Hobbing the Ring Gear


Work has begun on the fine down feed for my milling machine. This is an interesting machining project because it requires the construction of a jig to hold the gear blank while cutting the threads into it. The total time to build the ring gear including construction of the jig was around 5 hours. Most of this time was spent trying to figure out how to hold the gear blank.


This picture shows the blank being cut from a slab of 1/2" T-6 aluminum. The hole saw was a little dull, so it took a while, but this is a great way to cut a circle from a piece of aluminum.


Truing up the blank was another puzzle. Since the chuck on my HB 7x10 can only accommodate 3", I had to bolt the blank to the chuck mounting plate.


This is the fixture that I had to build to hold the gear blank. It is constructed form 5/8" and 1/2" plates. The shaft is 3/4" CRS turned down to fit snugly in the center hole of the chuck.


Another shot of the fixture with the blank in the jaws.


Setting up the fixture in the mill and aligning the tap with the radius that is cut into the blank.


The first few rotations were made by hand until the cuts were deep enough to self index. Then I fired up the mill on its slowest speed and cut the gear teeth all the way around. The tap tends to put a considerable amount of side load on the left fixture up right, which causes some flex in the setup. This made the tap wander to the right as it was cutting and caused the teeth to be slightly out of center, however, the whole process worked amazingly well. A gusset on the left upright would fix this problem.


The gear after polishing up the teeth and filing down the rough spots.


Finished product.