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Determining cut size for rings


#1

I appreciate any help you can provide on how to figure the proper
cut length for ring sizes. I have printed out various ring size data
sheets that I have found on the internet, but they don’t ever seem to
work right. Clearly, I never properly learned this and therefore end
up with far too many trials and errors!

For example, it seems that the cut length for a size 11 ring is
different for a wide 1/2 inch band that is 18 gauge thick versus a 6
gauge wire band that is about 13 gauge in thickness.

What factors do I use to determine the correct length the first
time?

Thanks for any advice!
Sally
www.naked-jewelry.com


#2

Hi Sally,

I suppose the easiest method is to get a piece of binding wire and
wrap it around the finger, I read it in a book someplace… can’t
remember which one.

I’ve got a spreadsheet that gives you the length of metal required,
that I whipped together, it gives you 1 and 2 metal thicknesses.

You’re welcome to it if you like.
Regards Charles A.


#3
For example, it seems that the cut length for a size 11 ring is
different for a wide 1/2 inch band that is 18 gauge thick versus a
6 gauge wire band that is about 13 gauge in thickness. 

When strip is bend into round or any other shape, the following
takes place: Strip thickness theoretically divided into 3 zones. The
outer zone stretches, the middle remains unchanged, and the inner
zone theoretically should compress. Practically compression is not
taking place and that causes outer zone to stretch more than theory
predicts. The amount of stretching directly relates to thickness and
indirectly to width, but still proportionate to it.

For a formula to take all this into account, it has to be quite
complex which would render it useless in practical application. So
the practical approach is to calculate length smaller and stretch it
to size after soldering.

Finger size in millimeters times 3 is a good starting point.

Leonid Surpin
www.studioarete.com


#4

A couple of things here…

POINT 1.

According to my tables, SWG 11 is 0.116" and SWG 6 is 0.192". A size
11 ring has an ID (internal diameter) of 0.811". Whatever material
thickness you use, the OD (outside diameter) will be the ID plus
twice the material thickness.

Since the OD is bigger than the ID, the length of material around
the outside will be bigger than the length on the inside. When you
bend the material into a ring, the material on the inside gets
squashed and that on the outside gets stretched. Somewhere on the
inside, a thin section remains the same length - its neither squashed
nor stretched. It is usual to assume that this thin section is
exactly halfway through the thickness, and is called the “neutral
axis” (NI). The length of material at the NI is the length you need
to cut off to make a ring of the requited ID. This length is PI times
the diameter of the NI.

If you imagine the finished size 11 ring, the ID is 0.811", but the
diameter of the NI is 0.811 + the material thickness.

For SWG 11, the length of the NI is PI times 0.811+0.116 = 3.1416 x
0.927 = 2.912".

For SWG 6, the NI length is PI times 0.811+0.192 = 3.1416 x 1.003 =
3.151"

POINT 2.

People’s skin continuously produces sweat and oils etc., but since
it usually evaporates fairly quickly the skin normally feels pretty
dry. Narrow rings don’t cover up much skin, so they don’t noticeably
hinder evaporation of the sweat. Wide rings are a different matter.
If they fit snugly, the sweat can’t escape and the skin underneath
goes white and puffy, rather like the skin under an Elastoplast or
Bandaid. If this is allowed to continue for weeks or months, the skin
starts to get itchy and and can get very badly infected - in extreme
cases this can end with amputation!

To combat this, wide rings are made rather larger than narrow ones.
The actual amount is difficult to determine, but 1/2 or 1 whole (USA)
size is not unusual. Unfortunately, because fingers are often thicker
in summer than in winter, the actual ring size is often a compromise.

IHTH
Regards, Gary Wooding


#5

I have always had good results using the table that Hoover & Strong
provides for the basic inner diameter sizes for American ring sizes-
this is compatible with my equipment.

Then, the formula is the ID + metal thickness (usually in
millimeters); add these, then multiply by 3.14. In other words:

(ID + thickness) x 3.14 = blank length.

Personally, I usually make rings about a quarter size small and then
stretch them to fit; this covers any small measuring errors and also
work-hardens the ring metal.

I do find that when i use square or rectangular wire, the inside
compresses while the outside gets drawn out, so after forming into a
ring, the cross-section is a trapezoid and may need to be corrected.

Wider rings need to be either comfort-fit or somewhat larger to make
them easy to wear, since skin needs to breathe. However, starting
out by getting your blanks the right length to make the size you’re
aiming for is the best first step, I think. :slight_smile:

Amanda Fisher.


#6

Sally,

Usually the charts give the base size for ring if you measured the
inner circumference, but they do not allow for the thickness of the
ring. Some tables are conveniently broken down into mm measurements
for thickness in addition to the initial length of the band. I have a
chart from one of my Stuller catalogs that is always correct for size
according to my mandrel.

If you only have the inner diameter measure, then you need to add
the thickness of the metal to that length. The reason I prefer using
mm measurements to gauge measurements is because you allow for
soldering layers on to the band.

Here is a chart I found that is not the mm measure chart, which I
prefer, but one with different measures based on the gauge of the
metal:

Hope this helps,
Melissa S.


#7

A while ago I constructed a PDF containing a table of American,
British, and European ring sizes. http://tinyurl.com/39ve3cq

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://tinyurl.com/39m27da 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


#8
So the practical approach is to calculate length smaller and
stretch it to size after soldering. 

yeah that works well for thinner, softer material. For thicker stuff,
and I mean ‘thicker’, if one cuts the free length to theoretical,
when one bends the straight wire into a circle one will usually find
that the ends just do not meet correctly and no amount of persuasion
will coax the ends to a complete clean butt. True, you can bend to a
D shape first but heavier harder material doesn’t like this too much.
You can choose to deliberately overshoot the target size, bend, then
cut thru the join to true it up, making it smaller in the process.