Regular and irregular FOS

[AndrewTT]

I understand the definitions of regular FOS and irregular FOS. Why is the distinction important? They both can be datum features. Is there something that one can do that the other cannot? I was posed this question at work the other day and I had no good answer.

example for discussion: If I have a octagon shape, stamped out of a thin part, and I want to use it as a datum feature, I will need to specify the circumscribed cylinder or the inscribed cylinder as my irregular datum FOS. If I have a round hole stamped I don't need such a note/indication since the hole is a regular FOS. Is this where the distinction ends between regular and irregular FOS?

Thanks!

 

[Belanger]

One distinction I can think of is regarding Rule #1. This "envelope rule" only pertains to a regular FOS. In other words, the form of a FOS is controlled by the size tolerance if it's a regular FOS. The form of the FOS is not controlled by the size tolerance if it's an irregular FOS.

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

 

[Tunalover]

Guys,
Please refresh my memory. What's the difference between a regular FOS and an irregular FOS? I don't have 14.5-2009.
Thanks

Tunalover

 

[Belanger]

In a nutshell, a regular FOS is one where there are directly opposing surface elements, such that the feature can be "pinched" across with calipers.
An irregular FOS is where there might not be directly opposing elements, but the surface elements can be enclosed by a cylinder, sphere, parallel planes -- thus treating it like a FOS.

An example of an irregular FOS is a 5-pointed star. The tips of the star can be encircled, thus creating a diameter. But it's not truly a "pinchable" diameter.

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

 

[mkcski]

AndrewTT:

Let me add my two cents: my understanding is that the irregular FOS "concept" is more related to establishing datums from a collection of features than defining the physical size and form of the "feature" itself.

The 2009 standard in paragraph 1.3.32.2 mentions "actual mating envelope". The form error allowed by Rule #1 is "ignored" when mating envelopes are invoked, as the envelope only contacts the "high" spots. I have had issues with how to measure the size when an irregular FOS that is defined by a diameter - see Fig 4-35 in the DATUMS section - and there are no opposing elements - pinching as Belanger called it - because the actual mating envelope does not check actual local size. However, with Fig 4-35 I resolved that the measured size the actual mating envelope (smallest circumscribed cylinder) is a reasonable facsimile of the collection of feature even though there are no opposing elements. Using position to locate the 3-pins could be used indirectly to control the size "around" the 3 pins.