How much is the Bonus Tolerance?

[Tarator]

Hi All,

Based on the measured values of the following external cylindrical feature of size on a rigid part, what would be the bonus tolerance for straightness?

Measured LMC = ø 8.006
Measured MMC = ø 11.527
Measured Actual Mating Size = ø 11.979
Measured straightness = ø 2.724

My answer is this:
1) Finding the size: Actual Mating Size - Straithness = 11.979 - 2.724 = 9.255
2) Bonus = MMC - Size = 12 - 9.255 = 2.745

Thank you.

 

[chez311]

Tarator,

Evaluating the bonus tolerance for MMC straightness would require comparison of the actual local size vs MMC size at each cross section.

This would of course be for an axis/resolved geometry interpretation. The current Y14.5.1-20xx draft only includes a surface interpretation for MMC straightness, so its not really necessary (nor is there any guideline to how this should be evaluated besides the verbiage in Y14.5) to do this calculation.

 

[Tarator]

chez311,

Thank you for the input.

So there is no one fixed-size value, from which we can calculate the bonus tolerance?

How are the reports done in practice? If you do not have one bonus tolerance value, how can one verify where it is in spec or not?

 

[Belanger]

The bonus tolerance idea -- and quantifying bonus in a table/chart -- is often useful to teach the general concept, since it's foreign to folks who only know the traditional style of ± tolerances. But as chez311 wrote, in the real world bonus on a form tolerance is usually difficult to quantify because the actual local size of the part is unlikely to be uniform/consistent.
You ask how one can verify it. The best way would be what's called a functional gage, and for the example you give the gage would be a sleeve (or simply a hole) with a diameter of 12.0 mm.
And I should also mention: If the drawing is using the ASME standard, and it's presumed to be a rigid part, then the straightness callout isn't needed because of what we call Rule #1 (automatic form control coming from the MMC of 12).

 

[Tarator]

Belanger,

It makes sense... Thank you.

 

[chez311]

Just to add - the only requirement for conformance to the surface interpretation is that no point of the feature violates your virtual condition. Hence why it can be checked with a hard gauge of fixed size.

 

[greenimi]

Chez311 and Belanger,

Why the "bonus" is not 12.000 - Measured Actual Mating Size = ø 11.979 = 0.021?
The FCF is nothing else than rule#1 explicit, isn't it?

If the FCF is removed (straightness Ø o at MMC) and the print is defined based on Y14.5, then I guess nothing change.
Am I correct?

 

[chez311]

greenimi,

Because use of the UAME to estimate size variation only works reasonably well if we assume there is very little form error ie: when we use it to calculate bonus tolerance/actual value for position. In the case of straightness, we are essentially trying to quantify form error - so wouldn't it be a bit backwards to make this assumption?*

The FCF is nothing else than rule#1 explicit, isn't it?

I agree.

*Edit: I should add as well that the text of Y14.5 explicitly states the actual local size in each cross section be used for this calculation:

As each actual local size departs from MMC, an increase in the local diameter of the tolerance zone that is equal to the amount of such departure is allowed. Each circular element of the surface (that is, actual local size) must be within the specified limits of size.

 

[greenimi]

Quote (Y14.5-2009 para 5.4.1.2)
As each actual local size departs from MMC, an increase in the local diameter of the tolerance zone that is equal to the amount of such departure is allowed. Each circular element of the surface (that is, actual local size) must be within the specified limits of size.

I do not think this statement is very clear. I think it's just a muddy one among the muddiest Y14.5 has.
But that does not mean you are not correct on what you wrote above.

 

[chez311]

greenimi,

You won't hear any disagreement from me. I think Actual Local Size is one of the murkiest can of worms in the standard. You simply asked why bonus tolerance wouldn't utilize the UAME size - part of my answer was just based on general reasoning (form control should take into account form errors) and the other part simply the verbiage which we are dumped with on the standard.

As noted previously this can pretty much be ignored for straightness at MMC since we can utilize the surface interpretation and skip calculation of bonus tolerance altogether for a real feature. If you want simply to know the maximum bonus tolerance (or maximum straightness error) possible for a given feature controlled with MMC straightness then of course the calculation would be very simple being the difference between MMC and LMC size plus the tolerance value stated in the FCF.

 

[Burunduk]

Y14.5-2009 para 5.4.1.2
Intends to say:

"As each actual local size circular element containing high points of the feature departs from MMC, an increase in the local diameter of the tolerance zone that is equal to the amount of such departure is allowed. Each
circular element of the surface (that is, actual local size) (of the feature) must be within the specified limits of size."

This is the only logical way to interpret the paragraph considering the actual local size definition.

 

[Tarator]

Burunduk,

How about a feature of size with 2 parallel planar surfaces (or also line elements)?

You cannot define a term based on one type of feature. You need to give a generalized definition that works for all features of size.

 

[drawoh]

Tarator,

Does your straitness tolerance do anything? Rule[ ]1 requires the part to be straight at MMC/MMB. Your questions and comments about bonus tolerance still are valid.

--
JHG

 

[Tarator]

In my opinion, the ASME committee needs to start using 3D sketches and diagrams and mention that each cross-section has an infinite number of actual local sizes (in theory). From which, we are interested in actual LMC and actual MMC (for each cross-section). Then they need to clearly define what an actual local size is (LMC, MMC, minimum circumscribed circle/parallel lines (external feature), maximum inscribed circle/parallel lines (internal feature), best-fit (Gaussian) circle/parallel lines, etc.). My point is whatever it is, it needs to be very clear.

 

[Tarator]

drawoh,

I only gave an example to start the discussion. If it helps, you can imagine the callout above as 0.1(M). My question would remain the same.

 

[Burunduk]

Tarator,
As you can see the original wording of para. 5.4.1.2 is mentioning diameters and circular elements and deals specifically with features of nominally circular cross-section. You can conclude what should be the basis for the calculation of additional tolerance of derived median plane flatness at MMC - by an extension of principles.