< Driving Wheel Cutting 1 >
The driving wheel diameter is 208 mm, which
is beyond the swing over bed of Myford Super
7. So the wheels must be turned in 'gap'
between the headstock and the bed. It causes
many restrictions. Also, phase difference
of the wheels is 120 degrees instead of 90
degrees. Moreover, axle diameter of the second
driving wheels is larger than that of the
first and the third driving wheels. I spent
much time for investigating procedures of
cutting the driving wheels.
I prepared the axles first. The procedure is the same as bogie truck axle. At the present stage, the second axle is still finished straight.
Full size driving axle has the thickest sections in its both ends where the wheels are pressed in. In case of model locomotive, the press fitting sections are normally turned down from a rod blank. As a result, the middle section of the axle is far thicker than the scale size. My loco's axles became 30 mm diameter.
The photo shows extended face plate for turning driving wheels. Laser-cut large disk is bolted down onto the standard face plate. The ring is also laser-cut and bolted onto the disk. And then, face of the ring is trued up by a clean cutting. The ring will hold the wheel's rim or back side face truly square.
I prepared wheel centering spigot. Steel round bar was silver soldered onto a MT2 blank arbor. And then the round bar is turned down to the second driving axle's diameter (thicker). Because of narrow 'gap', the spigot has to be removed in every time I exchange wheel castings. Every time I drive the spigot into the headstock, the center is kept truly. Incidentally the spigot has screw holes at both ends, so as to secure wheels by its center.
I started with the second driving wheels. First the reference plane is cut in the back side of the rim. When clamping, three small thin plates are sandwiched between the jig's ring and the wheel at every clamping positions, so as to avoid distortion of the casting. Because of huge diameter, I could not increase turning speed more than 50 rpm. So it took much time for one cut. I employed a limit switch to stop the machine at desired position.
Centering is done with dividers. I cleaned the boss center, scribed four arcs like '#' from rim's inner edge in four directions, found center from the mark '#' and drilled the center hole with a center drill.
A hard center in the tailstock pushes the wheel casting at its center against the jig, and then the wheel is clamped between its spokes. After that the center hole for axle is drilled and extended by a boring tool, using actual axle as a gauge. When I started turning with 250 rpm, terrible quake happened because of the casting's big counter weight. I had to reduce the speed to 160 rpm.
Before release, the boss surface is cut and finished. I cannot help cutting a part of counter weight, because outer end of the boss interferes with inner edge of the counter weight.
The rim is finished together with outer face of the counter weight. Note the casting is centered with the spigot.
So as to finish the counter weight surface without disturbing the boss, the casting is clamped eccentrically on the jig by screw stud and nuts
The casting is centered again with the spigot. Groove between the rim and the counter weight is cut by a parting tool. The parting tool is deformed to suit face cutting. That is to say, outer edge of the tool is ground diagonally.
Back face of the boss is cut by boring tool. After that, the center hole should be chamfered in order to clear small fillet at the axle's shoulder. Incidentally, I designed the boss backside height lower than the rim backside height, in other words, the axlebox will go into the wheel, so as to secure much side play of the driving wheels.
When finishing outer back surface, the casting is clamped at its center. The spigot is pulled from the head stock end, in order to avoid the slack by the top screw. Note the screw head between the spokes to prevent the job from slipping.
Back side recess is cut by a knife tool. Again the tool is deformed to suit for face cutting.
That's all of this month. It takes really much time to cut driving wheels...