DIMENSION TOLERANCES

We as a rule use architecture dimensioning and now need to use decimal format and I'm unsure as to if there is a standard practice for precision. If the main dimension is out two places should the tolerance not exceed that number of places? Is it also acceptable to have several levels of precision on the same drawing, ie. 2.5", 4.125" and 0.0312"? Thanks for your input it is appreciated.

 

Larry
I don't know exactly what you are drawing, so my comments may or may not be
right on. Geometric Dimensioning and Toleranceing (GD&T) is a whole new can
of worms for you, and depending on the importance of what you are doing, you
may want to hire someone familiar with GD&T to advise you. Also, the cost
of over (or under) tolerancing something can quickly cost your firm a lot of
money that didn't need to be spent.

To answer your question, yes you can show any number of levels of precision
on the same drawing. As for tolerances, you may have something dimensioned
as 1.25", your tolerance may be +/- 0.05, or it could be +0.1. It all
depends on how the part is interacting with other parts.

The overall shape of something may not be real important in terms of
tolerance, but the distance between holes may be (usually is) very
important. So think about how the parts go together and then consider if a
plus tolerance makes more sense than a minus tolerance.

The tolerance is entirely dependent on many factors. What is it mating up
to, does one piece go inside another, etc.

I know enough about GD&T to be dangerous, so I suggest again that you hire
someone for consulting and or training if what you are doing is of any
importance.

Kent Elrod

format and I'm unsure as to if there is a standard practice for precision.
If the main dimension is out two places should the tolerance not exceed that
number of places? Is it also acceptable to have several levels of precision
on the same drawing, ie. 2.5", 4.125" and 0.0312"? Thanks for your input it
is appreciated.

 

The "standard practice" for precision in drawings is based on the use and
desired functionality of the part. You need to know the function of the part
to properly dimension it.

Generally speaking, you want to tolerance something with the loosest
(greatest amount of deviation allowable) tolerances that will still maintain
the function of the part. Too tight of tolerances means higher bids,
machining costs, and inspection time. You can definitely "over" tolerance
something, and it's pretty common with people that don't actually design the
part or understand it's function.

A standard title block will usually have something like this...
..x ±.1
..xx ±.03
..xxx ±.010

Each tolerance applies to dimensions with that many places, except where
noted. The amount of places a dimension has is based on the parts function
and allowable deviation.

GD&T and things like Max Mat. Cond. & Position are a whole other matter, and
shouldn't be used unless you have a good understanding of their use and
implications. You can kill a part/job by improperly applying GD&T.

--
Robert Davis
QC/CMM Dept.


E.A.S. Manufacturing Co., Inc.
804 Via Alondra
Camarillo, Ca 93012
805-987-3665 Voice
805-987-7948 Fax
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www.easmfg.com - Web Site

format and I'm unsure as to if there is a standard practice for precision.
If the main dimension is out two places should the tolerance not exceed that
number of places? Is it also acceptable to have several levels of precision
on the same drawing, ie. 2.5", 4.125" and 0.0312"? Thanks for your input it
is appreciated.