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Dimensioning Principles

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Today we are going to talk about dimensioning. So when I've drawn the orthographic views and the isometric views that we have done so far, I've been able to see the shape of the object, some relative location of the features of the object and some relative proportions. But if I want to know exactly how big an object is, exactly exactly where the features are located or some special things about my features like exactly how deep does this hole go then I'm going to want to use use dimensioning to do that. A dimensioned drawing tells me not just what an object looks like but how big it is and where the features are. Dimensioning also allows me to communicate through my drawing how exact those dimensions need to be. So if I want my block to be an inch wide If I'm trying to make it exactly an inch wide I'll never succeed because no measurement is exactly perfect. What I need to decide is how close to one inch it needs to be. And that's what tolerance does for me. It tells me how close to a dimension I need the actual measurement of my object to be. Before we start talking about dimensioning there are a few assumptions that we need to understand. First, we need to understand over here on the left that lines that look perpendicular are assumed to be perpendicular. So if I draw two lines on my object and and they look like they're perpendicular to each other, I'm going to assume that they are. I don't need to specifically call out in my drawing that the two lines are perpendicular. I also have some assumptions with symmetry. So an object like this, if I'm not given any more information about it, I'm going to assume that its symmetric. So now that we have the first couple of assumptions out of the way, we can start talking about how we dimension some of our basic shapes, how we communicate to whoever is looking at your drawing how big that shape is. So the first shape we're going to be talking about dimensioning is a simple rectangle. So in order to fully define how big my rectangle is I have to give you three dimensions. I need to give you the width of the rectangle, the height of the rectangle, and the depth of the rectangle and here's where we start talking about a few of the choices we have to make. First, I could've put this dimension here up on this other view. But my default is usually to put dimensions on the front view, as long as the front view gives me a good idea of what's going on with that feature that I'm dimensioning, I'll often put it on the front view. I've also chosen to keep this dimension here because I also have another dimension on the front view. When possible, it's good for me to group dimensions on the same view. This makes it so that I get as much information as possible while just looking at one view. I also have another dimension over here. Now I couldn't put this on the front view because the front view doesn't show depth. I could have, however, tried to put that dimension down here. Or maybe over here. And there's a reason I didn't do that and that is I tried to keep my dimensions between the views as much as possible. And this keeps them all contained in the same space and again, makes them clearer and easier to find them. It would have been equally acceptable for me to have placed that depth dimension up on the top view, as long as I kept the dimension between the views. So let's look at an example where we are dimensioning some rectangles. I have this object here. And you'll notice that it looks like it basically has two notches cut out of it. So what I need to do for this object is to find how big it is overall, as well as the size and location of any features. So I'm gonna start by getting some of my overall dimensions in. So its 3 inches wide, 2 inches tall, and 1 inch deep. Now I'm going to start dimension to my features. So I figure out—So I place the dimension for the notch feature. And I place a few more dimensions for my notch feature. And then I have fully dimensioned this object. Now you may have wondered why I didn't dimension the notch as being this wide. And then leave this other dimension here so that I've dimensioned the width of the notch and the location of the notch like this. And there's a reason for that. When we started the video we talked about the fact that no dimension is exact. When I go to create this part it will never be exactly one inch. There's some sort of tolerance built in. So let's say that my tolerance is a tenth of an inch. Then this width could be as narrow as 0.9 or as wide as 1.1. This width could also be as narrow as 0.9 or as wide as 1.1. So that means that this overall dimension could be as small as 1.8 or as big as 2.2. Now if instead I dimensioned it the way that I haven't where I show this dimension And this dimension Now if I go back to my tolerance being a tenth of an inch, this dimension could be as small as 0.9 or as big as 1.1, but now we limit the tolerance of this dimension to only being 1.9 or 2.1. So I basically saved myself 2 tenths of an inch that could be off on this part. This sort of dimensioning is often referred to as datum dimensioning; where I choose one line or one place on my object and I do my dimensions relative to that line. If you noticed I've done datum dimensioning down here as well. When would I not want to do datum dimensioning? Well let's say that I have to fit an object in this space. Then it is really important to me how wide this space is. In that case I would care more about that width so I would include this dimension and get rid of this one. So it really depends on what's important to me. If I don't know the function of the object then it's a good idea for me to choose to do datum dimensioning. If I know the function of the object then I need to determine what dimensions are critical to that object And make