原臺灣大學土木工程學系教授 (Professor) 土木工程學系 (Department of Civil Engineering)

Hello, everyone, now I am going to introduce you the instruction for drawing circles.

There are several ways to draw circles in AutoCAD.

Here, "center, radius" "center, diameter". We can indicate center and diameter, then, we can draw a circle.

We can also indicate center and radius to draw a circle. We may also use "2-point" to define the diameter,

and draw the circle. We may also use "3-point" to draw the circle. As we have learnt in secondary school geometry,

we know we can define a circle with three points. Then, there is also "tan, tan, radius". We provide two tangent lines and a radius to define a circle.

There is also "tan, tan, tan" which defines a circle with three tangent lines.

There are several ways to draw a circle. Today, I am going to draw this interesting and cute diagram on the right.

Through drawing this diagram, I will show you how these commands work.

First, "center, radius". When you choose this command,

The first point you select will be the position of the center.

While the second point will be any point on the circumference.

Once we have the center and any point on the circumference, we can draw the whole circle, because now we know the radius.

Now, let's see some demonstration. We have selected a center point,

and we give a radius of 50 units, then, we can draw a circle with radius 50 units.

Then, we are going to draw a concentric circle, with a diameter of 70 units.

The radius is 35 units. Here I am going to use another command for drawing circles, that is, "center, diameter",

give the center and diameter. Here you will encounter a problem, how am I going to find the center for the 50-units-radius circle?

How do I get the same center point for both circles? This is what we can do.

A very convenient way is to use the "Snap cursor" to find the center point.

So here, open the menu of "Snap cursor" and select "center". After you turn on the "Snap cursor" function,

When you want to find the center of an object, you move your cursor to the circumference,

and you will be able to select the center point of this circle.

So, what we use now is the "center, diameter" command. With the "Snap cursor" we've found the center,

and we give the diameter as 70 units.

Now, there is a smaller circle of a diameter 70 units drawn inside the bigger circle.

Then, we are going to draw the four red arcs in the example.

To do so, first let us observe, these arcs have the point of tangency at these two 45 degree intersected lines.

As you can see the blue lines of diameter,

they made a cross here. With these tangent lines, we can draw four circles.

Let us see how we are going to use "tan, tan, radius" to draw this arc. So, let's see how we are going to do that.

First, locate the center point. We have done that, we use "Snap cursor" and locate the center.

When we move our cursor to the circumference, we can get the center.

Starting from the center, we are going to draw the tangent line at 45 degrees to the X-axis.

The length of the tangent line should be 50 units, if you still remember, the bigger circle's radius is 50 units.

So, with the degree, we can use polar coordinates, its 45 degrees with the X-axis,

we can draw this tangent line. Now, we move on to the second tangent line.

In the same way, get the center with "Snap cursor", draw the line from the center,

with polar coordinate, the length is still 50 units, and the angle is 315 degrees with the X-axis,

or you can say it is -45 degree, and we get the second tangent line.

Now, we are going to draw the circle with "tan, tan, radius". We select the first tangent line,

and the second tangent line, and what is the radius?

As we can see in the sample, it has written R35, so radius is 35 units.

key in 35 and we get our circle

with the function of "tan, tan, radius".

In the same way, we draw the tangent line on the top left, also use the polar coordinates, and radius 50 units.

Then, we use "tan, tan, radius" again to draw the circle on top.

Then, draw the tangent line on the bottom left, then. In the same way, "tan, tan, radius"

we can get the other two circles. Now we have completed the drawing of all the lines we need.

We have two concentric circles, we have four radius, well, actually, two diameters, and with these four radius as tangent lines,

we can get four circles with the function "tan, tan, radius"

We must have felt that there are a lot of unwanted objects here. The arcs are too much.

We have to trim it. Here I am going to show you another important command in AutoCAD,

the command for "Trim", you can see this icon for "Trim" here.

To trim objects, first select the boundaries, that is,

the boundary for trimming that object, then, press Enter, then select the object you want to trim.

So first the boundary, then, after you have an object as boundary,

select one or more objects that you want to trim part of it.

That sounds a bit abstract. Let us use the example. Now, I choose this circle as boundary,

the first big circle we have drawn. This is the boundary for us to trim our objects -

the red circles we want to trim. So, after selecting the boundary, we start to select the parts that we want to trim.

This arc, you can see a small cross here next to the circle, then, you can trim it.

This is quite simple, I hope you will practice it a few more times, since this is an extremely useful command that we often use -

the command "Trim". For those of you who are very sharp, you already realised that we have not yet completed the diagram.

Apart from the colour difference, we lack two straight lines, one diameter from top to bottom, another diameter from left to right.

This is for me to show you something.

We are going to use the "Quadrant" in the "Snap cursor" function to draw these two lines.

So, first, go to the menu for "Snap cursor", and turn on the function of "Quadrant".

Click on "Quadrant" to turn it on, there is a tick showing it's on. Then, you can now use "Quadrant"

When you move your cursor to the circumference, you will get the quadrants on the circle.

Quadrants, that is 2 ending points of the horizontal diameter, and the 2 ending points for the vertical diameter.

With the quadrants, we can draw these two lines very quickly.

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