After further thought I realized Jim is making a big of assumption when he
says 1/8 = 0.125.
If it is just a plain old decimal representation of a fraction, in other
word completing the division, then the answer is correct. But in machining
the answer may not be correct.
I wrote:
Much of the older drawings that the company I work for still uses shows
dimensions in fractions with a notation in the block that all fractional
dimensions have a tolerance of ±1/32 inches. This is very close to ±1.0 mm,
meaning a dimension of 1/8 inch (0.125 inch) can very anywhere from 0.09375
inch to 0.15625 inch (or 2.4 to 4.0 mm), thus as you can see 3 mm is an
acceptable alternative to 1/8 inch.
Any machine shop, today or 50 years ago, that does not pay attention to
tolerances will be laughed right out of business. I made no claim that
every application could have its numbers rounded to whole metric numbers,
but there are a number of them that safely can. It all depends on the
tolerances and if the rounded metric number falls with in the tolerances.
I've seen dimensions labelled as such: 0.500 in (-0.000 inch +0.015 inch).
Expressing this in millimetres, it would look like this: 12.70 mm (-0.00 mm
+ 0.38 mm). I see no reason it can't be expressed as: 13.00 mm (-0.70 mm
+0.00 mm) or even tighten the tolerance a bit and make it -0.50 mm instead
of -0.70 mm. The resultant part in metric would be identical to one
produced in inches.
Jim, no competent engineer would send a drawing to a machine shop in
fractions and expect to get a precision part back. If it has to be 3.175
000 mm then it would be stated as such with acceptable tolerances also
stated, not implied or assumed. If the acceptable tolerance is ±1.0 mm, and
the machinist chooses to be within ±0.01 mm, then all the better. If the
machinist makes the parts to 3.0 mm then there is no error and the part is
not rejected.
The machinist should be embarrassed if he didn't make the part to print not
for making it a rounded metric amount that would be within the stated
parameters on the print.
Dan
LOL! You would be laughed out of any modern machine shop in the world if
you tried to make a claim like that!
1/8" = 0.125" = 3.175 mm. The error of 0.175 mm is about 7 thousandths of
an inch, which is entirely "rejectable" for many parts, such as ball
bearings, precision shafts, connector pin spacings, etc. Any competent
machinist would be embarrassed to be "seven thou" off a specification.
