Page 17 - Engineering drawing from first principles using AutoCAD

10 Engineering drawinq [ro111 first principles

require so enter 15 and press <Enter>. Repeat the procedure at intersection 65,170 and
enter the other value of 25. You will appreciate that for drawing the same size circles at
different centre distances the left and right-hand mouse buttons can be used alternately.
The three tangent lines can be drawn automatically in turn. Disable the Ortho and Snap
features. Select Draw from the menu bar. Choose Line followed by Assist in the menu
bar. Select the Object Snap mode. The Running Object Snap dialogue box will now
appear (Fig. 1.10).

The command line now reads line From point: ddosnap. Click on Tangent and
press <Enter>. The command line changes to line From poin t: ddosnap From
point. Click on one of the circle circumferences and then read To point. Click on
the second circumference. The tangent line is drawn and having completed the operation
the computer awaits the next instruction and shows simply Command:.
Notice the upper- and lowercase letters in the command space and the choice given to
you. After making your selection, insert your option and press <Enter>.
Tofix each tangency point turn off Snap and Ortho. Choose Line from the Draw menu,
then Object Snap in the Assist menu. Click on the Centre option and press <Enter>.
The command line now reads line from poin t: ddosnap. Click the left mouse button
on the circle circumference. Notice that a line will now appear from the circle centre.
Click again on the Object Snap button in the Assist menu, cancel previous Centre
option and select Perpendicular. The command line now reads To point: ddosnap.
Click the left mouse button on the tangent line followed by <Enter> and the normal will
be drawn at right angles to the tangent. Repeat the process to establish the other five
points of tangency.
We now have construction lines in place giving three tangents and the positions of six
tangency points and it is necessary to finish our drawing to professional standards. BS 308
clearly states that drawings should be produced using two contrasting line thicknesses,
thick and thin in the ratio 2:1. Professional manual draughtsmen generally use pens of
thicknesses 0.35 and 0.7mm. The Standard also lists the types of lines to be used for
different applications and these are illustrated later.
Some experimentation may be required here as different plotters, printers, inks and
papers give slightly different results. I normally use a line thickness of 0.4mm for outlines

Running 0 bjeet Snap •

I
! :

,i
Min Max
1 c=J j
•__.w~._._,....w~~.~J
l~,_~~~..",~ ..m"... ,. •••~.... _. .... ~_•• _.... _ ....._,_._._•

Figure 1.10