After that, the opposite key groove is cut in the same manner with a different post. It means the each key groove is angled from the inner crank axle independently. The second photo shows the brass posts.
To tell the truth, I had made an irreparable mistake here. I will show it next month. If you look at above photos carefully, you may aware of the mistake.
Before dismantle the setup, I copied the quartering result (angle between two grooves) down to a jig as the photo. The jig consists of 30 degree slope base, sliding square bar and upper guide plate. The tip of square bar is cut to a push-fit to the groove. The upper guide plate pushes the square bar hard.
First and third driving axles are finished with the jig. After cutting a groove, the axle is reversed and its angle is fixed by the jig. And then the opposite groove is cut.
So as to secure the shafts of the cranked axle firmly, I drove 4mm spring pins in every connections.
Finally, middle part of the axle is cut by band saw, and finished by end mill.
The rectangle keys were made from square steel bar. The length is about half of the wheel thickness. Key's thickness is 0.1mm shorter than the height of key hole between wheel and axle. Key's width is first cut to 0.1mm larger than the key hole width, and then finished to press-fit to the hole with fine files. All keys are temporary hammered into the key holes and checked there is no play.
There are blow-holes in the wheel castings, particularly at the root of spokes. I repaired them by Devcon plastic steel putty. Before it, the wheels are cleaned and degreased by paint thinner and rinsed by acetone.
Masking with paper, I sprayed urethane aerosol paint.
Using the wheel turning jig again, rim and boss are polished. For boss, I employed sand paper sticked on a flat board. Note the counter weight is protected with cloth tape.