True Position CMM Results Evaluation Worksheet
A CMM report usually gives you X and Y deviations per hole, not a pass/fail on the position callout. This worksheet converts deviations to diametral true position, adds MMC bonus tolerance, and flags each feature in a pattern with percent of tolerance used and margin.
| Feature | X deviation (mm) | Y deviation (mm) | MMC modifier (1 = yes, 0 = RFS) | Departure from MMC size (mm) | Stated position tolerance (dia) (mm) | True position (dia) (mm) | Bonus tolerance (mm) | Allowed position (mm) | Tolerance used | Margin (mm) | In tolerance (1 = pass) | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
0.0466 | 0.03 | 0.18 | 25.9% | 0.1334 | 1 | |||||||
0.146 | 0.01 | 0.16 | 91.3% | 0.014 | 1 | |||||||
0.163 | 0 | 0.15 | 108.7% | -0.013 | 0 | |||||||
0.0601 | 0 | 0.1 | 60.1% | 0.0399 | 1 | |||||||
0.1669 | 0.005 | 0.155 | 107.7% | -0.0119 | 0 | |||||||
0.0197 | 0.02 | 0.22 | 9% | 0.2003 | 1 |
Units are millimeters but any consistent unit works. True position of a hole axis = 2 x sqrt(dx^2 + dy^2), the diametral zone convention in ASME Y14.5. Bonus tolerance applies only when the feature control frame shows MMC (or LMC with the departure measured toward MMC instead) and equals the departure of the actual size from MMC. Datum shift from a datum feature at MMC is not included, add it to the bonus column if your frame uses it.
Automate position checks from your CMM data
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How to use the worksheet
Enter one row per feature. Type the signed X and Y deviations from the CMM output (measured minus theoretical), the stated position tolerance from the feature control frame, and whether the frame has the MMC modifier. If it does, enter how far the actual hole size is from its MMC limit, which for an internal feature means actual diameter minus minimum diameter. The sheet computes position, bonus, allowed position, percent used, margin and a pass flag, then totals the failures and the worst case at the bottom.
- Use the same units for every column, the sheet does not convert.
- Enter 0 for the MMC modifier on RFS callouts, bonus is then zero.
- Enter the departure as a positive number, the sheet ignores negative values.
- Add or delete rows to match your pattern, totals update automatically.
The method behind it
ASME Y14.5 position for a cylindrical zone is a diameter, so the radial error of the axis from true position is sqrt(dx^2 + dy^2) and the reported position value is twice that. The feature passes when that value is less than or equal to the tolerance plus any bonus. Bonus tolerance under MMC equals the difference between the actual mating size and the MMC size, so a hole machined larger than its minimum gives extra position room because a mating pin still assembles. Percent used is position divided by allowed position, a quick way to see which holes are drifting before they fail.
Reading the results
In the example, H3 fails: its deviations of -0.071 and 0.040 give a position of about 0.163 against 0.150 allowed, because it is RFS and gets no bonus. H5 also fails at roughly 0.167 against 0.155, even though it has a small bonus. H2 passes at 0.146 against 0.160, but at about 91 percent used it is a warning sign. Look at the signs of the deviations across the pattern: if all holes shift the same direction the cause is probably a fixture or work offset, not random machine error.
- Percent used above 80 on several holes: investigate offsets before the next lot.
- Consistent signed shift across holes points to work offset or datum setup.
- Random scatter with no shift points to tool deflection or thermal effects.
Common mistakes
The most common error is comparing the radial value sqrt(dx^2 + dy^2) to a diametral tolerance, which understates the error by half. The second is applying bonus tolerance to an RFS callout. The third is measuring size and position at different temperatures or on different setups, so the size departure used for bonus does not match the part the CMM located. Finally, remember that when the frame references datum features of size at MMC, a datum shift allowance may add further tolerance that this sheet does not calculate on its own.
Frequently Asked Questions
Why multiply by two?
Position tolerance for a hole axis is a cylindrical zone specified as a diameter. The CMM deviations give the radial offset of the axis, so the diameter of the smallest cylinder containing the measured axis is twice the radial distance. Comparing the radius to the diametral tolerance would pass bad parts.
What if my CMM already reports true position?
Most CMM software reports position as a diameter already, in which case you can compare it directly. Use this sheet when you only have X and Y deviations, when you want to add bonus tolerance yourself, or when you want to check the software setup against a hand calculation.
Does this work for LMC callouts?
Yes, if you enter the departure of the actual size from LMC toward MMC in the departure column and set the modifier to 1. LMC bonus follows the same arithmetic but the departure is measured from the least material limit, so double check the direction on your drawing.
Does it handle polar or non-hole features?
It handles any feature whose position zone is a cylinder or circle from X/Y deviations, such as pins, bosses and slots measured by center point. Features with a two-parallel-plane zone, such as a slot width callout with no diameter symbol, are evaluated on one axis only, so use the larger deviation as twice the single-axis error.
If your CMM results already land in a database or ERP quality module, Netray can build a read-only agent that evaluates position against the drawing callouts automatically and shows the query it used.
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