GDT on machined extruded part 1

[rkrz]

We are making parts out of extruded stock and trying to redo the prints with GDT.
The part is machined all around and we want to use 2 holes as the secondary and tertiary datums.
However it is not a flat plate as usually shown in the ASME standard, and the hole (datum) needs to be controlled to the extruded 'step' to be in the right (y direction), with everything else being called back to it.
Attached is an example.
Would it make sense to instead just give a start +- on the 28.75 reference dimension to the 25mm hole and then use positions/profiles from there?
Just want to learn how to tackle this.

Thanks.

 

[3DDave]

J-P, seconded.

 

[Burunduk]

I also don't feel comfortable with the standard's wording, that a reference dimension is "usually" without a tolerance. A tolerance is what we can "tolerate" when evaluating acceptance - not something that a ref. dimension should specify. I would prefer a description that says that a reference dimension has a range (just as any other dimension), and if really necessary that range can be expressed as auxiliary information, either by using the +/- symbol or as limits within the parentheses.

A possible example could be a shaft with the OD specified as STOCK, in which case the tolerance for it is specified in a referenced standard. That OD is also used as a datum feature, let's say "A". A machined diameter on a portion of the shaft is specified with a size tolerance and a position or runout tolerance relative to "A". The drawing maker could want to provide the step dimension from the OD to the machined diameter, with the information about its possible worst-case variation, if he expects that this information may be useful to the drawing user. This way the efforts required for making the calculation based on a stock tolerance that needs to be looked up, the machined diameter tolerance, and the position or runout tolerance, can be spared from the user.

 

[3DDave]

B,

You should submit a change request and see what happens.

It is exactly how much can be tolerated - acceptance is a function, but not the only consumer of drawing information, regardless of the preponderance of inspectors writing the standard to their advantage.

Stock tolerances for aluminum extrusions are only suggestions, at least for the Aluminum Association standard, and subject to negotiation between the mill and procurer, so they don't necessarily appear in any standard for a given extrusion and would not be available to be "looked up."

Making it reference, with the associated tolerance, tells the fabricator that the dimension wasn't simply missed and the range of expected variation. It allows tooling to be designed ahead of the arrival of material. In the case of rolled material there can be internal stresses and it is handy to know how much material to remove to maintain the balance so the part does not distort.