Flatness control

[pyromech]

After reading several threads, flatness doesnt count on stacks unless leakage is of concern. Alex K. workbook does count it, see attached. The second stack shows a flatness 0.6 which makes me more confused. In my case, i am looking for the gap on top of the assembly far away from the flatness of the bottom washers. Do i care for flatness?

How about concentricity control? not most suitable but it appears in one of our drawing. I suppose it gets treated like position since is a location control.

thanks

 

[pylfrm]

pyromech,

In exercise 14-4, the value of 0.6 for distance A comes from the maximum flatness error allowed by the 10.6 - 10.0 size tolerance on the base plate.

In general, flatness errors in stacked components can allow them to overlap a bit. How you account for this depends on the tolerance schemes of the individual components.

As for concentricity, the answer again depends on the details of your application.


pylfrm

 

[Belanger]

In Exercise 14-3, the reason flatness is included in the stack is that the profile on the right side is referencing datum A. And a datum is defined as the theoretically perfect plane derived from the left side. But in reality the left side of the part could be concave, thus eating into the wall thickness.
And since the datum is formed from the highest points, the flatness error is only felt in the min direction.

If the thickness dim of 2.5 were not basic, but toleranced directly, then the flatness would not be considered in the stack.

In Exercise 14-4, the reason flatness is included is that the dim of 14.0-14.2 is not considered a feature of size, so Rule #1 doesn't apply. Any bumpiness on the table will indeed affect the unknown dim X. In the textbooks this is what they call an "offset" condition, where the stack is being done away from (offset from) the location where the parts actually touch. The flatness error shows up in both columns because the bumps could go down or up from the mating plane.

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

 

[pyromech]

Pylfrm

For stack 1, why is flatness applied at the min. wall thickness?

 

[aniiben]

In Exercise 14-4, the reason flatness is included is that the dim of 14.0-14.2 is not considered a feature of size, so Rule #1 doesn't apply. Any bumpiness on the table will indeed affect the unknown dim X. In the textbooks this is what they call an "offset" condition, where the stack is being done away from (offset from) the location where the parts actually touch. The flatness error shows up in both columns because the bumps could go down or up from the mating plane

J-P,
For fig. 14-4 then why 0.1 flatness is not included for the maximum X (14.8 maximum)?
I did not understand this fact. I am missing something, but I don't know what.
Thank you J-P.

 

[Belanger]

Because that flatness only applies to the two surfaces of the cover flange. And when that flange sits on the table, only the high points touch the table. Think of the flange surfaces as being either concave or convex, or even with irregular bumps: that wouldn't change where the 14.0-14.2 dimension comes from.

In my previous post I should have clarified that the "flatness" I was speaking of comes from the table (from its size tolerance). The actual flatness symbol on the flange and its 0.1 tolerance is ignored because of what I just mentioned -- only high point contact.

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

 

[aniiben]

Okay. I agree. The only caveat I will add (not sure if it is applicable, but I will add it anyway since it is Friday) is that when 14.0-14.2 is measured/ this dimension should me measured from the highest points (and not from the lowest ones)

 

[Belanger]

True -- which makes me wonder if this would be a good situation to use the dimension origin symbol (a little end circle at the lower side of the 14.0-14.2 dimension line).

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

 

[CheckerHater]

14.0-14.2 dimension doesn't look like FOS to me, so it must be local measurement, not "envelope" (it's still Friday )

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Belanger]

CH -- if you're referring to my musing about the dim origin symbol, it needn't be reserved to a FOS. See Fig. 2-5 of the standard.
Apologies if you're referring to something else; I didn't see any mention of an envelope in the discussion so I'm not sure what you mean.

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

 

[CheckerHater]

Belanger,
I saw you responding to allben's post about dimensions being "measured from the highest points (and not from the lowest ones)", so I wanted to remind that envelope requirement was not in play.
Sorry if it was confusing.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future