GD&T: Multiple Datum's Referenced, More Than One Datum?

Datums referenced on a drawing using Geometric Dimensioning and Tolerancing can be really confusing, even on simple parts! When you get right down to it Datums are not that difficult to understand when you consider what they are used for. I have received several questions about Datums, why and how they are used and why use more than one. I'm going to answer those questions using a simple part that has one hole and has three Datums referenced. But before I go into the explanation about multiple Datums I recommend that you read THIS post I wrote about a parts size and how the dimensions tolerances determine what a part can actually look like. For a quick review there are THESE posts that I wrote about Geometric Dimensioning and Tolerancing Basic concepts. This post is a quick overview of the basic concepts of using multiple Datums. There is a lot more to Datums than what I'm writing about here and I'll get into those more complicated concepts later!

     

Lets start with a simple drawing of a part that has one hole in it and the hole is positioned in relation to three Datums.

 

Simple Part with Multiple Datums

The drawing above has a hole in it that is True Position to three Datums in this order: A B C. The reference to Datum A is holding the location and orientation of the center axis of the hole to Datum A (bottom surface). Take a look at THIS picture from an old post if that doesn't make sense. Another way to look at this is Datum A is the surface that someone is going to measure the perpendicularity of the hole to. So Datum A in this case is a place to start taking measurements. We'll start taking measurements with Datum A because it's the first Datum and we will continue measuring the part with the remaining two Datums in order from left to right. The Datums don't have to go in alphabetical order, they are in used in the order that you reference them when you measure the part. The important thing to remember is Datums are used to measure a part.

   

So what are the other two Datums doing? I'll explain the below... ;-)

Plasma Actuator Airfoil Wind Tunnel Testing

This week I built a Plasma Actuator in the shape of a crude airfoil and tested it in my Homemade Wind Tunnel. Plasma Actuators are research devices that use an electric field to change the characteristics of gas flow near a surface. In this experiment I was trying to get the airflow over the top of the airfoil (wing) to adhere to the surface of the airfoil when it normally wouldn't. Separation of the airflow over the top of a wing causes the lift generated by the wing to be reduced resulting in a stall. Before I talk more about what I built let me put a couple of pictures that I took from my experiment to help explain what I'm writing about.

   

Airfoil in Wind Tunnel Plasma Actuator Off

   

Airfoil in Wind Tunnel Plasma Actuator On

   

The above pictures were taken one after the other with the same airflow, the only difference between the two is in the lower picture the Plasma Actuator is turned on. In the lower picture the flow over the top of the wing is much smoother than in the top picture and is adhering to the top of the wing. Read more below for some details about the Plasma Actuator that I built and a video of it running.

Geometric Dimensioning and Tolerancing: Position, Zero Tolerance and Material Condition

I have written a few posts about Geometric Dimensioning and Tolerancing (GD&T) basics and I want to write another one about a topic that is probably the most often question asked of me: Zero Tolerance dimensions. The name Zero Tolerance is confusing to some folks because as we all know "every dimension must have a tolerance" so what is a Zero Tolerance dimension? Does that mean that the part has to be perfect? Why would anyone put a Zero Tolerance on a drawing? I'm going to try and answer those questions in simple steps starting with a quick review of Material Conditions because you can't have a Zero Tolerance on a drawing without specifying a Material Condition. If you haven't read my other posts about Geometric Dimensioning and Tolerancing I suggest that you do by clicking HERE and reading the posts related to GD&T. At a minimum read the posts about Material Condition and True Position. One thing I want to mention is my intent here is to explain the concepts in simple terms and avoid all the esoteric 'advanced concepts' that you will find in other places on the net. I get questions from friends and colleagues who are not familiar with GD&T and I try to answer those questions as simply as I can. Once someone understands the basics the more complex concepts are easier to get. If you are not familiar with a Zero Tolerance here is a drawing that is using it.

Drawing With Zero Tolerance

Before I start talking about Zero Tolerances I want to talk briefly about Material Condition because the concept of material condition is key in understanding Zero Tolerances. I'm going to go fast so if you want a better and more detailed explanation of Material Conditions read THIS post that I did awhile ago. Here we go! The Material Condition concept is a fancy way to think about how big or small a particular Feature of a part is. There are three material conditions and two types of Features. The three Material Conditions are: Maximum Material Condition or MMC, Least Material Condition or LMC and Regardless of Feature Size or RFS. The two types of Features are: Internal (like a hole) and External (like a block). When considering Zero Tolerances the only Material Conditions that are important and MMC and LMC so I'll skip RFS here.

San Francisco Fleet Week

I like airplanes, ships and standing in long lines fun day trips and I got my fill this weekend. So I went to the 2011 San Francisco Fleet Week on Saturday. It was a lot of fun and I took a bunch of pictures, I'm going to put a few of the good ones here. The USS Carl Vinson (CVN-70) and the USS Bonhomme Richard (LHD-6) were two of the most notable ships there as well as the Blue Angles and many other aircraft. There are tons of pictures, news articles and info. about San Francisco Fleet week on line so I'm not going to put a lot of that here. If you want to know more about SF Fleet Week Google it. Because this Blog is about the things that I am doing I'll stick to what I did. BTW: I am still working on the wind tunnel that I wrote about last week but I don't have a lot of progress to write about.

      

To get to San Francisco we took the East Bay Ferry from Oakland to San Francisco. This is the way to go to avoid traffic and standing on BART. We left Oakland and went straight to San Francisco. On the return trip I took the picture below of a crane on the Alameda waterfront. I thought it looked neat with the moon right off the end of the crane boom.

Alameda Pier Crane

Old mechanical stuff like that looks neat and I really like the picture so I put this one first. It's rusted and looks like it's about to fall over but it appears to be in service. The Ferry leaves from the post of Oakland and I have only one interesting picture of the ship that is right next to the ferry.

Homemade Wind Tunnel for Plasma Actuator Testing

I finally got around to putting the wind tunnel together, well actually it's a breeze tunnel. There isn't much of a wind blowing through it! After looking at the performance of the Bladeless Fan I built I decided that it was going to  create a higher airflow than I need for the Plasma Actuator experiments that I want to do, so I decided to slow it down a bit and use a small DC fan. The DC fan is easier to use because I can control the speed of the air with a variable power supply and the airflow from the Bladeless fan just dropped off all together when I lowered the input air. This wind tunnel is only to do some experiments with the Schlieren setup I built and High Voltage and a low velocity and variable airflow is what I need. Before I built this wind tunnel I did a lot of reading on line and found some plans from a NASA wind tunnel site describing a small classroom wind tunnel for students to build as well as a lot of very technical information about "real" wind tunnel design. As I mentioned above my wind tunnel is more of a breeze tunnel so I didn't need a lot of the flow control, settling areas and velocity constrictions. I just need something to provide a smooth flow of air past a viewing area. Below is what I came up with.

Homemade "Suck Down" Wind Tunnel

In the picture above the airflow is right to left, the fan is on the left, the view area is in the middle where the glass windows are and the inlet air goes into the right side. Because the fan is downstream of the test area this is considered to be a "suck down" tunnel. One advantage of a "suck down" tunnel is the turbulence and chopped up air created by the fan isn't blowing over the test area. Smooth air is sucked past the viewing area. Click below to read more!