Page 18 - Engineering drawing from first principles using AutoCAD

First steps 11

and leave the default thickness for centre and construction lines. My Canon BJ-230 ink-
jet printer then gives acceptable linework.
Some parts of the circles are not required on the finished drawing but may easily be
removed. Select the Break option in the Modify menu. At the command line the
instruction reads Command: _break Select obj ect.
We need to choose the circle to be broken, then select a small part of the circumference
for the break. With a circle there are obviously two options. You can click on the
circumference and then the position of the second click can be clockwise or anticlockwise
in relation to the first point. Using one option close together and anticlockwise will
remove just a small arc. The other option with the second point taken in the clockwise
direction will remove the large arc. Just experiment here to prove the point.
To clean up the circle from centre H in Fig. 1.8 click on the circumference at about the 5
o'clock position and the circumference will appear in a dotted line. Click again anti-
clockwise at about 40'clock and the small arc will disappear. We now have a broken circle.
Using the BREAI( command again click on the circumference at the 5 o'clock position
and also on tangency point F,then repeat for the 4 o'clock position and tangency point A
and the required arc will be complete.

Clean up the required arcs from the circles with centres J and I( and the profile of the belt
is defined, but not with the correct line thickness. The thickness of lines can be changed
by clicking Modify in the menu bar and choosing Edit Polyline. The command line will
now read Command: _pedi t Select polyline:. We have three arcs and three
straight lines to edit, so click on anyone of them and the command line now reads:
Enti ty selected is not a polyline
Do you wan t to turn i t in to one ?<Y>
The answer is 'yes' so press <Enter> or click the right-hand mouse button and the
command line now offer many options as follows:
Close/ Join/Width/Edi t vertex/Fi t/Spline/Decurve/Ltype gen/Undo/
eXit.
The first thing is to establish a new line thickness so type W on the keyboard and press
<Enter>. Command line now reads Enter new width for all segments:. Type 0.4
and press <Enter>. The line thickness is increased and the list is repeated at the command
line:

Close/Join/Width/Editvertex/Fit/Spline/Decurve/Ltype gen/Undo/
eXit<X>
We have the result required, so just press <Enter> and the command line will clear and
await the next request. Repeat the procedure for the other parts to complete the belt
profile with a continous polyline O.4mm thick. Note again that by pressing the right-hand
mouse button immediately after you pressed <Enter> that the Modify options were
reinstated at the command line. Use of the right-hand button for repeat commands
will improve your speed of working.
If you later wish to thicken up the profile of a complete thin circle and are using an early
version of AutoCAD you may find when you select the Edit POLYLINE command that the
software will not accept your entry. The secret here is to make a small break in the circle
circumference and try again. You will be lucky this time, but the polyline is incomplete.
After the thick line is drawn the command line repeats the following instructions:

Close/Join/Width/Edit/vertex/Fit/Spline/Decurve/Ltypegen/Undo/
eXit<X>