Datum on hole pattern

[nmine]

Hi,

I want to use a pattern of holes (4 holes) as a tertiary datum (datum C). The catch is I want to control the position of two of the holes with respect to each other tighter than that distance between the other group of two holes. Can this be done using gdt? If so how?

 

[3DDave]

It can be done using feature control frames, not gdt.

Note that the callouts are not defined within ASME Y14.5, so it's not clear what the concept here is.

Have you taken a course in the topic or gotten a chance to read the standard?

 

[nmine]

Hi 3D Dave,

Yes I have taken a course a while back but I am definitely not an expert. I do not have the standard handy though. I'm perplexed at how I can use the four hole pattern as datum C but control the position of the green holes tighter than the position of the group ( a pattern within a pattern). If I use a composition position like in the picture below it would hold the position of all holes to 0.4 instead of what I want - 0.4 between the green holes and 0.8 between the two groups of green holes. (note: I had the wrong datum in the lower FCF)

 

[3DDave]

Just use two different single segments for the two pairs of holes and one single segment for all four holes.

This could, however, make things more complicated for manufacturing and inspection; they will probably just position all the holes to a diameter of 0.4 and ignore the larger tolerance.

 

[pmarc]

The proposal provided by 3DDave will work, but both single-segment FCFs of 0.4 will need to be associated with a SEP REQT note. Otherwise, the spacing between them, due to the default simultaneous requirements rule, will still be controlled within 0.4.

Of course, the datum feature C symbol should then be associated with the 0.8 FCF.

 

[3DDave]

Just beat me to an edit pmarc. In a mass production system those holes will be punched with one tool all at the same time, but just for making inspection have separate setups, SEP REQT would be the ideal addition.

 

[Burunduk]

SEP REQT doesn't require two separate set ups, it just means that the unconstrained degrees of freedom in a single set up can be used to allow variation between the two features or patterns in a way that ignores their mutual basic location/orientation. One way to think of it is the act of moving the tolerance zones overlay across the projection of the inspected part in an optical comparator without changing how the part is set up for measurement. With SEP REQT the features are not required to fall within their tolerance zones simultaneously. Note that if all degrees of freedom are constrained SEPT REQT won't do anything.