Let me expand a bit on my last post above. But let me start by saying that the metric system is great. It is easy to use and easy to 'visualize' dimensions.
I hate fractional dimensions in the English system. So I usually convert them into the decimal system first. Then it is much easier to visualize how much you are talking about. For example, what is the difference between 13/64 inch and 3/8 inch?
Can you really 'see' how much 13/64 inch is? I can see 3/8 but not 13/64, let alone what the difference is.
Now if you take .375 inch minus .203 inch or .172 inch, I can visualize how much that is. And I can measure it real nicely with my calipers (it has a dial indicator with markings down to a thousands of an inch).
If I have to convert a dimension of 1 and 13/64 inch to mm I don't use aps, I use my little calculator in the drawer in front of me:
(1+13div64)x25.4 = 30.559 mm. Of course, I would call 30.6 mm being close enough
If your model is in 1:48 scale, you take the full scale dimension (let's say twenty feet) and convert it to mm for your model by doing the same conversion on a calculator: 20 x 12 x 24.5 div 48 = 122.5 mm.
Finally let me give you another example how poor the fractional system is. If you are working with a line that is 0.50 mm thick and you want to know how much that is in inches, you divide the mm by 25.4 which is 0.020 inch. Now would you convert that to fractions of an inch? I don't think so.The closest I come up with is 5/248 inch, which is ridiculous.
BTW. In 1959 the inch was set to be exactly 25.4 millimeters. Prior to that, it was set at 39.37 inches per meter. The difference is only 0.0002%, or 2 parts per million. That's something we don't have to worry about. But it is interesting that the inch was set to that value, not the millimeter.
Edited by Modeler12, 27 February 2013 - 04:45 PM.