Measure Twice Cut Once, But What Is The Measure?

[camano ridge]

Actually Jim has a western holster making tutorial pinned on these forums. The first page shows how he uses the strip. I have used the method and it does work. http://leatherworker.net/forum/index.php?showtopic=19338&page=1

Edited June 24, 2015 by camano ridge

[RKCrowe]

Jim when I think real hard, I usually lube the brain with some Makers Mark. I think I will think hard tonight.

[byggyns]

If you want the math, take the diameter of your object, add the thickness of the foam x2, add the thickness of the leather x2, add 1mm for interference- nothing fits together exactly with no gaps. Multiply that sum by Pi ~ 3.14. That would get you a close to perfect butt joint. Since you want a 10mm overlap, add that 10 mm to the total, then add the leather thickness x2. That should get you pretty close to the measurement you want.

An example is a 70mm cylinder, 2mm foam, 3mm leather. Add them together with the allowance for inexact fit, you get 81. Multiply that times Pi, and you get approx 254 mm. This should be very close for a butt joint.

Add your 10 mm overlap and your leather thickness twice, and you end up with approx 270 mm.

The biggest issues most people have with the math are

1. forgetting to add the thicknesses of the wrap layers twice before multiplying: my normal mistake- I only add the thicknesses of each layer once
2. forgetting that the leather measurement should be for the outside of the leather, since the outside grain does not stretch much, but the flesh side will compress more easily- so they don't add the leather thickness at all before multiplying
3. not giving any allowance for the inexact fit between the layers
4. not taking into account the leather thickness for the overlap: not adding it at all, or not adding an additional allowance for the bend outwards

ETA: If you are overlapping the foam, you will have to add even more length onto the leather for that added thickness.

Edited June 25, 2015 by byggyns

[billybopp]

Doing the math is often the way to go, but sometimes the practical - simple approach is best!

It reminds me of a story that I've heard, but cannot say if it is fact or fiction. Thomas Edison came across one of his engineers hard at work with pencil and paper looking at the glass globe part of the prototype light bulb that was sitting in front of him. When asked what the engineer was doing he replied that he was trying to calculate the volume of the bulb. Edison grabbed the glass globe, filled it with water and drained it into a graduated cylinder and told the engineer "There's your answer".

Bill

[Jim]

And is why I use the KISS method;

Keep

It

Simple

Slim

Leather work has it's own rules of math that is not found in any books.

[byggyns]

Honestly, Jim, I use your method more often than doing the math. The practical, rather than theoretical approach tends to be faster.

Mostly, I use the math when making an initial pattern for a prototype. That way, I can layout any other pieces of the project and / or the tooling pattern to see how the composition will come together. Then, I revise the final dimensions with the test strap. The math also helps if you don't have the object on hand to test at first.

Mainly, I provided the math because he asked for a formula due to being frustrated with his own calculations. If he used the same calculations I provided and still came up short, then I can't explain that. It gets me pretty close each time, but as you said leather has its own rules and all hides are a little bit different in how they behave.

[RKCrowe]

Yep, byggyns, I left out the step of multiplying the sum by pi. I will use it for initial project planning and verified by Jims strap method.

[RKCrowe]

Well, I finished the koozie! I used the math formula and the strap method to come up with my measurements. I decided to use butted joint instead of the overlapped joint.

The project came out great, the can goes in/out with just a hint of resistance from air pressure/vacuum.

Thank you all for your advice, suggestions and experiences.

Richard

[ENC]

The formula is C= 2 x pi x R. where C is circumference R is the radius of a circle and pi is a constant, 3.14. So any change in radius is going to change the circumference by 6.28 times the change in radius, regardless of what the radius change is.

Ray

[silverwingit]

One thing to be aware of any time you intend to use a lining with a curve in the primary leather. You need to glue the lining to the primary in close to the same curvature. If you cement the liner to the primary when they're flat, the lining will wrinkle badly when you bend the primary leather to shape.

Michelle