Mar 29, 2008

Snap Mode

Status Bar Snap (right-click for settings)
Pull-down None

SNAP or F9

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 on and the Grid is displayed, the Snap and Grid spacings are the same and 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.

Command Sequence
Command: SNAP

Specify snap spacing or [ON/OFF/Aspect/Rotate/Style/Type] <10.0000>: (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 click the SNAP button on the status bar.

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

"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).

The "Type" option allows you to set the snap type to either Grid (the default) or to Polar. The Polar option can be used in conjunction with Polar Tracking so that Snap mode snaps along polar tracking angles rather than to the grid.

The grid snap is particularly useful if you need lots of modular objects such as bricks or paviors. In the illustration on the left, the Aspect option is used to set the X and Y snap spacings to the brick dimensions and the Rotate option is used to set the orientation of the bond. Once these settings are made, the bricks can be accurately drawn without any other drawing aids.

All of the Snap variables can also be set using the Drafting Settings dialogue box. Right-click on the SNAP button and choose Settings… from the menu.

Setting Grid Limits

Toolbar None
Pull-down FormatDrawing Limits
Keyboard LIMITS

Drawing Limits is used to define the extent of the grid display and to toggle Limits mode which can be used to define the extent of your drawing. The grid is displayed within a rectangle defined by two pick points or co-ordinates.

Command Sequence
Command: LIMITS
Reset Model space limits:

Specify lower left corner or [ON/OFF] <0.0000,0.0000>:
(pick point, enter co-ordinates or Return to accept the default value)
Specify upper right corner <420.0000,297.0000>:
(pick point, enter co-ordinates or Return to accept the default value)

Drawing Limits can also be used to turn Limits mode on or off. Limits mode can be used to control where objects can and cannot be drawn. 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

Limits mode is useful if you know the extent of your plotted drawing sheet and you want to prevent objects being drawn outside of this area. However, Drawing Limits is most commonly used simply to control the extent of the Grid.

The Drawing Grid

Status Bar Grid(right-click for settings)Pull-down None
Keyboard GRID or F7

The drawing grid is a regular pattern of dots displayed on the screen which acts as a visual aid, it is the equivalent of having a sheet of graph paper behind your drawing on a drawing board. You can control the grid spacing, so it can give you a general idea about the size of drawn objects. It can also be used to define the extent of your drawing. See, Setting Grid Limits, for more details.

Command Sequence
Command: GRID
Specify grid spacing(X) or [ON/OFF/Snap/Aspect] <10.000>: (enter grid spacing)

Grid spacing set to
10 drawing units

Grid spacing set to
5 drawing units

Although you can use the command line to control the visibility of the grid by using the "ON" and "OFF" options this is more easily achieved using the F7 key or, better still, by clicking the GRID button on the status bar. However, the command line does offer some additional options. The Snap option allows you to automatically set the grid spacing to the current snap spacing. You can also change the aspect ration of the grid. By default, the X and Y spacing of the Grid are the same, resulting in a regular square matrix of grid points. But 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 Drafting Settings dialogue box. You can access grid settings by right-clicking the Grid button on the status bar and selecting Settings… from the menu. You can also do this from the pull-down menu, ToolsDrafting Settings… and click on the "Snap and Grid" tab.

You may have noticed that the grid does not extend infinitely in all directions. In fact, it is only displayed within a finite rectangle. You can control the extent of the visible grid using Drawing Limits.

Mar 12, 2008

Ortho Mode

Status Bar Ortho
Pull-down None
Keyboard ORTHO or F8

Ortho is short for orthogonal, which means either vertical or horizontal. Like the other options on the status bar, Ortho is not really a command, it is a drawing mode which can either be turned on or off. Ortho mode can be toggled on or off in one of three ways. The quickest way is just to click on the ORTHO button on the status bar. The appearance of the button tells you whether Ortho is currently turned on or turned off. When Ortho is turned on, the ORTHO button appears pressed in. You can see how this appears by looking at the status bar illustration below. In the illustration, Ortho is turned on but Grid and Snap are turned off.
Status Bar
Ortho can also be toggled on and off using the F8 Function key (see Function Keys below for more details). Finally, you can also type ORTHO at the command prompt as in the command sequence below. Using Ortho is the equivalent of using your parallel motion and set square on a drawing board. With Ortho mode turned on you can only draw lines which are either vertical or horizontal. Turn Ortho mode on now and draw some lines to get a feeling for how it works.

Command Sequence
Command: ORTHO
Enter mode [ON/OFF] : (type ON or OFF)

Ortho mode is probably the simplest of all the drawing aids, and historically one of the oldest. It is either on or it is off and there are no special settings to make. Also, it does a very simple job; it constrains drawn lines to the horizontal or the vertical. You may not be surprised to learn, therefore, that its use has largely been superceded by more recent features, particularly Polar Tracking, described below.

Drawing Aids

Drawing with AutoCAD is really just like drawing on a drawing board. Most newcommers to Computer Aided Design assume that they will need to learn how to draw all over again. In fact, many of the drawing aids that AutoCAD provides are analagous to traditional drafting tools. Just as you have a parallel motion and set squares to help you draw horizontal and vertical lines on a drawing board, AutoCAD has similar drawing aids which can help you to draw horizontal and vertical lines on a computer. This means that in many respects, the drawing techniques are very similar. If you ever get stuck, think how you would complete a task on a drawing board and then look for a similar way to do it with AutoCAD.
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