Machining the Bull Gear
Copyright 2008
Precision Locomotive Castings

The first step to machining the bull gear is to cut off the casting gates if they are still on.

I chucked the gears up in the 4-jaw chuck in the lathe by the mounting boss on the back. Indicate the gear in by indicating on the face of the teeth near the OD of the gear, and then on the back of the gear just behind the large end of the teeth. You will probably need to go back and forth between the two surfaces in order to get the gear casting to run true. At this point bore the gear out, keeping all the gears with in .001 of each other on the ID.

Chuck up a piece of stock for the mandrel of the fixture and machine to print.

Chuck a piece of stock up in the 3 jaw for the machining fixture. Indicate the cross side in to the face angle of the teeth. I used a small sine bar to do this. (Fig. WBG-1). The magnet is holding the sine bar in position on the cross slide.

Machine the step in the fixture as shown on the print. This will be used to locate the conical surface that we are machining for the teeth to rest against from the face of the fixture. This will establish our distance from the apex of the gear cone to the surface that will be bolted to the wheel. Use a sharp cornered tool to machine the step in, and color the corner with layout fluid. Machine the cone until the layout fluid just disappears, and the apex to the surface of the fixture will be established. Finish machining the fixture and bolt the mandrel in. (Fig WBG-2) Make a plate to bolt on to the end of the mandrel.

At this point, you are ready to slip the gear onto the mandrel and machine the front and back of the bull gear mounting boss using the front of the fixture as the reference surface. (Fig WBG-3). Note in this image the end of the mandrel has not been cut to length or drilled and tapped for the plate.

From this point the gear casting can be faced front and back of the boss, and the OD turned. (Fig. WBG-4) Be sure to hold the dimension of the OD, since this fits into the wheel and locates the gear axis to the wheel axis.

After machining all the bull gears, the fixture can be pulled out of the lathe and clamped to the mill table to mill the drive lugs on the back of the gear and drill the bolting holes. (Fig. WBG-5 and WBG-6).

In the last operation I mounted the gear back in the lathe in soft jaws that were bored to fit the mounting boss, and a face cut was made on the front of the teeth as per the finish mark on the print.