306 AutoCAD 畫弧 (arc)

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Hello, everyone, I am going to introduce the command for arc in AutoCAD.

In AutoCAD, when you select the command for arc, you will be surprised by the number of different ways to draw arc.

Drawing arcs is actually more complicated than drawing circles.

That is because, apart from you need to know the center and the radius of the arc,

you also need to know the two end points of the arc.

So there are many ways provided by AutoCAD for drawing arcs. You can see the most common "3-Point" here.

"3-Point", that will be the two end points and the one point at the center.

Then, there is the "Start, Center, End", "Start, Center, Angle",

"Start, Center, Length", "Start, End, Angle", "Start, End, Direction",

"Start, End, Radius", "Center, Start, End", "Center, Start, Angle",

"Center, Start, Length" and also the "Continue", there are very many of them.

I don't think it is not essential to introduce each and every one of them.

It's not easy to remember them like that, and with many methods, you won't know when to use which one.

So, what I am going to do is use this example on the right. Well, we use this example on the right to introduce you the ways to draw arcs.

Usually, we will start with the easiest one.

So, let us learn two or three of these methods, and we can always use these ones.

When you are free, or when you face a situation when these methods are not very convinient, then,

you can come back and try other methods. However, generally speaking, these are enough.

Well, let us start drawing this diagram.

We see this example, an interesting example. We are going to study its structure carefully.

We see the outermost part is a circle.

And we need to draw two diameters, one is horizontal, another is vertical.

On the vertical diameter, we need to get several equal-division-points.

As you can see, there are three points on top, another three at the bottom. Or you can draw a six points in total. (Note: This is an error, total should be seven)

After you get the equal-division-points,

Then, you can use these points as your end points for arcs and draw the arcs.

So, let us now start with the circle.

Select the center, give a radius, which is 50 units. Then, remember we talked about Quadrant before.

So with Quadrant, we can draw the horizontal and vertical diameter. Now, after we completed this, we are going to learn something new.

The way to draw the equal-division-points. First, we select the diameter, the vertical one.

Well then, we are going to draw the points. So, select the vertical diameter,

and we want to divide it into equal parts, eight equal parts. Just now I made a mistake by saying six points, we need seven.

Here, we have the commad for "Draw". When you click on it, it will give you a collection of tools for drawing.

Select "Divide"

Well, you can see some information there, clearly, it is a tool for division.

Given any length of line and you can divide it into several equal parts. Okay, we select the command "Divide"

We are going to divide this line into eight equal parts. Select this line,

then, key in the number of equal parts that we want. It should be 8.

Now you see, 1, 2, 3, 4, 5, 6, 7, 8, that's eight equal parts.

and a total of seven equal-division-points. I think you get the idea.

So eight equal parts, we key in 8 here and it gives us 7 points.

Some of you may have noticed that, after I divide the line, you don't see the pretty x's that I have in my sample.

Now, you will need to set the style of these points, so that you can see your points.

Here, you se "Utilities" and there is the "Point Style...", after you select this, you will see several different styles that you can use.

Just choose the one. It will be good to use the one that we are using now,

so that you can draw the same diagram as the example. If you don't see your points,

maybe you did not choose a visible point style, or the points are too small to be seen.

So you go to the "Point Style" and adjust the size.

Well, let us draw the arcs now. Come to "Home", then, "Arc", and select "Start, Center, End".

This is a very understandable method,

but there is an extremely important concept that we have to know, that is, the arc is always drawn from the starting point

in an ANTI-CLOCKWISE direction. This is because AutoCAD follows the Right-Hand Rule.

So, the positive direction is always anti-clockwise.

So, the first point, the starting point, as you can see, it's at the bottom of the arc. The second point, the center, is at the center of the arc.

The third point, the ending point, will be at the top of the arc.

Coming back to our diagram,

let's assume that, our first point is on top, and the third point is at the bottom,

then, the arc will be formed on our left-hand side, because the concept is ANTI-CLOCKWISE movment.

