Datum Targets on a casting, reassign Datum "A" after cutting off the Datum Target Ears at

[bear9298]

I recently ran into a situation where a designer had added 2 ears about .75 below the top surface of the cylinder to establish Datum Targets A1, A2, and A3 on the under side of the ears, B1 and B2 were on the ends of the ears (opposite sides), and C1 was placed on one side of the ear to establish "C" Datum, all were perpendicular to each other. A 3 legged jig was made to contact Datum Targets A1, A2 and A3 giving 3 points of contact as required by ASMY14.5 GD&T. likewise for Datum Targets B1, B2 and C1. This allowed machining a bore in the top of the cylinder and additional features. Now, here is the interesting part or actual question? After machining the top face of the cylinder and the bore, he cuts off the ears but show's the casting and ears in phantom along with the Target datums as required by ASME Y14.3. But! then reassigns Datum "A" to the top surface that was just machined off.
For history and future castings the Datum Targets A1, A2 and A3 still exist and can be recreated in the future if need be, therefore the Top facing off should be assigned Datum "D" and all features referenced back to it and not have reassigned Datum "A". Am I incorrect in my understanding of casting Datums?

 

[drawoh]

bear9298,

This sounds weird. The whole point of casting in datum target features is to have everyone in the manufacturing process use the same jigging points. Consider the following process.

[ol]
[li]Design and document a casting, specifying appropriate datum features.[/li]
[li]Make the casting and inspect it.[/li]
[li]Document a machined part based on the casting. Call up the original cast-in datum features. Machine in some accurate features, and inspect everything.[/li]
[li]Document a second machined part based on the first machined part. Call up the accurate machined in features as datums. Machine off the original cast-in datum features. Inspect everything.[/li]
[/ol]

At end of this process, you have three drawing packages, and three parts each with their own number. If I were determined to do this in SolidWorks, the casting would be a SolidWorks part model. The second part would be an assembly consisting of the cast part, with material removed, presumably by machining. The third part would be the second part assembly, with more material removed. There is no problem preparing the drawings. If you set up the part's metadata properly, the BOMs on the fabrication drawings will work.

How badly do you need to remove the datum "ears"? What value is there to a two[‑]step machining process?

If you leave the "ears" in place, you can have a one[‑]step machining process. Almost certainly, this is good practise.

--
JHG

 

[3DDave]

On the one hand datum symbols and feature characteristic frames apply only to the drawing they are applied to, so [A] can be used on the casting and on the machining. This applies to separate casting and machining drawings.

The use of phantom features is a perennial favorite. Take the machined part to inspection and ask them to locate all the datum targets.

There is no shortage of letters and it's just a habit from the examples that "A" is used as a primary. What is a bit dumb about that is the FCF's determine priority unambiguously, so "D" is just fine and if it lowers the odds of confusion, use it.