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After nearly 2 years of retirement, my old mechanical stirrer was reinstated
for a new job. Bad-ass magnetic stirrersTM can't excel at everything!
I made this baby a long time ago when the only hotplate/stirrer I had was too
weak and feeble to stir an Al/Hg reaction. I didn't have any money, so I went
up to my friend’s machine shop, scrounged around the scraps, and pieced this
together.
First, I found a long rod (stainless would be best, but only normal steel was
available), then I drilled two holes through one end, cut a slot
perpendicular to the holes, drilled out the holes on one side of the slot a
little bigger, and tapped threads into the other holes. Then I wrapped a
couple of layers of teflon tape around the metal on each side of the slot and
a few inches up the rod. Then I found a sheet of flexible teflon that fit
snugly into the slot and cut out a paddle. Then the teflon tape covering the
holes in the rod was pierced and the teflon paddle inserted into the slot and
centered. Two holes were made in the teflon to allow screws to pass through.
Two stainless steel screws were inserted through the drilled out holes and
teflon, and screwed into the other side of the rod securing the paddle.
Next, an unidentified chunk of white polymer (hopefully fairly chemical
resistant) was turned down on the lathe so that it would fit snugly into a
24/40 GGJ. A hole slightly larger than the diameter of the stir rod was
drilled in the middle.
I decided to use an electric motor to power the stirrer, and began searching
the vast heaps of various electronics parts at the house of a friend. Finally
I found a decent motor which just happened to have a toothed pulley, matching
belt, and another matching pulley.
Back at the machine shop I drilled a hole in the extra pulley to accommodate
the stir rod. Then started thinking about how to mount the motor to the white
plastic part that is inserted into the neck of the flask. I found a piece of
metal (brass, I think) that appeared to have once had a rod of some sort
through it, and decided to use it for my purpose, even though it was ugly,
and bigger than necessary, because I was tired and wanted to do as little
machining as possible. I call this the motor mount. So the two holes that
originally caught my eye were drilled out a little larger to accommodate the
stir rod, the motor was attached to one end with one screw (to allow easy
belt tension adjustment), and the other end received two holes for screws
which were screwed into the white plastic part that plugs into the neck of
the flask.
Finally the stir rod was inserted through the white plastic piece and up
through the top of the tall metal motor-mount. The extra pulley was slid onto
the stir rod, the rod was positioned for proper depth, the pulley slid down
to the motor mount, and the set screws on the side of the pulley tightened to
secure it. I didn't bother making any kind of bearing for where the pulley
meets the motor-mount because I was tired. And it worked just fine if I
remembered to occasionally spray some lube there.
I found a transformer (looks like the kind of thing that charges your cell
phone) with a 12 volt output for powering the motor. I had planned on
including a dimmer switch for controlling stir speed, but decided that the
full power speed would work fine for all of my uses.
Here are pictures: (probably much more useful than my lame, verbal
description  )
  
The first pic shows the stir-rod with the teflon paddle. The paddle is about
2.5 inches wide, and a little under an inch tall. To fit it into a flask, the
"blades" of the paddle are loosely wrapped around the rod. After
insertion, they straighten out some, and when stirring is started, they fully
straighten out. When removing it, the taper of the top of the RBF (and the
minor bend still present in the flap) will cause the blades to curl/wrap back
up and easily be removed.
The second pic shows the spare pulley, belt, motor, tall metal motor-mount,
and white plug. I'm not sure, but I don't think the motor is spark proof. I
used it anyways and never had a problem, but if you want to be a good, smart
chemist, you might want a spark proof motor. Also, the assembly is not air
tight, so pressure reactions are out of the question. The hole in the white
plug was drilled a little larger than the stir-rod out of the necessity to
reduce fricton between the two. It was never a problem, though, even when
used with reflux. Little or no vapor escaped.
The third pic shows the fully assembled stirrer adjusted for, and inserted
into some glassware.
Unfortunately I don't have any kind of plans or diagrams because I put it
together on the spur of the moment with what was available at the moment with
no prior planning. I just thought this might be of help to anyone trying to
get started without much money, because unless you fork over a hefty chunk of
change, or happen to get lucky at an auction site or garage sale, most magnetic
stirrers really suck ass. Especially if you want to stir an Al/Hg!
Real
chemists can't smell.
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