The importance of
Engineering Drawing Standards cannot be overemphasized at least if you are
only concentrating on designs that you will be selling to the public. If
you are designing and building your own cabinets or other products, then
you will only need to understand and read your designs enabling you to
transfer your ideas into reality.
Engineering drawing
standards have been around for a very long time. Although there is more
than one standard, ANSI (American National Standards Institute) is the
preferred standard.
Drawings are in fact
specifications, specifications on how something is to be manufactured.
Limited Dimensioning
- Lately, there has been a trend towards limited dimensioning. Limited
dimensioned drawings allow the draftsman to show the part without showing
all of the important features and dimensions. While limited dimensioning
has it's benefits, I am not convinced that it's the right way to go. You
see I was taught long ago that the best way to show a clear drawing is to
completely eliminate drawings being interpreted.
Interpreting Drawings
- While we are on the subject of interpreting drawings, I'd like to clear
the air. I have seen countless employment ads that state, "must be able to
interpret drawings." To interpret means to read between the lines or to
infer. Thus drawings and the data contained within them should never be
interpreted. Instead, drawings should always be taken at face value,
meaning they should always be taken literally. I can see it now. Some
person is looking at one of my drawings and says "ah heck, just because it
says that this hole is .5 diameter, doesn't mean I have to make it .5,
I'll just make it .375. You can see from this example, how nothing would
be manufactured to print . . . and the resulting confusion that such
thinking would lead to.
Reference Dimensions
- From the countless TVA's (Tooling Verification and Authorization) and
PVR's (Part Verification Report) that I have done, I can tell you that the
one thing that I constantly run into is the lack of clearly represented
data on the drawings. One way to eliminate confusion is the use of
reference dimensions. The use of reference dimensions is always correct.
Anytime, we can prevent the need for me or any other engineer to pick up a
calculator to figure out a dimension or distance from one feature to
another, we are doing ourselves a favor and creating much less work as an
end result. Do yourself and your company a favor and start using reference
dimensions after all, drawings should always be CRYSTAL CLEAR!
Tolerances - The
use of tolerances is a fact of life. They are in fact necessary.
Tolerances provide a means of manufacturing a part as inexpensively or as
expensively as we allow. The one thing to keep in mind here is that the
tighter the tolerance, generally speaking, the more money that the part is
going to cost you to produce. If the feature being dimensioned to is not
critical, then limit the precision to two decimal places. You'll save lots
of money by doing that. I have seen and talked with many engineers who are
of the opinion that everything must have at least a precision of at least
three decimal places.
Geometric Dimensioning
& Tolerance - See above. If you don't need it, don't use it. Higher
than normal part price will be the result if you do use it.
Layering - In all
cases, each and every individual or company that produces designs on CAD
(Computer Aided Design) should have some form of a layering standard,
whatever they choose that standard to be. Layering and layering standards
ensure consistency.
