GD&T Concentricity, Is It Round?

I want to write about several simple Geometric Dimensioning concepts that I have seen misused, misunderstood and misapplied over the years. It all boils down to things that are supposed to be round. Concentricity, Roundness, Cylindricity, Axis as a Datum and the list goes on and on... Is it round? I think that a lot of folks get these concepts wrong because "round" seems so simple on the surface but when you get into the details it's not that straight forward... or is it? Actually it is pretty simple if you slow down and think about it. The first idea is: Where is the axis of something that is round? Take a look at the picture below and let me know if this makes sense.

 

A Right and Wrong way To Establish A Datum
(Click on the image for a bigger view!)

     

In the top of the picture there is Datum A that is placed incorrectly on the axis of the part. I have seen this done so many times and it makes no sense! In the lower picture the datum is placed on a diameter. Why is it wrong to place the Datum on the axis? In the real world there is no ONE center axis of any part with multiple diameters and there is no 'Datum Axis' in this case. Which axis is the datum? In the lower part of the picture above it is clear which axis is the Datum because the Datum is on a round surface. That surface is creating the Datum axis. Because there are several surfaces on this part there is no way to know which axis is the datum in the top picture. Have a look at the picture below to see what I mean.

Geometric Dimensioning and Tolerancing is all about measuring a part. In the case of the below part (exaggerated example) there are 4 axes established by the 4 surfaces. Unless you specify which one is to be used as the Datum there is no repeatable way to measure the part!

Lots of Axes!
(Click on the image for a bigger view!)

 

What is round? Something that is round is well... round! All the points on the surface have to be equal distant from the center. That does not mean that the object has a constant diameter. There are a lot of different shapes that are not  round but they have a constant diameter. A common one that you see with hallow parts that are turned on a lathe is a trilobed or pentalobe shape. That's because the chuck on a lathe usually has either three or five jaws. When a hallow part like a tune is chucked in the lathe the jaws squeeze it in three or five places. The part is then turned to a nice round shape and when it's removed from the lathe chuck it 'springs out' where it was being squeezed. What you end up with is a part that is either trilobed or pentalobe in shape. Parts like that will have a constant diameter but not be round!

          

What is Concentric? Concentricity is the coincidence of the median points of all diametrically opposed elements of a feature. That is a confusing way to say that if you measure across something (the diameter) and you find the middle of that measurement (the median point) and keep making measurements of the diameter all the way around... if all the middle measurements line up the shape is concentric. Concintricity is not the same thing as round. A rectangle is not round but it is concentric.

     

Concentric Rectangle

 

In the picture above there is a rectangle and it's obviously not round. Look at the first measurement across it's width (diametrical measurement) and the center (median point) of that measurement. Now look at the other measurements. All the median points (middle) of the measurements are in the same place (coincident) so the rectangle is a concentric shape. I have seen a lot of drawings over the years where the designer wanted something to be round and they used Concentricity on the drawing. They are not the same thing.

    

That is all I have time to write today. This post wasn't as 'in depth' as I wanted it to be but I hope you get the idea. I'll write more about this stuff soon! Meanwhile check out the Site Index of Projects link to see some other things that I have built or written about.