Pattern of Patterns/Repetative Patterns

[KENAT]

Tried a quick search, as I'm sure it's been done to death before, but it didn't show up.

I'm having a mental blank or something so would appreciate a hand. We often have situation where we have a 'pattern of patterns'.

Tec-ease has a suggested way of doing it essentially making a feature in each pattern a datum, and minimizing dimensioning by using 'INDIVIDUALLY'.

Alternatively, round here I often see more like this sketch

Can someone remind me the pros/cons of each way, or if they're even both legitimate to 14.5M-1994. If there's a relevant section in 14.5 that would be great too, as I had trouble finding it.

Simplistically in the tec ease way am I right in thinking the smaller holes follow the datum hole of each pattern, so slightly relaxing tolerances on those smaller holes relative to global datums, almost comparable to composite tolerancing?. Where as in my sketch all features are tied to same global datums so slightly more restricted.

Also, on my sketch, if I wanted to make the 1in hole the datum for each pattern, could I just make it 'D' in the detail view, reference the 4 holes FCF to D and leave it at that. Or would I need to add 'INDIVIDUALLY' next to the "4X DETAIL A" or something?

Thanks.

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[Belanger]

One difference I notice in the two examples given in your post is that the Tec-Ease example ties each pair of holes to its own datum for that local pattern, while the other example has all holes (large and small) tied back to A, B, C. So in the second example each of the 20 holes can do its own thing independent of the other 19 holes; but in the first example any movement by the large holes will influence the overall location of the two holes around it.

I notice that the 2009 standard includes a new picture of the concept you're asking about -- see page 128 (Fig. 7-37). I don't think that picture introduces a new idea, but it at least formally addresses the topic.

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

 

[KENAT]

Belanger, sorry that's what I was trying to explain and seeing if I'd missed any other implications.

I don't have the 2009 standard, and we still work to 94 anyway.

Thanks.

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[dingy2]

Kenat:

Fig. 5-39 on page 138 of the 94 standard reflects the thought of creating a datum on hole and placing a "8X INDIVIDUALLY" beside it. The problem in your situation is the fact that each of the 4 holes around that larger hole must be oriented.

If there was a greater relationship between the 4 holes and the larger hole, then I would use the example shown on page 138 of the standard. I would have the datum structure set up as primary A, secondary D (that is the larger hole) and a tertiary C. I need the tertiary for orientation.

If there is not a greater relationship, then I would suggest dimensioning the holes individually. The 16 smaller holes must then be dimensioned from B & C. I would also shown the larger hole as 4X (size) while the smaller holes would be 16X (size). I believe that is what JP suggested.

Food for thought.

Dave D.

http://www.qmsi.ca

 

[PeterStock]

I would make the 1.000 dia hole datum D with INDIVIDUALLY beside it. Then change the datums for the .125 dia holes to A|D(M)|B.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[nithinku]

As per my little understanding of GD&T the Attached Picture is correct way. We also Done some of dwgs Similerly.
I dont think Note "Indiviually" will come into the Picture here as Holes need to be orientaed and located to center hole (4X)

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[KENAT]

Thanks everyone for you input, and sorry for missing the '4X' on the .125 holes in my sketch.

Seems I had it about right.

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[PeterStock]

The attachemt is correct as shown, at lease I would be ok with it. But the GD&T controlls all of the holes as one feature. I am thinking that the four patterns has a part attached to it, not one part for all the holes. As such the suggestion I made would be the way to go. Each 1.000 dia hole controls the 4 .125 dia holes arround it. The control on the 4 large holes holds the patterns together. I am thinking that the tolerace value for the large hole would be larger than the tolerance value for the small holes.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[KENAT]

Thanks Pete, I should have said 'my understanding of the differences seemed about right'.

My sketch is just a really simplified example to help verify my understanding, the real application that brought this up is more complex.

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