Welcome to our UPM MOOC
TECHNICAL DRAWING FOR MECHANICAL ENGINEERING
We are now in MODULE 3
Where we are going to talk about part drawing
This module 3 will be divided in 3 units
Unit 1: we talk about constructive solid geometry
which is a methodology for part analysis
Unit 2: we will study components of a part drawing
Unit 3: we will take a view over
international standards of technical drawing
Now let's start with module 3, unit 1
Constructive solid geometry
part analysis methodology
What is a part drawing?
A part drawing is the document that describes
clearly and precisely the geometry
and way of manufacturing of a part
For this purpose we use views, sections and dimensions
that finally yields to the manufacturing
of our real part.
What is constructive solid geometry CSG?
The idea is to divide a part into basic elements
and to join them by boolean operations
Operations like union
that means adding
and difference (subtracting)
that constitute the principal operations
Geometric primitives (box, sphere, cylinder,...)
Are the simplest elements to construct a part.
The part construction is made by steps:
First we need to select the principal element
in this case the major cylinder
2nd we add in the top the small cylinder
The 3rd step is to subtract
cylindrical hole to the prior conformation
And finally we obtain the final part
What is CSG useful for?
We find often interpretation mistakes
in the technical drawings
This is because we don't know
the correct origin of the piece
In the top figure
it has a cylindrical hole that derives a distance
The bottom figure has a cut
by two planes at a distance that derives
a in circular imprint
It is completely incorrect
to make dimensioning of the distance
in the top view
and the circular measurement
in the bottom view
What is CSG useful for?
Well, it really allows us to follow the correct way
that sets the dimensions of the original primitives
and omits the derivatives dimensions.
In the case of the sphere with a cylindrical hole
what we have to represent
are the dimensions of the principal elements
In the other case, the sphere cut by two planes
we have the dimensions relevant to this situation
Let us begin with the fundamentals of CSG
original primitives sizing and location
We originally part from the original primitives
in this case a cylinder and a sphere
Then we need to establish the size
and location dimensions for these primitives
to constitute the first step of the analysis
Well, following the CSG fundamentals
we have the boolean operations
The first one is the union
that makes in an unique volume
the two primitives that before
are sized and located
Difference, in difference we subtract
to one primitive the volume o part of the volume
of the other primitive.
In this case we have
two possible solutions
depending on which element subtracts the other.
A less frequent boolean operation is the intersection
that means the common volume
between the two primitives
that we have to describe the part
One of the fundamentals of CSG
is that the boolean operations are no commutative
In this case we have a part
made of a sequence of operations
that is different if we change
the order of these operations
then we obtain a different part of the initial one.
Well, geometric primitives are
the basic elements for CSG.
These are the basic and regular geometric solids
box, cylinder, torus, frustum, etc.
Also, we have as basic element of CSG
the mechanical features
These are combinations of primitives that
for reasons of standards, performance or manufacturing
have a specific representation and dimensioning.
Repetitive elements, chamfer
fillet, dovetail, countersink
are some examples of mechanical features
Let us now to begin with the description
of the principal primitives
First the box
For sizing a box we need 3 dimensions
that is, height, width and depth
To represent the box
we need 2 views for placing
the 3 dimensions of the box
Linear location could need up to 3 dimensions
And angular location could need
up to 3 dimensions
but this is in very rare cases
The reference for location dimensions
could be corners or symmetry planes of the box
depending on the rest of configuration of the part
The second primitive, the cylinder
is the most frequent primitive in technical parts
Sizing needs 2 dimensions, diameter and height
And as a revolution body
its representation only needs 1 view
the parallel to axel one
Linear axle location would need
up to 2 dimensions
Angular axle location would need
up to 2 dimension
but angular position of the primitives
is a situation that hardly occurs
The reference for location dimensions must be the axle
It is not correct to refer to curved contour
Frustum
Sizing a frustum needs 3 dimensions
minor diameter, mayor diameter and height
As a revolution body
representation only needs 1 view
the parallel to axel one
As cylinder linear and angular location
would need each up to 2 dimensions
but linear occurs very frequently
and angular is a very rare situation
The reference for location dimensions
must be the axle
It is not correct to reference to curved contour
Torus, this revolution primitive needs
2 dimensions for its sizing
first. guideline diameter. and second. generatrix diameter
that is, the tube diameter
As a revolution body, we need only needs 1 view
The reference for locating dimensions must be
the center of the torus
Hexagonal prism
This is not a curved
this is a faced, planar faced geometry
but very used in the technical parts.
Its sizing needs 2 dimensions,
height and distance between parallel faces
because that fits the dimension of the spanner
For representation is obligatory
the view of the regular hexagon
and one view more for the height
of the parallel...
Hexagonal prism
The reference for locating dimensions
must be the axle
It is not correct to refer to vertex or hexagon sides
Sphere
Sizing a sphere needs only a dimension
Its location would need up to 3 dimensions
there is non-sense in angular location
To find a single sphere hardly occurs
Now let's summarize module 3, unit 1
What have we learnt?
A part drawing describes precisely the part
CSG methodology divides a part into basic elements
and join them by adding and subtracting
CSG allows the methodology the correct part representation
The basic types of elements
are geometric primitives and mechanical features
The common primitives, box, cylinder, frustum, torus,
hexagonal prism and sphere, have specific dimensions and references
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