Assembly Geometric Tolerances

[kkrahmer]

Hello all,
I am looking for help in setting up the best way to create assembly level geometric tolerances. This is not a problem when 3 orthogonal surfaces are selected, just create 3 datum planes, name them A, B, & C and set them. The problem I have is when you are using features of piece parts. For instance the mid plane of some central element (gold part), a hole for one axis (red part), and a surface for another plane (top of blue part). In this example all of the features are in different parts and these features are used as datums at the piece part level as well.



Karl Krahmer
Mechanisms and Structural Systems
ASRC Aerospace Corporation
Kennedy Space Center, Florida 32899
Mail code ASRC-5211
Office: 321-867-5052
Fax: 321-383-4144
-<">mailto:->




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[frederick.burke]

Hi Karl,



Just create the assembly datums in the assembly.



Then label them and refer to them normally.



Not the best practice to use a symmetry plane for an assembly reference.



Think of sliding the assembly against pins on a tooling fixture.



Two pins define one plane A, a large pin will slide into the cylinder B
and two pins define the top surface of the blue part C.



If you can picture your assembly tooling fixture in your head, you
should be able to crank out the tolerances.



Does this help or did I misunderstand the question?



Best regards,



Frederick Burke

[kdemont]

Karl, I agree with Fredrick (see below)that the datums will need to be created
at the assembly level, but I suspect that if you try to use all the same
datumsthat you had in each of the parts, there is a fair chance you will over
constrain the assembly.

Of course the datums need to make sense to the assembly.

One possibility could be to use the hole in the red part as primary,one surface
of the gold part as secondary (to stop rotation), and a second surface of the
gold part as tertiary (to stop sliding on the hole in the red part)
Another possibility might be to justcopy the original datums from gold part.
There are certainly many other possibilities. It really depends on how the
assembly is used at the next level.
"Just create the assembly datums in the assembly.
>Then label them and refer to them normally.
>Not the best practice to use a symmetry plane for an assembly reference.
>Think of sliding the assembly against pins on a tooling fixture.
>Two pins define one plane A, a large pin will slide into the cylinder B and two
>pins define the top surface of the blue part C.
>If you can picture your assembly tooling fixture in your head, you should be
>able to crank out the tolerances.
>Does this help or did I misunderstand the question?" - Frederick Burke
>

[frederick.burke]

Good input from Ken!



By the way... in the original question, you used the word "surface".



That implies "not necessarily something planar that we can butt up
against".



If that is the case, I would recommend adding an additional hole to the



assembly for the purposes of locating it in your assembly fixture.



The tooling hole that you added becomes another datum to tolerance with.



Make sure the location of the tooling hole makes sense. It can be
plugged later if necessary.



Of course, if this is a welding fixture we are designing, that changes
things again.



If you are welding to tight tolerances, it is best to leave a small gap
between parts



that will be filled in during the welding process.



Ideally your assembly fixture will look something like the mating
surfaces on the



next higher assembly. Try to keep it simple. There is no limit to the
amount of



time you can spend pondering how slop might change how the parts fit
together.



Fun stuff!



Let us know how it turns out.



Best regards,



Frederick Burke