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.
Symbol: |
 |
Datum Required: |
Yes* |
*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.
for the position of a surface, is the tolerance band distributed equally about the ideal surface or is it completely above the ideal surface? eg: take the case you have used, for a block of height 60 mm basic with a position tolerance of 0.5 mm, is the allowable deviation 0.25 mm above and below the ideal surface or 0.5 mm above ideal surface?
Hi Kaushik, as you say, the tolerance bank is equally distributed about the ideal surface. So in my example, it would define 2 planes around the ideal 60mm basic plane: one 0.25mm above the ideal, and one 0.25mm below the ideal. I hope this helps! Wayne
Hi there, i have doubt on If I had included the diameter symbol (Ø) in front of the 0.2mm for M3 drill hole.
Does the Ø0.2 is the tolerance start counting from M3 point ? which the tolerance point for my M3 is M3.2 ?
Looking forward to your explanation
Hi Karen,
I am so sorry for the delay in reply! Let me try explain your question. The “Positional” tolerance is completely independent from the “Size” tolerance. So that means the 0.2mm position has no bearing on the M3 tolerance. For example, you could have a thread tolerance (as per ISO 965) of M3-0.5 6g and a positional tolerance of Ø0.2mm.
The diameter symbol (Ø) in front of the 0.2 positional tolerance, would mean you want a circular tolerance zone. If you omitted the diameter symbol, that would indicate a square tolerance zone.
When indicating the position of a circle (or a threaded hole in your case), the position referred to is the centre point of the circle. In theory, this should be the drill point of the tool. However, in reality, holes are rarely round! So be aware that the centre point of the hole might not be in the middle. When measuring the position of threaded holes, it is common practice to install a threaded dowel pin into the hole and then measure the shaft of the dowel pin.
I hope all of the above makes sense!
Hi, thanks for these examples.
In the case of the example “Position of a hole pattern with no datum features” what would be the resulted tolerance zone for each hole?
Hi Kress,
No problem at all – let me try explain. The resultant tolerance zero for each hole would be a square of size 0.2. If I had included the diameter symbol (Ø) in front of the 0.2 it would have been a circular tolerance zone. The squares would be positioned relative to each other as per the basic dimensions (20 and 10).
In this example, the holes are positioned to themselves, but not the rest of the plate. In order to do this, you would need to add another dimension to one of the holes the outside of the plate. This type of dimensioning is useful when you are trying to mount something on a plate that needs tightly controlled holes, but you don’t really care where the thing is mounted on the plate. For example, mounting a motor to the plate – the motor mounting holes might need to be tightly controlled, but where the motor sits on the plate isn’t that important.
Any other questions, let me know!
Thanks,
Wayne
Hi there! Such a good article, thanks!
You are very welcome Serena! I will be writing up other GD&T topics very soon.
I never knew that! Good ape….