Page 76 - Engineering drawing from first principles using AutoCAD

Pictorial projections 69

Drawing notes for Fig. 3.1

Construct this drawing as an exercise. Draw a cube of any convenient size with one face
ACBD shown by the dotted line. Draw the dotted line AOB a suitable distance to the right
having projected the corners A and B. Add the other dotted lines in the right-hand view
to show the thickness and complete an end view of the cube. Imagine the cube to be
rotated in the left-hand view across the diagonal CODuntil angle AI-C-O is equal to 30°.
Complete this view with length AI-C equal to BI-E.
Project lines from points AI, BI and E to the right. Lengths can be trimmed later. You can
now physically rotate the cube in the right-hand view by the ROTATE command and
using the point 0 as the centre of rotation.

Simply turn the cube anticlockwise until the lines intersect as shown. You have two views
in first-angle orthographic projection which is described in a later chapter. The left-hand
view in isolation is an isometric drawing of a cube. Line your work in using polylines.
Note that in your isometric drawing, the true length of the cube ACactually appears as C-
Al, which is shorter in length. This fact is of academic interest only as we use the given
true lengths in isometric constructions. AutoCAD can reduce and also enlarge drawings if
necessary.

AutoCAD has some handy features to allow rapid draughting. The Drawing Aids box in
the Settings menu provides a grid of lines at 30° to the horizontal and vertical lines,
forming a grid of equilateral triangles so that lines can be positioned along the three axes
for accurate measurement.
In the Draw menu we also have an ELLIPSE feature which allows the quick insertion of
ellipses in the three planes. By using the <Ctrl> + E keys on the keyboard we can toggle
and obtain the cursor lines for each of the three planes.

As an exercise to demonstrate basic Isometric drawing, draw a piece of tubing of 40mm
outside diameter, 30mm inside diameter and 25 mm long in each of the planes (Fig. 3.2).
Open the Drawing Aids box from the Settings menu. Insert a SNAPvalue in the box
which suits the required drawing dimensions and in this case 5mm is appropriate. Insert
a value of 10 in the Grid box. Click on the Isometric Snap/Grid box in the bottom
right-hand corner and notice the 'X' value changes to 8.6603 in the SNAP box and to
17.3205 in the GRID box. Click OI{.

The command line reads Command: '_ddrmodes. Use <Ctrl> + E and toggle until you
read <Isoplane Left> at the command line and the cursor lines appear.
To draw the 40mm ellipse, select Ellipse from the Draw menu and the command line
reads

Command: _ellipse
<Axis endpoint l>/Centre/Isocircle:

Select Isocircle and enter I. The command line reads Centre of circle:. Click on
the chosen point and command line reads <Circle radius>/Diameter:. Enter 20
and the ellipse is drawn. Repeat the procedure and enter 15 for the internal 30mm
diameter. At a distance of 25 mm along the axis of the pipe and to the right, draw a
second ellipse for the end of the pipe. Use the TANGENT feature to add the outside
lines.

Choose Line from the Draw menu, then Assist and click on Tangent in the Running
Object Snap dialogue box. Repeat for the other tangent. Remove unwanted linework
and finish the drawing with POLYLINES.