Straightness with no modifier and inferred wall thickness

[brandnew1]

Hi all,

i'm confused, wonder if anyone can give me some guidance.

There is a straightness callout for an inner diameter 1.303/1.305 of .010 (see attached pdf). i'm uncertain how to calculate this properly: an external feature (ID will act as an external feature) or internal (since its an ID) to determine the virtual condition. If V.C. applies this will allow me to determine what the wall thickness can range between.

The ID of 1.303/1.305 is within an external diameter that is 1.495 +/- .010 but this ID has the .010 straightness callout, so my first question is:

A) Does a .010 straightness callout mean a +/- .005 boundary about the axis or does it mean a .010 max measured boundary even if it is only .001 on one side but .009 on the other.
B) Does virtual condition apply here given that i don't know the size the mating part will be nor is there a material modifier on the straightness callout?
C) There is a note on drawing indicating: runout on all diameters on a common centerline to be within .005 FIM. - Does this restrict this .010 straightness?

thank you

 

[CheckerHater]

Condition on your drawing is known as "straightness at RFS" and explained on the picture.

Yes, runout may restrict straightness, unfortunately, with "drawing note" runout it is not always clear, "common centerline of what?"

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[brandnew1]

Thank you CheckerHater for the attachment.

Based on the attachment (Fig. 6-2) saying, "the derived median line of the feature's actual local size must lie within a cylindrical tolerance zone". So then this is not a tolerance of a plus or minus .005 but total distance is .010 (see attached pdf which shows different possibilities for the location but so long as the median line is within the .010 the part meets the specification)

So that brings me to the question about wall thickness and determining the virtual condition based on this inner diameter. i don't know what the gage size will be so all i can base this on is just the ID.

For an external feature (pin/boss)- Your largest size (MMC) + your geometric tolerance
For an internal feature (hole) – Your smallest size (MMC) – your geometric tolerance.

Is this a fair assessment:
Based on a straightness of .010 the wall thickness of the ID (1.303/1.305) to the OD (1.485/1.505) can range between .085 to .106 (math is shown on the pdf). Again the straightness according to the blueprint applies only to the ID.
A side question. This note about runout of all diameters to be on a common center line of .005, does this go against this straightness of .010?

thank you again

 

[Belanger]

Your Question C:

"There is a note on drawing indicating: runout on all diameters on a common centerline to be within .005 FIM. - Does this restrict this .010 straightness?"

If that's what the note says then it's not really any help. Two reasons: First, a runout tolerance needs a datum. So when checking for .005 FIM, which centerline should it be rotating around? (Because in the real world they all won't be aligned with one another.)
Second reason: The word runout by itself is somewhat ambiguous, because there are two kinds of runout -- circular runout and total runout.

So you asked if runout would control straightness. Circular runout = no. Total runout = yes.

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

 

[brandnew1]

Thanks for the response Belanger; this is what the customers inspection indicated about that note:

General notes on drawings stating that all diameters on a common centerline must runout .005 FIM is usually looking for coaxiality or location to one another. Straightness would be held per GD&T Rule #1. We would ‘choose’ a diameter then insure that all others (on a common centerline) runout within the stated tolerance.

i read, "f a straightness control is applied to a feature of size, Rule #1 is overridden." (website: Straightness

Again, if i'm trying to evaluate this part based on the blueprint that calls for a straightness of .010 diameter for the inner Diameter only (Regardless of feature size) where the OD has a plus or minus .010 tolerance, if this part were to be taking up the full .010 tolerance (say one side is plus .006 while the other side is minus .004) how then can this part even meet this .005 runout?
Also i'm wondering if the virtual condition regarding wall thickness actually has a greater range due to this straightness callout.

Am i viewing straightness incorrectly here...please see previous attachments that show my rendition of how i'm viewing straightness.

thank you again

 

[mkcski]

brandnew1:

Be aware: you are using dial indicator readings on the SURFACE- incorrectly called runout readings by some - to determine the straightness of the AXIS. This is not a Runout Control inspection so forget FIM readings. This will entail a complex analysis of dial indicator readings from the surface to "find" where the axis is at each cross-section and then compare that to all other cross sections to find the derived medial line.

One method we used was to place dial indicators 180-deg opposite each other and traverse them simultaneously along the cylinder and record readings from both indicators at some predetermined distance - like say, every half inch. Rotate the part 90-deg and take another set of readings along the part. You will have two sets of readings at each cross section - and X offset and a Y offset. The difference of any pair of opposing readings is the radial distance from the axis of rotation to the center of the part at that cross section. Using the Pythagorean formula you can determine distance and direction of the axis at that cross section. None of the distances can be more then .005-inch from the axis of rotation.

Hope this helps.

 

[mkcski]

brandnew1:

Sorry I forgot to add something. Because there are no datums with form control there is no motion control (constraint). So, if you get out of tolerance reading you can move the part to establish a new axis-of-rotation and start the procedure over again. A spread sheet could be designed to automate the analysis.

