Profile tolerance : Offset surface

[Madhu454]

Hi All,
Have one question regarding profile of a surface tolerance. The tolerance zone is formed by sweeping a spherical ball of size equal to tolerance value over the toleranced surface to have the upper limit and lower limit surfaces.

1. Can’t we simply offset the toleranced surfaces to get the tolerance zone.
2. Want to know the exact idea of using spherical ball concept to define the tolerance zone. (Is there any manufacturing considerations)?

 

[CheckerHater]

1. How you "simply" offset the surface?
2. It's mathematical concept, not manufacturing. The ball is imaginary.

 

[pmarc]

The concept of profile tolerance zone formed by sweeping a spherical ball of size equal to tolerance value over the toleranced surface is ISO specific. In ASME both profile tolerance zone boundaries are offset in a direction normal to true profile of a feature.

The difference becomes visible when you consider a geometry like the one shown in fig. 8-12 in Y14.5-2009. If this figure was according to ISO, the all-around tolerance zone in the area of all corners would be rounded and not sharp.

 

[ajack1]

As I understand it that is the difference you get when you have sharp corners or a radius or another feature come to that smaller than the offset.

 

[CheckerHater]

If this figure was according to ISO, the all-around tolerance zone in the area of all corners would be rounded and not sharp.

Really?

 

[pmarc]

Really.
The figure from ISO 1101 does not show shape of the profile tolerance zone. It just shows which portions of the contour the all-around specification applies to.

 

[CheckerHater]

As I understand it that is the difference you get when you have sharp corners or a radius or another feature come to that smaller than the offset.

Yes, ASME and ISO approach will yeld different results in that case. Looks like the ISO does better job at "offsetting" the surface.

 

[CheckerHater]

The figure from ISO 1101 does not show shape of the profile tolerance zone. It just shows which portions of the contour the all-around specification applies to.

Well, here is another look at "all-around" tolerance zone. I still don't see any rounded corners:

 

[SeasonLee]

Madhu
Here is the basic concept of profile tolerance-ISO, hope this will help.

Season

 

[axym]

CH,

Pmarc is right, ISO 1101 specifies the swept-sphere method. The diagram you posted appears to be incorrect, because the corners are not rounded. Is it from a textbook?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

Evan,

Would you please show me where in ISO 1101 or ISO 1660 “swept-sphere” tolerance zone actually goes around the sharp corner?

Yes, the tolerance zone of a profile tolerance is defined by tangential envelopes on circles (profile tolerance of a line) or on spheres (profile tolerance of a surface) whose diameters are equal to the tolerance value and centered on the nominal form.

No, the form of the envelope of the circles or spheres (tolerance zone) between the specified points is not standardized.

So, I would greatly appreciate reference to any reliable source showing how said envelope behaves when going around the corner.

My source :

http://www.iso.org/iso/home/store/publication_item.htm?pid=PUB100296

The author: Dr. Henrik S. Nielsen, Chair of ISO technical committee ISO/TC 213

As ISO 1660 is currently being reviewed by the committee, it’s hard to tell what the future will bring.

 

[pmarc]

The main problem with sharp corners controlled by swept-sphere-based profile tolerance is the inconsequence in shapes of inner and outer boundary of the tolerance zone in the corner area. If we take the picture attached by SeasonLee, we will easily notice that the tolerance zone boundary that lies inside the material of the part has both corners sharp, while the boundary that lies outside of the material has both corners round. In my opinion this is logical consequence of swept-sphere tolerance zone approach - the inner boundary must be sharp and the outer boundary must be round.

Now, in ASME we will not have profile tolerance zone outer boundary round in the corner area (unless such a requirement is specifically stated on the drawing). And this is the difference between ISO and ASME I was talking about.

At the end of the day, I am afraid we will not get consensus, until some new/revised ISO standard(s) clearly show(s) how to deal with sharp corners controlled by profile tolerance.

 

[CheckerHater]

I agree, ISO should provide more clarification.

But I really see no difference between ISO and ASME approach. ASME definition can be interpreted to produce round corners as well .ASME just arbitrarily decided to extend surfaces until they intersect. ISO could do the same.

This is why I am really curious where Season's picture comes from.

 

[SeasonLee]

My reference book is Geo Tol Pro Fundamentals Workbooks by Al Neumann page 12-19
Here is his profile interpretation on ASME Y14.5-2009 standard.

Season

 

[CheckerHater]

Thank you Season,

According to ASME Y14.5.1M “A profile tolerance zone is an area or a volume generated by offsetting each point on the nominal surface in a direction normal to the nominal surface at that point”.
No mathematical definition is given to what is happening around sharp corners, or how to extend offset surface outside of its boundaries.
Even if we imagine that the sharp edge has some qualities of a surface, and we still able to draw normals to it in different directions, we end up with something like enclosed picture.
This is what I mean by saying that default ASME definition results in rounded corners as well.
On the good side ASME arbitrarily decided that offset surfaces have to be extended until they intersect each other.
On the bad side there is no definition on how to extend them. It looks easy with straight lines, but could be difficult with curves of higher order.
This is why I think both ASME and ISO “have some splaining to do”

 

[axym]

CH,

I agree that the current standards (ASME Y14.5, ASME Y14.5.1, ISO 1101) all have gaps in the definitions.

ASME in particular, states that the offset boundaries need to be extended but doesn't say how to extend them. We can make reasonable assumptions on extrapolation for flat surfaces and simple arcs, but extrapolation surfaces with non-constant curvature is a guessing game.

The ISO 1101 swept-sphere definition has its drawbacks as well, but I thought that at least the meaning was clear. You're saying that we don't know what the tolerance zone boundaries would look like in the vicinity of a sharp corner? I'm not sure I see the uncertainty.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

I'm not sure I see the uncertainty

The ambiguity actually starts with definition of "all-around"

 

[axym]

CH,

I don't have the ISO 1101 standard in front of me, so I'm not sure how it defines "all around".

The figure you posted looks like it is from a textbook, not one of the ISO standards. Is that correct? The way that the tolerance zone boundaries are depicted in the figure does not agree with the idea of sweeping a sphere along the true profile.



Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

There is still something I do not understand about your interpretation of ASME interpretation of what is happening in the vicinity of sharp corners, CH.

On one hand you say that:

This is what I mean by saying that default ASME definition results in rounded corners as well.

On the other hand:

On the good side ASME arbitrarily decided that offset surfaces have to be extended until they intersect each other.

How is it possible? What am I missing here? Why don't I see rounded profile tolerance zones in figs. 8-8 or 8-12 in Y14.5-2009 (for example), if this is default interpretation?

 

[ewh]

I too am confused; as CH states Y14.5.1M states in section 6.5(a) "Definition. A profile tolerance zone is an area (profile of a line) or a volume (profile of a surface) generated by offsetting each point on the nominal surface in a direction normal to the nominal surface at that point." Since a sharp corner has no one "normal" it seems to follow that a radius would be swept about that corner.
The example illustrations don't show that interpretation, leading to this confusion.

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