Real life/world design cases (DML, DMP, circularity and cylindricity)

[greenimi]

Someone had a question on linkedin regarding the derived median line straightness and derived median plane flatness.
In addition to his question, I would like to extend the same idea to circularity and cylindricity. In other words, I would like to toss it out there for some feedback on some design cases where circularity or cylindricity would be a good idea to be used (again in addition of DML and DMP)

Likedin thread stated:
“I think (but I am not positive), that I do have an "approximate" good understanding of the definition of the derived median line and derived median plane (controlling the derived median line of the feature to be within some cylindrical tolerance zone and controlling the derived median plane of the feature to be within two parallel planes within some tolerance apart, respectively), but I would like to get some good real life design scenario where the DML and DMP is a good idea to be applied.

In other words, where these “tools” provided by ASME-Y14.5 shall be used? Some design examples, where DML and DMP would be beneficial as appropriate callouts.
I know using DML and DMP is "a way" of getting rid of the "perfect form at MMC" requirement (aka Rule #1).”

 

[Belanger]

The concepts of circularity and cylindricity don't have anything to do with DML and DMP. Those acronyms are when speaking about form tolerance applied to a feature of size.
Circularity and cylindricity do have some application, but they are tricky to measure because they get into things like maximum inscribed cylinder, least-squares circle, minimum radial separation, etc.

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

 

[pylfrm]

Cylindricity is sometimes specified for test bars used to evaluate machine tool accuracy. It is also specified for setting masters where actual diameter either doesn't matter or can be measured and recorded.

- pylfrm

 

[greenimi]

J-P,
re: "The concepts of circularity and cylindricity don't have anything to do with DML and DMP"

I understand. But the main reason and my question was about some good applications of such of callouts independent to each other. In another words:
- good applications to use DML
- good applications to use DMP
- good applications to use circularity
- good applications to use cylindricity

 

[dgallup]

I've used cylindricity for extremely precise bores that need to have mating parts slide in them with a very precise fit. Mostly fuel injector nozzles. Circularity is applied when there only needs to be good precision at one location such as a conical valve seat that has to seal against a ball. There are specialized form testers that can measure cylindricity and circularity to 10 millionths on an inch.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Belanger]

Sorry about that, greenimi. I didn't understand what you meant by the parenthetical statement "in addition of DML and DMP."

Pylfrm has one application. And I know in the automotive industry circularity is used quite regularly on crankshafts and camshafts (as a refinement of circular runout). Cylindricity could be applied to the cylinder bores in an engine block.
For things like straightness and flatness on a FOS (which is where DML and DMP come in), I don't have as quick of an answer. Just about everywhere that I've used FOS straightness, it's been with the MMC modifier which changes the thinking from DML to a virtual condition.

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