# Dimensioning Principles

0 (0 Likes / 0 Dislikes)

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 outâ€”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 insiâ€”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.