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How to determine which feature defines a centerline?

Piper3T

Plastic
Joined
May 9, 2016
Hello,

My company makes several blocks that, functionally, are intended to be symmetric.
And every print has a "CL" centerline drawn.

My question is related to the fact that several different features are intended to be symmetric. Outside walls, inside walls, holes, etc. Which feature do I use to define the centerline?

The tolerance stackup allows for non-symmetry, and there is nothing on the print that actually calls for symmetry, except the centerline and several dimensions called from it. I've been told by both engineering and assembly the part functions best when symmetric, but they allow some tolerance in every feature for manufacturing. The problem I get back to is that since there are so many features, so much tolerance, which feature defines the centerline?

I ask because several feature locations are called off of centerline. But those dimensions are only called "once".

Centerline_Question.jpg

For example, a side wall from centerline is .500. That dimension does not call out 2X, nor is the other side wall dimensioned that way. Instead, the distance between both side walls is called out 1.000.

Another example, that really confuses me. The outside width is called out to be 2.000 with a basic dimension across both faces. There is another dimension from centerline to one of the faces at 1.000. This 1.000 dimension seems redundant to me as there are no additional tolerance restrictions. If that is the engineer's way of defining centerline, doesn't the tolerance allowed potentially shift the centerline away from the center defined by the 2.000 outside? And potentially away from the centerlines of the other features.

I would think the best solution for this part would be to use the symmetry GDT symbol. Define the 2.000 feature as datum A, call out other features symmetric to datum A. It seems to be the best compromise for manufacturing, design, and inspection.

I simplified some of the numbers and tolerances. But manufacturing sometimes has trouble staying to every requirement on the prints, related to this issue. The tolerances seem reasonable, but there are a a couple tight ones, that are in or out depending on how centerline is defined.

If anyone can shed some light on this issue it would be appreciated. All engineering tells me is that they are following the latest standard (defining centerline from an edge), but it would be nice to understand why it is defined that way.
 
For example, a side wall from centerline is .500. That dimension does not call out 2X, nor is the other side wall dimensioned that way. Instead, the distance between both side walls is called out 1.000.
Those dimensions are correct.

If you dimensioned each wall to the CL, you could not then dimension the distance between the walls. That would be providing two distinct specs for a single dimension, which is a NO-NO.

The proper convention, as apparently shown on your drawing, is (CL to one wall and wall to wall), OR (each wall to CL), but not both.

If you want to define the location of the CL, I suggest using a dimension from one external wall, with tight tolerance.


- Leigh
 
This whole discussion would be cleared up by use of GD&T. The centerline will be from whichever feature is your datum feature in the feature control frame.

Avoid using symmetry because symmetry and concentricity are difficult to inspect. There are relatively few cases where concentric or symmetry is really needed. Balance of mass is one. Use position or profile is preferred.
 
Thanks for the replys. I looked into it over the weekend and it seems true position is a good alternative. I'll suggest it to engineering. Unfortunately some of the in-print parts are not functioning correctly because they are not close to symmetric.
 








 
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