[modeleng] Re: Calculating gear form of very low tooth count gears

Are you trying to duplicate a gear you already have?  If so, how about photo 
etching its semi-twin from the appropriate thickness of brass?  You would 
probably have to finish the teeth with a fine file.  Charles
-------------- Original message -------------- 
From: "Chris Crosskey" <chris.crosskey@xxxxxxxxx> 

> Hi Folks, 
> I'm trying to clone a very small gear for a slightly off-beat lighting 
> project. I'm trying to clone an Optikinetics Total Eclipse (see 
> http://www.funky-parrot.com/totaleclipse.html for more info about this 
> rather insane lighting effect), and I've run into two problems, one of 
> which is the motor, it's probably about 8W, 12V AC and runs fairly fast, 
> the other is the sun gear in its planetary gear arrangement is 8-tooth 
> and it has a hole up the centre, it's about 8mm wide (the gear overall) 
> so it might well be 1Mod. If I could work out the profile without 
> measuring a rather worn one in my possession and trying to copy it it 
> would be very helpful. Is it worth measuring the planet gears (about 
> 25mm/25 teeth but I'm not being precise here)... it could just as easily 
> be 24DP, I will need to measure properly, but how would I go about 
> cutting such a gear?.... is there a way to generate a cutter for 
> this?... I've got the WPS book on gear cutting (one of Ivan Law's IIRC) 
> and I'm happy enough cutting other, rather higher toothed gears but I'm 
> not up to working it all out at those kind of levels.... Assuming it is 
> 1 Mod then what sort of pressure angle will get me in the ballpark for 
> an 8-tooth gear with a 2.5 to 3mm hole in the middle of it?.... 
> The effect works by having a pair of rather inefficient drive systems 
> from a single motor powering up both sides of a planetary gear system 
> where the gears are clear and have colour section son them. The whole 
> assembly is spinning so fast that it just colour mixes in graduated 
> bands that change due to differences in the inefficient power 
> transmissions (these can be varied by moving one of the foam covered 
> jockey wheels BTW)..... I'm thinking of cloning it without the 
> inefficient drives and just having a one-tooth variation in a normal 
> gear transmission system..... hopefully a 12V 500rpm sync motor will run 
> the whole thing fast enough to colour blend..... but I'm still stuck 
> with that minute gear in the middle of the planetary, if I go small 
> enough on the tooth form to get into sensible territory I'm down at 0.5 
> Mod and I'd rather not be cutting gears that fine in plastic then 
> whirling them round in the gate of a projector..... I guess I could try 
> it though....... 
> 
> Any thoughts on 12V AC motor sources would be welcome too, I've done 
> some googling but not turned up anything particularly useful.... And 
> Optikinetics (and their main dealers Optifanatics) have been very 
> helpful but simply don't have much information (or indeed spare parts) 
> for stuff that old.... plus the original designer (who was mad as a box 
> of frogs) died a fair while ago..... 
> 
> Any help gratefully received 
> 
> Chris Crosskey 
> 
> 
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