“Oh, what a tangled web we weave, when first we practice to deceive!” attributed to Sir Walter Scott, 1808.
Geometric Dimensioning and Tolerancing, or GD&T, is a system of symbols, definitions, and practices designed to eliminate miscommunications between the description of an item and its real-world implementation. It removes unintentional deceptions.
This was supposed to be sharing my understanding, instead it highlights, to me, that I do not fully understand.
This image shows all the sides of the object as well as two projection images. This shows us what the object should look like, but it doesn’t tell us how to make it or the actual locations of each item.
Here I’ve added a center line and a dimension. The dimension says that this is 2 inches from the bottom to the top.
So where is the centerline? Knowing where the centerline is important as it tells us where the slot will go.
Worse, the computer has a concept of absolute sizes. In the model and on the paper, that part is 2.0000000000000000 inches wide. It is not realistic to make something that is exactly on size. In the physical world, we need things to be close.
How close is “tolerance”.
Consider a hole that is marked as 0.2500 and a pin that is marked as 0.2499. First, those measurements are difficult to reach. The tighter the tolerance, the harder it is to hit and the more likely you are to scrap a part.
If those two parts were made to those specifications, the pin should go into the hole. There are still reasons why it might not, but it should fit. There is a gap between the walls of the pin and the walls of the hole of 0.0001.
For perspective, a piece of printer paper is 0.004 inches thick. So when we are looking at hitting a tolerance of 0.005 we are looking for about the thickness of a single sheet of paper. If we are looking for a tolerance of 0.0005, then we are looking for about a tenth of the thickness of a piece of paper.
With my equipment and at my skill levels, I can hold 0.001 all day long. 0.0005 is a bit harder.
As a manual machinist, if we need to hit very precise tolerances, we will often times use abrasives to take off the last 0.0002 of a part.
So the engineers provide a tolerance for the theoretical values they provide so that they are achievable.
Which takes us back to my silly drawing. To know where something is, you need to first specify where to measure from.
The A in a box with a filled triangle pointing to it tells us that this is a datum. The cool thing is that we can use that datum in many views. Since the dimension is based off the A datum plane, that means that is the measurement.
If the part was too thick, we would take it off the side that is NOT the datum.
(hey, these are just notes, don’t use them, look them up. The ASME Y14 spec is only a few hundred dollars.)
Here is the final top view. I have all the dimensions to machine this, but I don’t have the GD&T. I thought I understood, I don’t. I need to do some more reading. I know I need a feature control frame in there, I just don’t know where to put it and what it is supposed to say.
To be more clear, I know that I need to mark my datums. 3-2-1 rule applies, which I understand. I also understand the 6 Degrees of Freedom(DoF). I know how to mark the datums and to transfer those datum markings as required, to make it easier on the machinist.
I understand that I put the datums at the end of the feature control frame, so A|B would be a primary datum of A and a secondary datum of B. I understand how to make axis and centerlines datums. There is just so much going on here. Any way, another geek dump before I go look at Ezell