Flatness + Parallelism 1

[Rwelch9]

Hi,

I am looking to see if anyone can put into words how a Flatness error would/could affect the parallelism of two planes.



Thanks

Ross

 

[chez311]

Short answer - it won't.

Long answer - it doesn't matter if you're talking about surface or DMP (Derived Median Plane) flatness. Two perfectly flat surfaces (0 surface flatness error) could be tapered to the limit allowed by the size tolerance and result in a perfectly flat DMP (0 DMP flatness error). The taper or amount of orientation error of each surface would be equal to (MMC_size - LMC_size)/2.

 

[axym]

Rwelch9,

It depends on exactly what you mean by "the parallelism of two planes".

If you're referring specifically to the geometric tolerance described in ASME Y14.5 or ISO 1101, then flatness error can affect parallelism.

Let's say that one plane is the considered feature for a parallelism tolerance of 1 mm and the other plane is the datum feature. The considered feature must lie within a tolerance zone consisting of two parallel planes that are 1 mm apart, and exactly parallel to the tangent plane of the datum feature. If the considered feature had a flatness error of 1.3 mm, it couldn't fit in the parallelism tolerance zone (even if the orientation was very good).

I usually rely on pictures - these things are very difficult to put into words.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[chez311]

Evan is correct, I simplified it in order to get the point across that flatness does not limit the maximum possible orientation error - but there is more nuance than I may have initially conveyed since orientation as defined in Y14.5 limits form error ie: a surface with a 0.1 parallelism tolerance must also be flat within 0.1 - therefore a feature with 0.1 flatness/form error has taken up its entire parallelism tolerance and must therefore be perfectly oriented to your DRF in order to pass, however a feature which has 0 form error may still take up its entire 0.1 parallelism tolerance by having 0.1 orientation error.

See my simplistic sketch below - hopefully its clear that the orientation error shown is either parallel or perpendicular to some datum feature (not shown).