Dimensioning the outside of the part
The plate below has a surface that mates to another part. The four small holes locate the plate to the mating part by going over studs protruding from the mating part. The plate will go over all four studs at the same time and then sit flat on the mating part. Another part pilots into the large center hole.
The outside edges of the part are not critical to anything. They just need to be there so that there is material around the bolt holes.
How should we dimension the edges of the part?
The outside edges of the part are not critical to anything. They just need to be there so that there is material around the bolt holes.
How should we dimension the edges of the part?
The figure below shows the dimensioning for all of the critical features.
With the outside edges of the part being not critical, people are tempted to just throw on some coordinate tolerances.
But with the coordinate tolerances come issues:
We don't know whether to measure from the outside of the part to the holes or whether to measure from the holes to the outside of the part.
Why did we put the dimensions from the hole to the edge on the right side instead of the left side? What do we really want?
We have tolerance accumulation when we look at the tolerance between the holes and the right side of the part vs the holes and the left side of the part.
We don't know whether to measure from the outside of the part to the holes or whether to measure from the holes to the outside of the part.
Why did we put the dimensions from the hole to the edge on the right side instead of the left side? What do we really want?
We have tolerance accumulation when we look at the tolerance between the holes and the right side of the part vs the holes and the left side of the part.
The right way to dimension the outside edges of the part is with a profile of a surface. We locate the outside edges with basic dimensions. Then we use a profile of a surface with the all around symbol.
With the profile tolerance, we know exactly where to measure to and from, we have no tolerance accumulation, and one surface is not any more important than another.
With the profile tolerance, we know exactly where to measure to and from, we have no tolerance accumulation, and one surface is not any more important than another.
