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Datum Changes to ASME Y14.5 - 2009 (revised August 20/09)
The following are the revisions that comply with ASME Y14.5 - 2009. Otherwise the same as shown on the ASME Y14.5M-94 version shown in another page on this web site. Under ASME Y14.5 - 2009, Maximum Material Condition (MMC) can now apply to datums that are features of size and also surfaces. The 94 standard would only allow MMC on datums that were features of size and NOT surfaces. A feature of size is a hole or pin of any shape and also a width. In most cases in GD&T, the holes or pins are most important to assembly and are used a great deal as secondary and tertiary datums. Usually, the perimeter of a non-cylindrical part is not functionally important. There are certain cases where there may be a partial hole or cutout that is used in assembly and could now be referenced as a datum.  Maximum Material Boundary The Maximum Material Boundary (MMB) is a new term used in the 2009 standard and replaces the terms "Maximum Material Condition" and also "Virtual Condition Size" when referring to a datums referenced with the maximum material condition symbol.
In certain cases, MMB is the maximum material size while in other
situations, it is the virtual condition size. It depends upon whether the
datum is a primary, secondary or tertiary datum.
Let's review the MMB for datum G in the above example.  If datum G was referenced as a primary datum, the MMB would be the MMC size of the hole which would be the smallest allowable size of the 12 mm hole which is 11.6 mm. It does not make any difference whether or not the feature actually has a virtual condition size as shown, the MMB is still 11.6 mm.. In our example, datum G is referenced at MMC as a secondary datum so the MMB is 12 - 0.4 - 0.2 = 11.4 mm which is the virtual condition size of the hole. If the secondary datum did not have a virtual condition size, it would default to its maximum material condition size of 11.6. Datum H Reviewed  If datum H was referenced as a primary datum, the MMB would be its maximum material condition size or smallest allowable size - 8.6 mm.
If datum H was referenced as a secondary datum, the MMB would be its virtual condition size but, in our situation, we have two (2) virtual condition sizes.
The positional tolerance shown would give us a virtual condition diametrical tolerance zone size of 9 - 0.4 (MMC) - 0.3 (perpendicularity) = 8.3 mm. We
also have a refinement of the positional tolerance with a
perpendicularity requirement. In this situation, we have a virtual
condition size of 9 - 0.4 (MMC) - 0.2 (perpendicularity) = 8.4 mm.
So, if datum H was referenced as a secondary datum, one would use the perpendicularity refinement resulting in a MMB of 9 - 0.4 - 0.2 (perpendicularity) = 8.4 mm. In our situation, datum H is a tertiary datum and
only used for orienting (anti-rotation) the part about datum G so that
we are able to confirm all the dimensions. In our situation, we will
use the MMB of 9 - 0.4 - 0.3 (positional) = 8.3 mm which includes the positional tolerances rather than its refinement of a perpendicular tolerance.
Here we have 4 holes of 8 +/- 0.3 mm. The feature control frame reflects a positional tolera
 nce of a diametrical tolerance zone of 0.25 mm beyond the MMC referencing primary datum A (usually the mounting surface), secondary datum G at MMC (12 mm hole) and tertiary datum H also at MMC (9 mm hole).We have already discussed that fact that the MMB changes depending upon whether it is a primary, secondary or tertiary datum. If there is any doubt about the MMB, one can reflect the actual MMB size in the feature control frame as shown above using brackets about the MMB size. This method can also be used if MMB size differs from the calculated size. Let's say we wanted the MMB size of datum H to be its refinement size of 8.4. One would then replace the 8.3 in the feature control frame with the refined size of 8.4 and that superseded the calculated MMB size. Confirmation Method  The checking fixture (functional gauge) simulates assembly.
We have the tolerance shown at MMC and also the secondary and tertiary
datums at MMC. This indicates that we must have cylindrical feature
such as a bolt/shaft assembled through datum holes G and H. If the
holes are a bit larger than MMC (smallest allowable size), we gain
tolerances the same as one would during assembly.
Whenever a MMB is specified as a datum, it is conducive to the use of a checking fixture that sometimes is called a "functional gauge".
