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Calculating ring blank length


#1

Hi,

I wanted to make a worry ring, using 22 ga sheet for the base. I
knew that the final size was to be an eight, and since I was going to
plannish (? hammer the heck out of it) it, decided to start with a
size 7 1/4.My math is just plain wrong, and I need help please, as I
ended up starting with a size 6 and a half.Here’s what I did:The ring
blank was 9mm wide, and since I wanted a starting size of 7 1/4, I
took 54.7 mm and added 2(.73) for two times 22 ga thickness (this is
obviously where my math is wrong - ha! ), and got 56.16mm. I decided
to be a bit generous and round up to 58 mm, to allow for filing of
terribly crooked ends. Soldered the ring such and promptly ended up
with a 6.5… Can someone help me with the correct formula? Or
point me to a resource? I have a book with a table, but doesn’t go as
thin as 22 ga, so I still need to know how to calculate correctly.

Thanks soooooo much!
Ros


#2
Can someone help me with the correct formula? 

Use a digital caliper, or otherwise find the diameter of the ring
mandrel at the size you wish. Book charts of ring sizes also work.
Take this diameter, add to it the thickness of the metal. Take this
total and multiply by pi (3.14159, or use the pi key if you’ve got a
scientific calculator. The result should be your ring blank length.
For wider rings, add a smidgin more.


#3

Let’s take a look at math first: Length of the circumference of a
circle is diameter X 3.14. For better precision 3.14158 can be used.
That is the whole math.

In practice however, there are complications. First is thickness to
account for. General recommendation is to add one thickness to
diameter and multiply the resulting sum by 3.14. Sounds easy enough.

The problem is that when we bend something, we increasing it’s
length. By how much? It depends on technique. With very careful
bending, not very thick gage, and good pliers - very little. Heavy
gage; bad bending technique, pliers not suitable for the task, and
etc… and degree of deformation can be striking, so it is not
possible to give any meaningful recommendation.

Personally, I am using diameter + one thickness and the sum is
multiplied by 3, not 3.14. As thickness increases I may use 2.9 or
even less. The difference between 3.14 and actual multiplier is known
as “fudge factor”, and it is different for any goldsmith, and
situation. You simple have to find you own.

Leonid Surpin
www.studioarete.com


#4

This is a reprint of a post I made to Ganoksin last summer.

A while ago I constructed a PDF containing a table of American,
British, and European ring sizes.

The European sizes are very useful because they give the length of
metal for that particular diameter. For example, USA size 11 has an
ID of 0.811". The nearest European size is 65, which has an ID of
0.8146", ie less than 4 thousandths of an inch bigger.

A useful rule of thumb is: Measure the thickness of the metal in mm,
multiply by 3 and add it the the European size to get the length of
metal required.

For example, from the following table
http://www.ganoksin.com/gnkurl/5r

SWG 6 is 4.877mm and SWG 11 is 2.946mm.

Length of metal for size 11 in gauge 6 is 65 + 3x4.877 = 65 + 14.63 =
79.63mm, and for gauge 11 its 65 + 3x2.946 = 73.84mm.

Multiplying by 3 instead of 3.142 is quite accurate enough, and
tends to give a slightly smaller size that is easily stretched to the
required size.

Regards, Gary Wooding


#5

kay, got it - diameter plus one thickness times something around 3. I
must be a goof to have actually lost length rather than gained - ha!
Or measured terribly, terribly wrong (which is also a possibility).
Thanks for responding, I’ll be trying my hand at this again soon and
will keep your response handy.

Ros


#6

Ros,

Too much math.

My commercial mandrel has ethced marks for stock ring blanks. Not
dead accurate but close. Wrap a piece of wire round the mandrel at
the desired size and measure its length. Subtract a little for
hammering (gut feeling stuff). Wacking it with a hammer gives you
some give and take although 22 ga is already pretty thin for a ring
I think (straight MM here, gauge is for rail road tracks and is
slightly old)

jeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand


#7

I just had another thought - I am using a tapered ring mandrel -
would height (it was relatively wide, at 9 mm) of the ring blank have
anything to do with me having lost .75 of a size?

Cheers, Ros


#8

Thanks Jeff - I’m already behind on the times, and I’ve just gotten
going!

Will have to find a table, showing me mm equivalents for gauges (and
there seems to be a bit of a range…) Yes, 22 ga is probably
too thin, I’ll look at at 20 ga…errrrr…0.64 mm/0.63?/.61 mm :wink:
in the future. I had done the old wrap a piece of something else
around the mandrel and still found loads of room for measuring error.

Cheers,
Ros


#9

Hello Ros,

Yes - always reverse the ring and hammer again on the mandrel to
even up both sides of the ring. Plus the wider the band, the more
friction/suction is created between the band and the finger. I’ve
found that the final size should be about half a size larger than
what would be appropriate for a narrow (2-3mm) band.

Judy in Kansas, where it will be 72F today. Maybe all the snow will
finally melt before the next batch falls!!


#10
Too much math. 

