Position tolerance, radial dimension tangent to semicircle slot

[ScottEng21]

Need some thoughts on a positional tolerance.

The part is essentially a bolt circle pattern of holes with a center hole that has a diameter that cuts through the radial holes leaving semicircular slots around the bore. (see attached sketch for reference) There is an existing drawing that has a radial dimension from the center of the part to the edge (tangent) of the semicircular slot.

Is it possible to use this dimension as a basic dimension for the positional tolerance? If so what exactly does it mean? Would it be a cylindrical tolerance at that point (edge of slot) or still about the center of the semicircle?

Never mind on attached sketch, thought I could attach it directly to the post.

Thanks for any input.

 

[3DDave]

If there is some direction in which size cannot be determined physically ...

a) the feature is considered to have perfect size where it is incomplete
b) the feature size is based on assuming the feature is the size one might extrapolate based on the feature fragment
c) the feature is not a feature of size in that direction
d) the standard doesn't have an adequate interpretation of this condition

http://www.tec-ease.com/gdt-tips-view.php?q=132

says one thing, another place something else.

Majorly interrupted cylinders or widths result in majorly irregular tolerance zones. I prefer to think of features of size as being a special case, without irregular zones, as one would expect from opposed surfaces. In explanation of the mess they left, I'll say the committee did the best they could.

Here's a nice example: Take a cylindrical rod and machine two parallel flats on it so that it has an ob-round section. Is the size of the section the distance between the flats or the extrapolated diameter? In the direction perpendicular to the flats, would one suggest the item's size is controlled by the diameter? How would one verify the diameter size in the direction perpendicular to the flats? How much of the cylinder can be removed and still be used as a feature of size?

 

[Belanger]

Dave -- I want to make sure I understand your view here...
Take, for example, Fig. 4-16 in the standard. The 11 mm hole is interrupted; are you saying that this is an incorrect usage of the size/diameter callout, merely because it's not a complete 360-degree circle?

If we agree that the standard is OK in this usage, then we could extrapolate that idea (make the slot bigger and bigger until it eats up almost half of the circle). Where would we say that it can no longer use the diameter callout or any type of GD&T that's meant for features of size?

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