|
|
Degrees
of Freedom Shown
in the
Feature Control Frame
(ASME Y14.5 - 2009)
First
of all,
Geometrical Dimensioning and Tolerancing is a bit complicated and the
2009 version only adds to the confusion. Please, Designers, keep
away from
using degrees of freedom in the feature control frame unless you are
ready to receive quite a few phone calls. Use the datum structure as
noted in the 94 edition of ASME Y14.5M-94.
 One
may see a
feature control frame as shown here. These letters in the brackets
reflect the
degrees of
freedom which I hope to explain in this paper.In Quality, degrees of freedom refer to statistics and simply means one fair comparison but this does not apply in GD&T. First of all, let's determine the meaning of degrees of freedom relating to setting up a part. From Wikipedia, the free encyclopedia Six degrees of freedom
(6DoF)
refers to motion of a rigid body in three-dimensional space, namely the
ability to move forward/backward, up/down, left/right (translation in
three perpendicular axes) combined with rotation about three
perpendicular axes (pitch, yaw, roll). As the movement along each of
the three axes is independent of each other and independent of the
rotation about any of these axes, the motion indeed has six degrees of
freedom. Translational means
linear movement.
From the definition
above, a
degree of freedom constrains the part from moving and it refers to
translational in the X, Y or Z axis with the remaining three (3)
degrees of freedom constrain its rotation. Here we have a rectangular
part with the primary datum designated as datum A, secondary as B and
tertiary as datum C.
 How does one set up on such a part?  We have a three (3) point set up on datum A, two (2) point set up on datum B and a one (1) point on datum C. Each one of the set up points restrains the part in some way and if you add all the set up points, you will arrive at a number of six (6) set up points or 6 degrees of freedom. I am now going to number the set up points by placing datum targets on each one. After that, we will discuss the restraining ability of each datum target.   Constraining degrees of freedom will contain three (3) points or datum targets in our case that will constrain the movement of the part in the X, Y and Z axis. These will be called "translational degrees of freedom" and will be shown in the datum reference section of the feature control frame as "x", "y" and "z".  Let's look at the primary datum A in our example. Datum target A2 constrains the part up and down which is in the direction of the Z axis. In other words, datum target A2 is the translational degree of freedom for the Z axis and will be called "z".  Secondary datum B which has a two (2) point set up. Datum target B2 constrains the part in the X axis and is the translational degree of freedom for the X axis and will be called "x".  The tertiary datum has only one (1) datum target which constrains the part in the Y axis and, thus, is the translation degrees of freedom for the Y axis and will be referred to in the feature control frame as "y". Now we will review the rotational degrees of freedom.  Datum target A1 constrains the rotation about the x axis and is called "u". Sometimes one will go back to the sketch showing the degrees of freedom and visualize the constraint of the datum target (as I have had to do designing this page).  Here we have datum target A3 and I will again go back to the degrees of freedom sketch and realize that this datum target restrains the part around the y axis and is called "v". With the three (3) point set up on the primary datum A, we have one (1) trans  lational degrees of freedom and two (2)
rotational as shown.We will now look datum targets on datum B. We have already discussed that datum target B1 is a translational degree of freedom. We have B2 shown here and going back to the degrees of freedom sketch, it is found that B2 is a rotational degree of freedom about the z axis and is named "w". Let's review the degrees of freedom on the above part using the datum targets. Translational degrees of freedom: A2 - referred as "z" in the feature control frame B2 - referred as "x" in the feature control frame C1 - referred as "y" in the feature control frame Rotational degrees of freedom A1 - referred as "u" in the feature control frame A3 - referred as "v" in the feature control frame B1 - referred as "w" in the feature control frame We now have our full 6 degrees of freedom shown for the above non-cylindrical part. If one reviews a drawing and a similar feature control frame is evident, it is referring to degrees of freedom and one must determine where the X, Y and Z axis are on the part and could place directional lines showing x,y & z with labels such as shown above.. Again, Designers, please keep away from this concept otherwise the drawing may seem to become more complicated than is required. Think about an online seminar. It is much easier to present and also better from a participant point of view. Check it out. The schedule is shown here. To sign up for an online seminar at our secure site, press here. David DeLong ASME GD&T Professional - Technologist and also Senior |
