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DATUMN REFERNCE CONTROL 2

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waqasmalik

waqasmalik

Mechanical
Jul 18, 2013
177
Hello every 1,Hoping u would be f9 and having best of ur days.I have a question and its really confusing me.

when we apply a perpendicularity tolerance or parallelism tolerance wrt any datum refrence then should we specify the flatness of that datum reference?If yes,then is it mandatory that the perpendicularity or parallelism tolerance be more than that tolerance value?
 
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u can suppose a simple part looks like "L" Shape.Its horizontal surface is datum A and vertical surface has a perpendicularity tolerance being applied to it wrt datum A.If i give a flatness tolerance of 0.1mm for Datum A then will the perpendiculaity tolerance of vertical surface having value .05mm wrt A will be ok?
 
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waqasmalik,
No .05 of perpendicularity will not be OK. It has to be greater than the flatness tolerance.

On the hand, if there was no flatness callout applied to the vertical portion of L, but perpendicularity, let's say 0.1, the perpendicularity would indirectly limit flatness to 0.1.
 
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While it is not required that a datum feature surface have a flatness tolerance, it is common to do so (and good design practice, in my opinion). However, the flatness number has no bearing on the perpendicularity tolerance -- the perpendicularity could be more or less, depending on its functional requirements.
And, the perpendicularity tolerance of that vertical surface will control the flatness of that surface, by the way.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
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Sorry, I did not notice you said: "flatness tolerance 0.1mm for Datum A" and not "flatness tolerance 0.1mm for vertical portion of L".
In that case, I do not see any connection between perpendicularity tolerance value on vertical surface and flatness tolerance value on horizontal surface.
 
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@Belanger. Sir,then it means that 0.05mm perpendicularity tolerance being applied to vertical surface is OK with reference to datum A which is horizontal surface having flatness tolerance as 0.1???
 
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If our Datum A has surface irregularities and we seat it on the surface plate then that irregularities will not affect the orientation of vertical surface?
 
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Correct. Remember that datum A is not the bottom surface itself, but the perfect plane established by the three highest points of the bottom surface. Once that datum is established, the perpendicularity could be 0/05 or even 0.0005 if you wanted.
It all depends on the functional requirements.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
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thanks a lot sir, u have really helped me out and my concept clear
 
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JP,
I am with pmarc on this, unless there is some exemption like tangent plane specified or something else I missed too.
Frank
 
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waqasmalik said:
[when we apply a perpendicularity tolerance or parallelism tolerance wrt any datum refrence then should we specify the flatness of that datum reference?If yes,then is it mandatory that the perpendicularity or parallelism tolerance be more than that tolerance value?]

Whenever a planar surface is used as a primary datum feature, a flatness control is applied to the datum feature. This general rule doesn't state in the standard, I think the reason is this will improve the stability between the datum feature and the datum feature simulator, it will have a better repeatability as well.

Saeaon
Click to expand...
 
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thanx seasonlee. What about second part of my question
 
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If you are asking about the perpendicularity tolerance value on vertical wall, I will say its nothing to do with the flatness tolerance value on datum A, the perpendicularity tolerance value is depends on your functional/application requirements.

Season
 
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Parallel tolerance must be smaller than the position tolerance of its surface (from Datum A).

lightweight, cheap, strong... pick 2
 
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