Disclaimer: CNC Concepts, Inc. accepts no responsibility for the use
or misuse of techniques shown in this web page. We simply publish information
we feel will be of interest to CNC users. In all cases, the reader is totally
responsible for considering the implications, good and bad, of implementing one
or more of the techniques we show.
Which mode is better, inch or metric?
Most companies work exclusively in one mode or the other. If the bulk of
their prints are dimensioned in inch (as with most companies in the United
States), they program and run the machine in the inch mode. If they happen
across a print dimensioned in the metric mode, they convert all dimensions to
inch (by dividing all millimeter values by 25.4) and still work in the inch
If you are one of the many companies that still work exclusively in the inch
mode, you probably don't know about the accuracy advantage of the metric mode.
This advantage has to do with the least input increment of the input mode. The
least input increment in the inch mode for the vast majority of CNC machines is
0.0001 in. In the metric mode, the least input increment for these machines is
0.001 mm. 0.001 mm is less than half of 0.0001 in (0.001 mm is equivalent to
0.000039 in), meaning your CNC machine will have a much finer resolution when
the metric mode is selected.
To get an understanding of this implication, consider a common indexer. A
five degree indexer has 72 positions (360 divided by 5). A one degree indexer
has 360 positions. Though the one degree indexer is no more accurate than the
five degree indexer, you can program it with a finer resolution. You can, of
course, index 34 degrees with a one degree indexer and cannot with a five
degree indexer. One way to compare this to the inch/metric mode selection is to
say that working exclusively in the inch mode when the metric mode is available
is like having a one degree indexer but only programming it in five degree
Said another way, a ten inch long linear axis has 100,000 programmable
positions in the inch mode. In the metric mode, the same ten inch long axis has
over 254,000 programmable positions!
When can this help?
If the bulk of your work has wide-open tolerances, it is likely that the
inch mode will more than suffice. It would be like saying you happen to have a
one degree indexer but never have to program it to any finer resolution than
five degrees. However, as tolerances get tighter, there are times when you can
successfully machine workpieces in the metric mode when you cannot in the inch
Consider how you calculate coordinates in your CNC program. Most programmers
will program the mean value for dimensions. For example, if you have a
dimension of 3.000 plus or minus 0.001 in, the programmed value will be 3.000.
In the case of plus or minus tolerances, determining the mean value is easy and
can be easily programmed.
On the other hand, consider a dimension of 3.0000 plus 0.0003, minus
nothing. In this case, the mean value is 3.00015. If working in the inch mode
(with only four place input), you cannot even program the mean dimension. If
you convert 3.00015 in to metric, it comes out to 76.2038 mm, which must be
rounded to 76.204. This dimension is within 0.0002 mm (0.0000078 in) of the
desired mean value!
This accuracy advantage of the metric mode is also involved with offset
setting. A tolerance of plus or minus 0.0002 in has less than four acceptable
offset setting positions. The same tolerance in the metric mode is plus or
minus 0.005 mm, which allows ten acceptable positions in the offset table! And
for SPC purposes, the amount of offset change from one adjustment to the next
can be kept smaller.
While people who have never worked in metric mode will find the transition a
little cumbersome, if you do tight tolerance work, these accuracy benefits are
well worth the effort. In many cases, you'll be able to hold size (or make less
scrap) on workpieces that have been previously impossible to machine on CNC