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

At recent Midlands ME exhibitions I have picked up several 12 v motors. The 
next one (ME exhib) is coming up shortly so that could be a (another!) good 
reason to go.

As for the gears, gear cutting of the larger ones in metal isnt too hard, 
but I imagine that plastics, depending upon the type used, might make life 
difficult.
Can you not search round for something that will do the job?
This seems to be a case where a comprehensive "odds" box is worthwhile!

You can often find small gears in laser printers, 5 1/4" floppy drives, and 
similar older computer items.
http://www.alanstepney.info
Alan Stepney's Model Engineering, steam, and workshop pages.


----- Original Message ----- 
From: "Chris Crosskey" <chris.crosskey@xxxxxxxxx>
To: <modeleng@xxxxxxxxxxxxx>
Sent: Tuesday, September 25, 2007 9:40 PM
Subject: [modeleng] Calculating gear form of very low tooth count gears


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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