![Maximum Maximum](http://static.wixstatic.com/media/3a6acb_f527f8eb96354f5d8b6a93f6cf2a8453~mv2.jpg/v1/fill/w_593,h_330,al_c,q_80,usm_0.66_1.00_0.01/3a6acb_f527f8eb96354f5d8b6a93f6cf2a8453~mv2.jpg)
The tolerance of position tolerance is added (for an external feature of size) or subtracted (for an internal feature of size) to the MMC of the toleranced feature. In the example shown, the virtual condition boundary for the tab equals 7.4. For the hole, the virtual condition equals 7.2.
When Zero is MoreBy James KeithWe have been led to believe that more is better - more speed, morefilled seats per flight, more miles per gallon, more inspection tolerance, andso on.With tolerances, one might think that a large value in a feature controlframe is better than a small value. Whenusing Geometric Dimensioning & Tolerancing andtalking about position tolerances, the greatest allowable variation actuallycomes from the application of a “zero” tolerance on a Maximum Material Condition (MMC) basis.Okay, let’s putthis in simple terms: Tolerance isair!
We can see this demonstrated by theformula shown in the ASME Y14.5 (GD&T) Standards, where Tolerance = Hole– Fastener, or T=H-F.Example: assumea hole diameter of.260 -.270, and a fastener diameter of.250. Both H and F will be the MMC value of therespective features. So, H (.260) – F(.250) = T (.010). The problem with thismethod is that holes less than.260 (down to.250) are not allowed regardlessof their location.Now this iswhere zero is more! We are going tomodify the formula slightly to read: T(0) = H-F.
![Conditions Conditions](/uploads/1/2/5/6/125637438/671401303.png)
Now we have a value of zero that the MMC holeminus the MMC fastener must satisfy. H(.250) – F(.250)=T(0).In other words,we have modified the MMC hole size to equal the MMCfastener diameter. The new hole size is.250.270 diameter and the location toleranceis zero at MMC. Virtual conditionremains unchanged.
See the examplebelow.