Form controls for datums

[Madhu454]

Hi All,
I will be in doubt whenever I am assigning a tolerance to the form controls, one simple example is attached.
Please see the attached drawing (drawing is incomplete).

Assume the center hole needs to be positioned to a tighter tolerance of 0.1mm. As we know the positional tolerance also controls the perpendicularity to primary datum (A).

The design intent is to have the hole positioned and to be perpendicular to primary datum. Now the question is, the tolerance on the thickness is 1mm and there is no flatness tolerance applied for the datum feature A, Assume the datum feature A is produced with flatness variations up to 1mm. in this situation what is the perpendicularity of hole with datum feature A. If I increase the tolerance on the thickness say to 2mm with no flatness control specified for datum feature A, then what is the effect on the perpendicularity of hole to datum feature A. There must be some effect that is the reason people say often to qualify the datum’s with form controls.

If I want to qualify the datum, then what should be the flatness tolerance value I need to choose?
Does it should be less than the positional tolerance value? if so much times?

Please suggest

 

[drawoh]

Madhu454,

Datum[ ]A is defined as the three points that extend down the furthest and contact your reference table. I am not sure what happens if the face is convex.

How does your part work? If you need your hole to be perpendicular to datum[ ]A, you need datum[ ]A to be accordingly flat.

--
JHG

 

[CheckerHater]

Madhu,

You have to distinguish between datum, datum feature and datum simulator.

Your hole has to be perpendicular to “perfect” plane after you somehow rest your datum feature against it (both ISO and ASME suggest ways of dealing with datum that wobbles, if necessary).
The picture shows (exaggerated) part that will pass the checking fixture.

So it is still your choice how flat you want your datum feature to be.

 

[leeekim]

Madhu,

From your sketch, the axis of the hole must be within a 0.1 diameter tolerance zone located from the datums A, B, and C. Datum A, B, and C are theoretically flat planes. The datum feature A is the surface on your part. The form on that feature is not controled and that is why it can look like the picture from CheckerHater. Increasing the tolerance on the thinkness will allow for more possible distortion. However, the positional requirement is still located by the basic dimensions to the theoretical dautm planes. So, the tolerance zone for the hole will always be perpendicular to datum A but might not necessarily be to the datum feature A.

If you are concerned about the form, consider using a generic statment that makes all untoleranced dimensioned with a profile tolerance to the datums. Then refine the features further to your liking.

Hope that helps.

 

[SeasonLee]

Whenever a planar surface is used as a primary datum feature, a flatness control should be applied to the datum feature. You will find out there is always a flatness control on the primary datum feature as shown on 09 standard Fig.4-2, Fig.4-4 through Fig.4-9, this will improve the stability between the datum feature and the datum feature simulator, it will also make your manufacturing and inspection repeatable.

SeasonLee

 

[pmarc]

SeasonLee,
If amount of flatness error limited by size tolerance is OK from functional standpoint, flatness control does not have to be applied.
Refer to the second sentence of paragraph 4.9 of Y14.5-2009. This is exactly what we have got here.

 

[SeasonLee]

pmarc
The OP stock size has a tolerance 1mm (±0.5), its really not good enough for a primary datum feature, I should add one more sentence on what I said : Flatness control tolerance value should be less than the size tolerance.

SeasonLee

 

[pmarc]

SeasonLee,
Maybe it is not good enough, maybe it is. We do not know that. If the actual datum feature A is concave, I guess there will be no problem with inspection repeatability. On the other hand, flatness tolerance 0.2 or 0.1 may be too much if the surface is convex - the part will still wobble when in contact with datum feature simulator.

And even if we say that flatness value should be less than size tolerance, will that suffice? It is of course true (assuming Y14.5's default Rule #1 is in charge), but will flatness tolerance of 0.9 make you feel better in this particular case?

All I wanted to point out is that the statement: "Whenever a planar surface is used as a primary datum feature, a flatness control should be applied to the datum feature." is too absolute, in my opinion. And I think Y14.5 in para. 4.9 confirms that.

 

[SeasonLee]

pmarc

Agreed what you said, and you will find out the same statement from Alex's "Advanced Concepts of GD&T" at page 14-4 Design Tip.

SeasonLee