I can't get my hand around the units

I can't get my hand around the units

Postby guest 1 » Fri Feb 14, 2014 12:16 am

First I read the drawing in AutoCAD is unitless, then I come across the text that said, that you can assign feet-inch, metric, engineering units, surveyor unit etc. to the drawing. How can work with different units, when the drawing itself is unitless?
guest 1
 

Re: I can't get my hand around the units

Postby Guest » Sun Mar 02, 2014 4:04 pm

guest 1 wrote:First I read the drawing in AutoCAD is unitless, then I come across the text that said, that you can assign feet-inch, metric, engineering units, surveyor unit etc. to the drawing. How can work with different units, when the drawing itself is unitless?


Consider you are unitless at first, however when you need to dimension the drawing, you would want to have some units displayed i.e. ft-inch or meters etc. at that point you can tell whether the units be ft-inch or metric. Use the DDUNITS command to access the units functions.
Guest
 

Re: I can't get my hand around the units

Postby Moss R » Thu Nov 27, 2014 12:55 am

Some useful hints from CADD Primer:

Locating Points Accurately
In the previous exercise you entered a number of points to draw different drawing elements. The points were entered using the grid points as a reference. However, you can not be certain that the points were entered exactly on the specified grid points. Even if the points were off by 1/100th of an inch, this inaccuracy is not acceptable in a CADD drawing.

CADD includes separate functions that enable you to locate points with pinpoint accuracy. These are called object snaps that are included in a point-selection menu or toolbar. You can use a specific object snap tool button to locate a point at a specific point.

Fig. 2.7 shows an example of a point selection toolbar that shows a number of options to locate points. When you want to locate a point exactly at the end point of a line or arc, click on the end point tool button and then indicate a corresponding point in the drawing. The cursor just needs to be near the end point and it automatically snaps to it.

When you need to locate a point at the intersection of lines or arcs, you can use the intersection object snap. To locate a point exactly in the middle of a line, you can use the mid point object snap. Other options in the tool bar allow you to locate points perpendicular to a line, at the center point of a circle, snap to an existing point, etc.

Notes:

• Using object snaps is the key to creating professional CADD drawings. At times, you may feel that you quickly need to finish a drawing and you may save some time without using the object snaps. This will certainly result in an inaccurate drawing. When you dimension this drawing, it will create inaccurate dimensions.

• Most CADD programs allow you to set specific object snaps as a default. When a default is set, you don't need to click on the object snap tool button. The default is automatically used when you enter a point. For example, in the previous exercise, you can set existing point (or grid point) as a default and all the points will be entered exactly on the grid points.

• At times you will find that you are not able to locate a point when you have selected an object snap. It may be that you have selected an object snap, but did not point to the same condition in the drawing. If the computer cannot find the corresponding point, it displays an error message.













Fig. 2.7: Object snaps are used to precisely locate points in the drawing area.
The Coordinate System
The coordinate system is another method of locating points in the drawing area. It enables you to locate points by specifying distances from a fixed reference point. You can locate a point by giving its distance in the horizontal direction, vertical direction, measuring along an angle, etc.

The coordinate system is available when a function requires data input in the form of point locations. You may use it while drawing, editing or any time you need to locate a point. The most common coordinate systems are as follows:

• Cartesian coordinates
• Polar coordinates

Note:

The method of entering coordinates varies from one program to another. For example, in AutoCAD, the coordinates can be entered directly in the command window with the keyboard. In MicroStation and Cadkey, special dialog boxes let you enter the coordinates.

Cartesian Coordinates
Cartesian coordinate system is a rectangular system of measurement that enables you to locate points with the help of horizontal and vertical coordinates. The horizontal values, called X-coordinates, are measured along the X-axis. The vertical values, called Y-coordinates, are measured along the Y-axis. The intersection of the X-axis and Y-axis is called the Origin Point, which represents the 0,0 location of the coordinate system (Fig. 2.8).


Fig. 2.8: Measuring distances using Cartesian coordinates.

The positive X-coordinates are measured to the right of origin point (or Y-axis) and the negative X-coordinates are measured to the left. Similarly, positive Y-coordinates are measured above the origin point (or X-axis) and the negative Y-coordinates are measured below. To enter a coordinate, you need to enter both the X and Y values separated by a comma (X, Y).

Example: To locate a point three units to the right and three units above the origin point, as shown in Fig. 2.8, enter X = 3, Y = 3 or (3,3). Similarly, to locate a point three units to the left and two units below, you need to enter X = -3, Y = -2 or (-3, -2). You can use fractions (if required) to enter the exact coordinate values.

Polar Coordinates
Polar coordinates allow you to define a point by specifying the distance and the direction from a given point. This mode of measurement is quite helpful when working with angles. To draw a line at an angle, you need to specify the length of the line (distance) and the angle.