 

[pmarc]

brandnew1,
The thing is that per the current print, that is per the print with this ambiguous runout note [as explained by J-P (Belanger)], it is impossible to correctly calculate min and max wall thickness. It would require too much guessing and assumptions.

In your attachment you did not take any runout tolerance into consideration, meaning the numbers you got are most likely incorrect. Looks like you assumed that straightness tolerance controls locational relationship (axis offset) between both diameters. That is not true.

 

[pmarc]

J-P,

You said: "So you asked if runout would control straightness. Circular runout = no. Total runout = yes."

While this is true in case of straightness control of line elements of actual surface of a cylindrical feature, when it comes to controlling straightness of cylinder's derived median line, no matter which type of runout control will be used, it will always keep straightness of DML within that runout tolerance. Do you agree?

 

[brandnew1]

Thank you all for your response. So it sounds like getting wall thickness based on blueprint isn't really possible (but this all started with customer asking about wall thickness')

MKCSKI, thank you for your comment, "Because there are no datums with form control there is no motion control (constraint)". i did not know this but there are form controls that tie to the inner diameter but not the Inner Diameter, in of itself, with the straightness callout that i have been asking about (attached pdf). So based on this comment, if the customer just uses circular runout then theoretically even as you move along the diameter with the straightness rising to .010 max it still could pass. But if they use total runout then this wouldn't work, right, because you're incorporating the entire surface simultaneously.

PMARC,

i updated the attachment with a partial view (with modifications) of the blueprint which shows the straightness to be .010 diameter spanning the length of the part that has the diagonal lines. The only control besides straightness is the side note of .005 FIM but the other runouts are all called to be circular runout. Now outside of calculating wall thickness, just general view of straightness in the ID is the attached pdf reflecting the concept correctly (where you can see that wall thickness will vary along the path)

General question, if one wanted to calculate wall thickness on a blueprint, what information would be minimally required?
If this side note actually referred to total runout, than you really don't have a straightness of .010 (if i'm interpreting this correctly)? - side note: SQE had told me that it doesn't really matter if the part 'bows' so long as straightness is met.

Again thank all and hopefully this will be my last set of questions.

thank you

 

[pmarc]

Now outside of calculating wall thickness, just general view of straightness in the ID is the attached pdf reflecting the concept correctly (where you can see that wall thickness will vary along the path)

While it is true that straightness error may result in wall thickness variation along the length of the tube, the image you found online does not explain the straightness error concept correctly. First off, per Y14.5 straightness tolerance does not control form of cylinder's axis (because by definition axis is a perfectly straight line derived from actual unrelated mating envelope of the cylinder), but form of imperfect derived median line. Secondly, regardless of how we will call the centerline shown on that image, it is perfectly straight, meaning it has no straighthness error.

General question, if one wanted to calculate wall thickness on a blueprint, what information would be minimally required?

If I were to calculate wall thickness, I would only have to exactly know relative to which diameter both diameters in question - ID and OD - are controlled by the runout tolerance from the note. Perhaps OD is controlled relative to ID, or vice versa. At the moment, I am not able to tell.

If this side note actually referred to total runout, than you really don't have a straightness of .010 (if i'm interpreting this correctly)?

That is true provided that the ID is not a datum feature. The straightness tolerance of the derived median line will then be limited to .005. And actually the same will be true if the note meant circular runout, as I tried to explained in my post addressed to J-P. But if the ID was the datum feature, then we would still have .010 of straightness tolerance available on the ID. That is why I said above it all boils down to knowing which feature is the datum feature.

 

[mkcski]

brandnew1:

Regarding you General question:

Pmarcs comment "Looks like you assumed that straightness tolerance controls locational relationship (axis offset) between both diameters. That is not true." appears to be the root of your issue. There is nothing on the drawing that controls the coaxial position relationship (surfaces of revolution on the same axis) between the ID and the OD and there is not default tolerance to be applied. So wall thickness calculations are not possible.

You have been exposed to another example of one of the great misunderstanding of drawing interpretation: the drawing shows the perfect part but few drawings have GDT to define how much size, form, orientation, and location imperfection is allowed. The consequence of not knowing how "bad" the part can be, is you make the part as perfect as possible. This adds unnecessary cost (in many ways) and lowers your competitive position and profits.

For you to interpret the drawing to understand the requirements, GDT must be applied properly. In your example it appears a feature needs to be a specified as a datum axis (the origin of measurement that constrains motion) and a FCF applied to the other features that are to positioned coaxial to the datum axis. More specifically if the primary design intent is to control wall thickness, position at LMC concepts should be used to communicate the geometry of the product. Note that wall thickness it the thinnest when the ID and OD are at LMC size.

To farther your knowledge, it might be beneficial to review inner and outer boundary condition concepts in Y14.5.

Also: if the the tube is commercially available material then the ASTM standard or other industry standards/practices for the tube most likely define the tolerance on wall thickness variation (when you buy the tube).

 

[Belanger]

pmarc -- I agree. I tossed out a quick post and didn't distinguish between surface straightness and FOS straightness.

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