Computer Studies
 Knot Making

Our first drawing

This is where the fun begins! In this tutorial we will draw simple lines and go through the various ways we can input them. By the end of this tutorial you will be able to draw lines using ‘Cartesian’ and ‘Polar’ coordinates, both in absolute and relative form, and direct distance entry! Sounds like gibberish? Keep reading…

Set up the units and the drawing limits if necessary, as you learnt in the previous tutorials, zoom all and let’s get started. My settings are ‘Decimal’ for Length, ‘Decimal degrees’ for Angle and ‘Millimetres’ for Insertion scale, east is zero degrees as by default and limits are 420,297. Check ‘Before starting a drawing’ tutorial if you find any problem with those settings. One more thing you want to check for is the dynamic input, ‘DYN’ mode button for ACAD 2008 in the status bar. In ACAD 2009 you have to move your cursor over the various modes in order to check which one it is. Make sure that it is switched off before you start (older versions of ACAD do not have this setting, this tutorial is compliant with those versions anyway, just ignore this step). These settings will also be used for all the later tutorials unless stated otherwise.

Drawing in Absolute Cartesian coordinates

Let us draw the square shown in the diagram first and then examine how we did it. The command for drawing a line is ‘line’ or ‘l’ for short. You can also use the ‘Draw’ pull down menu or click the ‘line’ tool from the ‘Draw’ toolbar. In ACAD2009 the line tool is in the draw panel under the home tab. Follow the commands presented below:

Command: l
LINE Specify first point: 50,50
Specify next point or [Undo]: 150,50
Specify next point or [Undo]: 150,150
Specify next point or [Close/Undo]: 50,150
Specify next point or [Close/Undo]: c

The first step was typing ‘l’ in the command line. This has been confirmed by the message ‘LINE’ and you have been prompted to specify the first point. Here we have entered the data 50,50. It is important to input the data exactly like this, with the comma in between so that ACAD can recognise that those are two separate numbers. ACAD, like other CAD programs uses Cartesian coordinates in order to keep track of where to draw our lines. Cartesian coordinates also known as ‘x,y coordinates’ are the same as those used in mathematics to draw graphs. By entering ‘50,50’ we have informed ACAD that we wish to start drawing our line from a point which is 50 units away from the origin in the x direction and 50 units away in the y direction.

As soon as the first point has been specified, the drawing area is updated. It will show what is termed a rubber-band, fixed from one end at the point (50,50) and stretching to the cursor at the other end. At the command window, we are being prompted to specify the next point. Here we entered the data ‘150,50’. This specifies that the line will end at this point, 150 units away from the origin in the x direction and 50 in the y direction. The difference in the x direction being 100 units makes this line 100 units long. As there is no difference in the y direction, the line is also perfectly horizontal.

Notice that in this method of ‘Absolute Cartesian coordinate’ entry, we always enter the coordinates of the point we wish the line to be drawn to. Keep entering the data as shown until you get to the last point. To draw the last line of the square we have two options; we can either draw a line to the point (50,50), which is the point we started from, and then press ESC to conclude the line command, or we can simply input ‘c’. Notice that the prompt says ‘Specify next point or [Close/Undo]:’, which means you are asked either to specify a point or Close (by inputting the first letter ‘c’) or Undo your last action (by inputting ‘u’). By choosing to ‘close’, a line is drawn to the first point and the line command is terminated.

Drawing in Relative Cartesian coordinates

Guess what! We’re drawing another square. And I just happen to know what you’re thinking. Take heart, thanks to relative coordinate entry, we don’t have to find the coordinates of the four corners of the square in order to draw it. Just follow the commands shown below and read the explanation that follows as you did before:

Command: l LINE Specify first point: 200,50
Specify next point or [Undo]: @100,0
Specify next point or [Undo]: @0,100
Specify next point or [Close/Undo]: @-100,0
Specify next point or [Close/Undo]: c

The first point entered is ‘200,50’. Thus far all is the same as before, we have just specified that we want our square to start from 200 units away from the origin in the x direction and 50 units away in the y direction. The next point entered ‘@100,0’ is in relative coordinate entry form. This is denoted by the ‘@’ symbol before the coordinates specified. In the relative coordinate entry, you have to type ‘@’ followed by the distance in the x direction from the current point, a comma, and the distance in the y direction. Thus our input means that the line drawn will be 100 units long in the x direction.

