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FredMatthis1-Visitor
1-Visitor
August 21, 2014
Question

Symmetry

  • August 21, 2014
  • 7 replies
  • 3514 views
I thought I saw somewhere that if you have a block with the width called out as a datum
and you have a hole "shown" in the center without a dimension, that it is ok to assume the hole is in the center of the block width.
Does anyone know if this true and where does it say that in ASTM specs.


Fred J. Matthis
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7 replies

K
kaudibert10-Marble
10-Marble
August 21, 2014

you would need to apply a true position callout to the hole referencing the datum

P
ptc-19327451-Visitor
1-Visitor
August 21, 2014
I don't have the specification doc in front of me, but I think that would be stretching the concept.

I believe that symmetry is assumed for features, and not just bilaterial, but also rotational especially when relating to a pattern of features such as a bolt circle. I seem to recall that the spec says something like: 'neither scaling, nor assumption of a distance or size is permitted'. This gets tricky because features drawn at approx. 90 degrees as explicitly noted to be orthogonal.

In the case of a rectangular block with the overall width and height dimensions shown and a central hole shown, dims from one set of edges to the hole must be shown so as not to make assumptions of distance.

The perimeter of the block is assumed to be nominally at 90 degrees, and the hole is taken to be orthogonal to the view that the hole size dim is shown.



Christopher F. Gosnell

FPD Company
124 Hidden Valley Road
McMurray, PA 15317
P
pdkoren1-Visitor
1-Visitor
August 21, 2014
I was just looking thu ASME Y14.5M-2009 this morning for another issue and saw very close to exactly that. The key is that the Datum is tagged to the dimension and not an edge. Tagged to the linear dimension it is defining the center plane of that dimension. If your hole is ON that plane AND you have a true position callout on the hole, then you are fine, at least in the one direction. Essentially, with the true position you need basic dimensions to the reference datums, but the hole is 0.0 to datum in question...

Sadly, and almost expectedly, I cannot find the figure I noticed... maybe it was in some other reference... sorry...


K
kaudibert10-Marble
10-Marble
August 21, 2014

Paul just confirmed what I said but you do need a basic dimension for the height

F
FredMatthis1-VisitorAuthor
1-Visitor
August 22, 2014
Thanks to everyone.
I do appreciate the good responses I get from ptcuser's.
It seems that this is a common practice but is not really documented anywhere.
I also see that is used in a number of training manuals.
I will probably place a dimension when there might be some confusion,
such as when showing small geometry and skip the dimension
when it easy understood.
Thanks again.


Fred J. Matthis
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J
jjakubos1-Visitor
1-Visitor
August 22, 2014
Creo Gurus,

I am in Creo 2 and am trying to use Creo Simulate lite and this is the message it gives me. Could this be a software load problem or is configuration issue or something else entirely?



[cid:image001.png@01CFBDF2.DC01E240]


Regards,

Jim Jakubos
Project Engineer
Rexair, LLC
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231-876-3934
J
jjakubos1-Visitor
1-Visitor
August 26, 2014
Ronald,

Thanks for the response. So, is it likely that file was not loaded when Creo was installed or what. I searched for that file name in my Creo directory on my machine and did not find it.

Thanks again for your assistance!

Regards,

Jim Jakubos
Project Engineer
Rexair, LLC
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231-876-3934
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