Parallel vs. Flush to surface

[2xPixTim]

Hi there,

I have an interesting drawing callout that states the following
- Flush to surface +0.3/-0.1
- Parallel to surface within ± 0.19

There is no datum specified.

This is an assembly of two injection molded parts, assembled via ultrasonic welding. For the sake of this thread, we can call the main part Part A and the component that is welded into place, Part B. I cannot attach a screenshot of the drawing, but here is a quick sketch of the dilemma, for reference.

I understand that parallelism in the conventional way is not the same as one surface being flush to another surface within a given tolerance, unless that surface is also the datum.

I talked to someone who has more history with this part and he gave me the following summarized interpretation:
- Since no datum is clearly defined, the surface of Part A is the implied datum. Therefore, the first part of the note gives a tolerance band around the surface of Part A, that the surface of Part B can be located in. So surface of Part B can be 0.3mm above or 0.1mm below the surface if Part A. The parallelism is applied in the same manner, so surface of Part B can be located within a tolerance band of ±0.19mm around surface of Part A. Since these tolerance bands overlap, they could technically be combined into a single tolerance band of +0.3/-0.19.

I'm looking for feedback on this interpretation. Are these two call outs truly synonyms with only a difference in tolerance?

Thank you in advance!

 

[chez311]

2xPixTim,

Is your drawing compliant to any drawing standard, or are there any plans to make it conform to one - say ASME Y14.5?

"Flush" is not an accepted callout in any standard that I know of (one can deduce what you mean however my point remains) and there is no such thing as "implied datums" to my knowledge.

If we are to take some liberties with what you mean, and interpret it in Y14.5 language - "flush" would require location constraint as well as *form/orientation so profile of a surface would be most appropriate for the planar surface. Parallel, as its name implies, only requires orientation constraint. These are not identical callouts. **As an additional note, your smaller tolerance band for parallelism provides a refinement of orientation within the larger profile tolerance - combining them would eliminate this refinement.

*edited

**Added note

 

[tim_member]

That's one of the reasons why dimensioning and tolerancing standards have been developed - to avoid guesswork caused by notes like these.

Regular parallelism doesn't have sign (because it's not located to anything), so what if, for example, the intent of "+/-" in "parallel to surface within +/-0.19" was to say that the toleranced surface should be within 0.19 like this \ or like this / relative to the "datum" surface?

 

[ewh]

It seems parallel has been confused with coplanar in the original explanation given to the OP. As chez311 pointed out, parallelism is a refinement of the profile "flush" requirement and the allowable tolerances can't be added.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[2xPixTim]

Interesting! Thank you for the quick responses!

chez311: There is no GD&T standard referenced on the drawing. Thanks for clarifying parallelism compared to the ambiguous "flushness". Parallelism only defines orientation independent of location. "Flush" includes location. Makes sense. Flush can be interpreted as a profile call out, with the parallelism refining the orientation.

tim memeber: Agreed! Further, the ± implies a tolerance band of 0.38. Who's to say that was truly the intent. Is the customer after a 0.38 or 0.19 parallelism specification...

ewh: Good point!

I will request a drawing update from my customer to clarify the intent.

Thanks again.

Cheers