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...

 

[ewh]

1. Attaching it with an extension line is acceptable.
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?

 

[wes616]

Thanks ewh!

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. :-D

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...

 

[ewh]

I think you'll like Atlanta. I worked on a contract there for two years and fell in love with the place. Good luck!

 

[RubenGman]

Size tolerance will also control flatness. If the size tolerance value is the same or close to the flatness value, no need to add a redundant flatness callout....the end product will be the same. In addition to form, size tolerances may also control profile, parallelism and orientation.

 

[ewh]

Wouldn't the size tolerance control just the size? If you relied on that, you would also need to specify straightness, at least.
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.

 

[ctopher]

I would call out the flatness and any other FCF's on the part dwgs. Then call out angle tol between parts at the butt joints. Use a weld fixture to align the assy.

Chris
Systems Analyst, I.S.
SolidWorks/PDMWorks 05
AutoCAD 06
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716

 

[RubenGman]

Size tolerances may control straightness (form). In a case where you would want it "wavy", a tight size tolerance is not desirable.

 

[dingy2]

Flatness is on a single surface (plane) and is a unilateral tolerance. If one has an overall dimension to this surface with a tolerance range, the flatness must be inside the tolerance range.

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

 

[RubenGman]

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.

 

[dingy2]

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!!!!

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

 

[ewh]

So it would be better to dimension a hole pattern using +/- tolerancing than to use a positional tolerance? I have to disagree. Doing so would require you to use a tighter tolerance than is actually necessary.
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.

 

[RubenGman]

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.

 

[ewh]

OK, you got me there!

 

[MechNorth]

Per ASME Y14.5M-1994, Sect. 2.7 Limits of Size; "Unless otherwise specified, the limits of size of a feature prescribe the extent within which variations of geometric form, as well as size, are allowed. The control applies solely to individual features of size as defined in para. 1.3.17." In the case of the polypro panel box, the best method would be to tolerance the thickness of the sheet as indicated and put a note such as "THICKNESS ONLY", "EXCLUDES FORM", "RESTRAINED CONDITION" or equivalent under the tolerance, and put a flatness control on the surface with a (F) freestate symbol in the FCF.

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

 

[ewh]

How to you tolerance non-critical holes which are on the same centerlines as critical holes (which are located by basic dimensions)? Do you add an additional view with the same dimensions +/-(as opposed to basic)?

 

[CheckerRon]

RE: ewh and same centerline holes w/basic dimension. That is where I still use a position FCF but give the non-critical hole a liberal tolerance (with MMC of course).

 

[ewh]

Whew! I'm glad that I haven't been doing it wrong all these years!

 

[MechNorth]

ewh,
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

 

[ewh]

My question wasn't about which control to use, but how to avoid using gd&t and basic dimensions entirely to locate non-critical holes on common center or extension lines with controlled holes.