Fig. 2.9 illustrates the concept of angle measurement in CADD. The radial lines show the angles measurements and the concentric circles show the distances. To draw a line as shown in the Fig. 2.9, you can enter two points using polar coordinates. To enter the point on the left, enter 120 as the angle and 5 units as the distance. To enter the point on the right, enter 30 as the angle and 4 units as the distance.




Fig. 2.9: Measuring distances using polar coordinates.
The Formats to Enter Coordinates
Cartesian or polar coordinate values can be entered in two formats:

• Absolute
• Relative

Absolute format is a way of measuring distances from a fixed reference location (origin point), which is the 0,0 location of the coordinate system. Consider this point to be stationary at all times. You can use this point as a reference to measure any distance in the drawing. Absolute coordinates are primarily used to adjust the alignment of diagrams in a drawing, to align one drawing with another or to make plotting adjustments.

Relative format is a way of measuring distances from the last point entered. All measurements are taken the same way as the absolute coordinates, with the exception that the relative coordinates are measured from the last point entered. When a point is entered, it becomes the reference for entering the next point and so on. This mode of measurement is frequently used for drawing because it is always convenient to place the drawing components relative to each other rather than a fixed reference point.

Note:

Each CADD program uses its own annotations to enter absolute or relative coordinates. The task table located at the end of this chapter shows some examples from leading CADD programs.
User-Defined Coordinate System
CADD allows you to create a user-defined coordinate system that can help simplify drawing. When you need to work with a complex drawing that has many odd angles you can create a user-defined coordinate system.

To define a user-defined coordinate system, you need to specify where you want the origin point and the direction of X and Y-axis. Thereafter, the computer works according to this customized coordinate system.

Let’s say you need to draw or modify an odd-shaped diagram, such as the one shown in Fig. 2.10. It is very difficult to use cartesian or polar coordinates because they would involve extensive calculations. In this case, you can create a user-defined coordinate system that aligns with the odd angles of the diagram (28.5 and 118.5). Now, if you draw a 10 units long line in the X direction by entering cartesian coordinates (X=10, Y=0), it will be automatically drawn at a 28.5 angle.


Fig. 2.10: A user-defined coordinate system can be used to work with odd-shaped diagrams.

Important tip:

The user-defined coordinate system is especially helpful when you are working with 3D. In a 3D drawing, you need to define each point with three coordinates and work with various surfaces of a 3D model. The user-defined coordinate system allows you to align coordinates with a specific surface, greatly simplifying 3D drawing (illustrated in Chapter 7 “Introduction to 3D”).
A Prototype CADD Drawing
A CADD program is designed to meet the needs of a vast number of individuals. It comes with a number of options that can be customized. For example, architects, engineers and surveyors use different annotations to measure units and to draw dimensions. You may have a number of preferences regarding a specific style of text, standard sheet sizes, names of layers, symbol styles, borders, or line types to be used in a project.

When you install a CADD program, it does not have any of the defaults set as per your requirements. You can create a prototype drawing and specify all the defaults that you would like to use. This prototype drawing acts as a template that can be used for all future projects.

When you start a new drawing, you can specify the name of the prototype drawing and all the defaults are taken from this file. This gives you a drawing of a specified size, with border type and all the other defaults set in the prototype drawing. When you start working with CADD, it is better to follow a prototype drawing. It saves a significant amount of time in starting a drawing, because CADD automatically sets the drawing environment for you.

Most programs allow you to create more than one prototype drawing and save them under different filenames. This is particularly helpful when you are working on several projects that require different drawing standards. Whenever you need to use a specific standard, you can specify the name of that prototype drawing and those defaults are activated.

Note:

CADD programs usually come equipped with a prototype drawing that contains certain preset defaults. These defaults are too general and may not meet your requirements. In most instances, you will need to modify them.
What if You Made a Mistake?
If you make a mistake in issuing a command or in entering any data, you can fix it without many hassles. CADD provides a very convenient way to fix mistakes. Most CADD programs have a built-in function (commonly known as UNDO) that instantly reverses the effect of the last command entered. Many programs allow you to use this function multiple times, so you can go back many steps to fix the mistakes. However, it is always better to take quick action as soon as you realize that a mistake has been made, because the more you go forward with the mistake, the harder it may be to fix it.
When to Save the Drawing or Quit
When you feel that sufficient work has been completed, you should save the drawing. As a guideline, approximately a 15-minute interval is considered appropriate to save your work. This ensures that if you lose your drawing for any reason, you will only lose a maximum of 15 minutes worth of work.

You can save the drawing in two ways: save the drawing with the same name or save the drawing with a different name. When you save the drawing with the same name, it overwrites the old information on the disk with the current information. Before issuing this command, you must ensure that you no longer need the old information. When you save the drawing under a different name, it makes a copy of the drawing and leaves the original data on the disk undisturbed.