The two commands that follow are also in relative coordinate entry. @0,100 implies that the line is drawn vertically, 100 units long in the y direction and @-100,0 draws a horizontal line of 100 units. Notice the negative sign here that implies that the direction in which the line is to be drawn is towards the left. The same applies for the y axes, and if you input a negative number the line will be drawn downwards rather than upwards. The last input is ‘c’ again, which is still the quickest way to end your line command.

Save your drawing if you wish.

Drawing in Absolute and Relative Polar coordinates

Let us start with the Absolute polar coordinate entry. Start a new drawing and set the limits to -200,-150 for the lower left corner and 200,150 for the upper right corner. Zoom all (the coordinate system sign depicting the two arrows representing x and y should be in the middle of the screen) and we’re ready to start.

Hmm! More squares. Different coordinate entry systems are important so don’t start thinking of skipping parts of the tutorial yet. The time you dedicate now will all be re gained a hundred times over through faster production rates later on. Let’s go through these final squares:

Command: l LINE Specify first point: 100<45
Specify next point or [Undo]: 100<135
Specify next point or [Undo]: 100<225
Specify next point or [Close/Undo]: 100<-45
Specify next point or [Close/Undo]: c

The first point specified is ‘100<45’. This denotes that the point is 100 units away from the origin at an angle of 45 degrees. Quite simple isn’t it? The second point is 100 units away from the origin at an angle of 135 degrees and so on and so forth. Notice the last point entered is ‘100<-45’. Angles can also be input in their negative form, remember east is zero degrees unless you have set it to be otherwise in the ‘drawing units – direction’ (ddunits command) dialogue box. To summarise, in absolute polar coordinate entry, we specify the distance of a point from the origin and its angle.

As with Cartesian coordinates, you can also input both absolute and relative polar coordinates. With relative polar coordinates we input the distance and the angle of a point from the previous point entered, rather than from the origin. Follow the example given:

Command: l LINE Specify first point: 50,0
Specify next point or [Undo]: @50<45
Specify next point or [Undo]: @50<-45
Specify next point or [Close/Undo]: @50<225
Specify next point or [Close/Undo]: c

Starting to make some sense? The first point input is in absolute Cartesian coordinates form (50,0). Then the second point is in relative polar coordinates form. @ denotes that it is relative, followed by 50 which specifies the line should be drawn 50 units away from the first point and <45 denotes that it shall be drawn at an angle of 45 degrees. Follow the commands shown and finish off the diamond drawn.

For further practice you can also try to complete the drawing as shown in the diagram below. This time it will be up to you to decide which coordinate entry form is best for the particular line you need to draw. To terminate the line command without closing you can press Enter or ESC, or press the right mouse button and select ‘enter’ or ‘cancel’.

Now save your work!

Direct distance entry

There are actually more ways to specify where a line is to be drawn. The easiest way is to initiate the line command (input ‘l’) and click anywhere on the drawing area where the line should be drawn. The disadvantage of this method is the lack of precision but it may be preferred for some situations where speed is more important than precision. A slightly more precise way to draw a line is by using the direct distance entry method. Initiate the line command and specify the first point. Then point the cursor in the direction you wish to draw the line and at the same time, in the command window input how long you wish the line to be. This will draw a line precisely the length you have input in the command window, but at an angle which is not as precise as it is specified on screen by the cursor. In order to reap the best results out of this method it is sometimes combined with the ‘ORTHO’ mode. Click ‘ortho’ from the status bar or press F8 to switch it on. Start drawing a line clicking anywhere in the drawing area. You will notice that you are restricted to drawing horizontal and vertical lines only. Thus if you now point the cursor horizontally away from the point you started, the rubber-band is set perfectly horizontal and by entering a distance, for example 50, the line will be 50 units long and horizontal.

Dynamic input

At the beginning of this tutorial we have switched dynamic input off. Dynamic input provides the user with an interface next to the cursor so you can keep focused on the drawing area. This should not be thought of as a replacement for the command window, and if you wish to switch it on again, you should be aware that it will slightly alter the way you input commands. It is recommended that for now you stick to the methods explained and leave dynamic input switched off until you have enough confidence to play around switching it on.

Hang on! The days of the humble beginnings are over… polylines, circles, ellipses, arcs, here we come!

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