That is why it is now on the left-hand side. Now, we need it to be on the right-hand side,

and so, our first point need to be at the bottom, and the third point need to be on the top of the arc,

then, we can produce an arc on the right-hand side.

Well, now, we are going to draw this arc, here we also use the Snap Cursor,

and here, we select "Node" from the menu,

so that, when you are selecting your end points, it will be easy to lock it on the nodes.

As you can see, once we move close to the node, we see a green square, showing that we have lock onto the node. This is very convinient, makes it easy to draw.

So, the first arc, we select the first point, the starting point.

Which, as we said just now, is at the bottom. Then, select the center, that's the node above it.

The green box shows that you have already lock on that node.

The third point, the ending point of the art.

That will be the node above. Here, we lock on the Quadrant. So, we have input three points and we will get our first arc immediately.

I hope you are able to follow my instructions step by step, that is why I made the pace for explaining the first arc very slow.

For the other arcs of this diagram, it should be very easy to draw them folloiwing the same steps. Well, I am going to explain the other ways of drawing arcs.

Now, I select "Start, Center, Angle".

And I am going to draw the second arc on top here.

In the same way, we remember our starting point, and center point.

The arc will move anti-clockwisely from the starting point moving towards the center point. So, all the angles we use here is anti-clockwise.

Okay, we select the starting point, which is easy, the one we have already identified just now.

So, we have a starting point, this node that breaks the line,

and we are going to choose the center point, which should be easy to understand, the point at the center.

Then, the angle. To draw such an arc, what will be the angle? 180 degrees, isn't it?

It is a positive 180 degree, since it is anti-clockwise, so all we have to key in is "180",

then, the arc is drawn. Or if you click on it, you will also see that it is showing 180 as the degreee.

Well, we have completed another arc.

Now I am going to use "Start, Center, Length" to draw the third arc.

Starting point, that's easy, same, anti-clockwise movement, so we select the point down here as starting point.

Center point, that's straight-forward too.

Use this node as center point. Now, length, This arc happens to be half of a circle,

so the diameter is the length. You can either key in the length

or click to select it: we have already given the first end point, and we click to give the other end point. If you click with your mouse, autoCAD will key in the data by itself.

Or if you type manually, the length will be 75 units, and there is our third arc.

Then, we are going to continue using "Start, End, Angle" to finish all the other arcs.

Alright, we see the arcs on the left, so we should know whether the starting point is on top or at the bottom?

On top, right? Because if I start from the bottom,

you will see that, this direction is wrong. Of course, you may also choose to type -180 degrees.

But usually, we will choose our starting point correctly. So, let me delete this and draw from the other angle.

That is, we will select our starting point at the top of the arc. Just now I select the wrong point on purpose.

When you start from the top, you will see that the arc is correct, because, as we have said, the positive direction for the arc is anti-clockwise.

So, the angle is positive 180 degrees. Now, we have completed this arc. And the other two arcs, I think is easy to follow.

Just select "Arc", "Start, End, Direction",

Or, you may choose "Start, End, Angle", whichever way will do. I am just doing some demonstrations.

Here, same, starting point on top, then, end point there. I did it the other way on purpose. In that way, you can use 180 degrees.

Well, I think the concept is just the same. Anti-clockwise as the direction, and we give two end points, the starting point and the ending point.

I have demonstrated different commands for you, basically, the concept is the same, I myself did not memorise each and every one of them.

For as long as you have the concept, it should be very easy for you to use the commands.

Alright, let me show you one more. "Start, End, Radius". In the same way, starting point is on top, and ending point is there at the bottom.

What is the radius? You can key in the radius or you can identify the center of the circle

and autoCAD will calculate the radius itself. So, now we have all the arcs required by this diagram.

This is a simple example, whereby we have used many different commands so as to get you familiarised with the different ways of drawing arcs.

I hope you will practice more.

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