Finding the center of circle

Jewelry Discussion
1

Make a jig with a 5/8, 7/8 and 1 inch diameter depression. Make a
pilot hole in the center of each depression. Use the pilot hole and a
drill bit to align the jig and clamp it in a drill press.

Kay

2

Hi Karen.

I am a goldsmith in the UK and I often have to find the centre of
discs. The first thing you must buy yourself is a disc centre lining
tool, they are not expensive probably only a few dollars, they are
even easy to make for yourself. They are steel and consist of a right
angle with a straight edge disecting the right angle so that you can
scribe a line disecting the disc, and by turning the disc around and
scribing a second line you have a dead centre where the line cross. I
could send you a photo of my centering tool if you want. My second
suggestion is for you to make a marking jig, I have made these type
of jigs to speed up production when needed. Jig making method as
follows:-

  1. cut a square of base metal ( copper or brass ) slightly thinner
    than the disc thickness,then pierce out a perfect fit cavity circle
    for your disc size in the square centre.

  2. rivet the square base metal to a smae size piece of thin tool
    steel ( rivet in each corner ).

  3. do the geometry and find the centre of one disc, place in cavity
    and drill a hole in the steel

  4. anneal the whole jig and drop in water when dull red hot, this
    will harden the steel.

  5. clean up and you have a jig that discs will fit into while you
    can mark dead centre with your pendant drill.

Please let me know if you would like detailed sketches.

I hope this all make sense, good luck, regards James
see my work here if you are interested
https://www.ganoksin.com/orchid/jmdesign.htm

3

If theses discs you are drilling are all of the same size, then I
would make the following jig out of two pieces of scrap sheet. You
can use brass, copper, nickel, whatever you have lying around.

For an example, I’ll use 1/2" as a diameter of the disc. Carefully
cut a hole into one piece of sheet that is the SAME THICKNESS as what
your disc is going to be. Use dividers or a template to lay it out.
You want to make it a close tolerance, not quite a press fit, nor
sloppy when you drop in your 1/2" disc.

Back this plate with a solid plate. Thickness of this plate is not
so critical. You can solder the two plates together with low temp
solder or even rivet them together.

Now VERY carefully find your center in the 1/2" hole with the aid of
dividers and drill through the backing plate. It should be a simple
matter to slide your 1/2" disc into the hole and mark it from the
opposite side. Use a fine pointed scribe or an automatic center
punch. The automatic punch will probably need the point ground finer
than what most manufacturers put on.

You could also make a variation on this jig that could be clamped
into a drill press. The problem I can see with this - is how to keep
the disc from spinning in the jig while you are drilling? The first
remedy that comes to mind would be the use of a small pair of vise
grips with padded or soft metal jaws…

Brian P. Marshall
Stockton Jewelry Arts School
Stockton, CA USA
www.jewelryartschool.com