How to Create Parameters for Your Sketch or Model
Video: How to Create Parameters for Your Sketch or ModelCreating parameters provides you with in-depth training on CAD. Taught by John Helfen as part of the Up and Running with Autodesk Inventor
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In the previous movie, we dimensioned this Sketch. While it's completely dimensioned, and we could move on to part modeling at this point, I wanted to step back for a second and explain some things that are happening in the background. Because this is an introductory course, I think it's important that you understand some of that background nuance that's taking place so that you better understand things as you are learning and moving forward. We are going to go ahead and edit this sketch by double clicking on it in the browser. What I want to talk about is Parameters. Inventor is a 3D Parametric Modeling system.
What that means is the dimensions that are created are actually just representations of parameters that the system is keeping control of in the background. You might've noticed when I was placing dimensions in the previous movie that each dimension, in the Edit dialog box, has a name. If you double-click any dimension, you can see this. In this case, we are editing dimension d0. You can see in the title bar here, if I close this dimension, and I double-click the next dimension, you'll see d1 listed.
Now if you've been following along in creating parts and sketches along with me, your dimension names might be slightly different, and that makes no difference. You can change the name, and I'm actually going to show you how to do that as part of this movie. So if you see a different dimension than what I'm seeing on the screen, or a different dimension name more specifically, that's okay. You don't need to worry about that. What's important here is because they each have names, you can do special things with them. For example, d0, d1, d2, those are not very meaningful.
They are really just a way for the system to keep track of the dimensions that are created. This square would make much more sense if the dimension name was width and height. So, for example, the very first dimension, the d0 dimension, if I wanted to give this a name that meant something to me, I can actually hit the Home button on my keyboard and in front of the dimension, actually place a name. In this case, I am going to call this width. Now you'll notice as I type everything is in red which indicates something is wrong, and I purposely stopped here so that you could see that.
Just because you see red, doesn't mean that you can't make things work correctly. In this case, what it's telling us is it needs an equal sign, or you could remove that, and it would go back to normal, it just needs a value. But what I've done here is I've typed width= and some value. This is a way to name your dimensions on the fly. Now you won't always do this--and especially in an introductory course, it's not necessarily needed--but again as your things get more advanced, and as you start creating more complex designs, naming your dimensions can be very handy.
I am going to go ahead and hit Enter here, and it looks as if nothing has changed. But if I double-click that dimension again, you will now see it says Edit Dimension : width and then we're back to seeing the exact value of that parameter. This is exactly what we want. The reason for that is we can then use that name in other dimensions. For example, right now the two dimensions that define the overall size of the square are not related in anyway. If I double-click this dimension and change it to 2 and hit Enter, my square is no longer square, and that's part of what's powerful about a Parametric Modeling System is the ability to build rules and intelligence into the models so that when something changes, it always will, you have to do less work in going back and changing your model.
I am going to go ahead and double-click this dimension to edit it, but in this case I am going to do two things at the same time. I'm going to name this dimension and also give it a value. So in this case, this is the height of the square. I am going to name this height. I am going to make it equal some value. I could enter some value. In this case, I actually want to enter the name of the other dimension, which is width. I want the height and width to always be the same. This will ensure that my plate is always square, regardless of what value I enter.
So in this case, I am going to enter width, and you'll notice the red goes away because now I have a valid expression, and I can hit Enter on my keyboard. Now something else happened here when I hit Enter. The dimension changed a little bit. If I double-click it, you'll notice that it says Edit Dimension : height and the value for it is width. So what the system is doing is it's taking the value of this width dimension down here at the bottom, and it's entering that value in. The other thing it did was it indicated that a formula or an expression was controlling this by entering in fx: in front of the value in the dimension.
But it doesn't change the Dimension value, it just indicates that something is driving this dimension or controlling this dimension other than a solid straight value. When you rename things, and you start getting a little more advanced in your dimensioning, there's going to be times where you want to see all of the names of things or all of the expressions driving these dimensions. To do this, you can change the Dimension style. With nothing selected on the screen, if you right-click in the Graphics window, you have an option in your right-click menu which is Dimension Display.
And under this flyout menu, you have a few different options, you have Value, Name, and Expression. Those are the three I want to focus on. Tolerance and precise value are more advanced, and we won't be covering those in this movie. Right now, we have Value selected. If I select Name, you'll notice that all the dimensions change to show the name of the dimension rather than their value. You can see that we have our width dimension down at the bottom, we have our height dimension here on the side, you can see the name the system gave to the other dimensions as well. If I go back and right-click again and go back to my Dimension Display, I have the option for Expression.
Expression is a little more valuable in the sense that it shows you both the name and the value or the formula that's driving or creating the value. So, for example, here at the bottom, we have width=1.75. Over on the right, we say height=width, and we can very easily see the width value right here so that we get a better understanding of what the system is doing and how this is actually controlling the size of the model. Now to go a little further, I am going to go ahead and go back to my Dimension Display as Expression by right clicking on the screen, selecting the Dimension Display and returning to the Value option.
The next step in Parameters that really adds even more power to the system itself is the ability to create formulas. While technically, height = width is a formula, we can get a little bit more advanced. If, for example, we were to change the overall size of our square, let's say it's 1.75, you'll notice that our overhang gets squished--I know not a very technical term--but you see the result. The reason for that is the overall width of the square is not related to the overall width of our overhang at the same time.
So let me go back to set this to 1.575, and we are back to proper proportions. There is a couple of ways we can handle this, and this is a really important thing to keep in mind while sketching when you get to a point where you are going to start using parameters and formulas. Just because you have an overall dimension that you want to see on a manufactured part, doesn't necessarily mean you need to create those same dimensions while modeling the part, and I'll show you what I mean. In the case of this 1.88 value, we actually know that there's going to be some distance from the edge of the out hang to the side of the square, plus the overall width value.
I'm going to edit this value, and I'm going to enter width +, the difference between the 1.88 and the 1.575, which is actually .305. When I hit Enter, again the overall size doesn't change, but what's important here is I built into the sketch intelligence that says always make this value at the top, the total width of this square plus .305. So if, for example, now when I change this dimension to 1.75, the outcrop moves with the overall size of the square, and we don't have to change proportions.
Now another concern I have is back to what I mentioned that the dimensions you use in the overall modeling, might not be the exact same dimensions you use in manufacturing. And the reason for that is I could cancel this and actually delete this dimension and simply add a dimension to this outcrop to define .305. What I've done here is created the exact same physical result in two different ways, one using a formula, and one just selecting a different type of dimension.
This is the power of a 3D Parametric Modeling System, and it shows the importance of the designers sitting behind the computer itself.
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