Couple Flatness Questions 1

[ModulusCT]

I searched through the forums, but didn't find all of the information I was looking for.

For the sake of the following questions, assume a cimple rectangular part 1.000" high with a size tolerance of ±.005".

Why would one want to apply a flatness tolerance as a refinement to the size tolerance when you could simply reduce the size tolerance? If I have a flatness requirement of .003" on a surface with a size tolerance of .005", why wouldn't I want to just change that size tolerance to .003"?

When a part is at MMC, the flatness tolerance is .000" (per rule #1)? Is that in both directions? Or would the flatness tolerance now be +.000 / -.003?

The GD&T book I'm reading now says that a FCF with flatness per unit (t.g. 0.003" IN/IN) is a refinement of a non-unit flatness callout. Does this mean, I must include the overall flatness callout in addition to any unit flatness? Or can my FCF simply show the per unit requirement?

Thanks for the help...

 

[KENAT]

... Because reducing the size tolerance may have significant cost/schedule ramifications beyond just improving flatness.

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[ModulusCT]

OK, so basically we're saying then that we don't care as much about the overall size as much as we care about the quality of the surface with the flatness callout?

We could, in theory have a part that is made to 1.005 and from that surface a .003" flatness tolerance (in one direction only) would apply? Or would it be .0015"?

 

[ModulusCT]

I guess I'm confused about how Rule #1 applies to flatness... If a part is produced at MMC then the flatness tolerance doesn't seem to apply because a bidirectional tolerance zone must be maintained?... And yet, one could easily imagine a situation where, at MMC the surface can violate the flatness tolerance toward LMC.

How confused am I?

 

[KENAT]

Simplistically yes.

Flatness tolerance is in itself independant of size tolerance. Flatness is essentially comparing the deviation of the surfact to its self. Size tolerance is comparing it to some other point/surface/datum.

Take a look in the relevant standard.

I think maybe you're starting to think about what would more properly be parallelism of one surface to another.

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[ModulusCT]

I guess I'm just confused as to why the flatness tolerance zone doesn't become uni-directional when the part size it produced at MMC.

Anyone have any simple way of explaining that?

Thanks Kenat for your help... I'll see what I can glean from ASME Y14.5.

 

[ModulusCT]

ok - I'm a moron... At MMC, no portion of the surface being controlled by the size tolerance would be below 1.005" and therefore, the applicable flatness tolerance would be 0.000".

 

[KENAT]

6.4.2 is the section in 14.5M-94, page 161.

6.5.2.1 lsat para "Where the considered surface is associated with a size dimension, tha flatness tolerance must be less than the size tolerance."

I'm pretty sure I've done a lousy job trying to help so see if any of the this is clearer.

thread1103-204167
thread1103-211411

http://www.tec-ease.com/tips/july-99.htm

http://books.google.com/books?id=GQ...v=onepage&q=flatness tolerance rule 1&f=false

http://books.google.com/books?id=qv...v=onepage&q=flatness tolerance rule 1&f=false

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[drawoh]

ModulusCT,

The worst case with your part is that it can be .995" high at one end and 1.005" high at the opposite end. When you fixture the flat face perpendicular to a gauge, the height variation will not exceed .003". Note how the face is not required to be parallel.

A machinist probably will just machine it carefully, but who knows. There could be a bonus allowance in there somewhere.

JHG

 

[ModulusCT]

Thanks, I'll check that out...

One more question came up today about flatness... We usually specify our flatness tolerances per unit length. Well, that's what people here think anyway.

I was unable to find anything in Y14.5 specifying any unit other than area when using flatness... Is anyone aware whether or not a previous version of Y14.5 allowed flatness per unit length?

Is a flatness tolerance of .003 / 1 possible? Or must it be .003 / 1 x 1?

Lastly, when specifying that unit area, I'm guessing that any trailing zero's are not needed. So, in the above example, it's .003 / 1 and not .003 / 1.000 x 1.000. Correct?

 

[fcsuper]

I'm not sure about the older versions of ASME (pre-94), but it doesn't seem logical to have a per/unit on a flatness callout since flatness is the surface defined as being between to parallel plans, and that it goes across the whole distance of the surface OR a specific area that is delineated in the part view.

If it was "per inch", then where would be the origin from which the surface can deviate from the stated geometric tolerance? Which direction would deviation be allow?

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[KENAT]

-94 VERSION SECTION 6.4.2.1.1 does say to use unit area.

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[SeasonLee]

Yes, Unit basis flatness tolerance is specified very claer in Y14.5M-1994 on secton 6.4.2.1.1 (page 161), you can see the example at page 162.

SeasonLee

 

[fcsuper]

Sorry, was too lazy to look that up, but either way, isn't that for a specific area?

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[ModulusCT]

Yes, it does state that the correct callout is for a unit area. However, I've seen some older documents that show a unit length.

I haven't seen the '84 spec though, so I can't say for sure.

Thanks everyone for your help... I think I'm a little better prepared to offer a solution than I was before.

 

[powerhound]

The unit length would be used with a straightness callout, not flatness.

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