|Project by vipond33||posted 10-14-2012 09:06 PM||5227 views||22 times favorited||37 comments|
I would have thought that the LJ community was very well balanced and happily true following the last contest, yet here we are again, gruesome death to all Politics & Religion notwithstanding. For the current contest is to once more build something that balances, and yet again with not even a T shirt for a prize; it’s just for fun. So what, I had to do it anyway (though I do covet that shirt).
My entry is an analog math balance beam (no batteries required, no digital readout in sight), a simple visual teaching aid for very young children that shows them the relation between numbers. With it, a teacher can easily demonstrate the concepts involved in ordinary (small number) addition, subtraction and multiplication, or have the students try by doing it themselves.
Steel washers on the left must equal (proportionately) steel washers on the right -
or the beam goes tilt.
This is an adaptation (and I am indebted to Jim Makowicki) of a project from his book Making Heirloom Toys.
I did take some small liberties with the design and materials but stayed true to his overall plan.
My own build is with ebony veneer covering solid cherry, beveled throughout and with the tapered beam done in quartersawn stock to stay true. Cherry pegs marked with Letraset numbers (anyone remember?) complete it with a mixed bunch of polished brass fittings.
As usual, I wanted to make it a bit prettier than plan but I also had to build it somewhat sturdier too because it is for school use – that is to say by unknown pint sized assailants. Still, by design, it must always be a rather delicate piece so some supervision is required. The overbuild is mostly in the case for I could do little about the tremendous leverage possible in those arms.
Outside of the indicator to pivot block joint there is no glue in the assembly, just long screws in cups. Somewhere down the road it may have to be worked on by others so I left it so.
The pivot ball bearings riding on the brass axle are triple sealed, 3/4” x 5/16” class 5 type, rated to 25,000 rpm. A bit overkill maybe? but Grade1 kids can really get going sometimes you know.
To preserve it and store in the supply room I made a fitted BB plywood case with inlaid solid hickory edges, leather padded retainers and a small interior cherry box to hold the washers.
Detail showing how the main pivot block is adjustable for that critical centre distance and showing the overall balance adjustment bar mount.
It is interesting to note that any variations in weight of the components are of no account in this type of device as you may easily correct errors with the sliding adjuster. What can never be corrected for are the distances between the weight points (posts) so there was a whole lot of very careful layout and drilling involved.
Even still, with minute differences in weight, slop on the posts and inertia friction in the bearings the indicator will often not point perfectly to the true north strong and free. Canadian made, eh?
A locking pin on a keeper wire holds the beam steady while you or the students add weights. Pull it out to find if your answer is correct! Anti slip material covers the base and rubber O-rings sit at the bottom of each post to allow you to get your fingertips around the lowest washer. Steel glides are nailed on 4 box sides.
When finishing up, what was amusing to me was that the very last operation on the build was to attach the carrying handle to the case with the beam inside – not at dead centre of course but at the combined balance point.
This will be a lasting (hopefully) contribution to my daughter’s small school as I’m not very good at baking.
Shellac followed by pre-cat lacquer.
37” x 7” x 5 1/2” Beam
39” x 8 1/2” x 7” Box
About 38 hrs.
Build on LJ’s.
Only 4 entries in the challenge so far, so let’s see many many more (and all the previous contestants for sure!). I’ll arm wrestle anybody for the t-shirt if there ever is one.
-- firstname.lastname@example.org : dovetail free since '53, critiques always welcome.