My commercial mandrel has etched marks for stock ring blanks. Not
dead accurate but close. Wrap a piece of wire round the mandrel at
the desired size and measure its length. Subtract a little for
hammering (gut feeling stuff). Wacking it with a hammer gives you
some give and take although 22 ga is already pretty thin for a ring I
think (straight MM here, gauge is for rail road tracks and is >
slightly old)

Only quibble I have with Jeff’s advise about how most of us ~really~
make rings is that 22 ga. is WAY too thin. 16 ga. (1.5mm - gauges are
for trains) would be a minimum, for me. And for a peened ring maybe 2
sizes small, to start. You can always hammer some more… I’ve also
done it with a large ball bur, and then rubber wheel it all.
Laborious but then there’s nothing about hitting the right size.
Sometimes that’s the only possible way for various reasons.

I use the scale on the mandrel always and always have. Never
calculated a shank in my life. Never calculated a bezel, either.
Just wrap it around the stone, doh…


#11

Apx 55mm IS the starting length for FS 6 1/2 (before accounting for
gauge) 58mm would give you a 7 1/4 +/- I don’t believe its necessary
to go out two decimal places for a ring size Formula
schmormula…put dividers an a ring stick, add a smidge


#12

Hi Ros,

Somebody once said “The good thing about standards is that there are
so many to choose from.”.

This is particularly true of wire gauges. It wouldn’t so bad if the
actual standard was stated, but it seldom is.

What, for example, is 20ga? According to four “popular” standards,
it’s one of the following: 0.9144mm, 0.889mm, 0.8838mm, or 0.810mm.
You choose.

Here is a nice comparison table I found…
http://www.ganoksin.com/gnkurl/67

I hope it helps.

I personally stick to the metric system: millimetres are very
conveniently sized for jewellery; inches are too big.

Regards, Gary Wooding


#13
I use the scale on the mandrel always and always have. Never
calculated a shank in my life. Never calculated a bezel, either.
Just wrap it around the stone, doh..... 

I am sure that everybody has a favorite way to do things. That fact
should not obscure that there are situations that require bezels to
be made to exact specifications, and in these instances methods based
on eyeballing and rough approximations are not appropriate.

The following is addressed to beginners only:

Some goldsmiths wear their disdain for mathematics like a badge of
honor. Developing such an attitude should be avoided at all costs. No
serious work can be done without solid foundation in at least
elementary mathematics.

Leonid Surpin
www.studioarete.com


#14

Roz, the diameter of the ring size desired plus the wall thickness
times pi (3.14) will give you the length of metal to make a ring.

Have fun.
Tom Arnold


#15

Leonid

I am sure that everybody has a favorite way to do things. That
fact should not obscure that there are situations that require
bezels to be made to exact specifications, and in these instances
methods based on eyeballing and rough approximations are not
appropriate. 

I’m one of those nasty people who use cnc. .001 mm really does make
a difference, hand fab work and I eye ball it. Hammers etc cover all
sorts of stuff. No problems with eye balling, and an old habit of
jewellers which goes back forever.

JeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand


#16

I choose stock sizes in millimetres. No ambiguity then.

Jen


#17

Thanks Judy (it is so NOT 72 degrees F here today - hahahaha! Wind,
snow, ice…some more ice) - normally for a 9 mm band, I would
definitely make the ring a bit bigger, but when flare the ends out, I
find that the ring fits like a narrow band… I have read something
about wide band mandrels…and wide band ring sizers…Probably
easier just to add a half size, right?

Cheers,
Ros


#18

Ha, thanks so much Gary! I’ve book marked that page. I never thought
that I’d enjoy a tool like digital calipers as much as I do (though I
start to feel panicky if I press the wrong button and it spits out
fractions).

Cheers, Ros


#19

Thanks John - got it, loud and clear - 22 ga is too thin, will use
it rolling out thinner for bezels. Maybe some backplates too. But 16
ga? I can’t cut it with my shears (you’re going to tell me to cut it
with my saw, right? On a long length, I start to weeble and wobble
with the saw and lose interest part way down the sheet)…

Cheers, Ros


#20
their disdain for mathematics like a badge of honor 

Well, I dunno about that but I do put great stock in observable
fact.

Observable fact number one…mathematical calculations did not work
for the OP OF number two…this is a recurring question here and
obviously calculations don’t work for quite a few orchidites OF
three…there is no math involved with a ring stake and folks do
quite well with them.

One could counter that the problem is one of deficient math skills.
OK, show of hands here…who’s willing to take remedial math courses
just so they can make ring blanks?

Nowhere is it written in stone that a ring blank or bezel MUST come
out 100% correct the first time out. You can make em smaller you can
make em bigger, its not a sign of human failure to go back and
adjust. In fact its preferable. Example, you calculate and measure a
length of 4mm square wire stock. Math says its perfect. Then you
bend that puppy only to find ends that, gasp!, are not square and
leave a big tapered gap. A more efficient way would be to make the
length extra extra long, bend it around and overlap the ends(like
jump ring making) and then make a simple saw cut and you’re home. Now
you will have a ring blank that is not inherently stressed because
you had to beat the bulldinkies out of it to make it comform.