It is difficult to confirm the positional tolerances with a CMM
(coordinate measuring machine) since all the requirements are to be
confirmed simultaneously and in many cases, the CMM Technician disregards the MMC in the reference datums.
 The pin that protrudes into the feature of size is a cylindrical pin as shown. The size of the pins would be the MMB size that is calculated or given in the feature control frame with brackets. Here we see that the pin size of secondary datum G is a cylindrical pin of 11.4 mm diameter while the tertiary datum pin would be an elliptical pin or cylindrical pin of 8.3 mm. Surfaces Referenced at Maximum Material Condition Here we have the same drawing with the exception that 9 mm diameter datum H hole has been replaced with a radius of 10 mm that is controlled with a profile of a surface feature control frame. This is a practical situation if the radius cutout is clearance for, say, a bolt head. It is probably as important to the part's function as the 9 mm hole was in the previous drawing.  Maximum Material Condition on a Profile Tolerances - Profile tolerances are defaulted to a range of a bilateral tolerance. In our example, have an inner boundary at a radius of 10 - 0.2 = 9.8 radius and an outer boundary of 10 + 0.2 = 10.2 radius. Maximum material condition is feature where the part weighs the most. In our case, it would be the inner boundary which would have a radius of 9.8 mm. Think of a hole. MMC is the smallest allowable  size and this cutout (radius) is a partial hole or diameter.The MMB in our situation is a radius of 9.8 centred on the true (theoretical) centre shown with basic dimensions. How would one set up the part for confirming the four (4) holes if the tertiary datum is a radius shown at MMC? Whenever there is a geometrical tolerance such in positional shown at MMC as is in our case, it is conducive to a checking fixture. A checking fixture or functional gauge will confirm all the requirements simultaneously as required while a CMM (coordinate measuring machine) will confirm the locations of the hole centres in various heights. If one finds a different result from the CMM and the checking fixture, then the checking fixture will supersede the CMM.   Image trying to set up the part on a CMM. How could one move the part on a rigid setup to optimize the confirmation of the four (4) holes? This is easily done on a checking fixture as shown.  Here is another example where ASME Y14.5 - 2009 allows a surface to be referenced at MMC but this may not be as practical as the previous example of a clearance radius. As a matter of fact, one might have difficulty coming up with a practical shop example. The bottom surface has a true (theoretical) dimension of 200 mm with a profile of a surface requirement of 0.6 referencing primary datum A and secondary datum G at RFS (regardless of feature size) to qualify as datum H. Once the surface meets the profile requirement, it becomes datum H and will be used in our situation as a tertiary datum. Again, a tertiary datum is for orientation (anti-rotation) about the secondary datum B. MMB - Since the profile tolerance is a range of a bilateral tolerance with 0.3 mm above the 200 true profile and 0.3 below it, the MMB will be one where one would have the maximum material into the part. In our case, the MMB would be the largest size which would be 200 + 0.3 mm = 200.3 mm. Since the feature control frame for the four (4) holes references both secondary datum G and tertiary datum H at MMC, it is best confirmed with a checking fixture.  Depending upon the actual dimension of surface H, the part can now pivot around datum hole G until it fouls on datum H simulator (checking fixture block set at MMB of 200.3 mm). Regardless Material Boundary (RMB) The RMB is a new term and is really regardless of feature size as shown in the 94 standard. In other words, there is no bonus tolerance on the datum feature if the hole was made larger than its smallest allowable size. We would still set up on the actual inside diameter as long as its within size tolerance. In the 2009 standard one might see the symbol called the "translation modifier" in the tertiary datum reference frame.  What does it mean.? The tertiary datum is for anti-rotation and is most likely a hole. We are only interested in the side of the hole, the translation modifier allows the movement of the tertiary datum simulator (V Cone on the fixture) or the unlocking of the V cone to move up or down to fully seat the cone. Degrees of Freedom Shown in the Feature Control Frame  One may see a feature control frame as shown here.These reflect the degrees of freedom which will be explained separately in another section of this web page. I really hope that Designers do not use this method since most on the shop floor would not understand it. I hope that the above information is of value. You may not see any of the feature control frames on your drawing for a while but, at least, you should have some idea of their meaning. David DeLong ASME GD&T Professional - Technologist and Senior Levels |
