Hi guys...
Metric to Inches. Inches to Metric. Decimal inches to fractional inches. Metric MM to Fractional Inches! Geeeeez! I just wanna make beads! Well, sometimes ya just gotta do the math, or ask and wait for an answer, and hope it's right...
Lots of you don't need any help with this, so please feel free to skip it. Oh, you already have? See ya! But if you are still reading this, this is just for you. Rather than just throwing numbers and fractions at you, I'll try and give you an INTUITIVE feel for what's going on, and end the confusion for good, I hope.
Ok, this can seem confusing and complicated to some folks, but only until you get used to it. After that it's really pretty straightforward. I'll make it very slow, thorough, (therefore long, sorry,) and work a couple examples for you. I'll also talk a bit about figuring inside diameters of tubes given outside diameters and wall thicknesses, fits, clearance issues, and some other stuff.
If you are math challenged, get a calculator, or use your Windows calculator program. Work the examples with me. It'll help you understand and remember how to do it later so you can get your answer in 30 seconds rather than post a question and have wait every time you run into a similar conundrum.
The rules:
To get inches given mm, divide mm by 25.4.
To get mm given inches, multiply inches by 25.4.
To find a close fractional size, multiply the decimal INCHES, (not mm!) by 64, 32, 16, 8, 4, or whatever the reciprochal of the fractional size you're looking for is.
The metric system:
One centimeter (CM) is 10 MM (Millimeters.) One meter = 100 CM = 1000 MM = 39.37007874 inches
Some "yardsticks" for small numbers:
0.001 inch = one one-thousandth inch.
0.016 inch = 16 thousandths inch. (This is the wall thickness of 1/4 inch sterling silver tubing from Rio Grande, BTW.)
A piece of paper is about 5 thousandths inch thick.
A matchbook cover or cereal box cardboard is about 20 thousandths inch thick. 0.020 inch.)
Here's a trivial example that will help you keep it straight whether to multiply or divide:
We already know 1 inch = 25.4 mm, right?
So, let's convert 1 inch to mm: 1 * 25.4 = 25.4. Amazing.
Convert 25.4 mm to inches: 25.4 / 25.4 = 1 (inch.) Truly astounding! The music of the spheres! Oh, the harmony in this universe! But now you won't ever get confused about which to do, multiply or divide, if you just remember to do that test.
OK, so one inch is 25.4 mm.
Therefore, 1 mm is one part in 25.4, or 1/25.4 inches, or, (punch your calculator,) 1 divided by 25.4 = 0.039370079 inches.
To see how many (if any) 32's of an inch that is, multiply that decimal number by 32:
0.039370079 X 32 = 1.25984252 or, about one and a quarter 32's of an inch. But, hey, I bet you'd prefer some practical, and relevant numbers, right? OK, here goes:
Let's start by converting 1/4 to a decimal: (Again, just punch in 1 divided by 4, = 0.25 (Of course we all know that.)
But, how many mm is that? Since there are 25.4 mm in one inch, then 1/4, or 0.25, of that amount of mm should equal 1/4 inch.
So, that's just what you do, take .25 X 25.4 = 6.35 mm. Another way to say it is 1/4 of 25.4 = (1/4) * 25.4, or, just 25.4/4, = 6.35 mm. Same answer.
To go the other way, converting a given mm size to inches, we have to divide the mm's by 25.4:
Or, 6.35 mm divided by 25.4 = 0.25 inches.
Most often though, you won't have those nice well behaved decimals like 0.25, so, here's how to handle that:
Say you have a 5 mm thick bracelet, and you want the nearest fractional size.
5 mm / 25.4 = 0.196850394 inches. Yipes! That doesn't help much! Or does it? let's see if that decimal inch expression is close to an even number of 32's of an inch:
0.196850394 * 32 = 6.299212598 32's of an inch. We'll round that to 6.3 thirty-seconds inch. Hmm, that is closer to 6 than to 7, but depending on whether we're talking about a hole, or a rod, say a mandrel, we might want to round up or down...
