Recommend a GD&T 1

[LONDONDERRY]

So we have this part fabricated from 7075-t6 aluminum. It a 4"x6" flat plate. Because of the all the machining the part becomes warped. So on the detail drawing I want to specify a GD&T. I've read through numerous books and it seems that a straightness tolerance is the one to useo rpossibly a flatness.

 

[CheckerRon]

Thanks for the correction MechNorth, I was thinking parallelism of B wrt A, but wrote flatness. (would still need control flatness on surface A though)

True, Fig. 4-20 just covers A-B datum plane, and not necessary the alignment of the surfaces wrt each other, but obviously I was thinking of in-line surfaces with a feature between, and using that picture as an example.

I guess I would also prefer your suggestion of flatness on datum A & profiling datum B surface wrt datum A, rather than specifying profile similar to Fig. 6-21 (the double arrow callout).

 

[MechNorth]

It all depends on what your design intent is.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[JeffMathena]

MechNorth,

So I take it then that my interpretation of the standard is correct as I stated above. I've re-read 6.5.6.1 and my brief explaination above still seems valid to me. Of course additional information shown on the drawings to correctly establish which surfaces you're talking about in the callout are definitely valid and required for accurate assessment. Your method you described is what is shown in fig 6-20, but I'm sure there are other creative methods that might be used based on the uniqueness of the geometry itself.

 

[MechNorth]

Yes, Jeff, that's correct. The only clarifying point is that it's not called flatness when you've got more than one surface, it's referred to as coplanarity, which encompasses a net flatness as well.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[JeffMathena]

OK, I'm going to walk away from this one.... No need to turn this into a $%*&! contest in my very 1st post.

 

[KENAT]

Welcome Jeff to one of the more pedantic forums.

I've been on the receiving end a few times, don't let it get to you.

KENAT,

Have you reminded yourself of faq731-376 recently?

 

[JeffMathena]

Thanks KENAT, I figure I'll lurk around and see what I can learn and possibly offer back to the community.

It's funny that I never did call it flatness if it is an interupted surface, and we don't even know if the original poster had an interupted surface. Oh well at least there are a lot of references to the standard and people can figure it out for themselves.

Jeff

 

[MechNorth]

Sorry, Jeff. I must have misunderstood your posting "If your surface you want to control the flatness on is interupted, then ...".

One of the problems with this kind of forum is that what we type and what the readers interpret aren't always the same; there's a lot of subtleties in the spoken language that aren't picked up in text. I got corrected twice yesterday for stupid things (stupid as in on my end) where I'd left things open to interpretation or used common terms instead of proper ones.

Welcome to the club.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com