Transcript Computer Aided Design (CAD)

Computer Aided Design
(CAD)
Week 9 :: Involute Gears and Dimensioning
Gears - Introduction
Single Gear
Gear Train
Driver and Driven Gear Wheels
Types of Gears
Spur Gear
Rack & Pinion
Note: Not an exhaustive list.
Helical Gear
Worm Gear
and Wheel
Some Gear Terminology
Pitch Circle Diameter
Fillet Radius
Gear Data and Equations
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Pressure Angle: Normally 20º
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Pitch Circle Diameter (PCD): Module x No. Teeth
A circle representing the two gears as tangent cylinders.
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Addendum = Module
The part of the tooth above the Pitch Circle.
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Clearance = 0.25 x Module (1/4 of Module)
The gap between the tip and the root of two meshing teeth.
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Dedendum = Module + Clearance
The part of the tooth below the Pitch Circle.
Gear Data and Equations
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Circular Pitch = Module x 3.14 (Pi)
The distance from a point on one tooth to a similar on the next.
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Tooth Thickness = Circular Pitch / 2
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Fillet Radius (approx) = Circular Pitch / 7
Spur Gear Calculation & Drawing
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The method used here to draw a typical spur
gear is called “Unwin’s Construction”. It is
also called the “Approximate Method”
because it is only produces an Approximate
“Involute Curve”.
Tip: When drawing a Gear in AutoCAD –
make full use of Layers and Colours.
Spur Gear Calculation & Drawing #1
diameter.
Spur Gear Calculation & Drawing #2
a circle
Spur Gear Calculation & Drawing
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Now its your Turn ->
 Draw
a Spur gear with the following details:
Module: 2mm
Number of Teeth: 25
Dimensioning to BS308 Standards
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Each dimension required should only appear
once. There should be no more dimensions
than necessary.
Linear Dimensions are normally in millimeters
(mm). The symbol may be ommitted provided
the drawing carries a statement of the unit
used.
Angular Dimensions should be expressed in
degrees and minutes, e.g.:
20° 30’
Dimensioning to BS308 Standards
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Projection lines and dimension lines should
normally be placed outside the outline of the
view.
Crossing of projection and dimension lines
should be avoided.
Projection lines should start just clear of the
outline of the feature and should extend a
little beyond the dimension line.
Projection lines should normally be drawn
perpendicular to the dimension required.
Dimensioning Overview
Dimensioning Overview
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Dimensions should be placed near the middle,
and above the dimension line.
Larger Dimensions should be placed outside
smaller dimensions.
Dimensioning Methods
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Parallel Dimensioning:
Consists of a number of
dimension lines originating
from a datum feature.
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Chain Dimensioning:
Should only be used where
the accumulation of
tolerances does not
endanger the function of
the part.
Diameter Dimensioning
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Diameter dimensions of a circle or cylinder is
shown by the symbol Ø in front of the No.
Radius Dimensioning
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Radii should be dimensioned by a dimension
line that passes through, or is in line with,
the center of the arc.
The dimension line should have one
arrowhead only, that which touches the arc.
The symbol R is placed in front of the No.
Dimensioning Features

Dimensioning of equally-spaced features
may be simplified by using methids similar to
below:
Dimensioning Holes etc.
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Notes:
- The term “spotface” implies
that the depth is the
minimum necessary to
provide a machined surface.
- Chamfers at 45° should be
dimensioned as below and
not described by a note.
Dimensioning Holes etc.
Dimensioning Tapered Features
Dimensioning Screw Threads
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The letter M, standing for ISO metric thread,
is followed by, the nominal diameter and the
pitch, both in millimeters. E.g. M8 x 1.
The absence of a pitch (e.g. M8), means that
a course pitch is specified. (e.g. The course
pitch for an M8 thread is 1.25mm.
(See Page 7 in AutoCAD Notes part II)
Thread tolerance
6H is suitable for INTERNAL thread tolerance.
6g is suitable for EXTERNAL thread tolerance.
E.g. M10 x 1 – 6g
Dimensioning External Threads
Dimensioning Internal Threads
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Note the hatch lines and line weights.
Dimensioning Threads
Typical Dimensioned Drawing