Casting Definition via GD&T

[jimbod20]

I have questions regarding casting definition.

I will try and describe the issue and the questions without a picture or drawing. I think this will be ok.

I have a complex investment cast part/shape with primary/secondary/tertiary datums. Two lugs defined by a basic radius of .XXX establish the primary 'datum axis' E-F through the two lugs. A third lug defined by a basic radius of .XXX establishes the secondary datum G. A .250 diameter spot defined by basic X/Y dimension from one of the lug radii on a surface of one of the lugs establishes the tertiary datum H and constrains translation.

The engineering drawing consists of iso views which show the part and the datum structure with a note that states " Geometry defined by solid model XXXXXX-D-1 Rev X. Cast cores shall lie within a profile tolerance of .XXX relative to datums E-F, G and H. Other cast features shall lie within a profile of .060 relative to datums E-F, G and H.

My questions:

Can the cast datum lugs vary in size by the profile tolerance of .060?

The answer leads to this summary.
If I mount each part (I pick 20 for a sample) in a fixture which adequately locates on the lugs E, F, G and the point H (lugs E-F establish a datum axis E-F). I would expect any feature on the surface of all 20 parts to lie with profile of .060 relative to datum structure? This includes variation in datum size.

 

[KENAT]

I'm not sure I follow your description but I'm not sure the features you've identified are necessarily appropriate.

You might want to look at datum targets that correspond with the nominal centers of the lugs or some such.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Belanger]

Keep in mind that datums don't really exist on the part. Datums are theoretical, although they are initiated from the physical part. I'd have to absorb the question a little more, but my inclination is "yes" to your questions. That doesn't mean those datums are the best, so Kenat's advice is probably sound.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[MechNorth]

Probably movable datum targets (per Y14.5-2009) would be best. Post a pic of your drawing if you can.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[MikeHalloran]

I've always thought of datums as features of a mating fixture, not features of the part, so the size of the part features that contact the datums is, er, relatively irrelevant.

Mike Halloran
Pembroke Pines, FL, USA

 

[jimbod20]

Hello,

I provide an attachment to continue this discussion. I'm still looking for guidance/feedback.

I provide the original message text.

Two lugs defined by a basic radius of .450 establish the primary 'datum axis' E-F through the two lugs. A third lug defined by a basic radius of .500 establishes the secondary datum G. A .250 diameter spot defined by basic X/Y dimension from one of the lug radii on a surface of one of the lugs establishes the tertiary datum H and constrains translation.

The engineering drawing consists of iso views which show the part and the datum structure with a note that states " Geometry defined by solid model XXXXXX-D-1 Rev X. Cast cores shall lie within a profile tolerance of .XXX relative to datums E-F, G and H. Other cast features shall lie within a profile of .060 relative to datums E-F, G and H.

The part mounts to the next assembly via the mount lugs and surface H, which is why I chose datum axis E-F, G, and H.

My original questions:

Can the cast datum lugs vary in size by the profile tolerance of .060?

The answer leads to this summary.
If I mount each part (I pick 20 for a sample) in a fixture which adequately locates on the lugs E, F, G and the point H (lugs E-F establish a datum axis E-F). I would expect any feature on the surface of all 20 parts to lie with profile of .060 relative to datum structure? This includes variation in datum size. Each cast part must satisfy profile tolerance with variation in lug size.

 

[jimbod20]

I have another question.

Is there an inherent advantage in choosing the large, flat surface on the bottom of the casting as the primary cast datum? I do machine this surface flat, therefore, I would loose the original cast datum.

Is a surface inherently advantageous as a primary datum (cast or machined) given a surface is not a "feature of size".

 

[KENAT]

No attachment that I see.

It might be worth you looking at ASME Y14.8 to see how castings are dimensioned.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[jimbod20]

Thanks Kenat. I will look at ASME Y14.8.

I provide attachment.

 

[Tunalover]

I've established datums on drafted surfaces using what are called equalizing datums. In fact ASME has a standard for this. This prevents having to use odd, unnatural datum features.


Tunalover