Profile of a surface vs irregular shape as a datum 3

[michal77]

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

 

[chancey]

I'm not the expert here but I'd say no. What is the functional relationship between the datum and the feature? It would probably make more sense if the Radius was the datum. Curious what other may have to say.

 

[ewh]

It is allowed, provided that the irregular surface datum is itself related back to the three primary datum. I don't think you can use an irregular surface as a primary datum. See ASME Y14.5M-1994 [¶] 4.5.10.1 and [¶] 4.6.7.

When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - [small]Thomas Jefferson [/small]

 

[TheTick]

Even if you could, good luck measuring that.

If you MUST use that surface, Use datum targets to define datum "A".

 

[ewh]

That would only define datum A as a plane, not the irregular surface itself.
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. - [small]Thomas Jefferson [/small]

 

[powerhound]

You can only do this provided you use datum targets. See 4.6.7.

Powerhound, GDTP T-0419
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[ewh]

Not necessarily. You can use other types of datums as your three primary. [¶] 4.6.7 refers to establishing a datum from a complex surface, but [¶] 4.5.10.1 is more to the point of the OP as I understand it, using an irregular surface (not a plane derived from three points on the surface) as a datum.

When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - [small]Thomas Jefferson [/small]

 

[vc66]

Maybe this is a stupid question, but here goes...

What in the world would possess someone to use that as a datum?




V

 

[ewh]

In our case, we make lofted composite parts, and datums are required to locate other feaures, such as holes and cutouts, as well as the overall profile of the part. We will use the lofted surface as a datum, but only when it is referenced to datum points defining the three primary datums.

When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - [small]Thomas Jefferson [/small]

 

[michal77]

Thanks a lot for your support.
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

 

[michal77]

Now should be fine....

 

[ewh]

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?

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. - [small]Thomas Jefferson [/small]

 

[michal77]

ewh,

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

 

[ajack1]

That is certainly how it works in Automotive. Carline the master datum runs through the front axle of the car and down the centre of the car everything is referenced from there despite the fact that the datum might not be anywhere near a particular part.

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.

http://www.metris.com/industries/ap..._machines/west_bromwich_tool_and_engineering/

 

[ringster]

I have seen instances where an irregular surface, which had holes, utilized tooling balls to position the holes and hence establish a datum for measurement.

V-22 Boeing Phila.

 

[MechNorth]

EWH has the right idea in that an irregular surface can be used to establish a datum, whether primary, secondary or tertiary. Depending on the geometries (as defined by basic dimensions) of the irregular surface, that one surface could establish all three planes ... but where is the origin of measurement? Good question, and one whose answer may bother some people, but is nonetheless correct. YOU establish where the origin of measurement is. On the drawing, indicate graphically (using phantom lines labelled as "ORIGIN OF MEASUREMENT", and basic-dimensioned back to the defined geometries) where you want to take the measurements from...you would probably set a vertical location based on an offset from the tangent point of the highest point. The second and third planes would be established wherever you want them to be, but you need to document them on the drawing. I've used this on complex formed tubing and on castings.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[michal77]

ajack1 - my glass shape is in a 'car position' so on second thought, I'm going to use a carline master datum as you said. One thing which should be done, is to establish on the drawing the RPS points and points which should be measured on my 'red area'. Then I will put a table with coordinates (and also in my case with tolerances in Y direction) and everything should be OK. Additional question - do you think it is possible to measure a profile of a surface in relation to carline master datum? If yes, how to indicate this on the drawing (in my opinion it can be done)

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

 

[ajack1]

Michal77, yes it is possible to measure a surface from the master datum (carline) this is exactly how it is done on most automotive parts, for example body panels.

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.

 

[michal77]

ajack1, I've tried to find correct way how to indicate, on the drawing, profile of a surface in relation to car line datum. I only found one drawing, where profile of a surface was in relation to X,Y,Z. For me this indication is clear and it looks OK, but I'm not sure what standard states in this case. I couldn't find anything about this in ASME.

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

 

[ajack1]

That sounds right, the grid system goes back to the old days when the master was printed on film and would show a grind at 100mm with a balloon at either end giving the carline in the two axis relevant to that view.

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.