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.

 

[ewh]

Flatness

When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - [small]Thomas Jefferson [/small]

 

[CheckerRon]

Yes flatness, not straightness to cover the whole surface.

 

[LONDONDERRY]

So can you guys give me more of an explaination as to why flatness and not straightness?

 

[GBor]

Straightness is 2-dimensional, like a beam over a certain length. Flatness is 3-dimensional, like the surface of a plate.

 

[ewh]

Straightness will only control "flatness" in one dimension. You can have an extruded wave that will meet a straightness requirement.

When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - [small]Thomas Jefferson [/small]

 

[CheckerRon]

GBor and ewh just explained it well.

Read pages 157-161 of Y14.5M-1994 to see the differences---single individual elements for straightness vs. all elements (whole surface)for flatness.

 

[ctopher]

You can also call out flatness on certain areas and not the whole part if needed, to make machining easier.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Jul 13, 2008)

 

[JV1]

Londonderry,

The flatness or straightness call-out depends on the amount or warpage you are can allow and the material thickness tolerance. Remember rule #1 (perfect form at MMC). If the plate spec is .25±.01 thick, when the part is produced at .26, it must be perfectly flat. When the part is produced at .24, the part is allowed to warp by the difference between the actual thickness and the MMC: .02.

If you need the part flatter than .02, then you need to specify a flatness tolerance on one side and maybe a parallelism tolerance on the opposite side. The flatness tolerance spefication cannot exceed the thickness tolerance; anything more that .02 flatness will be incorrect (in my example). If the part is allowed to warp more that .02, you need to specify straightness to the feature of size (thickness)dimension. This will allow you to violate rule #1.

For comparison, if the part had a .01 flatness requirement,
when the part thickness is:
.26, flatness must be .00 (perfect per rule #1)
.25, flatness must be .01
.24, flatness must be .01

If you specify a .01 straightness to the thickness dimension, when the part thickness is:
.26, flatness must be .01
.25, flatness must be .01
.24, flatness must be .01

If you specify a .03 straightness to the thickness dimension, when the part thickness is:
.26, flatness must be .03
.25, flatness must be .03
.24, flatness must be .03

I hope this helps.

JV1

 

[PaulJackson]

JV1,

I disagree,

Rule #1 does not apply to flatness since it is a surface form control... (at least it does not yet with ASME Y14.5M-1994 but possibly with ASME Y14.5M-2XXX if flatness (M) is permitted to apply to a FOS "thickness").

Straightness although can be applied to a planar FOS in the current ASME standard by specifing it under the size callout. You are correct that it is a constant value if no (M) is specified in the tolerance but it is a variable value if the (M) is specified.

In your examples Rule # 1 does apply to size so a 0.25 +/- 0.01 thickness would be limited to 0.26 "perfect form at MMC" and flatness constrained to its limit on the specified surface(s).

Straightness (M) is not constrained by Rule #1 therefore if the straightness was applied to the size callout and thickness 0.25 +/- 0.01 and straightness tolerance was 0.03(M) then the overall effctive thickness would be 0.25 +0.01 +0.03 which is 0.29.

With straightness applied to the size without (M) the overall effctive thickness would be limited to +0.03 above the actual maximum thickness.

Paul

 

[dingy2]

Flatness is a surface condition and does apply in this situation according to ASME Y14.5M-94. Rule 1 is not applicable to a surface but only to a feature of size.

I would suggest, like the others, that you apply a flatness tolerance on the surface.

Dave D.

http://www.qmsi.ca

 

[CheckerRon]

LONDONDERRY:
It occurs to me the question none of asked directly is, "is your warpage exceeding the size tolerance of the 4 X 6 part"? What warpage your design stand and still be functional is going to determine if you go with flatness (prefect form @ MMC) or straightness @ MMC with a staightness limit you can live with.

 

[desertfox]

Hi Londonderry

If the fabrication is welded then that maybe one of the reason's for the distortion, as well as using GD&T you might want to look at stress relieving the part before final machining.


