Short version:

Jim is a hard act to follow, but going to the most absolutely basic
level of what dividers are good for and what kinds there are:

Long version:

Any time you want to transfer a measurement from one place to another
you use dividers. Especially if you want to transfer a measurement from
one place to several; even direct transfer is less accurate if you are
making more than one new mark. Set the dividers and scratch or poke a
mark. All the time, every day. I don't think I use any tool more often.

To divide a line into any desired number of equal spaces, in a human-
sized working environment, the easy and highly accurate way is to "step
off" the divisions: set the dividers to your guess of the desired one-
division distance, set one point at the start of the line, and step
them along the desired number of divisions. You will be a little short
or long, so correct the setting by your guess of the proper correction
and step off the distance again. Each repetition comes closer the
correct, and it takes very little practice before you have it as
perfect as humanly possible in two or three settings. There are more
purely geometrical ways to divide a line, but none faster, easier, or
more precise.

Of course you can scribe arcs and circles with dividers, but one
important property is that when you use a setting to scratch a circle,
then you can step around the circle with the same setting and you will
get six equal divisions. Elaboration of this fact is the basis for
roundels in chip carving, hex signs, and a lot of other basic
decoration.

In fact, dividers used with a straightedge are why rulers are completely
unnecessary as well as being extremely inaccurate.

Some other tools do specific work better; but, for instance, if you have
no marking gauge you can do its work with dividers, whereas you can't do
all the work of dividers with a marking gauge.

Often you need several dimensions, and you want to save each one for a
while; so you need several (in fact many) sets of dividers, and it is
good if you can tell them apart. This is OK because top-quality dividers
are common and inexpensive in flea markets, and the best ones cost no
more than poor ones (in my area I feel stupid if I pay $10, though $5
isn't common any more).

The usual dividers for woodworking nowadays are machinists' spring
dividers, which have spread to most of the trades that use dividers
(that is, to most trades that make things). My favorites for both
woodworking and bookbinding are Starrett 6" "Fay" spring dividers with a
"speed nut" similar to the speed nut on a good bench vise:

http://www.starrett.com/metrology/product-detail?k=77B-6

Starret makes three grades of spring dividers: the "Fay" with solid rectangular-
section legs and the screw passing through the leg, the inferior
"Yankee" with thin rectangular legs and the screw passing through a
stud sticking out from the leg, and the "toolmakers'" with round-
section legs and a finer screw. The "Yankee"-style are what you
commonly see, by other good and bad makers as well as=A0 by Starrett,
but they are weaker and less elegant; the "toolmakers'" are slow
because of the fine screw and their extra precision isn't needed for my
work. "Fay" are best. If you consider buying them new (I did, one time)
bear it in bind that they come with crude conical points--- you are
expected to shape the points to your own needs and tastes. I have
points with a range of sharpness for different uses, a few of which I
reshaped or corrected but many with the points as I got them. And I
have a good many 4" pairs (I used to prefer them) and pairs with solid
(slow) nuts and "Yankee"-type pairs. Almost all cost $10 or less,
except my first pair of mediocre common-hardware-store Generals (now
about $15) and the one pair of new Starrets (now $100). Really poor
spring dividers are rare, though they do exist (I mean original
manufacture, not condition; poor condition is common enough).

Wing dividers are also a good choice for woodworking, since (like spring
dividers) they will hold a setting reliably:

http://www.csosborne.com/no106.htm

The fine-adjustment screw outside the leg is a refinement, so don't
worry if a pair was made without it; but make sure the main screw is present--
it is a pain to try to replace it, and while wing dividers are less
common than spring dividers, they are common enough. I don't use mine
very often, as they feel a bit heavy in my hand, but that is more what I
am accustomed to for bookbinding; the extra sturdiness of wing dividers
is an asset for woodworking. Again, be prepared to reshape the points to
your taste and to bring them even, using files and grinding.

"Lancashire pattern" spring dividers (with both legs and the bow
forged elegantly as one piece) are often regarded as rare-and-
historic antiques:

http://www.davistownmuseum.org/bioStubs.htm

but I think they must have been made much later into the 20th century
than dealers and collectors want to believe; I have found them in the
wild, and the pair I keep on my benchtop was no more expensive than any
of my other pairs. Their drawback is that they are slow, since they
never have speed nuts and tend to be stiff.

There is a wide variety of elegant spring dividers made for mechanical
drawing, but I find them a bit too delicate for even bookbinding, much
less woodworking, and in general they have no advantage I can see over
machinists' dividers. They usually were made with alternate points for
ink, pencil, and double metal points, and usually you find them with
just one of the alternatives in place.

Friction-joint dividers were historically older, and they are still
made, especially for "hermaphrodite" calliper-and-divider combinations:

http://www.starrett.com/metrology/product-detail?k=243-6

I've never trusted them to hold their settings, so I don't have any
(except in sets of drafting tools, which don't count).

I'm running down, your patience is probably exhausted, and I don't
think I answered the actual question. But, for what its worth, there it
is. No tool is more necessary-- once you wean yourself from rulers.=A0
=A0 =A0 =A0

Tom Conroy


Ron Harper wrote:

"I realize that this is the post that will fill my quota for the day,
but alas I am in need of guidance. Where on the net can I find out what
I can do with dividers? As a power tool guy, I never used them."

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