sure that those dimensions are explicitly included. So where do I choose to put my dimensions? Well one important consideration is choosing the most descriptive view. So if I go back to this, notice that I've chosen to put all three of these dimensions on the front view, whereas I could have put them up here on the top view. And the reason I didn't put them on the top view is that it's hard for me to see in the top view what exactly I'm dimensioning. Down here in the front view I have a good picture of the contour of the object. So if I choose to put my dimension where I have a good picture of the contour its really easy for me to see what the dimensions are referring to. Another good rule of thumb to help me with this is to not dimension to hidden lines. Generally speaking, hidden lines are not going to be my most descriptive view I want to try to keep my dimensions between the views, And I want to try to avoid tolerance build-up. And that's why I use that datum dimensioning instead of chain dimensioning that I could've used instead. I also generally want to keep the widest dimension lines furthest from the object. This keeps my dimension lines from overlapping. If I go back here again, If I had put this 1 dimension out all the way out here instead of close to the part Then I would have overlapped with the 1.5 dimension and the 2 dimension. By choosing to put the 2 farthest away from the part then the 1.5, then the 1, I've made it so that my dimensions overlap as little as possible. Another thing I want to avoid when placing dimensions is putting them on the part. This can make the view of my part very confusing. It's hard for me to tell what is a dimension line and what is a feature on my part. So instead I put my dimensions outside the bounding box of the part. So in this case I would consider my bounding box to be something like this. And so nothing insi—no dimension should be placed inside that bounding box. So I wouldn't put a dimension in here. Because that would be inside the bounding box. Another shape that I will commonly want to dimension is a cylinder and before I go into how to dimension cylinders, we first need to make a distinction between positive cylinders and negative cylinders. So a positive cylinder is a cylinder like this outer cylinder here, where you can touch it and feel the outside of it. It's like a pencil or a rod. A negative cylinder is a hole. And how I dimension the two of those will often vary. So in this case I have dimensioned my positive cylinder over here in the right-side view. And that's this 2 dimension here. Positive cylinders are always dimensioned where they look like rectangles. So over here it looks like a circle and over on the right side it looks like a rectangle so that is where I placed my dimension. Any time I am dimensioning any type of cylinder, either a positive cylinder or a negative cylinder cylinder, I need to include my diameter symbol, right here. And that lets me know that that dimension refers to a circle, which is not always obvious when I am dimensioning a circle in the view where it looks like a rectangle. Negative cylinders are a bit trickier. The negative cylinder dimensioning convention depends the position of the negative cylinder. If the negative cylinder is concentric with a positive cylinder, like in this case where I have a hole that's concentric with the positive cylinder, then I will put the dimension for the negative cylinder in the location where it looks like a rectangle. And that just groups my dimensions together so that it's easy to see all of the cylinders that share a common axis. So here you can see that I've placed my dimension for my hole in the right-side view with the dimension for my positive cylinder. And again I have included the diameter symbol. And I have also included the depth of my cylinders. Note that because this is not actually referring to the cylindrical feature itself, it's referring to the depth I do not have a diameter symbol. I only use diameter symbols when they are actually diameter. Now If my negative cylinder were positioned in something like a square. So if this shape were a square instead of a positive cylinder Then my negative cylinder diameter would show up like this. So any time that my hole is not concentric with a positive cylinder I need to dimension it where it looks like a circle. And that is going to make things easier for whoever is machining the hole. So here you can see that I have my same dimension - it's still a diameter. I have a leader line pointing to the outside of my circle. I need to be careful with my leader line, that it is radial with the center of the circle. So here you can see that if I follow my leader line in it would touch the center of the circle. If my leader line is not radial then I need to re-position my arrowhead until it is. If my arrowhead is not pointing straight toward the center of the circle or it is not positioned radially, then I need to re-position my arrowhead. Arcs are another shape that I will frequently encounter. For an arc, rather than listing the diameter I list the radius. So the rule of thumb is that if my arc is less than 360 degrees then I will list the radius instead of the diameter. So here I have my radius symbol in front of the measurement. Notice that I am in an English drawing where I am using inches so there is no leading zero. If I were in metric I would be using a leading zero. And again my leader line is radial to the center of the circle. When I am trying to fully dimension my object, one of the things that I need to dimension is the width of my object. And so its tempting to put the width of my object, something like this. But what I need to bear in mind is that this radius of this arc is .75 and because of the centerlines I know that this arc 180 degrees. So this requires that my part is 1.5 inches wide. So I would not include this width dimension because it is already