Using GD&T Position to control the location of a feature of size allows 57% more positional tolerance than when using coordinate dimensions.
This increase is due to the shape of the tolerance zone. While coordinate dimensions result in a rectangular tolerance zone, the tolerance zone for GD&T Position is diametric. When inspecting the location of the feature, the method of inspection is the same – you still measure deviation in the X and Y directions. However, these X and Y deviation measurements must be converted into a single diametric deviation value.
We have created a handy calculator to do this conversion for you!
To use the calculator, enter the information from your drawing and input the measured values for your feature. The calculator will then output the total position tolerance, including any potential bonus tolerance, the calculated position error (diametric deviation value), and indicate whether your feature meets the drawing requirements for position. Below the calculator, you will find step-by-step examples of how to enter the information and the results to expect.
Download the Free Position Conversion Chart
If you would like a quick reference for converting to GD&T Position, we have a handy chart available for you to download for free! When measuring position tolerance with calipers or other simple equipment, this chart will quickly tell you the reportable diametric deviation for your feature. Simply use this chart to convert your X and Y deviation measurements into an actual diametric deviation, which you can then compare to the position tolerance listed in your feature control frame!
In addition to the Position Conversion Chart, you will also receive these helpful resources:
- GD&T Symbols Reference Chart
- Drill Tap Chart
- Orthographic Projection Chart
- ASME vs ISO GD&T Standards Comparison Chart
- ASME Y14.5 2009 vs 2018 Comparision Chart
Position Inspection Example for an Internal Feature at Regardless of Feature Size (RFS)
Let’s walk through an example of how to use the true position calculator for inspecting position of an internal feature at Regardless of Feature Size (RFS). In the drawing below, we have an aluminum channel with holes described by the feature control frame as having a position of .008 at RFS in reference to datums A, B, and C.
We will first enter the information pulled from our drawing.
Feature of Size: We are inspecting a hole feature, so select the radio button for “Internal Feature.”
Size Parameters: Our drawing indicates a hole size of .625 with a tolerance of +/- .005. In the “Size Parameters” section, input “.625” in the “Nominal Size” box and “.005” in both the Tolerance (+) and Tolerance (-) boxes.
Modifier Used: Select the “RFS” radio button. Our feature control frame does not have an MMC or LMC callout. When this is the case, the default is Regardless of Feature Size (RFS).
Position Tolerance: The last input from our drawing is the Position Tolerance. Input the .008 value found in our feature control frame.
Next, we will enter the Measured Values.
Location Measurements: Let’s say that we measured the location of this hole, and found that it measured 2.754 in the x direction, and 1.253 in the y direction. This is a deviation from true position of .004 in the x direction and .003 in the y direction.
Input .004 in the B(X-deviation) box and .003 in the C(Y-deviation) box.
Measured Size: Notice that there is an error warning for the "Measured Size" prior to entering the measured hole diameter. The calculator checks to ensure that the measured diameter is within the drawing tolerances. If it is not, you will see the box for “Measured Size” highlighted in red, with this error message for an internal feature, “Your measured size is out of tolerance. It must be less than the LMC and greater than the MMC.”
Let’s say that we measured this hole to be .627 in diameter. Enter “.627” in the “Measured Size” box. This is our last input into the calculator.
Now that we have entered our drawing dimensions and measurements into the True Position Calculator, it calculates the diametric deviation value (Position Error) and determines if it falls within the allowed tolerance zone (Total Position Allowed).
The position tolerance pulled from our feature control frame is .008. Because the feature is at RFS, there is no possible bonus tolerance. Therefore, the Total Position Allowed is equal to the position tolerance. The calculated position error equals .0100, which is greater than the total position allowed. Therefore, this feature fails inspection, as indicated in the Pass/Fail status in red.
Position Inspection Example for an Internal Feature at Maximum Material Condition (MMC)
Now, let’s walk through the same example, but this time our feature control frame includes the Maximum Material Condition modifier (indicated as an “M” inside a circle). This means that the holes are being described by the feature control frame as having a position of .008 at MMC in reference to datums A, B, and C.
Calculator Inputs: All inputs from the previous example are the same, except for the selection of “Modifier Used.” In the “Modifier Used” box, select the “MMC” radio button.
With this one modification, notice the change in the calculator outputs. Designing for Maximum Material Condition allows for Bonus Tolerance, which is equal to the difference between the MMC and the actual condition (size) of the measured part.
- For our part example, this results in a Bonus Tolerance of .007.
- When added to the Position Tolerance of .008, this results in a “Total Tolerance Allowed” of .015.
- The Position Error (diametric deviation value) of .0100 is less than the total tolerance allowed, thus the feature passes inspection. The calculator indicates “Pass” in the Pass/Fail status and highlights it in green.
Why does designing a feature to MMC allow for bonus tolerance? When designing to Maximum Material Condition, you are designing with assembly in mind. You are ensuring that the stack up of size and form of the shaft will always be smaller than the stack up of size and form of the hole that it will be inserted in. Therefore, as the size of the feature deviates from its Maximum Material Condition, you gain a bonus tolerance – the difference between the actual feature size and the MMC size.
For a more detailed explanation of Bonus Tolerance when using the Position control with the MMC modifier, check out our True Position symbol page.