I think i may have my ring formula muddled up.

Is it: (3.14 x inside diameter) + metal thickness?

Thanks,

Rhonda

I think i may have my ring formula muddled up.

Is it: (3.14 x inside diameter) + metal thickness?

Thanks,

Rhonda

`I think i may have my ring formula muddled up. Is it: (3.14 x inside diameter) + metal thickness?`

Almost.

The inside diameter plux the metal thickness, the multiply the sum

of those two by pi.

So: 3.141 x (inside diameter + metal thickness) = blank length

You can also use the outside diamber, subtracting the metal

thickness, as in:

3.141 x (outside diamter - metal thickness) = blank length.

Or, if you’re bored and want to play math games, you can also use:

(3.141 x inside diamter) + (3.141 x metal thickness) = blank length

(that’s the same as the first formula, just rearranged.)

The point is that both the inside diameter and the metal thickness

are multiplied by pi, where your version just adds it on without

multiplying it by pi.

Peter Rowe

`I think i may have my ring formula muddled up. Is it: (3.14 x inside diameter) + metal thickness?`

No, its 3.14 x (diameter + metal thickness). It makes the

(reasonable) assumption that the neutral axis is in the centre of the

metal.

Regards, Gary Wooding

Hi Rhonda,

`I think i may have my ring formula muddled up. Is it: (3.14 x inside diameter) + metal thickness?`

You are close. It is Inside diameter + metal thickness x 3.14. If you

wait until you have multiplied by pi to add the metal thickness it

will be too small as the metal thickness also needs to be multiplied

by pi.

Cheers,

Lona

Hi Rhonda,

The ring formula is: inside diameter + metal thickness x 3.14. You

can also do: outside diameter - metal thickness x 3.14. It depends

on what you are using as your measurement.

Cassandra.

www.cbjewelrydesigns.com

According to my the formula is:

(inside diameter + metal thickness) x 3.14

Good luck

Joel

`According to my the formula is: (inside diameter + metal thickness) x 3.14`

This is correct. If you don’t add the metal thickness, the ring

should only connect at the right size on the inside edge of your

metal.

Hi cassandra and Lona

Your formula is almost correct. When the formula is written down you

must have brackets (is that the proper English word?) like this:

(inside diameter + metal thickness) x 3.14, otherwise it is not

correct for gettting the length of the ring blank. Without the

brackets you will have a different result.

Per

Hi Per,

`Your formula is almost correct. When the formula is written down you must have brackets (is that the proper English word?) like this: (inside diameter + metal thickness) x 3.14, otherwise it is not correct for gettting the length of the ring blank. Without the brackets you will have a different result.`

I really don’t understand why the brackets would make a difference

in the result. I have been doing it this way and teaching my students

how to do it for years. I just put the numbers in the calculator and

it always works out fine. But thanks anyway. I guess if you were

writing it down for somebody it would make a difference in their

understanding the formula.

Lona

Lona, see what happens to your calculator result if you write it as

3.14 * ID + metal thickness. You will get the wrong result and your

student’s blank will be too short. Do understand why? I’m sad that

calculators are used as not a tool to accelerate processing, but an

alternative to teaching math (which I hated, but understand, sort

of).

`I really don't understand why the brackets would make a difference in the result. I have been doing it this way and teaching my students how to do it for years. I just put the numbers in the calculator and it always works out fine.`

Parentheses override the order of operations when evaluating an

arithmetic expression (or have highest priority in the order). Not

having the parentheses in the expression (inside diameter + metal

thickness) x 3.14 would yield a very different answer because,

working from left to right, the multiplication would be performed

prior to the addition. Not sure what your calculator is doing or

what you are teaching students.

Robert

`really don't understand why the brackets would make a difference in the result. I have been doing it this way and teaching my students how to do it for years. I just put the numbers in the calculator and it always works out fine. But thanks anyway. I guess if you were writing it down for somebody it would make a difference in their understanding the formula.`

Some calculators perform their calculations in the order the buttons

are pressed. EG. 3+4*2 is calculated as 3+4 (which is 7) times 2 =
14. Other calculators do multiplication and division before addition
and subtraction, so 3+4*2 is calculated as 3 plus 4*2 (which is 8),

which comes to 11. Most modern calculators do it this way, so you

use brackets to remove the ambiguity. (3+4)*2 says; add 3 and 4, then

multiply the result by 2.

IHTH

Regards, Gary Wooding

`I really don't understand why the brackets would make a difference in the result. I have been doing it this way and teaching my students how to do it for years. I just put the numbers in the calculator and it always works out fine.`

You’re calculator may be just by accident doing it the right way.

But brackets make a big difference. Brackets determine what gets done

first.

