Thread: Noob question about blade size

+1 To Glenn,
Consider also that a .7" blade is NOT a full .75" and the way measurements USUALLY work is you can go over, but you can't go under. A gallon of gas must be at least a gallon of gas, a 12 oz can must contain at least 12 oz, etc. They can over fill and put 12.2oz in a can, but if they get caught putting 11.8oz in they're in trouble.

So you could apply similar logic when measuring blades today. Particularly when looking at resale practices it avoids a headache to designate a .7" blade as 5/8 instead of 6/8.

Of course if you've ever bought lumber all of this goes out the window...

FWIW
Jim

It is of course very difficult to represent metric measurements in inches as rational numbers, particularly when traditionally we only really use 8 or 16 as the denominator. You can, of course, keep expanding the denominator geometrically (so, go to 32ths) but for some reason that doesn't seem to occur with straights, or at least it is very uncommon. Even if you did, you'd still run into the same issue.

So some data loss is necessary when converting from metric to inches (when you constrain the inches data to be rational). The data loss can take one of 3 basic forms:
(1) Rounding to the nearest lowest unit (eg 16th);
(2) Floor: take the number back to the largest previous integer (in the numerator) (eg: 13/32 becomes 3/8);
(3) Ceiling: take the number up to the smallest following integer (in the numerator) (eg: 13/32 becomes 7/16).

In my travels I have noticed that people tend to get upset if you overestimate or round up the width of a blade. Therefore what we most often see is the Floor operator being employed, at least in my experience.

So, for your particular example, 18mm = 0.7086614 inches. This is somewhere between 22/32 and 23/32. Using the Floor function, we take 18mm to be 22/32 = 11/16. So, if you are happy to talk about 16ths of an inch you could leave it there. If, on the other hand, you are prepared to go to 8ths, applying the Floor function again yields 5/8, as appears in the Wiki.

Considering that 1/16th of an inch is 1.5875mm, and assuming most people use a ruler to measure the width, and taking into account the various definitions of what "razor width" actually is, and given we really only care about these things in ballpark terms anyway, I'd say the 5/8ths of an inch definition in the Wiki is pretty OK, and in fact I would go further and say it is actually the most conservative, and hence honest, appraisal of the inch equivalent to 18mm. However, if you wanted to get pedantic about it, and you were very very sure about both the accuracy of your measurement device and the way you measured the width (as in, the definition of razor width - which, by the way, is not set in stone) then you might want to adopt the 11/16ths conversion.

James.

Where's Heisenberg when you need him?

Originally Posted by FoxAndFish

Measured the blade.

'Size doesn't matter it's the way that you use it' However, one of my female friends did say that nine out of ten women would disagree with that statement.