>I'm trying my hand at constructing a ratio based epoxy scale. I got the
>plans from the designer after reading an article about his balance
>(called the Super Epoxy Mixing Balance) in Sport Aviation a few months
>back. Will it work? I have no idea, but the project has gotten me in
>the garage and finding out how much fun it is to take a bunch of pieces
>and wind up with something that actually resembles the picture in the
>plans.
Steve - you may be interested in my thoughts in response to that article.
On 10th April 1998, I sent the following to the editor of "Sport Aviation"
but so far have had no reply, nor seen any response in the mag.
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I read with some interest the description of Melvin Levine's "Super Epoxy
Mixing Balance" in Craftman's Corner, Sport Aviation, Vol 47, No 4, p123ff,
as I have recently been comparing the merits of several other epoxy
balances.
Melvin's design seems simple to implement, and its resolution depends
mainly on beam length and pivot quality. Its accuracy, on the other hand,
depends on careful measurement of the distances (from the datum line
directly below the pivot) to the centres of the two cup positions. The
detailed construction manual which Melvin offers probably addresses
adequately the question of how to ensure that the pivot axle is exactly
above and parallel to the datum line on the balance beam. As a general
principle, I'd recommend keeping the pivot height above the beam to the
minimum - reducing this will improve sensitivity, but will, of course,
increase settling time.
However, I was intrigued by the distances chosen for the two cup centres.
Melvin suggests the use of a steel rule with 1/64" graduations, but one of
his dimensions (4.196") does not fall on a 1/64" interval, thus it will be
difficult to transfer that dimension accurately to the balance beam when
marking-out. Further, using this dimension will give very slightly less
than exactly the 43:100 ratio required - about 0.02% light on hardener.
However, I find that there is a pair of dimensions which both fall exactly
on 1/64" graduations, AND also give the exact ratio of 43:100. Make the
resin-only cup distance 4 44/64" (4.6875") and the resin+hardener cup
distance 6 45/64" (6.703125"). These are the only (imperial-fraction)
dimensions near to Melvin's original values which satisfy both these
conditions.
Another approach, probably more attractive to those outside USA, would be
to use a steel rule graduated in millimetres (mm). The nearest convenient
distances would then be 100mm and 143mm.
To obtain the best repeatability, it might be desirable to use a beam twice
or three times the length of that described, which would reduce in inverse
proportion the effects of small absolute errors in marking-out (and of cup
placement errors in use). For instance, those using SI metric dimensions
might like to use 200mm & 286mm or even 300mm & 429mm
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>Now to the point (sorry<g>). What are the mixing ratios of the
>expoxy/resin combinations for the Europa?
Resin to hardener - 4:1 by mass.
There are other balance designs (such as Jon Tye's, that I wrote up in EF
#12) which I feel are better suited to amateur construction, while
providing more than adequate precision. However, that design may not suit
everyone's mode of working.
cheers
Rowland
... that's Rowland with a 'w' ...
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