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Derived Median Line Straightness and Derived Median Plane Flatness 1
- Thread starter gabimo
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pmarc
Mechanical
- Sep 2, 2008
- 3,248
Good question.
Per the Y14.5-2009 standard DML Straightness and DMP Flatness tolerances are orientation refinements, or in other words, their values can't be greater than the orientation tolerance value applied to the controlled feature. Per para. 5.4.1.2 (explanation for DML Straightness):
"Where the straightness tolerance is used in conjunction with an orientation tolerance or position tolerance value, the specified straightness tolerance value shall not be greater than the specified orientation or position tolerance value."
Unfortunately from geometry (as defined in the standard) point of view this statement is incorrect. Imagine a banana-shape cylinder having a severe DML straightness error, yet having its actual unrelated mating envelope perfectly perpendicular to a datum plane. In that case even though there is a huge DML form error, orientation error will be zero.
Per the Y14.5-2009 standard DML Straightness and DMP Flatness tolerances are orientation refinements, or in other words, their values can't be greater than the orientation tolerance value applied to the controlled feature. Per para. 5.4.1.2 (explanation for DML Straightness):
"Where the straightness tolerance is used in conjunction with an orientation tolerance or position tolerance value, the specified straightness tolerance value shall not be greater than the specified orientation or position tolerance value."
Unfortunately from geometry (as defined in the standard) point of view this statement is incorrect. Imagine a banana-shape cylinder having a severe DML straightness error, yet having its actual unrelated mating envelope perfectly perpendicular to a datum plane. In that case even though there is a huge DML form error, orientation error will be zero.
CheckerHater
Mechanical
Shortly:
Form tolerances are conditions of surface, or derived medians.
Orientation tolerances are conditions of surface, or derived axis or plane.
When both control surface, some relation between the two is possible (hence para. 5.4.1.2)
When one controls curved median line and the other straight axis, there is no relation or ASME has some ‘splaining to do.
Same thing with curved median surface and flat derived mid-plane.
So, if we are talking about something “derived”, then – no. Derived median and derived axis have different meaning and are very little related.
Form tolerances are conditions of surface, or derived medians.
Orientation tolerances are conditions of surface, or derived axis or plane.
When both control surface, some relation between the two is possible (hence para. 5.4.1.2)
When one controls curved median line and the other straight axis, there is no relation or ASME has some ‘splaining to do.
Same thing with curved median surface and flat derived mid-plane.
So, if we are talking about something “derived”, then – no. Derived median and derived axis have different meaning and are very little related.
CheckerHater
Mechanical
pmarc said:
Could you be more specific? Last time I checked, Position may be defined both in terms of the surface of a feature and in terms of resolved geometry of a feature.
pmarc
Mechanical
- Sep 2, 2008
- 3,248
CH,
I would really like to have the sense of humour you have.
Position tolerance applied to a surface and position tolerance interpreted in terms of surface are two totally different things. I am more than sure that you know that, but I can't understand why you mix these terms in this case.
I would really like to have the sense of humour you have.
Position tolerance applied to a surface and position tolerance interpreted in terms of surface are two totally different things. I am more than sure that you know that, but I can't understand why you mix these terms in this case.
CheckerHater
Mechanical
Are saying that cylinder is not a surface? And you are totally serious, no humor involved?
CheckerHater
Mechanical
Let's play "simple answers":
Both Position, and Straightness tolerances can be applied to cylindrical surface. True or false?
Both Position, and Straightness tolerances can be applied to cylindrical surface. True or false?
pmarc
Mechanical
- Sep 2, 2008
- 3,248
There is no reason to play any game.
I can just repeat what I said. Para. 5.4.1.2 states:
"Where the straightness tolerance is used in conjunction with an orientation tolerance or position tolerance value, the specified straightness tolerance value shall not be greater than the specified orientation or position tolerance value."
This statement, at least to me, is clear evidence that per the standard straightness tolerance applied to a feature of size (to control its DML) is not allowed to be greater than orientation or position tolerance applied to the very same feature.
I say that this is incorrect, and guess why... Exactly because of the reason you described:
I can just repeat what I said. Para. 5.4.1.2 states:
"Where the straightness tolerance is used in conjunction with an orientation tolerance or position tolerance value, the specified straightness tolerance value shall not be greater than the specified orientation or position tolerance value."
This statement, at least to me, is clear evidence that per the standard straightness tolerance applied to a feature of size (to control its DML) is not allowed to be greater than orientation or position tolerance applied to the very same feature.
I say that this is incorrect, and guess why... Exactly because of the reason you described:
CH said:
Gentlemen,
Look what I found on linkedin:
"Bryan Fischer., President - MBD360 LLC and Advanced Dimensional Management LLC
Rich - right - it's not just straightness and flatness, but I think they cover it if the zone is the correct shape (cylindrical or parallel plane zone respectively for a cylindrical feature or a width feature). I purposefully didn't state which type of form error because any form error, local or global, straightness, roundness, etc. affects the calculation. Generally, for a cylindrical feature of size, we need to understand the perfect form boundary at the size that is specified (MMC or LMC) and the additional allowable form error at the other size (LMC or MMC). If we specify a straightness and flatness tolerance that controls the median geometry, then we need to consider the cumulative effect of the cylindrical or parallel plane tolerance zone controlling the median geometry, the local size error, and any modifiers, which yield the size and form boundary. Then from there we consider additional tolerance or additional form error. This ties into one of the debates that has been evolving in our standards development, whether the median form error allowed by such a form tolerance is a subset of orientation or location tolerances. How that is resolved affects this discussion.
What I meant by "in spec" is "within tolerance" (meets the specifications). Only a revocation of ASME's envelope principle (Rule #1) by a note, rule, or a straightness or flatness tolerance controlling the median geometry removes our perfect form at MMC (or LMC for features toleranced at LMC) requirement. Remember that only the two-point local size is currently legally invoked. 5.1;s spheres are not invoked without a special note. Note that ISO differs greatly in the last few bits... "
I don't know if helps much..............
Look what I found on linkedin:
"Bryan Fischer., President - MBD360 LLC and Advanced Dimensional Management LLC
Rich - right - it's not just straightness and flatness, but I think they cover it if the zone is the correct shape (cylindrical or parallel plane zone respectively for a cylindrical feature or a width feature). I purposefully didn't state which type of form error because any form error, local or global, straightness, roundness, etc. affects the calculation. Generally, for a cylindrical feature of size, we need to understand the perfect form boundary at the size that is specified (MMC or LMC) and the additional allowable form error at the other size (LMC or MMC). If we specify a straightness and flatness tolerance that controls the median geometry, then we need to consider the cumulative effect of the cylindrical or parallel plane tolerance zone controlling the median geometry, the local size error, and any modifiers, which yield the size and form boundary. Then from there we consider additional tolerance or additional form error. This ties into one of the debates that has been evolving in our standards development, whether the median form error allowed by such a form tolerance is a subset of orientation or location tolerances. How that is resolved affects this discussion.
What I meant by "in spec" is "within tolerance" (meets the specifications). Only a revocation of ASME's envelope principle (Rule #1) by a note, rule, or a straightness or flatness tolerance controlling the median geometry removes our perfect form at MMC (or LMC for features toleranced at LMC) requirement. Remember that only the two-point local size is currently legally invoked. 5.1;s spheres are not invoked without a special note. Note that ISO differs greatly in the last few bits... "
I don't know if helps much..............
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