First of all, 6/32 is 3/16, right? (We can always multiply the top and bottom of a fraction by the same number without changing its size, so (6/2) / (32/2) is the same quantity, 3/16. )
So, if I have a HOLE that is 5 mm, what fractional inch size would be a pretty good fit through it, in 32's inch language? The hole is 6.3 32's, so a 3/16 (6/32, same thing,) rod will fit through with some room to spare. How much room? About 1/3 of a 32nd of an inch. (The hole is 6.3 32's of an inch, right? Now try this: punch in 1/32 to convert that to decimal. You get 0.03125 inch. Now, divide that by 3 to see the decimal equivalent of one third of a thirty-second of an inch: 0.03125/3 = 0.010416667 inch. Let's shorten that to .01. That is One One Hundredth of an inch, or in my language, ten thou. (ten thousandths inch.) A piece of typing paper is 4 or 5 thousandths thick, so you'd have a snug fit with a piece of paper wrapped around a 3/16 mandrel going through a 5 mm hole, if that helps you visualize the fit. (.005 gap on each side, = .01 total clearance.)
The reason you have to be careful is, if you had a 5 mm ROD, and a 3/16 HOLE, the rod would be too BIG, by the same, albeit, TINY amount. That is an interference fit. Sometimes machinists do it on purpose, and force a slightly larger metal rod into a slightly smaller hole in metal, under huge pressure, so it sticks tight. We can't do that with glass, can we?
For instance, if you have a bracelet that is 5 mm, (which is slightly BIGGER than 3/16, remember?) and a 3/16 MANDREL, you would need to coat your mandrel THICKER than 5 thou to increase the bead hole at least that far PAST 3/16 inch to fit the 5 mm bracelet, or as some have said, remove some glass. This is why UNLINED beads made on 3/16 mandrels will fit the 5 mm bracelets, only if you coat THICK, or grind out a little glass.
We could have started with 16ths rather than 32nds in the example above:
5 mm / 25.4 = 0.196850394 inches.
0.196850394 * 16 = 3.149606299 SIXTEENTHS of an inch. (Don't get mixed up, now, That is NOT 3.1496.. INCHES!)
We are over 3/16 inch by aproximately .149 or about 0.15 parts of another 16th: 1/16 = 0.0625, and, 0.15 times 0.0625 = 0.009375, just under the figure we got above, of 0.01, but that is just because we rounded the numbers DURING the calculations. Had we not rounded, the numbers would match exactly. In general, you're pretty safe rounding to the nearest thousandth inch, (3 decimal places,) or two decimal places when working with MM.
OK, now for the 1/4 inch mandrels, and the silver tubing. (If you kept the faith and waded through it all, I salute you. )
OK, your mandrel is 1/4 inch, and your tubing is also 1/4 inch, and your bead release makes the bead hole even BIGGER than 1/4, so we can be sure the tubing will fit through the hole, no prob. But, how big will the hole on the INSIDE of the SILVER lining be, and will it still fit the bracelet? The tubing from both Rio Grande and Santa Fe is 1/4 inch OD and has a WALL THICKNESS of 0.016 inch. Now, that is off the RADIUS, not the DIAMETER. To figure the inside diameter (ID) of the hole, we just need to take the outside diameter, (OD) and SUBTRACT TWO times the wall thickness: 0.25 - (2*0.016) = .25-.032=0.218 inch ID.
How much is that in MM? 0.218 * 25.4=5.5372 MM Viola! The hole is 1/2 MM bigger than the 5 mm bracelet, a nice easy slip fit, which is why, of course everybody is happy about 1/4 mandrels for lined beads with these bracelets.
Just thought that might help some of you who are struggling with this or other conversion issues, FWIW.
I do hope that helps, and I didn't "talk down" to anyone. Some folks never do this stuff, so somewhere, the basics needed to be spelled out clearly, I think. Be looking for new bead lining and cleaning toys, and BIG SS MANDRELS, at a very affordable price, very soon, from Art in the Round!
Take care now, and do the math with confidence!
Dave
OD'ing on mm and inches? Tubing, Bead Lining for the math challenged...
