Using cast hole as a datum
Using cast hole as a datum
(OP)
I've got a part I'm trying to tolerance and I'm not sure the proper protocol of how to set this up. In the part, the hole in the center is as-cast. The O-ring groove and bolt holes all should be in relation to where ever the center of that hole ends up being. We don't really care about the size or the position of the cast hole, it is what it is, but everything else needs to be relative to that hole. Can I use this cast hole as a datum? What's the best way to tolerance this out? Thanks in advance for the help!





RE: Using cast hole as a datum
If you want the other features centered on the hole regardless of feature size then just making the hole a datum feature and invoking it Regardless of Feature Size (i.e. no circle m next to that datum in the Feature Control Frames) should achieve that.
By the way, you need 2X on the 63 & 66 dimensions or else extend the leader lines. Also you don't need the 33 & 31.5 dimensions if using appropriate GD&T controls on the hole locations.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Using cast hole as a datum
As for the dimensions, the 33 and 31.5 dimensions locate the bolt holes with respect to center hole. If I take those away, isn't the pattern just a free floating rectangle, then?
Thanks!
RE: Using cast hole as a datum
“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV
RE: Using cast hole as a datum
A crude rule of thumb in fixture design is that the fixture must be ten times more accurate than the thing being measured.
If you are using a feature of size (FOS) as a datum, it helps if the FOS is something accurate. A fixture that picks up an FOS casting will not be accurate or repeatable. Maybe, you can tolerance the sloppy datum!
I strongly recommend defining datum targets on your casting. If you can mould them into casting, so much better. With datum targets, you do not care how inaccurate it is. You do not care that flat faces are not flat. Everybody in the process is working off the same repeatable datums.
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JHG
RE: Using cast hole as a datum
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Using cast hole as a datum
Thanks,
RE: Using cast hole as a datum
A rough cast could throw off your hole positions.
Chris, CSWA
SolidWorks 14
ctopher's home
SolidWorks Legion
RE: Using cast hole as a datum
The basic dimensions are defined in the standard as being located per the feature control block, but an assumption would have to be made as to which features are being controlled without extending your centerlines or adding a qty to the dimensions.
I don't think extending your feature centerlines vs qty is addressed directly in the standard, but to eliminate any assumptions one or the other should be done.
“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV
RE: Using cast hole as a datum
But to handle the main question, try post the thread where Jim Sykes GDTP-S provided methods of using a tapered hole as a datum.
http://www.eng-tips.com/viewthread.cfm?qid=151290
Tunalover
RE: Using cast hole as a datum
I'd perhaps fall back on what is said in 2.7.3 but that's perhaps a little weak in this case.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Using cast hole as a datum