3D Point Tolerance on Part - Car Body Position

[njw5]

How can a tolerance be applied to a single point in space?
-Plastic injection molded part
-Mold built to 3D data with 0.006 shrink factor
-test/cmm fixture built to gage points on part
-part print came many months later with table showing car body position xyz points to measure with +/- tolerances in only one axis for that point on the part (tolerances vary from 0.05mm to 0.2mm) Each of these points are "numbered" and are called out to show where each point is at on the 3D view of the part on the 2D print.
-no datums

How can a tolerance be added to a single point in space? Don't tolerances involve at least 2 points or a distance from a point to a plane, etc?
Aren't tolerances relative to other features on the part and the tolerance level relative to the length of the dimension being called out?

We are all greatly confused on how to go about fixing all the "out of spec" data points ! Especially confused on how a single point in space can have a tolerance!

Any feedback and suggestions would be greatly appreciated - especially on how a single point in space can have a tolerance.

 

[ajack1]

Car companies tend to work in car line; this is a master datum point where any part can be measured from usually in the middle of the front axle so this gives a master datum.

For the measuring of individual parts they usually show three areas where the part needs supporting and then means of preventing rotation usually holes or slots. So now you have the part orientated correctly about the master datum, you will often see car line grid references as well XY in one view XZ in another. It is usual to then pick convenient points to reference from as you probably don’t have a CMM capable of getting a whole car in it.

From there it is a matter of comparing the model against the part, this is usually done as a general surface profile but sometimes specific areas require different tolerances.

Obviously that is only a general overview but I hope it helps, try doing a search for car line inspection or car line CMM for more details.

 

[HGMorgan]

The 3D Point Table is basically an Inspection Point Table, and the points have tolerance because they are either on a feature, or, at a theorectical location derived from features (C/L's and intersections).

Examples:
1. 3D point is on contoured surface of product.
2. 3D points are center points of a series of holes.
2. 3D point is at intersection of two each tube centerlines.

The designer wants the product features checked at those points, and, it has to check with-in the tolerance noted. The product Datum Axis should be the axis of the point table. A review of the model will show that, I.E.; the model should have the points, if not, key some in and see where they land on model.