Does Dia Tolerance Control Straightness 1

[swertel]

I have a tube with an inner diameter dimension consisting of a +/-.005 tolerance.
Does that tolerance also control the straightness of the tube?

From my interpretation, if the ID was a datum, then the theoretical datum surface would be a cylinder that fits within the tube. That implies straightness must be maintained. But, what I don't remember is if the same applies to the physical surfaces. I'm afraid that I will end up with a bent tube whose diameters at random places fall within the size tolerance, but will not functionally work with its telescoping mating tube.

(I'd love to change the print to include a straightness callout just to be sure, but this is a build-to-print job. I don't control the design.)

--Scott
www.aerornd.com

 

[chez311]

swertel,

As long as we are assuming (1) that your print is to ASME Y14.5-2009 and (2) there is no Independency symbol called out then any FOS is by default held to Rule #1. Read 2.7.1 for a full definition of rule #1 but the gist is that under this rule the surface of a regular FOS cannot extend beyond a boundary of perfect form at MMC, the allowable local variation of form is equal to the departure in local size from MMC toward LMC, and that if a geometric tolerance is specified at LMC perfect form is then required at LMC. See also Fig. 2-6 for a simplified example of this in effect.

The simple answer to your question is as long as (1) and (2) are true then yes - the variation in straightness (form) is by default controlled within your limits of size.

Also a note on your mention of datum features - I don't THINK the application as a datum feature or not puts any extra constraints on this (implied or otherwise) over rule #1. For example if the Independency symbol were applied to a feature with no other form controls, referencing it as a datum feature in a DRF I do not believe would apply a default straightness control. Someone can chime in and correct me however if that statement is incorrect.

*Edit: grammar

 

[swertel]

Thanks, chez311, re-reading 2.7.2 helped clear up my confusion as well.

We deal with custom made tubes as well as stock tubing. In one argument (stock tubing), straightness is not held based on the diameter tolerance per 2.7.2. In another argument (custom composite tubing), straightness is held based on diameter tolerance per 2.7.1. Depending on what material the person I'm arguing with deals with more often, that's probably the perspective they are going to argue over.

--Scott
www.aerornd.com

 

[TheTick]

If straighteners is that important, you should spec it. Total runout, perhaps?

 

[3DDave]

"Stock" tubing should be controlled by the straightness tolerance within the specification that describes it. It will often be found in documents from ASTM. The same applies for any number of other characteristics, such as material composition or strength. The "stock" diameter tolerance would not normally appear on the drawing at all.

 

[chez311]

3DDave is correct, I was under the assumption you were referencing a finished dimension - I should have mentioned the caveat about stock sizes.

If this is a dimension on the raw stock tubing, then as he said you are at the mercy of whatever standard that stock is held to. If this is a dimension on custom tubing you may have some flexibility to hold this to a different dimension/straightness tolerance - but this may be highly supplier dependent and you may get some pushback on this, as you noted.

You said this is not a design that you own so I'm not sure how much control you have over this but if this is a critical dimension and the stock tubing standards do not hold the size/straightness tight enough to your requirements it may be worth considering an additional finishing operation (ie: turning/boring) paired with an appropriately sized stock (if material needs to be removed by the finishing operation the stock size may have to change).

 

[swertel]

A few points of clarification.

1) This is not stock tubing. This is a custom composite (wound) tube, so the tolerance dimension does apply. Paragraph 2.7.2 does not apply.

2) I can't spec the straightness because I don't control the drawing. I can propose a change to the document owner, but I already know the where that effort will lead.

3) The argument between me, another engineer, the supplier, and quality/inspection (and so many more it's ridiculous) is with respect to whether or not the diameter tolerance implies a straightness over the length of the tube. I wrote here first before re-reading the spec. Thanks to chez311 for finding the applicable paragraphs for me.

Based on paragraphs 2.7.1 and 2.7.2 I can now see why each individual in the discussion regarding straightness of the tube thinks they are right. We have some people in the conversation that typically deal with stock tubing and pipe and others who don't. Now that I've been reminded of the difference, I can go back into the discussion with the proof necessary to get this project moving again.

--Scott
www.aerornd.com

 

[3DDave]

I was just pointing this out for future discussions with the "stock" tubing guys. It's a separate area with its own rules and those you are discussing this with should know the difference already. It gets even worse when dealing with extruded aluminum tubing as it is partly covered by Aluminum Association and, per their spec, by any other negotiation with the extruder.

Straightness is typically a refinement of the size tolerance. Or a substitute when the envelope principle is excluded.

