Profile of a surface vs irregular shape as a datum
Profile of a surface vs irregular shape as a datum
(OP)
I wonder if it is possible to refer profile of a surface to a datum which is an irregular shape. Please take a look on attached example. Let say that drawing is acc. to ASME (but in this case I'm not sure that there is a difference between ISO and ASME).
Many thanks for any remarks.
michal 77
Many thanks for any remarks.
michal 77





RE: Profile of a surface vs irregular shape as a datum
RE: Profile of a surface vs irregular shape as a datum
When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - Thomas Jefferson
RE: Profile of a surface vs irregular shape as a datum
If you MUST use that surface, Use datum targets to define datum "A".
RE: Profile of a surface vs irregular shape as a datum
Read the standard; it IS technically allowable to use an irregular surface as a datum, though I don't think it is very applicable to this situation, and QA is another matter entirely.
When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - Thomas Jefferson
RE: Profile of a surface vs irregular shape as a datum
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
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RE: Profile of a surface vs irregular shape as a datum
When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - Thomas Jefferson
RE: Profile of a surface vs irregular shape as a datum
What in the world would possess someone to use that as a datum?
V
RE: Profile of a surface vs irregular shape as a datum
When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - Thomas Jefferson
RE: Profile of a surface vs irregular shape as a datum
I would like to show you, why I need an irregular shape as a datum. On attached file you can see simplification of a glass shape which is completely irregular (don't take into consideration the borders, they are not important). You can measure only openings for bolts (R10). Blue area (R20) it's a contact area with other stuff, so it could be used as a datum.
In my opinion there are only to ways to control the real part. First is to apply profile of a surface to the entire body without datum (then measured body will be a datum to itself). I suppose that it can be done but I'm not sure about measuring in 'real life'. Second way is to measure only a red area (honestly it's only important for me) in reference to mounting areas (blue circles). In my opinion second way is much better but then I have to establish datums on irregular shape.
Do you thing it's possible to establish datums around the openings in my case? And if yes, using irregular shapes or datum targets?
PS - ewh you stated: we will use the lofted surface as a datum, but only when is referenced to datum points defining the three primary datums - I assume it means for you that area of irregular shape which can be a datum, must be correctly established by basic dims in reference to other (planes, bolts etc) datums. Am I right?
michal77
RE: Profile of a surface vs irregular shape as a datum
RE: Profile of a surface vs irregular shape as a datum
It is not practical to attempt to locate the irregular suface datum by dimension to the primary datums. This would be similar to trying to define a complex surface on a drawing with dimensions, and in our case, the primary datum points are themselves on the irregular surface.
We have used basic dimensions in the past to locate the primary datum points, but they do not have to be referenced to other datums. It would be difficult to relate the points to other datums when they are defining the primary datums. This is one of those weird exceptions to that practice in the standard. The only tolerances involved when using basic dimensions in this situation are standard tooling or gaging tolerances, which we do not specify. You can used toleranced, non-basic dimensions, however. See ¶ 4.6.2.
When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - Thomas Jefferson
RE: Profile of a surface vs irregular shape as a datum
Last question from my side, please don't get nervous :) You stated: ....the irregular surface datum is itself related back to the three primary datum.
Unfortunately it's little unclear for me...Is it mean that this surface is established in correct position in 3D and it's in relation to absolute axis system (I mean that we have exact coordinates of this shape)
Could you please tell me where can I find some draft which can show me this kind of solution, or where can I find more detailed info about it?.
Thanks in advance
michal77
RE: Profile of a surface vs irregular shape as a datum
For inspection purposes each part is "held" by control features and measured against the model.
There are a few examples on the link below, or do a Google search for carline.
htt
RE: Profile of a surface vs irregular shape as a datum
V-22 Boeing Phila.
RE: Profile of a surface vs irregular shape as a datum
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Profile of a surface vs irregular shape as a datum
MechNorth - very interesting way of setting up an origin using irregular shape. I have to think about it (it's fresh for me).
Many thanks
RE: Profile of a surface vs irregular shape as a datum
The normal way is to have bushes or blocks at nominal sizes near to the part say x500 y600 z 700 for the CMM to pick up on the checking fixture. The master model or drawings if necessary must show the areas you are using as datums to locate the part and also if these are clamped or "free".
Do a Google search for checking fixtures or Automotive checking fixtures for some examples.
RE: Profile of a surface vs irregular shape as a datum
On the mentioned drawing there is only a grid with coordinates in XYZ directions, so I think it's some kind of designation that car line datum is in use....
michal77
RE: Profile of a surface vs irregular shape as a datum
Some CAD systems will do this, not sure what system you are using.
As for ASME I know nothing about that, it is all ISO over here.