Five Points You Must Remember While
Dealing With Practical GD&T
This geometric dimensioning and
Tolerancing tutorial will discuss points related to the practical application
of GD&T. So you can use this article as the checklist for applying
tolerance.
As a mechanical
design engineer you might be pretty clear about the individual concepts of
all the five categories of GD&T, but while applying practically the
geometric dimensioning and Tolerancing symbols to your design, you may find the
task bit challenging. The following points of the GD&T tutorial will
help you strategize applicable tolerance for your design.
Key Points of GD&T Application
1. As soon as you are done with the
2D CAD drawing from the 3D CAD model, find out the feature
of size dimensions,where GD&T need to be applied. The fig-1 below
has a hole, for this purpose.
2. Next thing you have to do is to find out the
sufficient numbers of datum. We have used three (A, B, and C). Though we have
used the position tolerance symbol here, but the inclusion of
datum A in the GD&T frame is ensuring the
perpendicularity of the hole axis. So, from the above drawing, you will get the
positional tolerance of 0.01 at MMC and perpendicularity tolerance at MMC of
0.01.
3. If a feature of size does not contain any
GD&T symbol then the size as well as the form will be controlled by the specified
size tolerance (Rule#1). In the above figure, the datum surface B and
its opposite parallel surface is controlled by the dimension 10.050 –
10.020 and there is no GD&T symbol attached. So from the Rule#1 flatness
of datum B should not exceed 0.03.
4. Whenever you are looking for a clearance
5. fit, you should go for the MMC material
modifier and if you use the LMC modifier with your GD&T
value then the related fit will be interference fit.
6. Usage of MMC (or LMC) modifier with GD&T value,
allows utilizing the bonus tolerance. In the Figure-1, GD&T
is applied for MMC, means, for 2.040 diameter there
should be a positional tolerance of 0.01 and as the diameter increases the
positional tolerance also increase. For 2.060 diameter, the
positional tolerance become 0.03.
Conclusion
The concept of geometric dimensioning
and Tolerancing is important for a mechanical design engineer. Design engineers
sometimes find it challenging for applying the