I'm kind of surprised that an off the shelf gear can't be found, or one=20
of required pitch diameter, and even a matching mate, modifying hub on=20
both if need be. Many years ago, a welding shop needed to make 18" dia.=20
chain sprocket for a bulldozer. This was before computer graphics. I=20
hand drew in ink the shape which then was used by an optical eye type=20
acetylene cutting machine. It worked. :)

On 5/14/13 11:29 AM, Allen Mulvey wrote:
> We did this in college foundry work for investment casting.
> We mixed talc in with the plaster to make it easier to
> calcine (drive off the water). If I recall properly, each
> molecule of gypsum tends to retain two molecules of water so
> by reducing the gypsum (plaster) content you can reduce the
> bonded water significantly. I don't remember the proportions
> we used. Experiment a bit. You just need enough plaster to
> hold it all together.  Preheat the mold before pouring.
>
>> -----Original Message-----
>> From: piclist-bounces@mit.edu [mailto:piclist-
>> bounces@mit.edu] On Behalf Of veegee
>> Sent: Monday, May 13, 2013 10:08 PM
>> To: Microcontroller discussion list - Public.
>> Subject: Re: [OT] Making a new spur gear
>>
>> On 2013-05-13 8:57 PM, IVP wrote:
>>> I'm guessing you'd need to let it cure properly so the
> hot
>> metal
>>> doesn't heat any remaining free water and crack the mold
>> This is a well-known procedure. I'm not inventing any new
>> methods. After
>> the plaster hardens, it is to be baked in an oven for a
> few
>> hours to get
>> out as much water as possible. Then it's safe to pour
> liquid
>> aluminum
>> into it. But I'll be heating the mold with the blow torch
>> before doing
>> anything with the aluminum to make sure it's free of water
>> as an extra
>> precaution.
>>
>> I'll be melting outside, on a solid concrete slab, with
> the
>> mold inside
>> a hole of a cinder block.
>>
>> If it feels unsafe, I'll dig a hole in the ground, line it
> with
>> gravel
>> and use coal instead of a blow torch so I don't have to be
>> near the
>> aluminum while it's melting into the mold. Cover with a
>> concrete slab
>> with a hole to breathe and let it sit until the coal burns
> out.
>> Doesn't
>> get any safer than that.
>>
>> Aluminum melts at low temperatures, and this method has
>> been used to
>> cast alloys of much higher temperature.
>>
>> If it really becomes an issue melting the aluminum safely,
> I'll
>> just use
>> zinc and melt it under a candle or something outside, and
>> stay far away
>> so as not to breathe any fumes (zinc fumes are toxic).
>>
>> But the volume of metal I'm dealing with is so incredibly
>> small that I
>> very highly doubt that I'll run into any issue safely
> casting it.
>> I just made the plaster mold and it turned out much better
>> than I
>> expected, even after creating too thick of a mix. The
>> precision of the
>> plaster getting into all the notches of the gear is
> incredible.
>> --
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