Vary wall thickness relative to tube OD

[1asdf]

I wish to specify the OD of a tube with a tolerance, the wall thickness of the tube with a tolerance, but add +tolerance to the wall as the OD varies from LMC to MMC.

This large tube fits inside a tube, and another tube fits inside the tube in question.

I have reviewed Y14.5, but I am still stuck. Input is appreciated.

 

[PeterStock]

I would suggest you specify the OD with its tolerance and the ID with its required tolerance. The thickness would become reference if you even put it on the drawing. If you are using a stock size tube, you should note the actual tolerances specified by the manufacturer. If you check the tolerances and you are outside the range you need for your application, you may need to use a larger size and thickness and machine down, or get fancy with your dimensioning. If this is the case what you might do (just of the top of my head, I have not seen this), I would give the dimensions as specified by the manufacturer as reference. Then I would give your required OD and ID dimensions with a reference to a note that machining to achieve the tolerance is permissible.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[Belanger]

Also keep in mind that the coaxiality (position/concentricity/whatever) of the two diameters will impact the wall thickness. I think this tolerance needs to be addressed to fully answer the question.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[1asdf]

I appreciate the input, thanks.

I wish I could let the wall thickness go and use only the ID and OD tolerances, but I need to tightly control the wall thickness as well.

The large diameter tube is custom made, it is extruded and then expanded to meet the spec. As such, concentricity is basically not an issue in this case.

The only options I envision so far:
A. Three overlapping tolerances (very ugly to achieve, probably flat-out wrong)
B. Specify only lower limit for the ID and upper limit only for the OD (is that even allowed?).
C. A wall thickness +tolerance bonus chart or table in the notes.
D. Some GD&T symbology that still escapes me.

Any further input is appreciated.

 

[dgallup]

I would use a toleranced OD, a min ID and a min wall thickness.

 

[1asdf]

I need to control the upper limit on the wall thickness as well.

I would actually like to avoid any ID spec and rely on OD with tolerance as well as wall thickness with tolerance. This is because the OD and wall thickness are easy to measure.

 

[MintJulep]

I'm struggling with the concepts of LMC and MMC given your known production method.

Since you're not using a process that removes material I don't see how they could apply.

I'm thinking profiles for both inner and outer surfaces should fully constrain everything.

 

[1asdf]

Sorry if I am not explaining well. I use LMC and MMC in the context of GD&T, but I could have just as well stated from lower tolerance to upper tolerance.

I need a lower ID constraint, an upper OD constraint, and +/- constraints on the wall thickness. I can add + bonus tolerance to the wall thickness as the OD goes form lower tolernace to upper tolerance. It is possible to meet both the ID/OD constraints and have the wall thickness too thin or too thick.

I have never used profile GD&T, but my materials seem to imply that I could not use it with a modifier for bonus tolerance.

 

[MintJulep]

Sometimes just spelling it out in a note on the drawing is best.

"Part must conform to ID and OD tolerances and at the same time at no point may the wall thickness be thicker than X or thinner than Y."

Maybe someone will be able to conjure up a set of GD&T symbols for it, but you'll get a phone call "What the heck are you asking for?" and you'll have to explain it anyway.

 

[1asdf]

MJ/Everyone,

I am reassured that I am not missing an ANSI method of achieving this, thanks.

Hoepfully they can also contend with an equation to calculate the added wall bonus tolerance as it relates to the OD.

 

[KENAT]

Not sure that whether or not to specify MMC & LMC really varies based on manufacturing method. It's a way to express geometric constraints not manufacturing method.

I thought I'd seen LMC used in similar applications but the details don't spring to mind right now.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[flash3780]

If I understand what you're trying to do properly, I believe that an appropriate method of tolerancing your cylinder would be to call the OD a datum (held to a +/- tolerance) and use a positional tolerance on the ID with an LMC modifier relative to .

This will allow the position of the inside hole to float as the hole moves away from LMC, which ensures that you maintain your minimum wall thickness.

You could also call the ID a datum and position the OD relative to it with an LMC modifier. Tomato-tomahto.

 

[dtmbiz]

Maybe I am over simplyfing or you are over thinking this, however it seems as if you specify the O.D. with appropriate tolerance, control the wall with the appropriate tolerance and use the straightness callout (with diameter in tol block of FCF) this would easily satisfy your fit requirements. Why is TOP (Tolerance of Position) even being discussed as required in this case? The I.D "location" is a result of wall thickness.

BTW
Concentricity is used in high speed rotating part applications.

 

[AliThePro]

You can specify the tolerances for both outer and inner diameter and a tolerance for wall thickness as long as there are ranges that all three are valid. The tolerance for thickness will be a limiting value for one diameter with respect to other. This is not a contradiction or over constraining.

 

[1asdf]

dtmbiz,

The allowable wall thickness varies with the OD. Maximum thickness is desirable for manufacturability, but is limited by the ID and OD constraints.

AliThePro,

I may end up taking the suggested approach, it is reassuring that it is not "wrong" to specify all three ranges.

In the mean time, I thought it best to specify the OD with tolerance, then calculate the wall thickness upper tolerance with a simple formula based on the measured OD. This would leave the ID open (no need to measure), though effectively constrained.

 

[dtmbiz]

My 2 cents, Ali...

Double dimensioning:

A feature shall not be located / defined by more than one toleranced dimension in any one direction"

Reference dimesion:

"A dimesion shall be enclosed in parentheses () when it is (a) repeated on the same drawing, (b) specified on a subordinate document, (c) an accumulation of other dimensions, or (d) shown for informational purposes."

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This is basic drafting / dimensioning regarding "double dimensioning" and when to use a reference dimension. I would strongly disagree that tolerancing all 3 features(ID, OD, wall) isn't "wrong". It definitely does not adhere to good basic drafting. I would argue that it is "wrong".

Always keep in mind that normally, many people will be involved with interpreting the drawing.

I would agree with your last posting to dimension OD and wall. Easy enough to interpret.

 

[PeterStock]

Think of it this way, the tolerance on the thickness serves to locate the ID relative to the OD.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net