Circular Symmetric Parts - General Tolerancing ISO 2768

[Storaker]

Hey,

First time poster here at eng tips here is a question on general tolerancing according to iso 2768:

For circular symmetric parts using general tolerancing according to ISO 2768:
Will the circular run out criteria combined with the flatness criteria indirectly control the perpendicularity of part surfaces normal to the axis of revolution? (Run out controls that each measured circle on the surface is perpendicular to the axis of revolution while, while flatness prevents cone shape (which is not controlled by circular run out).

Example: ref attached figure

For Surf A and B: ISO 2768-mK for applicable size gives general tolerance:
Flatness: 0,1, Circular run out:0,2 Perpendicularity: 0,4
Will the circular run out combined with flatness indirectly control perpendicularity of surf A and B relative to center axis Ø25 (the longer of the datum’s) to:

​

Within the run out criteria, (0,2)

​

Within the run out + flatness (0,2+0,1=0,3)
Or is 0,4 the still the allowed perpendicularity

Best regards,
Audun Storaker,
Mechanical Engineer

 

[CheckerHater]

Surfaces A and B as shown on your drawing are parallel.

Could you be more clear about what should be perpendicular to what?

 

[Storaker]

Surface A and B are perpendicular to Ø25 and Ø15, marked surface C and D in the updated drawing. The reason I am not calling out datums is because these are general tolerances. With no designated bearing surfaces, I believe the standard calls for the longer of surfaces to be used as the datum, in this case surface C.

Audun

 

[CheckerHater]

OK, very good, we check A and B against C.

So, the surfaces A and B are perpendicular to C within 0.4.

The flatness tolerance zone of 0.1 is allowed to float and "wiggle" inside of perpendicularity tolerance zone.

Now remember that circular runout does not control the entire surface. It only applies to "circles" and tells us how much every circle is allowed to "wiggle".

The final result looks somehow like the enclosed picture. I only showed one surface for clarity.

The resulting surface lays within 0.4 perpendicularity zone. Perpendicularity zone controls orientation.

It also lays within 0.1 flatness zone, but the zone itself is allowed to float (flatness does not control orientation, only the form). They say flatness is a refinement of perpendicularity.

The circular runout creates additional restriction. It tells us that any circle on the surface is not allowed to "wiggle" more than 0.2.
It indirectly controls how much our flatness zone can "wiggle" so it refines the flatness requirement.

 

[Storaker]

Thank you, that is a really illustrative description and figure!

Just one thing I am not completely sure of; in the figure it looks like sum of the flatness tolerance and the circular run out tolerance refines the zone within which the surface lies. If flatness and run out is within speck then the measured perpendicularity will never be more than flatness tolerance + run out tolerance = 0.2+0.1= 0.3 for the example.

I added some colors to your figure (attached). Am I right to understand that material is allowed to be in the green zone but not allowed in the red zone, or are there other factors that still mean the material surface can be anywhere within the full 0.4 perpendicularity zone?

Audun

 

[pmarc]

CH,
I may be mistaken, but I think Audun's question was about something else.

Per my understanding it boils down to a question: Are you able to provide a sketch that shows a surface with flatness error = 0.1, circular runout error wrt C = 0.2, and perpendicularity error wrt C = 0.4 all at the same time?

 

[CheckerHater]

Did you look at my sketch?

 

[CheckerHater]

@Storaker:

The "green" zone that you have created is still allowed to float within "blue hatch" zone. So, no, material IS allowed to be in "red" zone.

 

[CheckerHater]

Or rather "red" zone doesn't really exist.

 

[pmarc]

Of course I did.
But I don't see this surface having 0.4 perpendicularity error wrt C.

 

[CheckerHater]

Why should it?

 

[Storaker]

So the reason why I am trying to figure this out, (besides trying to understand the basics) is because of a stackup, much simplified figure is attached.

I will use the perpendicularity in addition the size tolerance to calculate the extreme values of the gap however it seems to me that using the 0.4 perpendicularity from ISO 2768 is overly conservative, all tough, my confidence is that last statement is not very strong right now ;-)

 

[CheckerHater]

The necessary warning about using ISO 2768:

This standard is not intended to support sloppy design work.

If there is a possible issue with form, fit, or function of the part, one should explicitly specify size / geometrical requirements on the drawing.

It can be smaller or larger than ISO 2768 value, but if it provides proper function of the assembly, or reduces cost, it should be specified.

