Effective Training Inc. Message Board

Can a half cylinder be considered a FOS?

I would say yes. I would say its what we would class as a normal FOS (pin) but interrupted, for which the standard has many examples, checkout this link http://www.etinews.com/techpaper_FOS_ASME.pps, for which Alex Krulikowski says the standard needs to give us more definition and clarification on this issue.

The standard defines a FOS as
1.3.32.1 Regular Feature of Size. regular feature of size: one cylindrical or spherical surface, a circular ele- ment, and a set of two opposed parallel elements or opposed parallel surfaces, each of which is associated with a directly toleranced dimension

Well it is a cylindrical surface and it would have a directly toleranced dimension, so yes its a FOS in my opinion.

Fig 4-30 in the 2009 is the closest to a half pin example i've seen in the standard

Thinking about it though, your problem would be however depending on how the cylinder was produced, it may or may not contain opposed points (eg if it was just made slighly shy of a half pin it would not contain opposed points), so even though technically it is a FOS at a half pin or bigger, best treat the half as a surface and not a FOS and look at profile of a surface instead of position (if your looking to locate it)