Desktop Gear Hobbing

Model Engineers' Workshop|May 2020

Desktop Gear Hobbing
Toby Kinsey has designed this fascinating piece of gearmaking equipment
Toby Kinsey

One of my occupations is as a model maker for a small toy and games invention company. We often have to construct prototype toys and use small Module gears for various gearboxes and mechanisms. The deadlines are often quite tight and to avoid the wait for gears to be delivered, the company stocks a range of small plastic moulded gears in different sizes, from 9 teeth up. These are mainly 0.5 Module, but we do keep some bigger module gears as well. These are generally sourced from companies such HPC (ref. 1). But due to the way the world seems to work it is invariably the case there isn’t a gear with the right number of teeth for a particular job or there is only one gear left when you need two!

Another issue is the bores of even the small gears are quite large, 3 or 4mm being common. This leads to lots of machining up of little sleeves to reduce the bore down to suit the 1 or 2mm shafts we use.

I also work on my own personal projects at home, but lack of deep pockets has stopped me from building up a stock of gears in the way that a company can afford to do. This was a limiting factor on what I was able to do at home as again, you never seem to have the right gear for the job in hand. I was faced with the choice of having wait while I ordered a particular gear or “borrowing” gears from work.

The solution to the gear issue I came up with is desktop gear hobbing with the Mini Electronic Gearhob Setup, or “MEGS”, photo 1. Pushing a few buttons, photo 2, you can select the number of teeth of the gear you desire, and it will produce you a gear up to 30mm in diameter. See ref. 2 for a video of MEGS in action.

Gear Hobbing

The journey that led to MEGS began when I thought would be useful to have a way of making gears myself. Of the ways to make gears I thought, a small gearhob would be the thing.

As many of you will know, a gear hob works by rotating a blank of material at the same time as a cutter or hob is rotated about an axis 90 degrees to the axis of the blank. The hob has spiral groove cut round it like a thread, with gashes cut to provide a cutting edge, photo 3. The blank is rotated in proportion to the cutter, i.e. cutting a 10-tooth gear the blank needs to rotate at 1/10th the speed of the cutter spindle. The beauty of hobbing gears is you only need one cutter to cut any size gear for any given module, unlike cutting gears on a mill using a dividing head and involute cutters. Where each cutter will only produce the right teeth form when cutting gears with a limited range of number of teeth. You can get an involute cutter that will cut a gear of between 12 and 14 teeth. You will need another cutter to cut the next few sizes up and so on. Which all means you will need a range of cutters to cover all eventualities. Another advantage of producing my own gears was that I could produce a long length of a particular gear and then part if off to just the right thickness, as and when needed.

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May 2020