December 7th, 2009
The following is posted with the permission of the author, David DeLong, who is a ASME GD&T Professional (GDTP) at Quality Management Services, Inc.
Datum Changes to ASME Y14.5 – 2009
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.
For further details, please see the full article at Datums 2009.
Categories: ASME Standards, Drawing, Part |
Tags: datum, David DeLong, GD&T | No Comments
July 24th, 2009
ASME Y14.5M-2009 has been out for a little while now (after almost a year’s delay). There are significant improvements and clarifications. One addition in particular caught my attention, the ALL OVER symbol. When applied to a Profile of a Surface, it pretty much defines the entire shape of a part in every direction (not just ALL AROUND which applies to the profile of a surface along a particular plane).
The symbol is either a double circle at the vertex of the associated bent leader, or the words ALL OVER placed immediately below the feature control frame.
The symbol indicates that a profile tolerance or other specification shall apply all over the three-dimensional profile of a part. It is applied as “unless otherwise specified” to allow for other existing dimensions and tolerances to take precedence.
The advantage of using this symbol is that it provides control of surfaces over an entire part without regard to part orientation, thus allowing us to directly reference the CAD model as basic and fully controlled, while still detailing critical dimensions and tolerances. This may help companies better parts where they rely on the CAD model to provide complete specification. In fact, where a CAD model is declared basic, companies may be able to effectively place the Profile of a Surface FCF with the ALL OVER symbol right into their drawing title blocks along side other tolerancing information.
Categories: ASME Standards, Drawing, Part |
Tags: all around, all over, FCF, GD&T, profile, profile of a surface, tolerance, tolerancing | 2 Comments
January 11th, 2008
 [moved from comments to its own posting]
Ahhhhhhhhhh!
That is the sound of actually completing my preperation for 2 break out sessions for SolidWorks World 2008. I hope that all attendees will not only learn but will have a lot of fun.
Also, feel free to let me know if I am cracked.
Anyhow, my presentations are :
1. Teaching GD&T using the power of SolidWorks.
The goal is for instructors and corporate trainers to use SolidWorks and all of its glory to help clarify what GD&T is and how it is applied. Eventually I will be submitting course wear to SolidWorks EDU for all instructors to use and this will be included.
2. Making life simpiler with Design tables (Formerly Design Tables Paris Hilton could do).
This is a unique presentation where we will look into “Company X†(I used to work there) and see how design tables could simplify their lives in their product design. Another added bonus for this presentation is one of “Company X’s†lead designers helped me out with this and will be helping me out.
All in all – SWW2008 looks to be like a fantastic event. I hope that I get a chance to finally meet some of the BLOG community over a beer or 2 or. …
Until then – Have a great week.
Categories: News, SolidWorks World |
Tags: Design Tables, GD&T, presentation | 2 Comments