Any reason straightness is an issue? Is there a reason that you could not create a paragraph in the procurement document to add the desired straightness as long as the part also meets the original drawing? I would gather there's already a named price and no harm should be done by making a product with less variation than the drawing allows. Honestly the latter is done all the time; just not put in writing. I've seen it happen where a part is sourced from a new place and the parts don't work even though they are compliant, where the previous maker had no problems - due to phone calls and just taking it upon themselves to tighten tolerance allocations.

 

[djhurayt]

But, what if I have a piece of tube that in every location the diameter is spot on, but the tube is bent into an "S" shape, that definitely not straight . . .

???

 

[3DDave]

djhurayt - if the drawing uses asme y14.5 it is rejected.

 

[Kedu]

if the drawing uses asme y14.5 it is rejected

Maybe the drawing has a note such as " Perfect form at MMC is not required"

Just saying......

 

[chez311]

djhurayt,

You are right - that would not be straight. Whether its rejected or not depends upon what you mean by the dimension being "spot-on" (MMC? LMC? nominal?), whether there is a straightness tolerance applied, whether the dimension is a "stock" dimension or subject to rule #1, or if the Independency symbol is specified (and if not at MMC, not a stock dimension, and no independency spplied - how much deviation is present). The rules clearly state how much deviation is allowed in each case, not what shape is allowed.

If we are to assume that what you mean is a part at MMC, with only a size dimension and no straightness tolerance applied, and rule #1 is in effect (NOT a stock dimension and NO independency symbol) - then 3DDave is correct. Rule #1 states that no form deviation is allowed at MMC and this part would be rejected. Other cases are slightly more complex.

I would suggest reading through the applicable parts of Y14.5-2009 regarding rule #1 and straightness/flatness.

 

[3DDave]

kedu - the only spec was the s-shape and diameter. Given those two facts there is no value in further speculation beyond the applicable standard, which covers the case you mention, but was not part of the problem statement.

No need to be clever.

I thought about this more:

Maybe the drawing has a restraint note because the tube is corrugated.

 

[swertel]

A little more background information. I didn't realize this topic would be so interesting.

The discussion started because the composite tube manufacturer wanted us to give him a straightness value for his mandrel. This is a very long tube and the mandrel will deflect under its own weight. The supplier, essentially, wants to know how much deflection in his mandrel is allowable while still meeting the requirements of the end item drawing.

Of course, during this phone conference with the supplier, everyone in the room had their own opinions and we sat arguing about it for several minutes because "we had to solve this problem immediately." Well, we ended the meeting with no solution. I took the action item to review ASME Y14.5. I posted a question here first, expecting it to take a while to get a response, and was going to dig into the spec. I got a response via eng-tips before I could get passed the introduction. You guys are awesome.

--Scott
www.aerornd.com

 

[3DDave]

Orient the mandrel vertically? Add a horizontal torque load perpendicular to the axis to the ends of the mandrel to offset the gravity induced deflection?

 

[djhurayt]

When I posted:
But, what if I have a piece of tube that in every location the diameter is spot on, but the tube is bent into an "S" shape, that definitely not straight . . .

What I was getting at is:
1) the OP asked "Does dia tolerance control straightness"
2) the general consensus seem to be YES, via Rule #1 and other conditions such as LMC/MMC

And I think this applies, if what we are talking about is a piece of tube that is generally "straight", as in a piece of pipe taken off the rack. So, I think what I was trying to saying with my above post, is that while in the OPs situation this is indeed true via Rule #1 ..., diameter tolerance does not ALWAYS imply a straightness requirement.

True: Yes/No

Edit: and let me preface this with "I know only know enough about the intricate details of GD&T to be very dangerous"

 

[chez311]

swertel,

Always glad to help clarify things - I find with many topics with GDnT what appears to be a simple problem on the surface can be deceptively complex once you really sink your teeth into it. This is luckily one of the more straightforward concepts for the most part. Hopefully you're now armed with the information you need to make your case!

 

[swertel]

djhurayt, so you're referring to having bent tube, like an automotive exhaust or similar?

In that case, the designer would want a consistent diameter throughout the length of the tube/pipe, but it is definitely not straight. I think Rule #1 would still apply to any segment that's implied to be straight. I also assume that the drawing would define the bend radii and locations, so straightness would be "overruled" in those areas by the other dimensions.

--Scott
www.aerornd.com

 

[swertel]

3DDave,

The composite tube is made out of wound tow fiberglass filament. It's a horizontal winding machine; can't change the orientation.

I think they may have the option to include a support on the opposite end of the mandrel so it's not cantilevered, but there is still the potential for deflection at the center. We're talking a "straightness" of .0025 over ~50 inches at LMC. I don't know the mandrel material.

--Scott
www.aerornd.com

 

[djhurayt]

...bend radii and locations, so straightness would be "overruled"...

That makes sense, thanks swertel