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Sunday, June 5, 2011

Geometric Dimensioning and Tolerancing (GD&T) Introduction

As a mechanical person I get asked a lot of questions about drafting, mechanical drawings and Geometric Dimensioning and Tolerancing or GD&T. I have written emails over the years  to friends and colleagues explaining some of the concepts and recently I have begun to help some folks learn it from scratch. Because of this I have decided to start writing some posts about this topic for some of the following reasons. One reason is this blog is about the things that I am doing and GD&T is one of the things that I do as a mechanical designer. Another reason for writing about this topic is the same reason I had in creating this blog: People ask me what I'm doing and building and it's easier to answer them in a blog post and let them all read about it in one place rather than me emailing a bunch of people over and over again. Also I'm writing about this topic because this is my blog I can do whatever I want
As I write more posts about this topic they are not going to come in any particular order and are not really intended to be a instructional course, they are just answers to questions that I get. As always with anything that I write about if you have any questions you can email me at or leave a comment. I'll be glad to answer any questions that you may have.
Geometric Dimensioning and Tolerancing
If you are interested in and not sure what Geometric Dimensioning and Tolerancing (GD&T) is check out this link HERE on Wikipedia. There is a general overview of GD&T and a neat interactive chart that is fun to play with. I'll summarize the topic generally by saying that GD&T is a language of mostly symbols used on mechanical engineering drawings that communicates the design intent to a person who is using the drawing to make something. GD&T is a international standard (or several actually) that controls all the physical aspects of what the drawing is describing in a single, concise and unambiguous way. From the drawing you should not only be able to make something but you should also be able to check and measure the thing that has been made to see if it's correct. Specifically Geometric Dimensioning and Tolerancing is a way to use dimensions to describe exactly what you want a thing to look like when it's made and how much that thing can deviate from exactly perfect and still be correct. The reason for this is you can draw something perfectly flat, round, 6 inches long etc...  but when it's actually made it can never be EXACTLY those things. Even machines can't cut or shape something to be exactly perfect, there has to be a tolerance with every dimension. For example, how much out of round is OK? How warped and not flat can a surface be? How much longer or shorter than X inches is OK? How do I read the drawing in a way that lets me check the parts dimensions? How do I know if the thing I am drawing will fit together with something else? Geometric Dimensioning and Tolerancing answers those questions and a lot more.

One thing to consider before talking too much about dimensions with tolerances is what are the important aspects to something mechanical. One way to think about this is what is important to any feature on a mechanical part. I have heard these different 'aspects' referred to as Levels Of Control and depending on where I have heard it there are any number of them. In my experience there are essentially four fundamental aspects to something that you have to pay attention to and think about when making a dimensioned drawing. I'm going to simplify them a bit with pictures and my own words so if you are looking for the official definitions buy the ASME Y14.5-2009 specification ;-)
Level 1 SIZE
The first thing to consider is somethings size. Size in this case is frequently confused with Shape (which is something different and I'll talk about below). The important thing to remember with Size is that it's a 2 dimensional aspect to something. Here is what I mean, below is a picture of a cylinder (1a) and there are a couple of sizes associated with it, the diameter and the length. If this were a metal rod that you wanted to put in a hole and you already had the hole drilled in something you would want the diameter of the rod to fit nice and snug in the hole and not rattle. That is the one way to say that you want the diameter of the rod to be D +/- a little bit (no smaller or bigger) than a certain size. If you stop and think about this for a second there are really 3 dimensions here; the Diameter, the Diameter Plus a little tolerance and the Diameter Minus a little tolerance. Since we are really only worried about the rod being too snug or too loose the only dimensions that really really matter is the the Diameter Plus a little tolerance and the Diameter Minus a little tolerance. The nominal diameter itself is not important if you think about it. The nominal dimension is really just a starting point and the 'Plus Tolerance' or Maximum size and the 'Minus Tolerance' or least size is what matters.
Maximum and Least are the terms to use for the extremes of Size, not maximum and minimum or biggest or smallest. The sizes are referred to as Maximum and Least with a M or a L with circles around them (see 1c in the picture below) . These actually refer to Maximum Material Condition or Least Material Condition but for now just think of them as the Maximum size and the Least size. Later on (in another post) I'll write more about this and it will get a little more complicated but for now just think of Maximum and Least sizes.
Anyway back to the rod and the hole that we want to put it in. As I mentioned above too big in diameter and the rod won't fit, too small and the rod will rattle around so there are only 2 sizes to be worried about. But wait! Not only that, lets say that you are not sure yet how far in the hole you are going to put the rod so you want to be sure the rod is the right size along it's entire length. To see if it's the right size along it's entire length you would have to measure the rod a whole bunch of times. A way to imagine that is if you cut the rod into really thin discs as in 1b and measure each one of them to see if each is not too big or too small - in other words within tolerance.

