I am a Hobbing Machine - fear me!

Usually a hobbing machine is a tool that cuts gears. In my case, I am a hobbing machine that hobbles gears by cutting them poorly.

Turns out, gears aren't just square teeth; if that were the case the driven gear would jitter - the speed would vary as the teeth slid by each other. This is what mechanical engineers like to call "bad".

As you can see from the diagram on the right (from the wikipedia Gear entry), there are many parameters that need to mesh in order for a spur gear to not bind, break, slip, backlash, splinter, wobble, or catch on fire topple over and fall into the swamp.

The real key to the kingdom is the curve on each face of the gear tooth. This curve is called the involute curve (click link to see animated curve).

See how these gears mesh; the velocity is kept constant, and the point of stress slides along nicely:

(Click to see below pic nicely animated)


Mine will probably look more like a lurching, lumbering epileptic one-legged ex-movie starlet going through detox, but I'm fine with that - should help the cheering when it inevitably gives up the ghost and sinks into the morass of the Baltimore Inner Harbour. Yeah me ;-)

And remember kids: Try this at home!

Useful links uncovered in this sub-quest:

• Chesapeake Area Metalworking Society - great list of local resources
• MITCalc - semi-free (30 days) Excel based spur gear profile calculations