The three mutually perpendicular planes are called a Datum Reference Frame. Note that we are measuring from the planes, we are not measuring directly from the sides of the block. Our strategy is to set the block on and/or against three mutually perpendicular planes. We want to measure where the hole is, and we want the measurement to be repeatable. When this is done, there must be a note specifying exactly how the clamping is to be done so that the deformation will be exactly the same each time the part is measured.Ĭonsider the part on the right side below. This is frequently done with sheet metal parts or other non rigid parts. But since the sheet metal is non rigid, we may be able to force all four feet to rest on the plane by clamping them in place. If we were to make a sheet metal statue of the dog, once again the four feet would never rest in the same plane at the same time. Likewise when measuring a part, if we set the part on a perfect plane, it will rest on the three high points. So the dog statue at any one time would rest on three of the four feet. But if we were to make a rigid statue of the dog and set it on a perfectly flat plane, we would never get all four feet to be in exactly the same plane. All four feet touch the ground because the dog adjusts how he stands. The ground acts as a datum.Ĭonsider some additional issues. The dog stands on the ground, and we measure up from the ground. Parallelism should never reference three datums, and this is because parallelism only controls orientation, not location.If we want to measure the height of a dog, we don't measure directly from the bottom of the dog's feet. To summarize, the parallelism callout is used with one or two datum references, depending on the type of feature to be controlled. The axis cannot be parallel to the third datum plane, and it is, in fact, perpendicular. Now, the axis cannot rotate, and the orientation is fully constrained. If we want to lock down the orientation of the axis fully, we must add a reference to a second datum plane, C. Note that the axis can still be rotated in a plane parallel to datum plane B. Now, imagine that the hole axis must be parallel to one of these planes, B. Visualize three mutually perpendicular datum planes A, B, and C located along the coordinate system X, Y, and Z axes. However, we are not controlling size or location with this constraint, only orientation. The diameter symbol in this feature control frame indicates that the tolerance applies to a feature of size. The feature control frame for this callout will contain the parallelism symbol, a diameter symbol followed by the tolerance amount, and two datum plane references. Going back to our parallel surface example, we can allow the surface to be translated parallel to the datum plane or rotated about an axis perpendicular to the datum plane and still accurately define the desired parallelism tolerance zone.Ĭonsider a second example, where the orientation of a hole is controlled with a parallelism constraint. Parallelism controls the orientation of a feature, but it does not control location. Any additional datum planes referenced would not be parallel to plane “A” and would only serve to confuse the reader.Ī single datum plane reference is sufficient for a parallelism constraint for a surface because parallelism does not require us to constrain all six degrees of freedom. The parallel planes that form the tolerance zone boundaries are also perfectly parallel to datum plane “A.” As you can see, datum plane “A” provides all the information we need to define the parallelism constraint. For example, think of one surface of a part, and impose a parallelism constraint of 0.030, with respect to a datum plane “A.” The tolerance zone in this example is defined by two parallel planes 30 microns apart. The feature control frame for this callout will contain the parallelism symbol, the tolerance amount, and a single datum plane reference. To explore this in more detail, we will first consider the parallelism of a surface.
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