| Don't Add Tolerance
by Rounding When Converting Dimensions and
Tolerances.
A lot of folks these days are
converting dimensions and tolerances from millimeters to
inches. When doing this, it is very common to round off
incorrectly. In fact, many CAD systems do not round according
to the standards. The result is that the conversion/rounding
process often adds tolerance to dimensions. Here is a better
way. First, everyone should know that 1inch exactly equals
25.4 millimeters. If you are using 0.03937-STOP. This
conversion value is not exact. When dealing with tight
tolerances, such as those encountered with grinding
tolerances, you may calculate incorrect values using 0.03937.
Don't multiply by 0.03937; divide by 25.4.
Once you
convert, it is important that you round off correctly. Take a
dimension and tolerance of 11±0.3mm. The limits of the
dimension are 11.3mm and 10.7mm. When these limits are
converted to inches, the limits will be .444881… in. and
.421259… in. If you round to 3 decimal places for common
machine shop work, the limits would be rounded to .445 in. and
.421 in., using the methods taught in most schools. These
values have been represented graphically. As you can see, the
conversion and rounding process has actually added tolerance
to the limits. The ANSI/IEEE 268 standard, referred to in
Y14.5, tells us that "where the original limits may not be
violated" this method is unacceptable. According to the
Standard, in such a case you must always round the upper limit
down and the lower limit up. In other words, you should round
both numbers "in" towards the median or center dimension. This
method results in tighter tolerances but guarantees that you
never open up a tolerance when converting units.
Of
course, if it is designed in mm and you build in mm, this is
not an issue. But, if you insist on converting metric values
to inches, do it right. |
