For hundreds of years dividers and calipers relied on a
single adjustment process to hold the legs in place. This usually involved friction at the main joint or a
thumb screw bearing down on a wing. It’s unclear when a second adjustment step
was added to the tool whereby the user could fine tune the spacing of the
points once they were roughly set. This is almost always seen in wing dividers. My guess is this option became
more common during the 1600’s. A 1709 print by Nicholas Bion shows a pair of
wing dividers (“d” below rt.) with a fine adjustment wing nut attached to the end of the
wing. If you look at the wing dividers in the painting “God the Geometer” from
c.1250 there is a wing nut on the outside of one leg which could have been for
fine adjustment. As you might predict, there are numerous ways that fine
adjustments were carried out. The focus of this post is to look at some of the
ways this occurred.
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God the Geometer
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Bion: Mathematical Instruments
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Likely the earliest and most widespread technique to fine tune the points was to extend a rounded section of the otherwise flat wing through
the leg opposite the leg with the locking wing thumb screw. If this protruding end was threaded, the wing could be moved a small distance in or out with a
wing nut. For this technique to work effectively there needed to be a way to put
pressure on the wing on the inside of the fine adjustment leg. Typically,
this was done by attaching a slightly curved piece of spring steel on the inside
of the fine adjustment leg near the joint with the other end pulling on the wing, exerting a force
opposite that of the thumb screw on the outside of the leg. This style has been in use for several hundred years. It’s common to
find examples of dividers where this piece is missing or broken.
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Sullivan's 1880 pat.
The most common form of fine adjustment |
In some early dividers a second locking wing nut passes
through the fine adjustment leg and presses on the wing extension within the leg and
locks the wing in place once the fine adjustment is complete. In the second photo below the main wing locking screw is on the left leg and not visible.
J. Stevens #61 |
J. Fenn, London, 1821-1873
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A later development was to replace the curved spring
steel piece with a small, coiled spring wrapped around the end of the thin wing
extension which pressed against the inside of the leg and against a stop on the
wider part of the wing. These variations are best understood by referring to
the photos rather than my convoluted descriptions.
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| Starrett #85 |
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Stanley #57
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What follows are a number of examples with various kinds of fine tuning mechanisms.
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Unmarked with "roller nut"
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W. Schollhorn "Pat. Appplied For"
The fine adjustment is on the left leg. |
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W. Schollhorn 1866 pat. +
1890 pat. for fine tuning on right leg + patented pencil holder
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Starrett #62
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Early example with threaded insert in top leg
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An interesting unmarked piece with an internal spring in the main joint to maintain tension.
Likely a user made caliper with Rube Goldberg fine adjustment in the upper left
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Rare to find wooden dividers with a fine tuning option
(doesn't work very well)
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Top thumb screw locks legs. Central knurled knob fine adjusts.
A final technique I'll mention for fine tuning dividers utilizes
eccentric lower leg points. Since the points are not in a direct line with the
legs above them, by rotating a lower leg one can change the separation of the points slightly. This can only work on dividers where the lower legs can rotate freely from the upper legs and lock . In the Stevens # 62 below the eccentric points provode a second fine tuning option that supplements the primary mechanism in the upper left leg.
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| Eccentric points on a J. Stevens # 62, 1890 pat. |
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