The Ortho Command

Status Bar
Pull-down	None
Keyboard	ORTHO

Command Sequence
Command: ORTHO
ON/OFF <OFF>: (type ON or OFF)

The Ortho command is very simple, all it allows you to do is to turn Ortho mode on or off. It is rarely used because there are easier ways to toggle Ortho mode. You can use the F8 key (see Function Keys below). You can also toggle Ortho by double-clicking "ORTHO" on the status bar. When Ortho is off "ORTHO" on the status bar is greyed out. When you double-click to turn Ortho on, the word is highlighted and will remain highlighted until Ortho mode is turned off. Notice that Ortho is only one of a number of drawing modes which are indicated on the status bar, see the illustration below. Ortho mode can also be set using the Drawing Aids command.

The Grid Command

Status Bar
Pull-down	None
Keyboard	GRID

Command Sequence
Command: GRID
Grid spacing (X) or ON/OFF/Snap/Aspect <10.000>: (enter the required grid spacing)

Although you can also use the Grid command to control the visibility of the grid by using the "ON" and "OFF" options this is more easily achieved using the F7 key or by double-clicking "GRID" on the status bar. The Snap option allows you to automatically set the grid spacing to the current snap spacing (see The Snap Command below). By default the X and Y spacing of the Grid are the same resulting in a regular square matrix of grid points. However, you can display a grid with different X and Y spacing by using the "Aspect" option. Grid mode and X/Y spacing can also be set using the Drawing Aids command. The extent of the visible grid is controlled using the Drawing Limits command.

The Drawing Limits Command

Toolbar	None
Pull-down	Format/Drawing Limits
Keyboard	LIMITS

The Drawing Limits command is used to define the extent of the drawing, the grid display and to toggle Limits mode.

Command Sequence
Command: LIMITS
Reset Model space limits:
ON/OFF/<Lower left corner> <0.0000,0.0000>: (pick point or enter co-ordinates)
Upper right corner <420.0000,297.0000>: (pick point or enter co-ordinates)

The command is also used to turn Limits mode on or off. Limits is turned off by default which means that there is no restriction as to where points can be picked and objects drawn. When Limits is on, AutoCAD will not allow points to be picked or co-ordinates entered at the command line which fall outside of the specified drawing limits. If you try to pick a point outside the drawing limits when Limits mode is turned on, AutoCAD reports to the command line:

**Outside limits

The Drawing Limits command is most commonly used simply to control the extent of the Grid.

The Snap Command

Status Bar
Pull-down	None
Keyboard	SNAP

Snap mode takes AutoCAD one step further than the drawing board. With Snap mode turned on AutoCAD only allows you to pick points which lie on a regular grid. The Snap grid is completely independent of the display grid. However, the Grid spacing and Snap spacing are usually set to the same value to avoid confusion. You can force the display grid to conform with the snap grid by setting the display grid spacing to zero. The display grid will then automatically change each time the snap grid is changed. When Snap mode is turned, the Grid is displayed and the Snap and Grid spacings are the same, the crosshairs will jump from one grid point to another as you move across the screen. This makes it very easy to draw objects which have a regular shape. The Snap command is used to set the snap spacing and to toggle Snap mode.

Orthographic Projection

Isometric Projection

Command Sequence
Command: SNAP
Snap spacing or ON/OFF/Aspect/Rotate/Style <5.0000>: (enter the required snap spacing in drawing units)

Although you can use the Snap command to turn Snap mode on and off, it is much more efficient to use the F9 function key on the keyboard or to double-click "SNAP" on the status bar.

The "Aspect" option can be used to vary the horizontal and vertical spacings.

"Rotate" is used to set the snap grid to any angle.

You can also set the snap style to either Isometric or Standard (the default) using the "Style" option. The Standard style is used for almost all drawing situations including detail drawings in Orthographic Projection. The Isometric style is specifically to aid the creation of drawings in Isometric Projection (see the illustrations on the right). You can also set all of these variables using the Drawing Aids command.

The Drawing Aids Command

Toolbar	None
Pull-down	Tools/Drawing Aids…
Keyboard	DDRMODES

The Snap, Grid and Ortho mode settings, can also be made from the "Drawing Aids" dialogue box, illustrated below. You can also use this command to control a number of other drawing modes such as Blips, Highlight and Solid Fill. The various options in the "Modes" area of the "Drawing Aids" dialogue box are described below.

