Page 257 - Engineering drawing from first principles using AutoCAD

250 Engineering drawinq [rom first principles

body is a metal casting and the flanges at each side are designed to be welded into a
pipeline. The valve seat (part 4) is a tight fit in the recess at the centre of the body. The
valve (part 3) is free to lift up and down in the seat, and is also guided at the top by the
spindle which projects into the 10mm bore in the valve cover.
As a draughting and an assembly exercise, draw all of these dimensioned details then
save them on to a separate file and assemble the components as previously described.
You will see from the solution (Fig. 10.33) that the internal valve would not be drawn in
section. Make sure that you use the SNAPfeature to move the valve over the centre line
of the valve seat. You will need to remove the chamfer lines on the seat. Turn SNAP off
temporarily and engage ORTHO while you move the valve vertically down onto the
seat.
Youwill also need to remove two lines in the recess to house the seat in the body and then
the previous sub assembly can be lowered into position. Finally screw the valve cover into
position and note that when threads are assembled, the cross-hatching is removed in the
area where the threads are engaged.
In service, flow will be possible from right to left where pump pressure lifts the valve.
Flow would not be possible from left to right since the valve would be pushed firmly
against its seat.

Exercise 8: safety valve

The component parts of a safety valve for fitting to a pressure vessel are detailed in Fig.
10.34. If the air in the vessel exceeds the design pressure, the valve is pushed away from
the conical seat in the valve base and air is vented through the hole in the body. The valve
is held against the seat by a spring, part 3, which is wound from 8mm diameter wire with
an inside diameter of 22 mm. Draw a sectional view through the valve as shown in Fig.
10.35. The standard for a spring consists of one and a half turns at each end and the centre
lines through each side. Springs are normally wound and then ground at each end to give
a flat surface for accurate seating. The parts at each end are designed to locate and guide
the spring. You can construct the spring from circles and semicircles. Draw one of each
and cross-hatch them. Use COpy and MOVE. Alternatively, just construct the spring at
one end, then take a copy, use ROTATE and reposition at the other end. Adjust the cross-
hatching to suit sizes of components.
Note that if the components had been assembled so that the vent hole in part 4 had been
positioned at the top, instead of at the side, and a similar section drawn, then the centre
line of the hole would have been positioned on the cutting plane. Figure 10.36 gives the
construction for the profile of the sides of the hole, which needs to be plotted, since the
sides lie on curved surfaces. The part plans of the holes are divided into six equal
segments and plotted as shown. If the points which lie on the hole profiles are lined in
by polylines you will get a succession of straight lines between the intersections. How-
ever, if you now click on the Edit Polyline feature in the Modify menu the following
options appear at the command line:
Close/Join/Width/Editvertex/Fit/Spline/Decurve/Ltypegen/Undo/
eXit<X>:
Choose Fit and type F. The straight lines will be converted to the best curve through the
points. This is a very handy feature for drawing graphs, developments and irregular
curves.