 

[Storaker]

So design purpose of what I am doing is to make the gap size as small as possible, with no collision and zero added cost. That is why want to use ISO 2768 for tolerances. I can off-course just increase the gap size to something huge, just wont look as good, or add a perpendicularity tolerance, but that wont teach me anything new. Anyways the real purpose is to really understand the standard and design the part so that non critical features does not need extra tolerances.

 

[pmarc]

Why should it?

Because the picture, as it is know, does not answer the original question. I thought your intent was to aswer that question, wasn't it?

So design purpose of what I am doing is to make the gap size as small as possible, with no collision and zero added cost. That is why want to use ISO 2768 for tolerances. I can off-course just increase the gap size to something huge, just wont look as good, or add a perpendicularity tolerance, but that wont teach me anything new. Anyways the real purpose is to really understand the standard and design the part so that non critical features does not need extra tolerances.

I would not like to start another discussion about inherent ambiguity of ISO 2768 standard, however I must say that this standard has actually very little to do with product definition basing on functional intent, design purpose or however we call it. And I think that your example is pretty nice example of that. By relying on general tolerances you will most likely satisfy manufacturing (as they will have to meet tolerance values seen as costs treshold in late 80's, but for many shops considered at least funny these days) and inspection (because they will have less job, since features produced with "normal" proces capability do not have to be controlled rigorously, if at all), but not function of the part.

Just one example - based on the description and sketch provided it looks to me that functionally much better option would be to control orientation of surfaces A and B relative to datum axis derived from surface D, not surface C. This would reduce orientational error accumulation between surfaces A and B on one hand and axis D on the other in comparison to the current situation. However, without explicit specification of datum feature D and proper perpendicularity tolerances relative to D this will not be possible, as CH noted.

 

[CheckerHater]

Storaker,
If you still thre, could you please clarify, did I answer your question or not?

 

[Storaker]

Hey,

If I got my question answered, well somewhat:

So my original question was how flatness combined with circular run out would refine the perpendicularity for circular symmetric parts. From the discussion and some thinking on my own: My impression now is that it will refine it to the axial component of the flatness tolerance + the run out tolerance.

Since angles are small, the axial component of the flattens tolerance almost equals the flatness tolerance. (as in sin(x)= x for small angels)

From this I would say that the maximum allowed perpendicularity for the for a circular symmetric part defined by iso 2768 would be the smallest of the following:
Perpendicularly as defined in the standard
Sum of flatness tolerance and circular run out tolerance

For the example, that would be 0,1+0,2=0,3 vs. 0.4

I believe this might contradict what was said about the shifting green zone inside the blue zone (ref previous figures), but if the zone is actually shifting then my interpretation is that this would not be a part of the perpendicularity bur rather size or position.

Now weather or not this correct, or would make sense if I explained it to a manufacturer that is a different story

 

[CheckerHater]

Here is an afterthought:

The part may look slightly different, like the one on the enclosed picture. In this case flatness tolerance zone may actually have less floating space.

So the general idea stays, the circular runout affects how much the tolerance zone can "wiggle", but not in simple straightforward manner, so we cannot really simply add 0.1 to 0.2 and get 0.3.

For whatever it's worth...

 

[Storaker]

Hey CH,
Thank you for your reply’s, It has definitely made me think. I have had a second look at the figures as tried to do some sketching on my own.

In the last figure you sent I believe the perpendicularity zone is to large. I have uploaded a modified figure with perpendicularity zone in orange. The zone should be perpendicular to the axis of revolution and contain the material surface. I don't think it needs to encase the flatness zone.

My thinking is more in line of the worst cross section that I can make within the criteria of flatness and run out and see what the resulting perpendicular is. I tried to sketch up a few different cross sections, see attached, to see if I could come up with any that would give perpendicularity larger than flatness + run out but as of now I still have not found any. (That is of course not proof that none exists, but rather the limit of my imagination)

http://files.engineering.com/getfil...4487-b3de-f691ce113230&file=ScreenShot987.png

http://files.engineering.com/getfil...4e0d-af2a-36647b8cc062&file=ScreenShot986.png

Audun

 

[CheckerHater]

Maybe you are putting too much thought into it

Worst case scenario will probably never exceed 0.3, and like I already mentioned, if for any reason you want to control this surface, you do so using GD&T symbology.

So, yes, indeed, perpendicularity requirement doesn't add much. And if surface in question was of any concern, something like total runout could control it nicely.