There are times when you may not want to save a drawing at all. When you make an irretrievable mistake, you may not want to save it. If you save, it will replace the last updated drawing on the disk with the damaged drawing. If the drawing seems to be damaged, it is better to just exit the drawing. You will lose the work you did since the last time you saved it, but it may be better to revert to the last version of the drawing.
Automatic Data Saving Features
CADD comes with a number of built-in safety features that can help safeguard data. Most CADD programs automatically create a back-up file when you save a drawing. The back-up file contains the original information that was in the file before you saved it. So, if you save a drawing and later realize that you need the earlier version, you can retrieve the original information from the back-up file.

Notes:

• The same back-up file keeps updating when you save the drawing, so you can only retrieve the most recent version. As soon as you realize that you have made a mistake, retrieve the information from the backup file. Do not save the drawing again, because it will overwrite the back-up file.

• You can also set CADD to automatically save your drawing at preset intervals while you’re working. This feature keeps on saving the current drawing at regular intervals. If you ever forget to save your drawing and it is lost, you can retrieve it from the automatic save file. The automatic save feature saves the drawing in a separate file without disturbing the original or back-up file.
CADD Filing System
The computer stores information in blocks of data called files. Each drawing created with CADD is stored as a separate data file. Similarly, if you create documents using other programs such as a word processor or spreadsheet, then these too are stored as separate data files.

In addition to data files, system software and application programs have hundreds of files of their own. The computer can be used to store and manage thousands of files depending on the available memory. To manage a large amount of files, good file organization is very important.

Files are organized in directories that make it easy to locate them. A directory is like a file folder that may contain any number of files and sub-directories. Each file is identified by its name, directory and sub-directory.

Fig. 2.11 illustrates the concept of creating directories and sub-directories on a hard disk. As shown, there are separate directories created on the hard disk that contain different programs, drawings and other documents. The drawing directory has three sub-directories under it that contain Architectural, Engineering and Surveying projects. The architectural directory has three sub-directories under it. Finally, there are drawings under project directory PROJECT 1001.

Fig. 2.11: CADD drawings are stored as individual files and organized into directories.

There are certain conventions used to name and store drawings. The operating system and the application program you are using establish these parameters. For example, when you are using the Microsoft Windows operating system, a file is located by specifying its name and directory path. To refer to PLAN 01.DWG in the example above, the directory path will look like as follows:

C:/DRAWINGS/ARCHITECTURAL/PROJECT 1001/PLAN-01.DWG

Notes:

• The three letters following the period after the drawing name is called the “file extension”. The file extensions are used to specify the kind of information a file contains. The extension .DWG is used to identify drawing files in AutoCAD and in many other programs. MicroStation uses .DGN and Cadkey uses .PRT as drawing file extensions.

• The number of directories and sub-directories that should be created on a disk depends on the number of files that are to be stored. The objective is to group similar information into directories so that the files can be easily located. You may develop any logical system to organize your files. You may decide to organize files according to project titles, dates, geographic location, name of the user, client or project manager.

• US National CADD Standard committee has developed standards to name drawings. Information on these standards can be obtained by logging on to a web site at http://www.nationalcadstandard.org.
A Word about Data Security
The most important task while working on a computer is to safeguard data. Data may be lost due to any number of reasons. Your hard disk may be damaged or data on it may be corrupted. You may forget to save your work and turn off the computer. You may erase a file when you meant only to move or copy it. A computer just follows instructions and takes action in a split second. You can lose hundreds of files within seconds just by pressing a wrong button.

The most important way to safeguard data is to organize your data and take preventive measures. You need to develop standards for naming and organizing files and directories. You need to create directories and sub-directories to store different projects. You need to segregate old work from new work and classify files according to dates, projects, user names, etc.

Make back-up copies of whatever you do. After each working session, store your work on floppy disks as well as the hard disk. Keep a written record of what the files contain. Develop a timetable for how often you will back up your entire hard disk to a back-up tape or other storage device. Keep multiple back-up copies according to dates and time. Store them at a remote location for safekeeping.
Getting Help
One of the first functions you should learn when starting to work with CADD is how to get help. Most CADD programs come with built-in help features that quickly get you started. Getting help is as easy as typing the word Help on the keyboard or selecting it from the menu. Most CADD programs provide help in the following ways:
• You can choose to use a basic tutorial, which is usually provided with the program. This tutorial gives you a quick tour of all the functions in the program.
• You can display an index that lists all the commands available in the program in alphabetical order. You can view topics on any of the commands listed in the index. These topics display the basic capabilities of the command and the steps required completing it.
• You can usually access task-based help in on-line help provided with the program. Most modern programs also provide context-sensitive help, such as help with dialog boxes, which provides the information you need when you need it.

AutoCAD, MicroStation & Cadkey Terms
The following are the important terms used in leading CADD programs:
(The exact procedures vary from one program to another) Topic not included.
Moss R
 


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