How to calculate wall thickness

[Andera]

In order to calculate X min. do I need to use the profile tolerance from the general note?
Profile|0.5|A|B| is a location control, but runout isn't (as previously discussed below).

What do you think? Which FCF should I use to get the correct min. wall?

https://www.eng-tips.com/viewthread.cfm?qid=430633

https://www.eng-tips.com/viewthread.cfm?qid=460097

That is what I get from Evan's replay:
"gabimot,

Short answer - total runout does not control the radial location of the surface (assuming that I understand what you mean by "radial location").

One problem here, that Y14.5 perpetuates, is the use of terms such as "location" to describe what is being controlled. As we have found in this thread, the terms "radial location", "axial location", and "surface location" may mean different things to different people.

I have found it very useful to describe constraint in terms of the transformations that are allowed and not allowed. So I would describe a Total Runout tolerance zone in the following way:
-The tolerance zone has perfect form.
-The zone cannot rotate relative to the datums
-The zone cannot translate relative to the datums
-The zone can "progress" (that is, offset away from the nominal surface while keeping the distance between the two boundaries constant)

So the Total Runout tolerance zone in your Surface_location example is two coaxial cylinders 0.02 apart, that are perfectly coaxial to datum axis A. The zone cannot translate or rotate relative to datum axis A. But the zone can progress (the diameter of the two coaxial cylinders is variable - they just have to maintain the radial distance of 0.02). This property is what makes the Total Runout zone not control size. This would be controlled by the 7.780 +.025 -.012 size tolerance.
Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca"

 

[chez311]

Andera,

Evan's reply attempts to categorize the different things we might mean when we refer to "location". The standard attempts to claim that runout controls the location of the axis of a feature, however with some clever geometry we can show that the axis of a feature with form deviation actually can deviate more than suggested by the runout tolerance.

To get to the core of your question, since directly toleranced diameters are involved - this in conjunction with the runout tolerance will give you the minimum wall thickness. The general profile tolerance note does not apply to the surfaces in question. Both surfaces include a directly toleranced diameter, which counts as "Otherwise Specified" (UOS = Unless Otherwise Specified). If a profile tolerance were instead applied, it would depend on what datum features were specified as to whether your runout tolerance would have any impact on minimum wall thickness.

Your minimum wall is (31.6-20.13-.1-.06)/2 = 5.655

 

[Andera]

If a profile tolerance were instead applied, it would depend on what datum features were specified as to whether your runout tolerance would have any impact on minimum wall thickness

So, if the minimum wall thickness is between the same inside diameter (Ø20.00-20.13) and Ø36 (basic) feature, then are you saying that runout (circular within 0.06|A|B|) could be involved along with the general profile
profile|0.5|A|B| and also profile|0.2|C|?

 

[Burunduk]

chez311, do you include the 0.06 runout in the calculation to account for form variation of the ID? I suppose that the coaxiality between the ID and datum B should not affect the wall thickness.

 

[chez311]

Andera,

Again, the 0.2 profile to |C| counts as "Otherwise Specified" - the general profile note would still not apply. As I show below, the circular runout still provides a limit on possible form deviation. If it were not applied, maximum form deviation of the inner bore (20 dia) and therefore minimum wall would be dictated by rule #1 (or if the runout tolerance was greater than the form deviation allowed by rule #1).


Burunduk,

Yes, though it seems my initial calculation might have been slightly optimistic on the amount of form variation allowed by circular runout. I think the calculation should instead be:

(31.6-20-.1)/2 - (20.13-20+.06*2)/2 = 5.625

Note that the reason why the .06 is doubled in the second term and not the first is that for the second you're trying to find the maximum radial distance a section of the inner bore at LMC may be from the UAME (dictated by MMC size) allowed by the .06 runout to |A|B| (the reference to |A|B| isn't important in this calculation - it can be considered to any arbitrary reference). This would be the UAME (at MMC) at full .06 runout deviation in one direction and an LMC section at full .06 runout deviation in the opposite direction.

For reference, if rule #1 dictated maximum form deviation, I believe the calculation would be as below:

(31.6-20-.1)/2 - (20.13-20) = 5.620

I could be wrong though, if someone wishes to check my work I welcome it.

 

[Burunduk]

chez311, I haven't thought deeply into it yet, but what seems counter-intuitive to me is that you get a smaller Min. wall thickness with runout than you do with the full form error allowed by rule #1 alone. The maximum allowed for error with rule #1 is larger (0.13 versus 0.06 with runout) so I would assume it would affect the Min. wall thickness to be smaller. I might be wrong.

 

[chez311]

Thanks for catching that, I accidentally added .005 instead of subtracted it for the rule#1 case. The equation is correct, the answer wasn't - I've updated it.