In the case of this formula, (diameter+ metal thickness) x pi, which

is the correct formula, whether you add the two numbers together and

then multiply the whole by pi is quite different from without the

brackets, or the other position of the brackes, which would multiply

pi by one number and then add the second number to the result.

For example: Just numbers for this, not realistic ones, but to

illustrate the math.

diameter = 5

metal thickness = 5

with pi x (5+5) you have the equivalent of pi x 10, or 31.4159.

if you had the brackets around pi and the diameter instead, then the

formula would be (pi x 5) + 5. That is (15.707) +5, which equals

20.707. Very different result, depending on the position of the

brackets.

It’s a question of what the order of operation is. If you multiply

first, then pi gets multiplied by only one of the measurements. If

you add first, then pi gets multiplied by both.

With your calculator, you’re probably entering:

diameter + thickness x pi. No brackets. The calculator is simply

performing the operations in the order you entered them, which ends

up correct.

If you entered the same numbers as:

pi x diameter + thickness, you’d get the wrong result.

But it’s intuitive in this situation to enter the measurements

first, so you got lucky and did it right by accident.

Adding brackets to the equation means there is then no longer any

ambiguity as to which operation (multiplication or addition) is done

first (within the brackets is always done first), so the results no

longer vary according to which direction you write or enter the

equation. Simpler calculators don’t have bracket keys, though, so

then you simply have to enter the numbers in the right order.

Peter Rowe

You need the brackets to force the “order of operations” (good old

BEDMAS - Brackets, Exponents, Division, Multiplication, Addition,

Subtraction) without the brackets the first step is multiplying the

metal thickness by pi, THEN adding the inside diameter

example:

2.5 + 0.2 x 3.14

2.5 + 0.628

= 3.128

But with the brackets the addition happens first:

(2.5 + 0.2) x 3.14

2.7 x 3.14

= 8.478

A considerable difference, I’m sure you’d agree. The first one is

barely over the diameter of the ring, certainly not the

circumference. And here I thought that I’d never need math in the

’real world.’

Cheers,

Mark Wells

`I really don't understand why the brackets would make a difference in the result. I have been doing it this way and teaching my students how to do it for years. I just put the numbers in the calculator and it always works out fine`

There is such a thing as precedence of operations. Depends on a

calculator it may make a difference. Besides, some people do not use

calculators, so brackets are very important.

Leonid Surpin

www.studioarete.com

Hi Lona

I am certain that you are getting the right length of ring blanks

when you are working. You add inside diameter and material thickness,

and multiply that sum with 3.14. You are in fact using the formula

(inside diameter+thickness)x3.14

I’m having a little trouble in explaining this in English. Perhaps

someone can help me out? I just wanted to show you that there IS a

difference in these formulas. There is a difference in the order of

operations without the brackets.

I’ll give you an example when we compare the two different formulas:

Let’s say we have a ring with inside diameter, Di= 20 mm, and

material thickness, t=1mm

(Di + t) x 3.14 =(20+1) x 3.14 = 21 x 3.14 = 65.94 mm --> length of

ring blank

and the other formula without brackets:

Di + t x 3.14 = 20 + 1x3.14 = 23.14 mm

or if it is written down in this order:

t + Di x 3.14 = 1 + 20x3.14 = 1 +62.83 = 63.83 mm

I just wanted to make the point that the brackets are important if

the formula is written down in a instruction manual for example. I

hope I made it clearer now.

Per

Hi Per,

You are right. This formula needs brackets. I am confused myself too

because Carles Codina in book "The Complete Book of Jewelry Making"

p.31 made the same mistake.

Irwan

If you do it another way, and you know the inside circumference and

the metal thickness, how do you work out the length to cut? You could

divide the circumference by pi, but is there a shortcut to this

method?

With very basic equipment you can measure your finger with a strip

of paper.

Hoping for a simple and elegant solution!

Tamizan

Well, I have certainly learned something today about brackets but

would just like to say that my calculations always get me the result

that I need. But I would just like to say “thanks” to everybody who

gave me a math lesson today. This is one I won’t forget very soon.

Lona

Finally, a use for my mathematics degree

(inside diameter + metal thickness) x 3.14

The parens are a signal to perform the action within them FIRST,

THEN the action(s) outside the parens. If you have always been

getting the correct results without considering this fact then you

have been lucky - both you and your calculator are performing the

addition BEFORE the multiplication, which the parens are indicating.

Of course to anyone who has not been made aware of this usage, parens

don’t mean anything.

To those familiar with the terminology it is a clear message **NOT**

to multiply metal thickness by 3.14 and THEN add inside diameter,

which would yield the wrong result (too short).

Mary Partlan

White Branch Designs