Some time ago, I came across an imperial site
where an engineer waxed lyrical about thous (1/1000 of an inch) used
in lathe work. As expected, he sang the praises of these obsolete
units. They are much more handy in machine work than mere
millimetres and microns, he states. Allowing the operator to work
easily to exact tolerances, something appearantly not so easy in
metric. Being a musician, I have no first hand experience in this
field, so I took it with a grain of salt. Not long ago, I came
across this piece burying yet another imperial myth. CNC stands for
Computer Numerically Controlled machines.
These are
excerpts:
CNC Resources
CNC Tip:
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 mode.
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 increments!
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!
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 work pieces that
have been previously impossible to machine on CNC machine
tools!
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