Straightness

[Andera]

Straightness on a cylindrical feature

I am a little confused with the following statement from Section 5 Tolerance of Form ASME Y14.5-2009
5.4.1.1. “Since the limits of size must be respected, the full straightness tolerance may not be available for opposite elements in the case of waisting or barreling of the surface. See Fig. 5-1.”

Why fig 5-1 shows the form error as being available for the opposite elements (top and bottom surfaces in figures 5-1 (b) and (c) shows the same curvature)

Please, could someone clarify my misunderstanding?


And second part of my questions about form tolerances: how to understand “ as each actual local size departs from MMC, an increase in the local diameter of tolerance cylinder is allowed which is equal to the amount of such departure” statement found in fig 5-3?

Does it mean that the tolerance zone won’t be cylindrical? If no, why not?

 

[Andera]

Let me clarify a little my first question: what full straightness may not be available for the opposite elements in Fig 5-1/2009?
0.11 or 0.02?

 

[Belanger]

In Figs. 5-1 (b) and (c), the size tolerance allows the part's diameter to be anywhere from 16.00 to 15.89. So it's OK to have the full straightness error on top and bottom elements curve in opposite directions because even with "waisting" or "barreling" that only impacts the overall diameter measurement by 0.04.
If the size tolerance were revised to be 16.00 max and 15.98 min, then waisting or barreling might not be allowed; the elements would have to curve in the same direction to still be within spec.
For your follow-up question: the straightness they refer to is the geometric tolerance: the straightness symbol and its accompanying tolerance.

 

[chez311]

JP beat me to the punch - I'd just like to add the below

If the size tolerance were revised to be 16.00 max and 15.98 min, then waisting or barreling might not be allowed; the elements would have to curve in the same direction to still be within spec.

I don't know if it has to be said, but just for clarity I would add that waisting or barreling might not be allowed to the full .02 straightness tolerance. You could have .01 deviation (Edit: in the manner shown by Fig 5-1 b and c - waisting/barreling) on either side (Edit: ie - half the tolerance zone on either side) and still be within spec. But yes, if the deviation was to utilize the entire .02 tolerance zone it would have to be in the same direction (think of the bent shape in Fig 5-1a) to be within spec.

 

[Sem_D220]

Regarding your question about the straightness of surface elements:
Imagine the cylinder from fig. 5-1 produced close to the upper limit of its' size tolerance: let's say the diameter varies in the range of 15.97 - 16. In this case, anywhere on the feature, the radial distance from the surface to the limiting boundary of the Maximum Material Condition is not greater than 0.015. Therefore the maximum barreling (per illustration C) allowed by the size limits is only 0.015, while it could be 0.02 (per the straightness tolerance) if the feature was produced smaller in size at its' ends.
Analogically if the feature was produced near the lower size limit, it is possible it would be more restricted in waisting than the straightness tolerance alone suggests.

As for fig. 5-3, this one is a little bit more difficult: ignore for a minute the illustrated bent cylinder of constant size and imagine instead a cylinder with a wavy surface, and it's diameter increasing and decreasing along the feature. The straightness of the derived median line is dependent on all the local center points, and it will be restricted in the following way: where the local diameter is smaller - the center point of that section area has "more room" for various locations, it may dislocate as much as it doesn't cause a "bump" on the surface of the cylinder that will prevent the entire feature from fitting into a 16.04 diameter boundary. Where the local size is exactly 16, the center point of that local section area is only permitted to be within a 0.04 diameter local tolerance area. So, to your question - yes, there might be a situation where the tolerance zone for the median line is not cylindrical. However, if the cylinder is of a constant diameter cross-section, and only bent like illustrated in the figure, then the tolerance zone is cylindrical and its' diameter is dependent on how bent the cylinder is allowed to be by the combination of its' produced size and the straightness tolerance value.

 

[Sem_D220]

If the size tolerance were revised to be 16.00 max and 15.98 min

Please remember that ASME Y14.5 says that for a cylindrical feature with a surface straightness control, the straightness tolerance must be less than the size tolerance. Since the straightness tolerance is 0.02, the specification of 16.00 MAX 15.98 MIN shouldn't be on the drawing in the first place.

 

[Belanger]

True, Sem_D220. I was just trying to fire off a quick answer

 

[Sem_D220]

Belanger, that's no big deal.

Actually, there is an inaccuracy in my explanation too - when I said: "let's say the diameter varies in the range of 15.97 - 16. ... the radial distance from the surface to the limiting boundary of the Maximum Material Condition is not greater than 0.015." This calculation is not true for all cases and I should have said it. It is only relevant to the specific cases where the derived median line is very close to being perfectly straight and there is this barreling or waisting phenomenon exactly like shown in fig 5-1 illustrations b and c. If the feature was produced bent like in illustration a, the radial distance from the surface to the MMC boundary can equal the diametrical difference between the MMC boundary and the produced size.

 

[Andera]

Chez311, Belanger, Sem D220,
Thank you for the feedback.

Are you saying that if straightness tolerance would have been 0.06 than there is no way to get waisting or barreling and the part to be per the print (fig 5.1) with maximum straightness allowed (of 0.06 per my hypothetical case).

 

[Sem_D220]

Andera,
According to your new case, we keep the size limits as 15.89-16 and change the straightness to 0.06. Let's see what can happen for the "barreling" scenario: if we assume for "barreling" that the median line remains (almost) perfectly straight, you have only 0.055 surface straightness deviation available for the feature to still remain within its' limits of size. So once again you can't utilize the whole straightness tolerance of 0.06.