Most modelers do not know how gears are made. I didn't know until I decided I wanted different gear ratios for the Trex. After making a homemade machine to hob gears I decided to buy this real gear hobber. It is a 1943 Barber Colman gear hobber, completely rebuilt in the early 60's and then left essentially unused until I bought it. After cleaning off 50 year old rancid, high sulfur cutting oil, the machine looks brand new.
The cutter is called a hob, which is essentially a large bolt with cutting edges in the thread. The cutting edge profile is the shape of the spaces of an equivalent gear rack, and the rotating engagement between the gear blank and the hob creates the proper tooth form. There is a standard gear tooth profile, but the advantage of the gear hobber is it can create standard tooth forms and non standard tooth forms. For
everyone who has wondered how Hirobo could have 95, 96, and 97 tooth main gear for the Freya without having adjustable spacing between the engine and main shaft, it is because the 96 tooth gear is a standard profile, and the 95 and 97 tooth gears are non-standard profiles that are made to the same diameter as the 96 tooth. The same explanation applies to the 17 tooth and 18 tooth tail rotor pinions that Hirobo uses, both of which have the same diameter.
The Barber Colman #3 gear hobber.
The cutters (hobs) for Module 1 and Module .7 gears. The leftmost arbor is the standard Barber Colman one for more industrial size gears than we use in the models, the middle one is the Module 1 cutter, and the right one is for Module .7. Module 1 is the usual gear size we use, and Module .7 is the smaller tooth the Trex 700 uses.
The Raptor G4 115 tooth main gear, 20 degree left hand helical. The two oddly placed holes were for attaching the blank to the spindle of the machine.