GD&T Help (Y14.5M-1994) - True position callout box on two dowel holes

[ginn68]

Can someone help me understand how a true position callout box on two dowel holes have the follow:

The dowel holes have one box that relate to both bores.
The top box is a Tp 0.6 m back to A M-N
The bottom box has a Tp 0.025 back to A

The datum A is the Surface that these bore are perp. to.
The datums M & N are raw bores.

The question I have is what does the bottom tp box apply to? (perp. back to datum A, or the span bettwen the two dowel bores)

Thanks to anyone for their help!!

[Jim Howe]

Any way to attach the actual drawing or a sketch of it?

[cbehrens]

Perp. back to -A- only! The first note covers the relationship between the two bores.

[ginn68]

Jim:

I tried to attach the file to this reply.

[Courier]

Not seeing it, My Guess is first or top call out is for the tp of the hole dimentioned from datums and the lower or 2nd callout tollerance is for the hole to hole dim distance from the first hole. so what i am saying is first one is for the absolute postion from datum and second is for relative distance from first thats why its a smaller tolerance given.

[Paul F. Jackson]

Ginn68

Since both dowel holes are controlled by one GD&T callout they are a pattern and the bottom callout controls not only the perpendicularity to A but also the spread between the holes. There are two cylindrical tolerance zones for the axis of each hole spaced the basic dimension between them and those zones are perpendicular to datum feature A. It controls both the spread and the perpendicularity simultaneously.

Paul

[ginn68]

Paul:

Thanks for your reply! If the 2nd callout (0.025 back to Datum A) is for the span or the position back to Datum B then wouldn't the callout be label like 0.025 -A- -B-??? I have tried to ref. this back to the Y14.5M standard with no success??


Michael

[ginn68]

Tried to attach file again!! It is zipped, and in Autocad Lt format (.dwg)

Ginn68