The Ortho option is used to turn Ortho mode on or off, see The Ortho Command for details.

The Solid Fill mode option controls whether objects such as Polylines with widths and Solids are displayed filled or in outline. This mode can be controlled from the command line using the FILL command.

When Quick Text is checked, all text objects in the drawing will be displayed with empty boundary boxes. This option is often used to increase Regen and Redraw times when the drawing has a lot of text. This mode can also be set using the QTEXT command.

The Blips option controls whether temporary blips will be displayed when points are picked or entered. Blips can be removed from the display using the Redraw command but they can only be suppressed completely by turning Blip mode off. This mode can also be set using the BLIPMODE system variable.

Highlight mode determines whether objects are highlighted or not when they are selected. You can also control highlighting using the HIGHLIGHT system variable.

The Groups option turns automatic Group selection on and off. When Groups mode is turned on, selecting an object which is a member of a previously defined group will automatically select the whole group.

The Hatch option determines whether the hatch boundary is selected when a hatch object is selected or not. When this mode is turned on, the boundary is automatically selected with the hatch.

The Snap and Grid options in the "Drawing Aids" dialogue box act in the same way as those in the respective commands, see The Snap Command and The Grid Command for details. If you are creating drawings in isometric projection, use the "Isometric Snap/Grid" option to change the grids from the standard orthogonal square grid to a 30 degree isometric grid.

Standard orthogonal grid		Isometric grid

The Function Keys

The F1 key on your keyboard brings up the "Help Topics: AutoCAD Help" dialogue box. You can use this dialogue box to search for help on any AutoCAD command or topic. Simply follow the instructions in the dialogue box and when you have found the item you wish to view, click on the "Display" button. You will usually be given a list of options in the "Topics Found" dialogue box, select the most appropriate and click the "Display" button to see the item.

The F2 key is used to toggle (turn on and off) the AutoCAD text window. This is a floating version of the command window which can be resized to suit your requirements. The text window contains the whole command history from the beginning of the drawing session. If you wish, you can scroll back to see which commands you have used. The text window is also useful for viewing the results of commands like LIST which report to the command line on a number of lines which may scroll off the command window and make them difficult to view.

The F3 key displays the "Osnap Settings" dialogue box. See the Osnap tutorial for details on the use of the DDOSNAP command.

The F4 key on your keyboard toggles tablet mode on and off. This only has an effect if a digitising tablet has been calibrated.

The F5 key cycles through the Isoplanes, this only has an effect if "Isometric Snap/Grid" mode is on. The options are Left, Top and Right. The different options describe the plane in which Ortho mode works. It also affects the orientation of Isocircles drawn with the Ellipse command. The illustration on the right shows a cube with isocircles drawn on the top, left and right faces. Each isocircle was drawn using the corresponding isoplane. See Tips & Tricks for a worked example using isoplanes.

The F6 key is a three way toggle which changes the co-ordinate reading in the status bar. By default the status bar shows co-ordinates using the Cartesian system. You can use the F6 key to turn the co-ordinate readout off and to change to the polar system when you are in pick mode. For a more detailed description of AutoCAD's co-ordinate systems see the "Using Co-ordinates" tutorial.

The F7 key is used to toggle grid mode on and off. When grid mode is on a grid of dots is shown on the screen as a drawing aid. You can set the grid spacing by using "Drawing Aids" from the "Tools" pull-down. The grid points do not necessarily reflect the snap setting, they can be set independently, however, you can force the grid to reflect the snap setting by giving the grid setting a value of zero. The word "GRID" is highlighted in the status bar when grid is set on.

The F8 key on your keyboard can be used to toggle Ortho (orthogonal) mode on and off. When Ortho mode is on AutoCAD will only allow you to draw either vertical or horizontal lines. You can think of it as being a computer version of the parallel motion on your drawing board. You can see if Ortho mode is on by looking at the status bar. The word "ORTHO" is highlighted when Ortho is turned on.

The F9 key can be used to toggle Snap mode on and off. Snap makes the crosshairs jump to points on a defined grid. The snap spacing can be set using the "Drawing Aids" dialogue box from the "Tools" pull-down menu. You can also see if Snap mode is on by looking at the status bar.

