Total runout or position + perpendicularity + cylindricity

[sharky98]

Hi all,

First posting here. I've been asked to review tolerancing of a part. It uses concentricity and we have some issues. We are looking at changing this (and also adopt ASME Y14.5-2018). We are not using a lot of GD&T in our parts (that's another story), so we do not have lot of experience with it (I do have some from my previous job, but nothing expert or even advanced level, maybe beginner/intermediate).

I can't really show the drawing or disclose the part design (you know the drill I'm sure! ), I'll try my best with vaguely describing words. So, the part is basically a tube with multiples inner diameters, but has no outside diameter that is safe enough to be used as a datum (being a casting left as-is in the final part). As such, my A and B datum are the end face and with some holes (fully constraint the 6 DOF). From there I have a requirements of some of the inner diameters to be coaxial together. I was thinking of using one of them with a mix of position, perpendicularity and cylindricity with my datum A and B, calling my C datum and then from there having my other coaxial features using runout about that datum C.

Where I am unsure if that although a total runout will control position, perpendicularity and cylindricity, it seems to me that it constraint it very strictly (a cylindrical enveloppe around the theorial perfect shape as I understand it?). That mean I'll have to go with my thighter requirements (let's say 0.001"). However, in my design requirements, the coaxiallity is more focussed on the axis than the cylinder faces. As such I might want to have tighter tolerances on the position and perpendicularity of the axis (something like position DIA 0.002" with perpendicularity DIA 0.001"), while the cylindricity is less of an issue, as the mating part is less critical on it's mating diameter (the cylindrical enveloppe could be something like 0.005" around the perfect theorical DIA).

In that kind of situation, is the combo (position + perpendicularity + cylindricity) could be a solution or you would still be using the total runout?

As a side note, I was also looking at using profile of surface instead of previously describe solution, would that be a good idea (it seems to be that it mean the same thing as the total runout...)? It seems that there is many way to obtain (somewhat) the same intended result, but not having expert-level knowledge, I fear that I might be overlooking the implication in manufacturing and quality control of each methods.

Regards

 

[drawoh]

sharky98,

Are you designing this part, or fabricating it? If you are designing, consider adding some datum target features to your casting. That way, the foundry and the machine shop will be working off the same features.

I have not used run-out on anything. Run-out implies that your part rotates about something. A three[‑]point datum[ ]A feature, a two[‑]point datum[ ]B feature and a one[‑]point datum[ ]C feature do not provide a good rotation point. There is nothing to stop you from selecting a fixturable round feature, locating from A, B and C by positional tolerances, and designating it as datum[ ]D.

If you are managing a casting, you can do it in three steps.
[ol]
[li]Cast the part. Inspect the casting.[/li]
[li]Machine the part to create datum features. Inspect the part to verify the datum features are in the right place WRT the csating.[/li]
[li]Machine the part to create everything else. Inspect the part to your machined datum features.[/li]
[/ol]

Datum targets are the better way.

--
JHG

 

[Burunduk]

You are saying that your company considers adopting ASME Y14.5 which means you don't work according to it yet. So considering this it's hard to provide you any useful advice because it's impossible to know which conventions from the standard are relevant to how the tolerances are perceived in your workplace and which not. It doesn't help that we don't know anything about the function of the part either, although I understand your need for being confidential. One thing that comes to mind is that if you worked per ASME Y14.5, I would recommend considering controlling the coaxial holes for position and perpendicularity only, without specifying cylindricity, depending on whether the form control imposed by the size tolerance and Rule #1 is sufficient for your needs (from your description it sounds that it might be but I don't know what the size tolerance for the holes is). That would indeed be less restrictive than total runout which controls coaxiality, orientation, and form all within one tolerance value. To your side note, the difference between total runout and profile is that profile normally would also control the size of the feature while total runout does not. It would be equivalent though with the dynamic profile modifier according to ASME Y14.5-2018.
Hope this helps somewhat.

 

[sharky98]

Thanks for the help. It got me farther in my thinking. I was able to make a dummy part that has similar feature than mine. It is a tube-like shape. The outter surface is not usable as a datum (not machined, casted). The goal is to constraint coaxially three portion of the inner cylinder (the 10, 8 and 9 diameters). The coaxiallity and perpendicularity must be tight, while the position of the whole can be slack (see first picture). I'm also trying to minimize the measuring error when quality control will check it (as one axis will be far away from the initial datum A), while keeping the design intent.

Design intent

Top View (I am not certain about datum B if having a 0 MMC is the right choice...)

Idea
Here I was thinking of getting the first hole as a datum C and then getting a position of both second and third hole in regards to that datum C. However, I think doing it this way would mean in fact having the purple cylinder (in the design intent) parallel to the green cylinder. Am I correct on this interpretation? If so, it isn't my design intent.

Idea 2
Here I think I capture the design intent. But I am not sure if I did something "illegal" according to Y14.5-2018 or should have used composite segment instead of single segment for the position. As I said, I'm far from being expert level.

Regards!

 

[Burunduk]

There is nothing illegal at any of the two ideas but idea no. 1 doesn't reflect the requirement that each axis must be perpendicular to A within .001" (that you stated in the first image). Only 1 of the 3 feature axes is controlled for that - the unrelated actual mating envelope axis of datum feature C.

 

[Tmoose]

Are there any bores for ball/roller bearings ?