determined by the arc. Similarly when I am trying to dimension the height I have two choices. I could dimension from the bottom of the part up to the centerline of the arc and then use the radius of the arc to figure out the height of the object. I may also be tempted to provide the overall height. Now this is wrong for two reasons. One of them is I've already given this overall height by the height to the center of the arc and then the radius of the arc. There's a bigger reason that this dimension here is incorrect though. And that is that I never want to dimension to a curved edge like this. So this line here is actually the extreme element of this arc. I don't want to have to take this measurement between the bottom and the top of that extreme element. So I will not choose to use this overall height. Because I don't want to dimension to a curved surface. Instead I will use the height of the center of the arc and then calculate the overall height from the radius. Now when I go to dimension a part like this I know that I need to get my overall height and width. I know that I need to get the location and size of this hole. But there are actually two different ways I can do it. The first way, over here on the left, does not assume symmetry. So it explicitly locates, using datum dimensions where the circle is located within the block. Even though the circle is in the middle, I explicitly dimension it as being one inch away from the edge. And even though it is one inch down from the top, I explicitly dimension that as well. So these two dimensions I put because I'm not assuming symmetry. However, I could indicate the planes of symmetry like I've done over here on the right. So here I've indicated a plane A, so I show the two axes I would see based on that plane and I've indicated a plane B. So, again, I would see plane B in two of the views, the front view and the right-side view. So by stating that these two planes are planes of symmetry, it is understood then that this hole is at the center of the object. So I no longer have the dimensions locating the hole because I know that its in the center. I still have the overall width, and the overall height, and the overall depth, and the size of my feature. But I do not need to locate my feature because of the symmetry, if I'm going to assume symmetry. A quick note on scaling: So when I look at a drawing, I know that that drawing has been scaled somehow. It may be a full-size drawing so that the thing I see on my piece of paper is exactly the same size as the real-life object . If may be half-size so that the thing I see on the piece of paper is half as big as my real-life object. Or a quarter-size. No matter what the scale of the sheet is the dimensions will always be full-size. So here I have a pencil drawn big, I've drawn it half as big, and I've drawn it half as big again. But notice that my dimension always reads 6.5 inches. So no matter what scale I have for my sheet, all my dimensions will read as if they were full-size. So a 13-inch block will always be dimensioned as 13 inches, no matter whether its full-size, half-size or quarter-size on the sheet. SKIP TO 19:26 Now I am going to want to give the location and size of this circular that goes through the object. To give the location, I am going to place it relative to one of these holes. And again that's because I know pretty precisely where the location of the hole's gonna be because its going to be bolted into the bolt patter underneath it. So that gives me a better definition of exactly where this central feature is going to end up. So I place my dimension there And now my circular feature is located. To give the size of the hole that goes through the circular feature, I'm going to dimension it where it looks like a circle. So I am going to click on it and I am going to give that dimension. The overall cylinder is a positive cylinder so I am going to dimension it down where it looks like a rectangle. So I've located and dimensioned the size of my holes. I've given the size and location of this central feature. The only dimensions I'm missing now are the location dimensions for this base feature relative to the top and bottom of my cylinder here. I'm going to do that down here in this front view because, again, there's no real advantage to using the right-side view. And I am going to choose to use datum dimensioning. Because I do not know exactly what is the critical dimension for this part. I'm not sure of the width of this base part is going to be important. Or if the distance between the base and the bottom of the cylinder is going to be important. I don't know. So since I'm not sure, the safest thing for me to do is to minimize tolerance buildup. So now I have placed all of my dimensions. I'm going to wanna make sure that they're nice and neat. So I'll click out of dimension and I'm going to want to keep them not too close together but they also don't need to be spaced out quite this much. You can look in the book if you would like more precise definitions on how far apart the dimensions should be from one another. So at this point the dimensions are nice and neat. No dimensions are overlapping. I can tell which dimension arrows each of these numbers is associated with. I'm going to want to go back through and double check all of my annotations. Do I have all of my centerlines? So I do in this view. I do have centerlines in this view. But if I look over at my right-side view I'll notice that I'm missing the centerlines for my holes. So I'm going to go back in and add those to make sure I have all of the centerlines for my drawing.

Video Details

Duration: 22 minutes and 38 seconds
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Language: English
License: Dotsub - Standard License
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Views: 1
Posted by: raghadkod on Aug 30, 2019

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