Tolerance value for FOS straightness

[Belanger]

I stumbled across a statement in Y14.5-2009 that doesn't sound quite right at first. Check out paragraph 5.4.1.2 on page 91, where it says: "Where the straightness tolerance is used in conjunction with an orientation tolerance or position tolerance value, the specified straightness tolerance value shall not be greater than the specified orientation or position tolerance value." Here's why that statement seems strange...

Think of a pin attached to a flat base, where straightness and perpendicularity are both applied to the FOS. I want to disagree with their statement because a straightness tolerance zone is controlling the derived median line, whereas the perpendicularity tolerance zone is controlling the axis of the UAME. The toleranced items aren't fighting each other, so why does the standard make the rule about the tolerance values?

The only thing I can think of is that having a larger straightness value would create an outer boundary that is larger than the perpendicularity's outer boundary. While not illegal in itself, having the form's boundary be larger than the orientation's boundary seems to violate the "hierarchy" of things, thus their rule. Or do you all have any other thoughts?

 

[Kedu]

The statement make sense at MMC but does not at RFS.
Pmarc and Evan have a nice explanation on one of the threads here on eng-tips. If I find it I'll post

 

[Kedu]

https://www.eng-tips.com/viewthread.cfm?qid=302375

Straightness of an axis as a refinement of position tolerance

OP: pmarc

 

[3DDave]

5.4.1.2 - proof that someone in the page layout department needs to learn about paragraph breaks. But no orphan at least.

 

[Belanger]

Yep -- I knew I was on to something. Thanks, Kedu.
The paragraph should have been qualified to exempt RFS from that blanket prohibition.

 

[3DDave]

For the RFS case - is this a representative model?

https://files.engineering.com/getfi...751-8c94-80fa5b5d1250&file=axis_vs_median.ggb

 

[Belanger]

Yes, your thumbnail graphic from the GeoGebra is indeed what I was thinking. The axis of the UAME could be within a very tight tolerance zone (not shown), while the D.M.L. is bent within your green zone (larger than the perp zone).

Evan's reply in the other thread is almost exactly what I was stating, but I still have a pestering question: Who cares if the outer boundary for straightness is larger than the outer boundary for perpendicularity? The statement in paragraph 5.4.1.2 that I focused on seems like it's solving a problem that isn't there (I know you'll be sympathetic to that comment, Dave).

 

[3DDave]

There's a thumbnail?

I'm working on getting better with GeoGebra and am not to the point where it would be easy for me to create a shear transformation.

It occurred to me that the restriction should be removed from that paragraph and given its own as it applies to more than cylinders/straightness and is also applicable to flatness**. It could also include the reasoning for why it is stated that way.

I expect the main reason is that it saves a lot of writing throughout the standard that the MMC/MMB might be size + orientation/position or might also include form tolerances. For example, 2.8.3 would need to add a reference to straightness along with size.

My gut feeling is based on the idea that an orientation/location tolerance should limit all the median points and that allowing straightness to be larger than orientation/position would violate that, but it also seems like it's just a convention and a change of rule to include straightness as part of the calculation would allow it.


**Speaking of which, is there any reason one cannot derive a median plane from a wedge and therefore control its flatness? And if the wedge was defined with a profile tolerance, would MMC controls be applicable to that flatness? This must have come up some time.

 

[Belanger]

Re: thumbnail -- yes, your attachment unzipped as several files. I didn't look at the XML spreadsheet data but I simply opened something called "geogebra_thumbnail.png."

 

[pmarc]

Seems like what Evan said in the other thread will be added to new version of Y14.5 that is going to be released this year.

Unless something has changed last minute, the standard will say that if DML straightness tolerance RFS is used in conjuction with an orientation or position tolerance RFS, there will be no restriction for the straightness tolerance value.

If, however, DML straightness tolerance at MMC is used in conjuction with an orientation or position tolerance at MMC, the straightness tolerance value will not be allowed to be greater than the orientation or position tolerance value.

 

[3DDave]

Ahh - I only see one file; I guess they followed the Windows xlsx/docx/pptx model and zipped them together.
As far as I know the data isn't just a spreadsheet, but includes the geometric constraints, the display defaults, and so forth.