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Concerning extension of center lines

Concerning extension of center lines

Concerning extension of center lines

(OP)
Hi,
I am using the ISO standard. I have a question about extending center lines, i have been told that you can do that, but i am uncertain. The idea is that you have several holes above each outer, and that you by extending the center lines makes it so that it is shared for all holes.

I present the question in more detail in the picture i have attached.

 

RE: Concerning extension of center lines

Tartof,

Extending center lines will not assure that the holes will be treated as one feature. This will help in reading the print, and for that reason I would extend them, but will not guarantee what you want to achieve.

With ASME Y14.5 standard this functionality could be achieved in several ways (e.g. positional tolerance 0 at MMC without any datum reference, continuous feature modifier <CF>).

With ISO I would think of CZ modifier application. ISO 1101:2004 shows an example (figure 26) for nominally planar faces, but IMO you could also incorporate it for your case.

By the way, I would also think about a mutual relationship (distance) between 4 left holes and 4 right holes. At the moment this seems to be missing on your print.       

RE: Concerning extension of center lines

Well, let's focus on one pair of holes at the very left of your print for a moment. The way I saw an application of common zone modifier was something like:
1. Remove datum feature symbol A.
2. Apply prefix '2X' to ø60 dimension.
3. Apply ø0.2 concentricity tolerance with CZ modifier to ø60 dimension, however without any datum reference in feature control frame (FCF). For this dimensioning scheme you would not like to have datum reference A, because CZ modifier would create one common cylindrical zone for both holes.

The next step would be to define mutual relationship between left pair of holes and remaining two pairs. I would try do it in a following way:
4. Add datum feature symbol A to concentricity tolerance FCF described in point #3.
5. Define 2X ø60 dimension for center and right pair of holes.
6. Define basic (theoretically exact) dimensions between axis of left pair of holes and axes of two other pairs.
7. Assign ø0.2 positional tolerance with relation to datum A to both ø60 dimensions described in point #6.

There is also another way, much easier, and absolutely legal per ISO standards. You could apply single ø60 dimension with prefix '6X', tie the axes of holes together by basic dimensions and assign ø0.2 positional tolerance with or without any datum reference, depending whether a control of orientation and/or location of the pattern relative to any external features is needed. With this approach 6 holes would be treated as a single pattern so the relationship of coaxiality within each pair of holes would be automatically controlled. The advantage of such scheme would be that none of the pair of holes would be favoured over the other two pairs, which I think would better reflect the functionality of the system.   

RE: Concerning extension of center lines

(OP)
Thank you very much, that is appreciated. I have two questions,

In the latest picture I send there where 12 holes; are you referencing to 6 holes in your picture? so if there where 12 holes as in my latest picture you would have added 12xØ60

Secondly you attached the datum feature in the right picture with the FCF. i have seen that in old books but i am not sure if you can do that these days. If i understand it correctly it is referencing to the centerline?
 

RE: Concerning extension of center lines

Question #1: Yes, if there were 12 holes then I would add prefix '12x'.

Question #2: Yes, you are right, attaching datum feature symbol to FCF means that in this case a datum will be an axis (centerline) derived from both holes assigned as datum features.
This way of datum feature symbol presentation has been quite commonly used - at least on ISO prints that I have seen and been working with. You can find it in ISO 5459 issued back in 1981 as well as in ISO 2692:2006. I do not have a copy of 2011 edition of ISO 5459, but I think this is a place you should look too, if you are really concerned about legality of this method.

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