straightness / flatness tolerance

[pete6696]

I am trying to place a straightness / flatness tolerance on a flat circular rubber part (dia 40mm) to constrain the amount of warpage allowable. The part is 2mm thick with size tolerance of +/-0.1mm. It is acceptable that the part warps +/- 1.5mm (z-axis) in either direction for it to pass. 1) Is the straightness / flatness tolerance allowed be greater than the 'size tolerance' or do I need to use a different tolerance type? 2)I am unsure whether I should use straightness / flatness, what determines which one I use?

 

[SeasonLee]

1. The flatness on the surface is controlled by Rule#1, for your case 2mm±0.1mm, the maximum allowable flatness error on the top is 0.2mm (±0.1mm).
2. Yes, you are right, flatness control tolerance value must be less than size tolerance.
3. For your case, I don’t think the warpage tolerance ±1.5mm is correct for a 2mm thickness part, I would rather say ±0.15mm, so the flatness tolerance will be 0.3mm which is great than size tolerance 0.2mm, its illegal and you have to make some changes.
4. Its better to use flatness control for your case.

SeasonLee

 

[ctopher]

If the part is molded, take in account the tolerances of the mold also.

Chris
SolidWorks 09 SP4.1
ctopher's home
SolidWorks Legion

 

[fsincox]

I was reviewing the new 2009 GD&T standard today and they have added allowing flatness to be added to the size dimension to exempt from rule #1. Hooray, I believe that is a blow in favor of logic, I always thought there was a logical inconsistency there, apparently, the ISO people thought so too as that is the way they do it.

 

[Belanger]

Pete--
First determine which edition of the GD&T standard your specifications will be designed to: 2009 or 1994 (or 1982; I suppose there are folks that still cling to that!).

If 2009: use a flatness tolerance applied to (or under) the size dimension. If 1994/82: use a straightness tolerance applied to (or under) the size dimension.

In either case, it's perfectly fine that the 1.5 is greater than the size tolerance. The rule that geometric tolerances must be less than size tolerances only applies when the geometric tolerance in question is applied to a surface.

Lastly, the flatness or straightness concept would probably not be controlled automatically by Rule #1, because the part you describe is a flat rubber part.
Rule #1 does not apply to "parts subject to free-state variation in the unrestrained condition." (2009 std, para. 2.7.2(b)

John-Paul Belanger
Certified Sr. GD&T Professional

http://www.gdtseminars.com

 

[dingy2]

Belanger:

You stated:

"If 2009: use a flatness tolerance applied to (or under) the size dimension. If 1994/82: use a straightness tolerance applied to (or under) the size dimension."

If we are talking about a surface, I would not suggest that the flatness tolerance be placed under the size dimension. That could be construed as requiring the centre plane to be flat rather than the surface. Place the flatness requirement on the surface (leader or extension line) if that is the need.

The ASME Y14.5 2009 revision now allows flatness of the centre plane and also at MMC which is similar to straightness in the 94 standard. Rule #1 does not apply in such cases.

I would not suggest using straightness in your situation but if one did place it under the size, it would be the straightness of the axis rather than surface. I do believe that we are talking about a surface and not an axis in your case. Again, I would not suggest using straightness.

Rule #1 does apply if the concern is a surface rather than a plane or axis.

Lastly, all parts are confirmed in a free state (unless otherwise specified) and I do believe that the part in question would be relatively rigid and also be confirmed in a free state.If the part in question is a piece of jelly, then rule #1 does not apply.

The 2009 std., paragraph 2.7.2 (b) refers one to 5.5 which details the application of free state. The example here is a thin wall product where the wall flexes.

Just another point of view.

Dave D.

http://www.qmsi.ca

 

[Belanger]

Dave,
The original question stated that he was interested in controlling warpage. That sounds like centerplane control, not a surface control. So the flatness symbol should be used on the feature-of-size dimension (or straightness on the FOS if it's not up to the 2009 std).

This is why I suggested flatness applied to the FOS, as allowed by the new standard. Whether the MMC modifier is used or not depends on the function of the part. I imagine he's worried about warpage for reasons of internal stress or deformation of the part, not just sliding it into a slip-fit assembly.

As for Rule #1, you are correct that a flatness or straightness tolerance on the FOS overrides the rule, but my comment about the free state was in relation to SeasonLee's post that flatness was already an automatic thing per Rule #1.

I guess we need Pete's input as to whether the part qualifies as a non-rigid part per paragraph 5.5.

John-Paul Belanger
Certified Sr. GD&T Professional

http://www.gdtseminars.com

 

[MechNorth]

There's also the option of adding "PERFECT FORM AT MMC NOT REQUIRED", which then allows a flatness tolerance in excess of the size tolerance to be applied.

As for using flatness (2009)/straightness ('94) of the derived median plane instead, it's ok as long as it's at MMC; at RFS, it's likely prohibitively expensive to verify as you have to average opposed points to determine the derived median plane, and if there's warpage then that's going to be significant work.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[dingy2]

Jim:

Flatness on the derived median plane shown in the 94 edition? I don't think so but it is included in the 2009.

Straightness is included with or without MMC but flatness is applied to a surface using the 94 edition.

Now, I could be wrong so you let me know where in the 94 edition I could find flatness on a derived median plane.

Dave D.

http://www.qmsi.ca

 

[fsincox]

Dingy2,
Flatness is not allowed on a median plane in the previous ASME Y14.5 standards untill this year (2009). I have stated my opinion on that and I am sorry if I confused the issue here. It has allways been allowed in all of the ISO standards I have seen. I presonally always felt it was a violation of logic.

 

[MechNorth]

Dave, pls read what I <u>actually</u> wrote; "flatness (2009) / straightness ('94) of the derived median plane". In '94, the straightness of a derived median plane was meant to read as "flatness" of a center plane; for confirmation, pls see 6.4.l1.1.3 ('94) and A.7.2 in ('09).

The flatness control was not allowed on the derived median plane in '94, but the straightness control applied to the feature of size acted as and was meant to be a flatness control.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[dingy2]

Jim:

I stand corrected on this one.

Dave

Dave D.

http://www.qmsi.ca

 

[MechNorth]

No problem. Coffee next time!

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com