Bull Gears - Part 2

Once the sides of the gear blank were machined to size, the gear blank was clamped to the table of a Bridgeport milling machine. The original dowel pin hole was used to find the center of the gear blank and the digital readout (DRO) was zeroed for both axis.  Using x-y coordinates, the location of the eight mounting holes were found and drilled. A special block was machined to mount to the top of the rotary table via the “T” slots in the top of the table. This block had a ½” diameter dowel pin in the center and 8 corresponding tapped holes to bolt the gear blank firmly in place. A large 4-flute end mill was used to machine the blank to the appropriate outside diameter for the gear tooth cutting process.

 

The actual gear tooth cutting process is not black magic nor should it scare you from trying it. There are cutters that have the exact shape that is needed for any given pitch diameter and tooth size. They are called “involute” gear cutters. . An involute cutter cuts the root of the gear tooth and the sides of adjacent gear teeth. The main concern is not to cut too much metal at one time.

 

The K&T was set up as a typical horizontal milling machine and the rotary table was disconnected from the power feed. An indexing plate assembly was mounted to the rotary table so the gear blank could be rotated in precise increments. A 1/8” wide slitting saw was used to remove a major part of the material where the root of the tooth would be. This lessens the amount of material that the gear cutter has to remove. Two passes were made with the gear cutter to reach the desired depth and profile. The K&T has a power feed on the knee. So I would index, flip the power feed lever to up, the stop would trip the power feed handle out. Then I would rapid traverse the knee down and start the process all over again. Meanwhile I would work at something else for the tractor. I guess you would call it multi-tasking. Believe it or not, when I finished machining the two bull gears, I almost had a full 5 gallon bucket of cast iron shavings.