Flatness in assy
Flatness in assy
(OP)
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.
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...





RE: Flatness in assy
2. The size tolerance is not related to the flatness tolerance. The size tolerance locates the plane of the flatness tolerance, while the flatness tolerance determines hoe accurate that plane is.
How's Texas treating you?
RE: Flatness in assy
Texas is pretty big country. I however, am not staying! I have gotten a nice job offer in Atlanta, and I'll be starting out there right after the thanksgiving holiday...
I'll check it out for a few months, and re-eval then...
It seems that all the jobs in Texas are contract jobs... and I need AT LEAST dental insurance.
Wes C.
------------------------------
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...
RE: Flatness in assy
RE: Flatness in assy
RE: Flatness in assy
You could call out a tightly conrolled size dimension on a part that is supposed to be slightly "wavey", and it would not force it to be flat.
RE: Flatness in assy
Chris
Systems Analyst, I.S.
SolidWorks/PDMWorks 05
AutoCAD 06
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716
RE: Flatness in assy
RE: Flatness in assy
One cannot have an overall tolerance of +/- .030, as an example, and have a flatness tolerance of .075. It would have to be within the size tolerance range other we would have a conflict.
If we took the whole flatness tolerance of .075 we would be outside the size limits - either on the minus or plus.
I would say that most designers do not really look at this but I could be wrong on this.
Hope this helps.
DD
RE: Flatness in assy
Dingy,
You have the concept correct. The rule of thumb I follow is to apply Form, Orientation, and Profile controls to components or assemblies that would prohibit the use of tight size tolerances such as seperable and non-seperable assemblies. If you're working on parts with tight size tolerances, you may want to reconsider the use of additional callouts as they may be redundant.
RE: Flatness in assy
If one has a size tolerance, it is only checked or confirmed in a couple of spots at best using an appropriate measuring instrument. The surface is not scanned. If that surface must contain a flatness tolerance because of its function, then the shop floor people in Quality and Processing are aware of the situation and will scan the area for flatness.
DD
RE: Flatness in assy
A +/-.005 tolerance equates to a diametral tolerance of .014, thus using positional tolerancing will allow a larger tolerance "window", allowing more parts to pass inspection and still fullfill their intended function. All you need is an educated workforce.
RE: Flatness in assy
EWH,
I have to disagree along with you. Use Position callouts when controling holes that require a Fixed or Floating case. Mating holes benefit from this formula. You've answered your own question.
However, non-mating holes used for weight reduction or holes used for venting may not require the use of positional tolerancing.
RE: Flatness in assy
RE: Flatness in assy
RubenGman has the right idea about not using positional tolerance for non-critical holes ... going one step further, that's the right use for a surface profile which will control the size, form, orientation and location.
Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
RE: Flatness in assy
RE: Flatness in assy
RE: Flatness in assy
RE: Flatness in assy
Unfortunately, surface profile has to indicate a surface, so an additional view would be needed. Indicating the diameter in a topographic view would just show an edge, and therefore a surface profile could not correctly be used to control that surface in that view. All features would be located by basic dimensions. The critical features would have tighter locational and size tolerances as appropriate; my preference there would still be the surface profile. The less-critical holes would use a looser surface profile.
Don't use linear tolerance on the location DIMENSIONS on a cylindrical feature because the dimensions (and therefore the tolerance) applies to a theoretical axis, plus they will give a square tolerance zone. The Position Tolerance applies to the axis DERIVED FROM the feature of size, and should use a cylindrical tolerance zone.
Again, locate ALL features with basic dimensions, and then control the size & location by appropriate tolerances based on individual feature requirements.
Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
RE: Flatness in assy
RE: Flatness in assy
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
RE: Flatness in assy
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.
RE: Flatness in assy
Jim
Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
RE: Flatness in assy
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)
RE: Flatness in assy