GD&T Level 1 Size Control

Each one of those disc measurements could be thought of as individual 2 dimensional measurements of size even though we are talking about a long rod. Each one of the measurements along the length would have to be within the Maximum and Least allowable sizes for the rod to be OK as in 1c. The Maximum Size and the Least Size are in this case perfectly round circles of two different sizes. Does that make sense? Size dimensions are 2 dimensional and you usually need to consider more than one, a lot more than one in many cases to measure or describe something that is in 3 dimensions like the rod is. The same is true for the length of the rod in this case and you can look at the picture and imagine what I mean. Imagine measuring the length of the rod with a whole bunch of measurements, 'cutting' the rod along it's axis into long sticks. There are potentially a lot of length measurements to do too if the length is important and you care to measure it. In this example I'm talking about a round cylinder but it could be a rectangular block or any shaped thing for that matter.
Level 2 SHAPE
The next thing to think about is the shape or Form of something and this is of course interrelated with it's size. In the example of the rod the shape is a cylinder and there are many cylindrical shapes that it could be. Have a look at the picture below to see what I mean. In the top picture below the rod is a cylindrical shape and each Size measurement made above is exactly the same but the rod is not straight. Not only is the rod not straight it's axis is also not straight. Measuring a rod like this for Size as we did above would only tell us that it's perfect in Size (within tolerance) but it's not perfect in shape.
GD&T Level 2 Shape Control

In the middle and lower views of the above picture there are a couple more examples of possible shapes the rod could have and still be OK in the Size measurements. Notice that in both these lower views the axis of the rod is straight. In the middle view you can see that the rod is getting smaller in the middle and bigger on the ends. The reverse is true in the lower picture with the middle being bigger than the ends. Both of these examples could be OK for all the Size measurements made previously if the smaller portions are all within the Least Size tolerance and the bigger portions are all within the Maximum Size tolerance. In both cases the Shape of the rod might not be OK.
Measuring something for Shape can be a complicated thing to do depending on what the Shape is and how much it can deviate from perfect. I won't go into that too much right now because this post is just about the different concepts. What I will say is to measure the Shape of something you would compare it to a perfectly shaped (cylinder in this case) area that is at the Maximum size. In the case of the rod if we could place it in a perfectly straight and round hole drilled at the Maximum size that is the same length as the rod, the Shape is OK. More about that in another post...

At this point we have just been concerned with the object - like a rod - all by itself. For the next two levels of control we need to add something else, anything else actually, to compare it to.

Orientation is how something is oriented to something else! Isn't it bad form to use the word you are defining in it's own definition? Have a look at the top view in the below picture of the rod and the block with the hole in it that we want to put the rod into. In the picture (and this example) the rod and the block are two different parts but they don't have to be. The rod could be the same part as the block, just a block with a rod protruding out of it. The concept is the same if it's one part or more than one. Orientation is how the rod is positioned relative to the block. If we want to put the rod into the hole, after checking the Size and Shape, we would then orient the rod so it's able to go into the hole. In this case the X and Y angles would have to position or orient the rod so that it lines up with the axis of the hole. If the rod were part of the block and just sticking up out of it somewhere Orientation would be the angles it was sticking up. Notice that in this case the rod is a cylinder and the rotation of the rod isn't important. If the rod were square and the hole square as well then we would need to dimension and control the Orientation rotationally of the square so it lined up with the hole. To keep this example simple I didn't make it that way ;-)
GD&T Levels 3 and 4, Orientation and Location Control

By now this should be obvious but in case it isn't I'll write something about it. Location is where something is located relative to something else. This, like all the other levels of control can get really complicated, but for now just think of Location as where something is relative to something else as in the lower view of the above drawing. The X and Y dimensions are now locating the rod right above the hole that we want to put it into. Again in this example the rod and the block with the hole are two different parts but they don't have to be. As in the case of orientation the block and the rod could be one part, just a block with a rod sticking out of it and the location is just where the rod happens to be sticking out relative to the edges of the block.
I mentioned somewhere at the top of this post that this is just about the concepts and not the exact definitions. If you are interested in what the actual definitions are refer to the ASME Y14.5-2009 specification. I wrote this mostly because I get asked about GD&T and I thought I'd start writing about it from my own interpretation of what I have learned over 20 some years of designing stuff professionally. Also when friends ask me questions I can refer them to my blog because I'm a little lazy and writing emails over and over is tiring ;- )
I have skipped a lot of the details of the levels of control because they aren't necessary in understanding the basic concepts. Once you see what these are used for the finer details will become important and understanding them will be easier at that point. Dumping all of the concepts details all at once usually causes information overload for folks so I have found it easier to explain things in smaller bites and that is what I have tried to do. Besides all the details don't make much sense without explaining them in context and that is hard to do and still keep it simple. I intend to write more posts about this topic in the future and hopefully the usefulness will become apparent and all the nitty-gritty details will be explained fully. Check back often for more posts about GD&T and email me or leave a comment if you found it useful (or have questions!)

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