POL versus POS (profile of a line versus profile of a surface) and their measurements issues

[greenimi]

When to use POL is a question that has come up at my work a few times b/c no one really understands it here. I could totally be wrong but my understanding is if you use POS then the entire surface of revolution must be checked. If you use POL then you check individual line segments. How many line segments do you need to check? I believe that it is up to the inspector unless there is written instructions (on the drawing or some other controlling document).

Yes, I would say absolutely design the part based upon its intent but the part also has to be made and inspected.

I would appreciate any input from someone who has an opinion/knowledge on POL vs. POS.

 

[axym]

greenimi,

Profile of a surface controls the entire surface with one tolerance zone. Profile of a line controls cross-sectional slices (line elements) of the surface independently, with a separate tolerance zone for each line element. The difference is roughly comparable to the difference between total runout and circular runout, or the difference between cylindricity and circularity.

As with any geometric tolerance, there is no definition of how many points need to check or how many line elements need to be check. For profile of a surface, the requirement is that the entire surface must be within the tolerance zone. For profile of a line, the requirement is that all line elements (in theory, an infinite number) must be within their tolerance zones. Deciding how much of the feature to check is part of quality planning.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[KENAT]

thread1103-394884 gives an example of when you might use it.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[pmarc]

I would like to add that in some (many?) cases profile of a line tolerance does exactly the same thing as profile of a surface, thus its usage does not really make sense in certain applications.

Take fig. 8-7 in Y14.5-2009 as an example. Changing profile of surface callout to profile of a line in this case wouldn't really make sense, because both callouts would constitute identical requirement.

So I would say that although there is an analogy, the profile of a line vs. profile of a surface comparison is a little bit more tricky than comparing circularity with cylindricity or total runout with circular runout.

 

[AndrewTT]

If you only had equipment to measure POL and didn't want to spend the money to be able to inpsect POS and if POL & POS give you the equivalent tolerance zones (or equivalent enough for your purpose) then why not use POL? I am envisioning a surface of revolution with a profile callout that could be measured on an optical comparator (POL) vs having to be scan/CMM inspected(POS).

POS requires the inspector to verify that the entire surface is inside the tolerance zone. POL allows the inspector make a judgement that enough complying line elements have been observed so no more of the part needs to be inspected against this requirement.

Does this make sense? I'm a noob to GD&T so I may be off base here.