Page 80 - Engineering drawing from first principles using AutoCAD

Pictorial projections 73

Youwill find it advantageous to always use the ZOOMfeature to draw on the largest screen
area possible. There is no point in struggling in a small area as errors become obvious to
you when enlarged. Remember also to keep the SNAPbutton engaged if you can.
Figure 3.4 shows an isometric view of part of a circular bar. Copy the given view and
prepare the pictorial base with lines at 30° to the horizontal. Distances DC =EF =70mm.
DA in the pictorial view =DA=95 mm in the orthographic view. Distances EF and AB in
the pictorial view give the dimensions for the minor and major ellipse on the sloping face.
It is not necessary to use the ISOMETRIC SNAP/GRIDfeature as lines at 30° can easily be
drawn.
Both ellipses can be drawn using the Ellipse feature from the Draw menu. The Com-
mand line will read
Command:
ELLIPSE
<Axis endpoint l>/Center:

Click the left mouse button on point B and the command line reads Axis endpoin t 2 :.
Click on point A at the other end of the major axis and the command line changes to
<Other axi s di stance> /Rota tion : . Click on either end of the minor axis across the
sloping face and the ellipse will be drawn.
Repeat for the base circle and use the BREAI( feature to remove the unwanted part.

Oblique projection
In this form of pictorial projection, one face of an object is drawn in its normal front
view and then its thickness is given by projection lines generally at 45°. This is
probably the most common angle for convenience of drawing but others can be
used. Figure 3.5 shows two illustrations of a cube. You will note that the right-hand
cube in cavalier projection looks out of proportion if the lines on the oblique plane are
drawn true to size. To improve the appearance, these true lines in cabinet projection
on the left-hand view are halved in length. The problem arises due to lack of
perspective in this style of drawing. To introduce perspective would complicate and
considerably extend the draughtsman's work and make the job uneconomical. Cabinet
projection is generally the accepted choice. If curves are present on one face it is often
possible to present them in the foreground view but if they appear on two or three
faces then they need to be plotted as shown in Fig. 3.5.
The example in Fig. 3.6 shows a cabinet projection of a cube and the method of drawing
circles on each face. The cube has sides of length l20mm with a circle of 100mm
diameter. The three corner lines of the cubes on the receding faces can be drawn as
described before for a typed line input using @60<45 at the command line. The dimen-
sions for the distances between ordinates will be 10mm measured along the 45° lines.
Note that for the curve construction on the right-hand face, you will need to measure
distances of 5, 10,20,30,10,50 and SSmm. For convenience, I find it quick and easy to
do this by drawing circles from point A, using the intersections with line AB to insert the
ordinates, and then erasing the circles afterwards.
I have projected the construction lines from the front face giving sufficient points to
construct the curves, and have lined in one of them. This curve is not an ellipse but
can be constructed by drawing a polyline from point to point with a a succession of
straight lines. Select the Edit Polyline feature in the Modify menu. The command
line will read