Common GD&T Student Questions: Identical Virtual Conditions for Mating Boss and Bore Features

[@sem7207]

Nice explanation. But functionally, wouldn't it be more likely that the boss and bore are used as the secondary or even primary datum features, and the patterns of holes are positioned relative to them and not the other way around? I think usually clearance holes and tapped holes do not locate parts like these to each other. The boss and bore often do.

[@Gdandtbasics]

Great comments! In scenarios like this, you are right, there could be the discussion of whether to use the bore/boss scheme or to utilize the tapped holes and pattern of thru holes. The correct decision usually lies somewhere in how much clearance does each combination have? If the bore/boss is a press fit type of situation then it would definitely be utilized before the pattern of holes.

[@QwertyCanada]

You're the best.

[@Gdandtbasics]

Aww, thanks for the kind words!

[@AlexLapugean]

I think this video is a bit misleading, it seems to suggest maximum material conditionis (MMC) is needed for the parts to fit and to determine a maximum material virtual size (MMVS), when it's actuially not. The maximum material condition simply allows the position to be worse than specified, in the case that the diameter is smaller than the maximum material size (MMS), by the same amount it is smaller by (so you get a bonus tolerance), as you know that you can allow a larger geometrical deviation in case you have a smaller diameter, without the part violating that maximum material virtual conditionl. So the part would be perfectly funcional without the MMC, you'd just potentially reject parts that are actually functional.

[@Gdandtbasics]

You are correct here. However, the ASME Y14.5 standard does define virtual condition as a boundary that is only defined when the material modifiers are applied. Otherwise the boundary is simply called "inner or outer boundary". So to stay true to the original question we made sure to include the material modifier in our example. So to use the correct terminology if the drawing does NOT utilize material modifiers you could easily set the "inner boundary" of the bore to match the "outer boundary" of the boss.Thanks for your comment!

[@AlexLapugean]

@@Gdandtbasics Sure, I agree with you. But I myself find it challenging to make beginners understand the maximum material condition, and even more so datum shifts, so over the years I tried to be more and more careful to how I phrase things to not create extra confusion. Good videos BTW, had a good time looking trough them, gave me new ideas how to better explain things to others.