You Don’t Always Need a Hard Gage to Evaluate the Surface Method – Part II (#71)
(In accordance with the ASME Y14.5-2018 standard)
(In accordance with the ASME Y14.5-2009 standard)
(In accordance with Y14.5M-1994 standard)
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Tip 70 showed the use of the surface method to evaluate a geometric tolerance that is applied at MMC or LMC. A simple built-up gage, or measurement with a CMM, were two methods shown for verifying that the feature’s surface does not violate a virtual condition boundary. For the surface method this is the only requirement that must be met to evaluate the position tolerance. A hard gage can be used for surface method evaluation, but we’re going to continue talking about methods that don’t require a hard gage.
This month, we’re expanding on this topic by showing additional ways to evaluate a geometric tolerance at MMC or LMC using the surface method. Our example has position tolerances at MMC applied to two cylindrical holes.
For the two holes on the drawing shown, first the size tolerances are measured and the datum reference frame established, let’s say by using a CMM (Coordinate Measuring Machine). To evaluate the position tolerance for each hole, the CMM probe may then be moved to its true position, as defined by the basic dimensions. From there, radial distances from true position to the feature surface can measured in as many places as is needed. The number of radial distances that should be measured, which should be specified in your measurement plan, depends upon how imperfect the parts and how tight the tolerances are.
The virtual condition (VC), combines of the effects of the size and position tolerances. It is equal to the MMC size limit minus the geometric tolerance, so:
VC = Ø20 – 0.1 – 0.6 = Ø19.3 mm.
Each radial distance must be greater than or equal to half of the diameter of the virtual condition, which is 9.65 mm. Our result for this radial distance measurement method will be a “pass” or “fail” outcome. If the shortest measured distance is greater than or equal to 9.65 millimeters, then the result is a “pass”, if it less than 9.65 millimeters it is a “fail”.
ASME Y14.45-2021, “Measurement Data Reporting”, provides a way to evaluate the surface method that also does not require a hard gage. This method provides a variable reported value instead of the attribute (Pass/Fail) result we get from the radial measurements method.
The Y14.45-2021 surface method reported value for position at MMC for an internal feature is:
Reported value = T + (VC size – LCMME size)
Where T = Specified tolerance; VC = Virtual Condition; & LCMME = Location Constrained Measured Mating Envelope
Next, we have an illustration of the radial measurements for the first method we covered, and the LCMME used for the second method. Do you see a relationship between those two items? The LCMME size is the shortest radial distance multiplied by 2.
With these methods there is no need to make a dedicated hard gage, and also no need to find the axis of the hole and determine if it meets the position tolerance.
To summarize, the options for evaluating the tolerances at MMC or LMC, using the surface method, but without the need for a dedicated hard gage, include:
1) A simple built-up gage, as we showed in last month’s tip #70, or
2) The smallest radial distance from true position to a sufficient number of points on the feature surface, to ensure the VC is not violated by the feature’s surface, or
3) Y14.45’s reported value which uses the specified geometric tolerance value and the “LCMME”. That relationship between the shortest radial distance and the LCMME that we pointed out in the illustration above shows us that both methods shown in this Tip are driven by the same measured parameter. Only the reported outcome differs between the two methods.
If the cost can be justified, and you prefer to use a hard gage instead to evaluate your tolerances at MMC, then you would just need to make a gage that has datum feature simulators, as shown in the CMM illustration above, and gage pins located at the true position for each hole, with each having diameter equal to the virtual condition. For more information about how to tolerance those gage elements, see ASME Y14.43, “Principles for Gages and Fixtures”.
In accordance with ASME Y14.5M-1994, Y14,5-2009, and Y14.5-2018
This tip was originally released in October 2003
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Tip added Feb 2025