Flatness in assy 2

[wes616]

Hi everyone,

Could you please give me a quick hand. I am checking the drawings of another engineer, and there is something I don't understand.

the basic problem

We are designing a big box from flat 1/2" polypro panels, which get welded together with but joints basic tolerancing for the assembly is +/- .030. On the assembly drawings, there are fcf's [flatness | .10] comming off of every assembly surface with an extention line.

Now I am pretty unfamiliar with the flatness feature and it's usage, but from what I have looked up in Y14.5
1. it is applied with a leader from the surface
2. it is usually toleranced L.T. the size tolerance on its applicable surface.

I must assume this is on here to control the warping of these 1.2" polypro panels during the welding process, but isn't there a better way to control this.

Q1. am I correct that his application is wrong, or should I just shut up.



Wes C.
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When they broke open molecules, they found they were only stuffed with atoms. But when they broke open atoms, they found them stuffed with explosions...

 

[MechNorth]

My apologies, ewh, but I'm not sure why you would want to mix linear tolerancing & GD&T when GD&T is already being used. The two typical reason given for using non-GD&T (linear) tolerancing are that (1) GD&T costs more, and (2) GD&T should only be used on CRITICAL features (presumably because it is believed to cost more).
It is a MYTH that GD&T costs more. Provided that the designer uses appropriate controls and tolerances, the costs will be the same or lower especially once you include scrap reduction. The myth is largely borne out of designers blindly convertig old linear tolerances to GD&T (which may result in actually tightening tolerances), and by inadequate understanding of GD&T in the shop. I have been as guilty as any other designer of blindly converting tolerances, and it always came back to haunt me. I can site examples of clean GD&T drawings that the shop didn't understand and the anticipated costs skyrocketed. I found that a few things were going on in the shop; (1) they didn't want to change the way they did things, and this was their way of protesting (2) they didn't see a need for change; (3, and many designers are equally guilty of this) they didn't understand that GD&T is just a communication tool that tells them exactly what the designer is expecting, be it a tight tolerance or a loose one.
It seems that nevitably designers bear the burden of educating the users of their drawings to properly understand them. Eventually, both sides will get to a better understanding of GD&T, but the path is very rough.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

 

[ewh]

MechNorth,
I agree with you. I was trying to understand the position of other posters who feel that non-critical features should not be located with basic dimensions, such as "GD & T should never be placed on a drawing unless there is a definite function or relationship that is needed. No function and relationship - no GD & T!!!!"
Provided the education is present, GD&T should actually save money.

 

[MechNorth]

Tks ewh. I was afraid...very afraid!
Jim

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

 

[ctopher]

Jim,
About the myth part. GD&T costing more is not a myth to a lot of purchasing people and some small machine shops. The one's that do not understand it, add cost because they think it's going to create more work for them. This is from my own experiences.
I have had engineers and managers come to me and tell me to remove GD&T so they can get a better guote! I stand by it and will not remove it.

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)

 

[CheckerRon]

Last word (for me) on GDT of holes. I didn't mean to imply that +/- should be used for non-critical holes in other areas on a dwg---especially on the same dwg with other true position hole callouts. Thks MechNorth for bringing that up. Personally, I and my colleagues were bummed out when 1994 ed of Y14.5 started paragraph 2.1.1.1 out with "Preferably". We were hoping for a mandatory statement for GD&T positioning of holes. Maybe next time? But, don't hold your breath