Thornton Miniature Gear Cutters

It might be worth adding a sacrilegious view of cycloid gears. This shows the a pinion in mesh with a wheel and the pitch circle diameters.

The pinion flanks are radial - part of the "aim" of cycloid teeth. If the gears mesh like that on purely radial points there is no pressure angle which is reckoned to cause wear on the shaft that carries them. These have the full profile though. If the other parts come into action bang goes no pressure angle and also no sliding contact.

The wheel flanks are partially radial because having exact radial teeth on all tooth counts would mean that a different cutter would be needed for each of them. So that is only usually done on low tooth count pinions. Rather than having the curve down to Beelzebub on the end the wheel teeth ends are just radiused. This gets them out of the way neatly and can be done on pinions too, in some ways these would give the pinions a better action. They can work out with lantern pinions too.

Like all practical gears they need some clearance. Not reckoned to matter on clocks etc as they only rotate in one direction but departures from true curves will interfere with all aspects.

Then they are depthed to run nicely, what ever that means and then supported in holes which in engineering terms might be called rather big for the shaft that is in them. Friction?. Codge wallop Any clearance will result in point contact unless carefully sized to take account of specific lubricants and load and speed ranges. So bang goes the careful depthing unless it isn't that critical but that would upset the other aspects of this type of gear anyway.

Couple of links

**LINK**

All true but for best action involutes do need correct depthing. Cycloids are often claimed to not suffer from this problem but that's clearly a load of bull. I have seen claims that a 14 1/2 degree pressure angle came from figures initially found on practical cycloid gearing presumably made by engineers. 20 degrees has since been found to make very little difference. When what this means in practice is looked at that is hardly surprising.

Also when rounded end pinion teeth can be used and other aspects.

**LINK**

Round over cutters are sometimes seen on ebay but making a cutter to do that wouldn't be difficult and the rest can be taken out with slitting saws. Thin one for the flanks and a fatter one to clear out the middle.

John

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Michael,

Just being nosey and curious as to what you are making with these. - Any chance of a write up thread as I need to cut some gears soon and have never done it.

Cheers, Nick