416 Machine Drawing
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Objects may be selected in any order to unite them with the union command and when it
is required to subtract one region from the other, first select the region from which it is required
to subtract. Condition: these regions exist overlapping, earlier to the operations.
(a) Union (b) Subtract
Fig. 21.17
21.13.2 Solid Modelling
A composite solid can be created from both solid primitives and swept solids or by extruding a
2D object, Boolean operations can be used.
Options:
Union — allows combining the volume of two or more solids into one. Select the
objects to join and AutoCAD creates a single composite object.
Subtract — allows to remove the common area shared by two sets of solids. One must
first select the solid from which to subtract and then the solid (s) which
are to be subtracted.
Intersect— allows creating a composite solid that contains only the common volume
of two or more overlapping solids.
21.13.3 Mass Property
Mass prop calculates and displays the mass properties of 2D and 3D objects. For solids, it provides
volumetric information such as mass, volume, centre of gravity, principal axes and moments of
inertia.
Command: MASS PROP
1. Select objects: pick the solid model (s) to be analysed. This will display all the properties
of the object(s) on the screen
2. Write to a file : if one wants the information written to a file, type Y and provide
a file name.
For regions, the mass properties displayed are area, perimeter, centroid, moment of inertia,
product of inertia and radius of gyration.
21.14 SECTIONAL VIEW
When a section plane cuts a part of the solid and the remaining part of the solid is projected,
that view is known as sectional view. Sectional views are chosen to reveal the inner details (hidden).
One may use full section, half section or off-set section to reveal the hidden details of the object
