MMC- Applying bonus tolerance using multiple Datums.

[Ghigan0]

TP|.005|A mmc|B mmc|C mmc

I understand i can use all the bonus tolerance from Datum C, but are their limits to how much bonus tolerance I can use from the secondary and tertiary Datums? I thought i remembered that you get 100% of the last Datum called out (C), and only half of the second (B). Is this correct??? and then what about the 3rd Datum?

Please help explain!

 

[Belanger]

There is no reliable answer to this question because it totally depends on the situation. We can't make a general statement about that.
(FYI -- the variation from the datums is not called bonus, but instead it's more properly called "shift" or "datum shift.")

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Ghigan0]

let me rewrite that...


TP|Ø.05 m |F m| E

My hole size is Ø 0.33 ± 0.05
actual Ø measures 0.332. the MMC for this Ø would be 0.052.
Which can be added to the 0.05 tolerance about, corret? and what about Datum F? Let's say Datum F is also a Ø.

 

[Belanger]

The datum shift cannot be added to the TP. It doesn't make the tolerance zone get larger; it allows the tolerance zone to shift. We'd have to examine the entire layout of the drawing to see the datums and how they relate to your hole.

Also, if a hole size is Ø0.33 ± 0.05, notice that its MMC is 0.28. Thus if the actual hole is 0.332, the position tol would grow to 0.102. Any datum shift would allow that zone to move, but perhaps only in a certain restricted direction.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[drawoh]

Ghigan0,

On your datums, MMC/MMB is not meaningful unless each one is a feature of size. I cannot visualize a design with three FOS datums.

--
JHG