The F10 key is used to control the display of the status bar, the display can be toggled off and on.

Tips & Tricks

• One of the most difficult aspects of drawing in isometric projection is the correct representation of circles. Obviously a circle in isometric projection looks like an ellipse (see illustration) but knowing exactly what aspect ratio to draw the ellipse at is difficult. Fortunately AutoCAD makes the whole process very simple. When "Isometric Snap/Grid" mode is turned on, the ellipse command gains an extra option, the "Isocircle" option. Follow the exercise below to draw a cylinder in isometric projection.

Step 2 - Setting the drawing aids
Start the Drawing Aids command by selecting Drawing Aids... from the Tools pull-down. In the "Drawing Aids" dialogue box, turn on Snap, Grid and Isometric Snap/Grid by clicking once in each check box. A small tick will appear in each box when it is turned on. Click on the "OK" button to confirm these mode changes. The graphic window now displays a grid of dots arranged at an angle of 30 degrees and the crosshairs will jump from one dot to another. Notice also, that the crosshairs are oriented in the left hand isoplane.

Step 3 - Setting the correct isoplane
In this exercise, we will draw a cylinder which stands vertically. The circles which we draw must, therefore be drawn in the "Top" isoplane. Use the F5 key on the keyboard to change the isoplane to "Top". AutoCAD reports to the command line:

Command: <Isoplane Top>

Step 4 - Drawing the base circle
Circles in isometric projection are drawn using the Ellipse command. Start the Ellipse command by clicking on the button or by selecting Draw/Ellipse from the pull-down. Now look at the command line:

Command: _ellipse
Arc/Center/Isocircle/<Axis endpoint 1>: I (choose the Isocircle option)
Center of circle: (pick a point in the lower half of the graphics window)
<Circle radius>/Diameter: 30 (enter a radius of 30)

Your drawing should look like the one in the illustration above.

Step 5 - Copying the base circle
Start the Copy command by clicking on the button or selecting Modify/Copy from the pull-down. Now look at the command line:

Command: _copy
Select objects: (pick the circle)
Tip: If you find picking the circle difficult, use the F9 key to turn off Snap.
Select objects:
<Base point or displacement>/Multiple: (pick the grid point in the centre of the circle)
Note: Use F9 to turn Snap back on if you turned it off.
Second point of displacement: (move the crosshairs vertically by 6 grid points and pick)

You should now have two isometric circles, one above the other.

Step 6 - Drawing the sides
Start the Line command by clicking the button or selecting Draw/Line from the pull-down. Now look at the command line.

Command: _line
From point: (use the Quadrant Osnap to pick the lower point of the left hand line)
Tip: There are a number of ways to invoke osnaps, they are available from the Osnap toolbar and from the keyboard. However, in this case it may be simplest to select Quadrant from the cursor menu. To do this, hold down the Shift key on the keyboard and click on the right hand mouse button. A menu will appear at the crosshair position. Simply select Quadrant from the menu. Now, move the crosshairs near to the left hand point on the lower isocircle. You will see a yellow diamond appear at the quadrant point (see illustartion). Pick the point.
To point: (use the Quadrant Osnap again to pick the left hand point on the upper isocircle)
To point: (to end the Line command)
Now repeat this process to draw the right hand line or use the Copy command to copy the left hand line to the right. Remember to use the Quadrant Osnap to pick points whichever method you use. This will ensure that the line is drawn or copied in exactly the right place. When you have completed this step, your drawing should look similar to the illustration on the right.

Step 7 - Trimming the circle
To complete the drawing we will remove the upper half of the lower isocircle to give the impression of a solid cylinder. To do this we will use the Trim command. Start the Trim command by clicking the button or selecting Trim from the Modify pull-down. Now look at the command line:

Command: _trim
Select cutting edges: (Projmode = UCS, Edgemode = No extend)
Select objects: (pick the two vertical lines)
Tip: You may need to turn Snap off (F9).
Select objects:
<Select object to trim>/Project/Edge/Undo: (pick the upper arc of the lower isocircle)
<Select object to trim>/Project/Edge/Undo: (to end the Trim command)

The isometric cylinder is now complete. Use F7 to turn the grid off and your drawing should look similar to the one in the illustration at the beginning of this example.