Unit 4: Basic CAD Drawing Techniques for Mechanical

Overview

Most mechanical drawings use decimal form of units. You will first learn how to set decimal units and precision of units commonly used in mechanical CAD drawings. Next you will set up a template drawing. This template drawing will be used in later units to speed up and standardize the drawing process.

Objectives

Introduction

Most mechanical drawings use a decimal type of scale, such as 4.25". Even though you will set your units to decimal for mechanical drawings, AutoCAD allows you to use any type of units in your drawing. If it is more convenient to enter information in feet and inches, for example, you can change your units at any time. Changing units or entering units in a different format will not affect any other objects in the drawing.

A template drawing normally contains values for limits, grid, and snap. After setting the correct units, you will create a template drawing containing the values for limits, grid and snap commonly used in mechanical drawings. template drawings will be created for Metric as well as English drawings.

Section 1: Setting Display Format and Precision (UNITS and DDUNITS)

To set the units, access the Units Control dialog box. This can be accomplished by:

While AutoCAD had five different measurement report formats, decimal units are generally used in mechanical drafting, because ANSI Y14.5M Dimensioning and Tolerancing standards specify that decimal inch or metric units in millimeters be used in engineering drawings. AutoCAD can display a maximum of eight decimal places. Decimal is the default type of unit in AutoCAD.

After setting the measurement report format to decimal units, the next step is to set the display precision. AutoCAD rounds the value of the display, so two digits may be shown as 2.88, three digits as 2.875, and four digits as 2.8751. Regardless of what you set the display precision at, AutoCAD is accurate to 14 decimal places.

For mechanical drawings, three to four digit display precision is normally used for inch drawings. For metric drawings, one- or two-place decimals are generally used, such as 16.5 or 16.50. This difference is due to the greater distance represented by inches as opposed to millimeters.

Angular Measurement

Angular measurement is also set in the Units Control dialog box. AutoCAD also has five different angular measurement options. Decimal degrees are generally used in mechanical drafting, which is the default type of angular measurement in AutoCAD.

Decimal degrees displays angular measurement as real numbers with up to eight decimal places, such as 42.500. Two-place decimal degrees are normally used for mechanical drawings.

Tutorial 4.1: Setting Units and Angular Measurement

In this tutorial you will set the units and angular measurement for a Metric drawing.

  1. From the Standard toolbar, select New. If prompted with the Save Changes dialog box, select No. In the Create New Drawing dialog box, enter the proper path, name the drawing MDEFAULT, then select OK.
  2. Choose Format/Units.
  3. Set the units to Decimal, zero digits for Precision.
    Set the angles to Decimal Degrees, precision 0 (AutoCAD default).

    Your Units Control dialog box should look like fig. 4.1

     

04fig01.gif (7790 bytes)

Figure 4.1 The units and angular measurement for a Metric drawing.

  1. Choose OK to close the Units Control dialog box. The next tutorial will continue from here.

Section 2: Working with template Drawings

A template drawing may be defined as any drawing file that you want to use as the template for the new drawing you are creating. Proper use of a template drawing can save a lot of work. When you begin a new drawing you normally set different variables, such as the limits, snap increment, and grid spacing. The advantage of using a template drawing is that it frees you from having to change these same settings every time you start a new drawing.

Tutorial 4.2: Creating a Metric template Drawing

In this tutorial you will create a metric template drawing. You will set the limits, grid and snap to values commonly used in a metric A size drawing.

  1. Continue from the previous tutorial.
  2. Choose Format/Drawing Limits.
  3. At the <Lower left corner><0,0>: prompt, press Enter.
  4. At the Upper right corner <12,9>: prompt, type 230,170 and press enter.

To show the entire limits within the view window, you issue the ZOOM ALL option:

  1. From the Standard toolbar, choose the Zoom All icon.

After the limits are established, you set the grid to 4 and the snap to 2:

  1. Choose Options/Drawings Aids to access the Drawing Aids dialog box.
  2. Enter 4 for the grid X and Y spacing.
  3. Enter 2 for the snap X and Y spacing.
  4. Check the Snap on and Grid On check boxes. Your dialog box should appear as shown in figure 4.2

04fig02.gif (11368 bytes)

Fig. 4.2 Setting the Grid to 4 and the Snap to 2 for a Metric drawing.

  1. The MDEFAULT template file is now ready to be saved. To save the drawing, click the SAVE icon on the Standard toolbar.

Section 3: Coordinate System Basics

Even though you may have set grid and snap modes in your template drawing, any coordinate entered through the keyboard overrides these settings. This is true even if the desired point location does not fall on the grid or cannot be snapped to, or if the To point: location is at an angle to the previous location with ortho mode turned on. Remember you can switch the coordinate entry method at any time during a command sequence.

Assignments

For the mechanical drawings you will first create a template for an Engish drawing. You will then use that template to create a simple drawing.

    Creating a Mechanical template drawing and a simple drawing using Object Snap and Object Tracking.