A circle is one of the most basic objects you can create.
There are several different methods you can use to create circles that are tangent to other objects. The first method is called Tangent, Tangent, Radius. You can find this method in the Circle split-button in the Draw panel of the Home ribbon. The Tangent, Tangent, Radius method creates a circle with a specified radius that is tangent to two existing objects. Click to select that method.
Suppose that you want to create a circle that is tangent to two lines. In the command window and at the dynamic command prompt, you can see that the program is prompting you to specify a point on the object for the first tangent of the circle, and as you move the cursor anywhere over one of the lines, you see a small glyph and a tooltip appears that says Deferred Tangent. This is an object snap that automatically pops up and allows you to snap to that geometry. Click to pick a point on one line. Then move the cursor over to the other line. The prompt changes. It now asks you to specify a point on an object for the second tangent of the circle and again you see the same glyph and tooltip. You can click to pick a deferred tangent point on the other line.
You still do not see the circle, but now the command is prompting you to specify the radius of the circle. There is a default value from the last time you used the CIRCLE command. You can either pick points to specify the radius or type the value. Type in a value of “1.5 “. As soon as you press ENTER, the circle is created. Notice that the circle is tangent to the two lines. It is not necessarily drawn at the points you specified, but it is tangent to the lines you specified. If you had specified a smaller radius, the tangency points would be closer to the intersection of the two lines.
Create another circle tangent to those same two lines. Go back to the Draw panel in the ribbon. Notice that the Tangent, Tangent, Radius method is now the default method shown in the Circle button. Any time a button includes a flyout like this, whichever option you choose from the flyout becomes the new default method assigned to the button. It floats to the top. So you do not have to click the arrow; this time you can simply click the button.
Again, click to select the first line, but this time click to pick a point near the end of the line. Then, click to select the other line. Next, when the program prompts you to specify the radius, type “.75 “, half the size of the previous circle. Notice that the new circle is still tangent to the two lines, but now it touches them much further from the points you used to select those lines.
You can use the Tangent, Tangent, Radius method to create a circle tangent to any two objects. Click to select each of the lines, and then press ENTER to accept the default radius value. Again, the program creates a circle of the desired radius tangent to the two objects you selected.
Consider the Tangent, Tangent, Tangent method. Suppose you want to create a circle that fits exactly inside the triangle so that it touches and is tangent to all three sides.
In the Draw panel of the Home ribbon, expand the Circle split button and choose the Tan, Tan, Tan tool. Pick a point anywhere on one side of the triangle, then anywhere on the second side, and then anywhere on the third side. The program automatically calculates the radius of the circle that fits perfectly inside the triangle and is tangent to each side.
Source: Autodesk
