GD&T Positional tolerancing 5

[rwild]

Can you have a positional tolerance feature control frame on a hole without referance to any datums? If this is a correct way of dimensioning, then what is it controling? Perpindicularity, hole to hole, something else?

 

[dakeb]

You are of course correct pmarc, there is a risk the lack of perpendicularity of the fixings and holes could disallow assembly. If there is a risk the parts will not go together the designer should specify a tolerance.

However, we also need a bit of common sense to avoid building in cost. If I am bolting together two thin plates for instance, I wouldn't bother with a perpendicularity tolerance. So we must allow pos tols withot datums where appropriate.

 

[dingy2]

dakeb:

You, of course, would assume perpendicularity with the face. I would suggest that the face should then become a datum and should be referenced in the feature control frame. It may not have any real relevance but would be part of the datum development.


Dave D.

http://www.qmsi.ca

 

[ewh]

As has been said many times before, common sense isn't all that common, and we know what "assume" can lead to.
In a well done drawing per the ASME standards there is no room for assumptions.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[dakeb]

Yes, ideally we should fully define every element of a part. Ideally we would like to be able to check that the part is delivered to the drawing. In reality though, we are not gonna cmm some M6 clearance holes going thru a 3mm sheet for perpendicularity. So it's pointless putting it on the drawing.

Envisage we are fitting, say, an electrical module with four M6 tapped fixing holes, into a fabricated 3mm thick drawer base in a 19" rack. We don't care exactly where on the drawer base the unit is sited, so standard linear dimension tols will do for group position to the first hole. The hole centres need to be controlled to match the cots item, so a pos tol with no datum is fine.

Perpendicularity will not be a problem because it is harder to drill a hole at an angle than it is to drill it face-on. Clearances in the holes will be specified sufficiently to allow for a reasonable lack of perpendicularity in such a thin sheet.

My point is if I specify a pos tol with a datum face called up for perpendicularity, no matter how big a tol value, the manufacturer will add a few £££'s just for the hell of it.

Dave

 

[KENAT]

Some vendors will add a few $$$/£££'s for every FCF or similar.

I'd suggest the solution, if they wont educated them selves/be educated, is perhaps to find new vendors.

Though I realize that's a bit of a pipe dream in many situations.

If it's a functional requirement, then having it on the drawing allows you to reject parts that don't meet the requirement at the vendors cost.

However, the debate of if GD&T is really worth it, or only for critical applications etc. has been beaten to death here before, though not for a while so maybe we're due.

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[PaulJackson]

I lost this no datum on a single segment non-coaxial pattern control with Don when we last debated (see thread1103-2146890) but I thought that subject was further debated prior to the newest standard release... just don,t know the outcome.

 

[fsincox]

Sounds like the exact same conclusion and the exact same clarity as it was then, too. I see no difference fundamentally from the "position" that I stated before. Composites was an easy call.
Frank