Last updated on 17th June, 2020
Position, or True Position, is one of the most useful tools in the Geometric Dimensioning and Tolerancing (GD&T) system. It it typically used on hole patterns but can be used to dimensioning surfaces, lines and points.
Position can be used on any Feature of Size (FOS) or None Feature of Size (NFOS). It often has many benefits over a standard linear dimension when used on a FOS as I go into on my introductory post to Geometric Dimension and Tolerancing.
*Generally speaking, you would always use position with datum features. However, it is not always required. Position can be specified using only ‘Theoretically Exact Dimensions’ or TED. This is rare but sometimes used where the hole pattern is used as the primary datum.
For many things with GD&T, I think it is best learnt with examples of its use. So, without further ado, here’s many examples. If there’s an example you’d like to see, please comment or contact me and I’ll see what I can do!
Position of a hole pattern with no datum features
As mentioned above, it is very rare to use position without a datum (but allowed!). Generally speaking, you would only use this type of scheme when you are using the hole pattern as the primary datum – as I have done in this example here.
Another point to note is that I have not included the diameter symbol (Ø) here in front of the positional tolerance value. But more on that later.
Position of a hole pattern with one datum feature
Here we have an example of position use on a cylindrical pattern of holes. The only (theoretically exact) dimensions needed here are: the angle between the holes and the circle diameter on which the holes lie (sometimes known as the pitch circle diameter or PCD). Since only one datum has been used, the positional requirement effectively defines the entry point of the holes. It doesn’t specify how deep or straight the hole are.
As before, I have not included the diameter symbol (Ø) here in front of the positional tolerance value. But more on that later.
Position of a hole pattern with two datum features
Similar to the previous example, this example of position use on a cylindrical pattern of holes. But this time, with two datum features. Datum B has been added as the surface in which the holes are drilled. Adding this extra datum basically controls the perpendicularity of the holes relative to the surface B.
Position of a surface using one datum feature
This is an example of the the position of a surface using one datum feature. This means that the indicated surface must always lie between two constructed planes parallel with datum A.
Position of a surface using two datum features
In this example we have two datum features which have been tied together using a perpendicularity tolerance. This now means that the indicated surface must lie between two constructed planes parallel with datum A and perpendicular to datum B.
Position of a surface using three datum features
In this example we have three datum features which have been tied together using a perpendicularity tolerances. This now means that the indicated surface must lie between two constructed planes parallel with datum A, perpendicular to datum B and perpendicular to datum C.
Use of the diameter symbol (Ø) in front of the positional tolerance value
When using position to define a point (like a hole centre point) you can specify the position value using a diameter symbol or without one. The difference is the resulting tolerance zone. Using the diameter symbol, indicates that the tolerance zone is a theoretical exact circle. Without the diameter symbol, the resultant tolerance zone would be a rectangle.