It will always have some kind of waviness, bumps and valleys.
Placed on the feature control frame itself Example.
A, datum is a perfect point, line, plane or surface but only exists theoretically.One position tolerance controls the location and orientation of the slot to the datum reference frame.You can think of them as an anchor for the entire part; where the other features are referenced from.Uncategorized, by, gladys on t 10:23.Here is another case where angularity cannot fully define the feature.From Theoretical to Real, in the example above, the Datum is the green line and the Datum Feature is the imperfect black line that forms. .In the above drawing this surface may be simulated by a near perfect granite slab.Yet, to do this fastening correctly, one of the most important information we need is the tolerance of the circular hole, and most of all, of the slot.To form this theoretical plane in the real world, measurement surfaces or points are used.A profile tolerance controls the size and form of the slot all around.Datum Features Datums, a datum feature is usually an important functional surface where all other dimensions need to be in spec with reference.The standard uses two ways to tolerance slots.
To keep the slot in line with the datum axis, position needs to be applied to the slots size dimension.
The reason they are not equal to each other is because the part surface is never 100 perfect.
The other method would work for any internal and external shape, including slots.
Commonly in manufacturing and design you would say datum to cover everything just for simplicity.Angularity would assure that the slot is at the 60 basic angle but ignore that the slot center plane should pass through the datum axis.The drawing shows that the center of the slot should be in line with datum axis.To establish a datum axis of a feature such as a hole, the symbol can be placed in a few ways.On a point or holes axis.On a surface, the symbol must be placed on or with one single extension line out of the datum surface.GD T Basics is now offering an affordable training program for individuals and companies looking to gain a foundation in.How Datum Features are Shown on a Drawing.