I was asked the other day about how to draw an arc with a specified length. Well, CAD provides many ways to draw an arc using point, center, and angle. Using arc command will not specify the arc length, but we can use fixed-points, radium and so on to draw an arc with approximate length and use LENGTHEN command to modify the arc length.

When drawing line segment, we can directly specify the distance of the line segment, but similar practices are not very often in drawing arc, so LENGTHEN command is not a commonly-used command. Actually, there are some to explore in LENGTHEN command, for example, we can set the parameters: percent, the total length and dynamically lengthen the arc. OK, Let’s explore all these together.

Firstly, draw something simple, a 100-long line, an arc and a polyline will do, as shown in the following picture. After your drawing, take a look at the length of these graphics in the Properties panel (CTRL+1), as shown in the following picture.

Set the Delta for the LENGTHEN command

1.Enter LEN and press Enter (or execute the LENGTHEN command at the menu and Tools panel), the Command line shows related parameters, as shown in the following picture.

2. Enter DE, press Enter, enter 30, press Enter, when prompted to select an object, click the side of the line that needs to be lengthened, as shown in the following picture.

The line is extended 30 from the side that you click the mouse, the Command line shows prompts to select the object to be modified, that is to say that you can continue to lengthen other graphics at 30 and if you select the wrong object, you can undo it by entering U, as shown in the following picture.

3. Try clicking one side of the arc, press Enter to terminate the LENGTHEN command, as shown in the following picture.

The Delta can be negative numbers, for example, we can enter -30 to shorten the graphics.

Set the Percent parameter

Percent (P) is to how much you want the original graphics to be modified, for example, if you want 100-long graphic to be 80-long, then enter 80, that is 80% of the original graphics; if you want it to be 150-long, then enter 150, that is 150% of the original graphic. But please note that we are talking about the percent before and after the modification instead of the increments.

Set the Total parameter to lengthen an arc length to specified length

The following steps show how to lengthen an arc to the length that you want.

  1. Enter LEN, press Enter, enter T, press Enter, enter 100, press Enter, and click one side of the arc, as shown in the following picture.

2. After the modification, we can make sure if the arc lengthen has already been modified by using Dimarc command.

The Dynamic parameter in LENGTHEN command

Dynamic (DY) in LENGTHEN command is to use the cursor to specify the position of one end of the graphics. This is not a commonly-used practice, but you can try for yourself.

Lengthen command is indeed not commonly-used, for we can use TRIM and EXTEND command to edit graphics when needed, but as far as I know, LENGTHEN command is more convenient when it comes to draw an arc with specified length.

Please note that LENGTHEN command applies to line, arc, polylien except Spline.

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