At their most basic, dividers consist of two legs hinged at one end. The hinge joint has to serve two somewhat opposing functions; it has to be loose enough to allow the legs to be moved apart and tight enough to prevent the legs from too easily moving when set. Many of the variations in construction of dividers focus on resolving these opposing actions. The hinge end of a pair of dividers is typically forged into two or more plates or leaves which are then pierced and fastened together with a rivet or less often with a threaded nut. For the moment I am going to exclude wooden, bent-wood dividers such as the traditional European cooper’s dividers and metal spring dividers from this discussion.
The flat hinge leaves can slide past each other and allow
the legs to move. At the same time the contact surfaces provide friction which
keeps the legs from moving too freely. The greater the surface area in contact,
the greater the potential friction. The most common way to increase the surface
area is to increase the number of leaves that make up the hinge. The simplest hinge with two plates or leaves
is called a lap joint. This is typical of craftsman-made wooden and metal dividers,
flat stock calipers and many more recent dividers such as those made by
Starrett. Two-leaf examples are generally supplemented with stout outside
washers.
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| Two-leaf hinge with washers |
A three-leaf hinge has two leaves on one leg and one leaf
on the other leg, creating a box or knuckle joint. This has the advantage of
enclosing the end of one leg within the end of the other leg which keeps the
hinge more secure and doubles the surface area and thus the friction provided
by the leaves. Without a doubt, this is the most common hinge type found on
antique dividers.
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| Three-leaf hinge with brass spacers |
I have yet to see an example with four leaves. This arrangement would have the disadvantage of offsetting the contact of the legs by a significant amount. A more complex variation utilizes five leaves. Again, the more leaves in contact, the more friction available. In a five-leaf hinge one of the leg ends is split into three leaves, the other into two leaves. It strikes me that this is probably not an easy task for the blacksmith, but then my forging background is non-existent. In constructing the hinge the maker can flatten one leg end and split it or saw it twice, the other leg once. Alternatively, three separate pieces of iron could be welded to each other along most of their length leaving three flattened ends. The early types of five-leaf hinge often show what I believe is termed a “coldshut” or crack at the base of the leaf extending a bit down the leg. The presence of a coldshut would argue that the leaves were not created by sawing. A blacksmith out there will have to explain to us how these hinges are created. A well-wrought five-leaf hinge is a significant plus in the value of a tool. A recent responder to my post on the Old Tools Listserv mentions he has seen seven-leaf hinges (4+3). These have to be extremely uncommon. Is there a law of diminishing returns when it comes to the number of leaves in a hinge of a pair of dividers
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| Five-leaf hinge with coldshut |
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| Five-leaf hinge, sawn |
Washers are often added under the rivet ends of two, three and five leaf dividers. In exceptionally well-crafted five- leaf hinges, brass washers are inserted between the leaves. The brass in contact with the iron provides even greater friction than iron on iron and reduces wear that can occur when two surfaces of the same metal (except hardened steel) are in contact. It is common to find leaf hinge dividers flattened at the top of the joint by a hammer in a crude attempt to tighten the hinge.
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| Flattened hinge |
The shape of the outside surface of the hinge rivet is often a good indication of the age of the tool. Brass “ball joint” (square joint) dividers where the rivet faces are rounded and often faceted are typical of the 18th century and earlier. In the late 18th and early 19th century iron and steel, flat “rule joint” dividers appear, especially in England. The name derives from the hinges resemblance to a traditional boxwood folding rule joint. They often have concentric rings on the flat outside surface of the hinge. Cone shaped three-dimensional joint ends show up in the 19th century and possibly earlier.
 Ball, rule and cone shaped hinges |
An alternative to the use of a rivet to hold the joint
together is found in many early drafting set dividers and some larger non-drafting
examples. This type uses what is now referred to as a “2 hole pig nut” screw/machine
bolt. These fasteners allow the joint to be loosened or tightened with the use
of an appropriate two pin key (often missing from the drafting set). Collecting
drafting instruments is a field all to itself. William Ford Stanley’s Mathematical Drawing and Measuring
Instruments (1900, 7thed, Spon, London) gives a detailed account
of the many different kinds of drafting instruments. This source is available
in its entirety on line (www.books.google.com).
Drafting dividers free from sets often turn up in sales. They are often quite
complex and generally finely crafted of brass, steel or even precious metals.
 "Pig nose" screws |
Today's Mystery Dividers No. 2
Twelve inch, inside calipers, unmarked. Most likely user made, not old, but very well constructed, possibly by Rube Goldberg and Associates. Fine adjustment knob to the left. Middle knurled knob on right allows for transfer measurement.
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| Mystery dividers #2 |
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