Position vs. Concentricity vs. Runout - Measurement of a Tube

M

mitchkerbs

I have a drawing from a customer that specifies the position of the outer diameter of a tube be 0.020 with the inner diameter of the tube being the datum. Customer has stated the the OD of the tube "needs to be concentric to the id of the tube with .010. Rest the tube on 2 “v” blocks near each end and indicate the id while rolling the tube."
I am puzzled by the drawing calling out position, the customer referencing concentrcity, and then telling me to measure runout.

Can someone help me understand why my indicator reading can only be half of the position callout?

Thanks a million!
 

Michael_M

Trusted Information Resource
This seems to me to be one of those call outs where the designer did/does not know exactly what they wanted. True position, concentric, and run-out can be used interchangeably in some cases depending on exactly what they want.

I believe the reason they are cutting your tolerance in 1/2 is based on the common rule for true position, Square root of (error in 'x' squared + error in 'y' squared). Then multiply by 2 since most of the time you are dealing in radius not diameter when working in this formula.

I would contact the customer or just reduce the run-out to .010.
 
Hullo Mitch,

Rest the tube on 2 ?v? blocks near each end and indicate the id while rolling the tube.
Which would mean using the OD as datum instead of vice versa... Just to be sure: Would it be possible to show us the drawing requirement? I feel a bit puzzled myself right now, but I agree with Michael about the tolerance cut in half. :)

/Claes
 
T

True Position

Your indicator will read a (otherwise perfect) difference of 0.010 absolute position somewhere at +0.010 and -0.010 rotated 180 degrees.
 

Reluctance

Involved In Discussions
I agree with Michael_M on the customer possibly not understanding what they are looking for on the print.

The true position tolerance called out could possibly be done using a dial indicator but not with very good repeatability. First of all they have switched the datum as Claes has already pointed out. Second, the readings from the dial indicator using the setup the customer describes are telling you more about wall thickness which is more like a profile tolerance than true position tolerance. Profile tolerance can give you true position, but it is more restrictive. For example, in manufacturing your part could be made up of two ovals with their major axis at 90? to each other and still be within the true position tolerance. However they would fail at this measurement method that is giving you profile tolerance.

Using the indicator you could try to account for this by recording the indicator reading at set points around the circumference of the part and calculate true position with the addition of an outside diameter measurement of the part at the respective points measured using the indicator. Or you could just have this done automatically on a CMM.

Measuring runout requires keeping your part stable with respect to an axis of rotation. Rotating a cylinder on v-blocks does not keep the axis of rotation in the same position unless the cylinder is perfectly round. So you really are not measuring runout using the recommended setup either.

It is probably time to sit down with the customer and determine what it is they are really looking for from your product. If they want to ensure control of the size of the outside diameter with respect to the inside diameter they can continue to use the true position tolerance, but they need to find a more appropriate measurement method. Another alternative would be to change the tolerance to a concentricity tolerance, but again the measurement method would need to be changed to suit the tolerance. If they change to a profile tolerance the recommended measurement method could be potentially be kept but there is some cosine error that would need to be accounted for.

Isn't GD&T fun? :D
 
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