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Assembly Dimension - reference dimension with tolerance or dimension with tolerance 1

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underqualifiedengineer

underqualifiedengineer

Mechanical
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Hi - new here/first post.

I'm being asked to specify in an assembly drawing that two parts should be X distance away from each other within Y (±Y) tolerance. This is the overall dimension. This dimension is to be inspected (to ensure the assembly is assembled correctly). If it doesn't meet this dimension, it means the assembly is assembled wrong - and needs to be redone / reinstalled.

Is there a correct method for this? My company has strict ASME rules (y14.5 and y14.100).

My concern is this dimension might be considered a machined dimension ( there are other machined dimensions on the drawing already ).

Thank you!
 
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There is no machining required. The parts just have to be screwed on a certain distance to meet this overall dimension.
 
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Is the distance being required between parallel faces of the two parts or is it maybe centers distance such as between two axes of pins?
 
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The former. It's three parts, a threaded rod, and two threaded flanges being positioned on the rod and torqued together.
 
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A toleranced dimension on an assembly should not cause any issues, as it is part of the assembly definition. If there is confusion between assembly dimensions and dimensions of features machined at assembly, a flag note or some other means of identification should be used (though the differences are usually obvious).

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV
 
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underqualifiedengineer said:
My concern is this dimension might be considered a machined dimension ( there are other machined dimensions on the drawing already ).
Click to expand...

Don't tell your assembler how to do their job. Your dimension specifies a requirement that must be met, and that you will inspect for. They will machine it if they have to.

If you place functional dimensions on an assembly drawing, state your dimensioning and tolerancing standard (ASME[ ]Y14.5?), and then follow you standard. Reference dimensions should be in brackets. Any dimension that must be achieved by the assembler, must have a tolerance of some sort.

--
JHG
 
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It's an assembly drawing. No machining allowed unless explicitly stated.
 
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ASME Y14.5 and related standards do not differentiate between assembly and detail drawings when it comes to the meaning of directly toleranced dimensions, or the meaning of dimensions and tolerances in general. Note that even if what you actually mean to control is the distance at which the components are assembled, specifying X +/-Y between the opposed faces will mean size. And size implies both Rule#1 and actual local size, and two reported values.
You may as well consider other Y14.5 means to control distance, such as profile of a surface.
 
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Using surface roughness symbols you can specify "material removal prohibited" (circle inside a vee). Wouldn't hurt to add a note to reinforce the point. I don't think you could expect a manufacturer/assembler to make the correct assumption.

As others have mentioned though I would consider why you need to control the assembler's process. If it's to protect performance, e.g. if strength could be negatively impacted by material removal, then maybe a note such as above is warranted. Otherwise you may be imposing controls that are not required.
 
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Hi, underqualifiedengineer:

As ctopher indicated, show the assy on the dwg with a distance dimension with tolerance. Add parallel/flatness/profile etc as required.

I assume one part is fixed, and the other part is floating within a range (clearance between fasteners and holes).

What happens if you can't achieve the distance you want in the assembly? It does not sound like you have a mechanism to control this distance.

Best regards,

Alex
 
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