John B. and other diallists (love the pun)
As an amateur machinist interested in dials I have always been
curious about how the early makers actually made the ring dials with
the slider.
If I was asked to do it from scratch, I would have to think a fair
bit about it. Is the slider dovetailed in the groove so that it can
slide around the circumference of the ring but not fall out? If so,
how do you get the slider into the groove?
I cannot visualise how I would machine a dovetailed groove on the
exterior of a ring. I could do it on a flat strip. No real
problem with a dovetail cutter in a milling machine, even a
small one like my Unimat 3. Once the groove is cut, curve the strip
and solder into a ring. Presumably I would put the dovetailed slider
into the groove BEFORE soldering it all together. (And trust that I
was a neat worker and my solder didn't run everywhere!
A simpler way would be to cut a square-edged groove (i.e. not
dovetailed) on the outside of the ring (a trivial lathe
operation). In the floor of this groove, cut a narrower slot
breaking thru the ring (again trivial, BUT not in a lathe! Why?
Because you have just parted the ring in two! Better to use chain
drill and file, or use a milling machine). You need this anyway
for the spot of light to shine thru the pinhole in the slider, thru
the ring to the graduations on the other side. Place the slider in
the wide groove and hold it in place, but slidable, with a couple of
screws or rivets placed from inside and running in the "thru slot".
Is this what they did???
Before you start any of this you have to know where to place the
groove in relation to the graduations. PLUS, the correct position of
the zero mark on the side of the groove so that you can adjust the
slider to the correct latitude.
Some of these apparently simple dials offer a challenge if you
actually want to make one.
Anyone got any thoughts???
Cheers, John
Dr John Pickard
Senior Lecturer, Environmental Planning
Graduate School of the Environment
Macquarie University, NSW 2109 Australia
Phone + 61 2 9850 7981 (work)
+ 61 2 9482 8647 (home)
Fax + 61 2 9850 7972 (work)
