Perpendicularity Stack-up

[DJL Eng]

Can perpendicularity tolerances stack-up? See the attached picture for an illustration. I could not find the answer on this site or elsewhere. I am a GD&T noob so I apologize if this is a silly inquiry. Assume compliance with ASME Y14.5-2018.

 

[3DDave]

That's how that analysis is done. The next level is to find the probability of that amount of error. If it is injection molded and a single mold cavity is used, that might happen to all the parts and if they are all stacked the same way they could add up just like that. Some other process may see random variations, but that analysis serves for the worst case.

 

[Belanger]

But... if the height tolerance (±y) is smaller than 0.02 total, then it would control the leaning-tower stack result, rather than the perpendicularity numbers (because that becomes an implicit parallelism control).

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

 

[3DDave]

Which leaning tower stack result? The amount of lean or the height?

 

[geesaman.d]

Yes, this is common with vertically-shafted industrial agitators. The shafts are hanging rather than stacked, but because they are rotating the straightness might be more crucial.
Good agitator shafting has the coupling faces extremely square to the overall centerline of each agitator shaft section. This way the perpendicularity error stackup is minimized. When overall straightness is critical, squareness of coupling faces is arguably more important to maintaining overall straightness than straightness of any one shaft section.

FWIW if you can assemble and measure runout before putting the unit into full service, it's very possible to test the couplings assembled at different orientations to cancel out the perpendicularity errors. Then match-mark them so they only get re-assembled the 'best' way in the future.

The good news is its easy trig to calculate worst-case perpendicularity errors and resulting straightness error.

 

[Belanger]

Which leaning tower stack result? The amount of lean or the height?

I was referring to the amount of leaning, but actually both. Notice that datum feature B is the left side. What would that have to do with stacking the parts in the manner shown?
Nothing -- so the stack for leaning is really about parallelism. That's controlled by the perpendicularity tolerances going in opposite directions, but also the size tolerance.


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

 

[3DDave]

This could represent a series of very long parts that are shown reduced in height to make the sketch manageable and datum feature B is the most important for each component; merely controlling the parallelism by itself won't improve that situation.

 

[Belanger]

The question pertained to the sketch that was provided.
Datum B plays no role in that stack; it's all about how the two main faces interact with each other.

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

 

[3DDave]

Then I propose that the side surfaces, currently identified as [B} not be used and allowed to be 85 degrees to the top and bottom faces, but still used to align the sides.

If plays no role, then there won't be any change to the result.