Regards

desertfox

 

[JeffMathena]

Hi,

Just a secondary point. If your surface you want to control the flatness on is interupted, then you would need to use the profile callout without any datum reference. Also you will need to callout how many surfaces it applies to.

Jeff

 

[MechNorth]

Jeff, to show the coplanarity of the interrupted surfaces you would have to show a phantom line between the surfaces to establish the relationship and then control the relationship by putting the leader of the surface profile to phantom line; otherwise, you're just controlling the flatness of each of the surfaces individually and not a collective coplanarity.

JV1, good explanation above.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[KENAT]

MechNorth,

I'm looking at 6.5.6 & 6.5.6.1 of ASME Y14.5M-1994 and don't see where it says you have to have the phantom line or that the FCF has to point at it.

Certainly it is shown that way in the examples (figs 6-20 & 6-21), and I can see that otherwise there'd be potential for confustion but I don't see it explicitly stated.

Where I had a part with something like 100 faces (interupted by small grooves) that were nominally coplaner I didn't show the phantom lines, just the number of surfaces below the FCF.

Is this a case where for it to be an individual requirement you'd have to explicitly state this?

KENAT,

Have you reminded yourself of faq731-376 recently?

 

[MechNorth]

Good observation, Kenat, and one I've raised myself. The standard does not explicitly preclude putting the leader anywhere else, and indeed shows both in Fig. 6-21.

Fig. 6-21 shows the relationship (phantom) line between the individual surfaces, and the two datum surfaces being coplanar by means of leaders directly to the surface. In this instance, where the features are separated by other features, the only option would be to use leaders directly to the surfaces. The two center surfaces are shown coplanar & located wrt the datum by means of the leader directly to the phantom line.

So, as long as the leader goes to the phantom line relating the features, or the leaders go to all of the related features you would get the same meaning.

The problem that I see most often is that someone will use (1) flatness on 2 surfaces thinking that they get coplanarity, or (2) a surface profile on an extension line leading to only one surface, which leaves it ambiguous as to which surfaces are intended.

Tks for keeping me honest, Kenat.

The situation you mention is fairly common. I've dealt with it in a few ways; without the phantom lines,
(1) add a note "ALL SURFACES AT THIS LEVEL", if all surfaces at that level are to be included,
(2) after indicating the number of surfaces add a note "MARKED 'X'" or "EXCEPT MARKED 'X'", when only some of the surfaces are to be considered,
(3) combinations and variants of 1 & 2, which describe which features are to be considered / excluded.

Also, the same methodology can be used to control the co-cylindricity of nominally-cylindrical coaxial features.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[CheckerRon]

Kenat and mechnorth: I run into this a lot too, and I really prefer the phantom lines ala Figures 6-20 & 6-21.

Another frequent case is Fig 4-20, interrupted surfaces, picture wanting a coplanarity on these surfaces.
Could do it by flatness, datum B to datum A, but maybe Profile, 2 surfaces like Figure 6-21 also? Maybe not from an inspection point of view. What do you think?

 

[KENAT]

Basically Mech North you peed in my cheerios and I'm moping;-).

I thought I'd done really well to use surface profile instead of flatness and then you bring the phantom line thing up:-(

There was no way I could use phantom line in my case, the item was only 12" OD and had like a spider web pattern of small grooves. However your note (1) may have been a good idea.

I'm not sure I'm going to go force a rev of the drawing but I'll keep it in mind for next time.

Thx

KENAT,

Have you reminded yourself of faq731-376 recently?

 

[MechNorth]

Ron, unfortunately flatness only controls one surface at a time, and does not relate that surface to anything else (each could be flat, but inclined at 37.89-deg to each other...random number). They specifically laid out Surface Profile as the way to establish coplanarity.

For Fig 4-20, you'd have to establish flatness on whichever was to be the primary datum (A?), and then locate the secondary (B?) to the primary by a surface profile wrt Datum-A. In fact, on Fig. 4-20, there is no indication of a coplanarity requirement, so the surfaces could be stepped (by design) by 5mm, and you could still (theoretically) use compound datum plane A-B.

Sorry about the Cheerios, Kenat. Buy you a round at the pub to make up for it?

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com