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| Proportional Dividers
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What are Proportional Dividers; what use are they?
There are two major uses for proportional dividers in Shipmodelling: changing the
scale of drawings, and fitting planks.
Anyone doing a lot of either is advised to invest in a pair of these.
Proportional Dividers are often to be found on
eBay (Search for "proportional
divider"), but vary greatly in quality and price (as I write this, there are 3 listed on eBay,
from $9.99 to $30, but prices of $50 are not uncommon).
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| Proportional Dividers (after Underhill)
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These dividers are double-ended, and pivot on a central screw which slides
along a graduated scale on the body of the instrument. Moving this screw up or
down the scale varies the relative opening of the points at opposite ends.
Different graduations are available, but the most common is that which
indicates the number times the wider end is greater than the smaller. Thus if
the screw is set to "5" on the scale the ratio between the two ends will be 5
to 1, so that if the small end is opened to 1 in., the: points of the other end
will be 5 in. apart. The marking of the scale is however not really important,
for any ratio, no matter how unusual, can always be obtained by trial and
error. For example, if you should want a ratio of 1 in. to 1 5/16 in. (say,
when scaling from a photograph), it is only a matter of setting the points of
the large end to 1 5/16 in, and moving the screw along until those at the other
end are 1 in., and clamping it there. After which any measurement taken by the
small end will be reproduced 1 5/16 times larger at the other.
Scaling Drawings:
(This operation used to be much more common in the days BC - Before Copiers)
The use of this instrument in, say, enlarging your model is obvious, simply set
the screw at the enlargement you require, measure from your plan with the
small end, and set out the work with the other. If you are reducing the scale,
then the dividers are used the other way round, measuring with the large end
and setting off the work with the small.
Fitting Planks:
(This discription is derived from Plank-on-frame Models, by Harold Underhill.)
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| Using Proportional Dividers to fit planks (after Underhill)
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The fitting of the plank to the underside of the wale and into the rabbet of
the stem is done first, and the lines representing the centers of the
frames crossed by this plank are drawn on its face. Let us assume
that it has been decided to fit eight planks between the underside of the wale
and the center of the first batten, then you set the center screw on your
dividers to "8", which means that there is a ratio of 8 to 1 between the two
ends of the instrument, so having fitted the upper edge of your plank to the
wale, you simply place one point of the long end of the dividers against the
underside of the wale in the rabbet of the stem, and the other where the
centerline of the first batten enters the rabbet, turn the instrument over and
lay one point on the upper edge of the plank, and the other point will indicate
the depth of the plank at that point. Mark this then move to the first frame,
open the wide end to the space to be covered on this frame, and with the short
end mark the width of the plank on the pencil line representing the crossing of
that frame. Repeat this on each frame in turn and you will have a series of
dots which represent the true shape of the lower edge. The sketch will make all
this quite clear.
There is another advantage in using these dividers for setting out the
planking, and that is, should any slight error be made in the actual cutting of
the planks after marking out, the dividers will automatically correct this in
succeeding strakes, since when once the first strake has been laid fore and
aft, the dividers will be reset to the number of strakes remaining to be fitted
in that particular belt. In the example used here, that means that when fitting
the second strake down from the wale the dividers would be set at "7", for the
third strake "6" and so on. In other words, each space is treated independently
and on its own merits.
See also the
Shopnote on making your own Proportional Dividers.
{John O. Kopf}
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