Computer Aided Design (CAD)

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Computer Aided Design(CAD)

• Week 9 Involute Gears and Dimensioning

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Gears - Introduction
Single Gear
Gear Train
Driver and Driven Gear Wheels
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Types of Gears
Spur Gear
Rack Pinion
Helical Gear
Worm Gear and Wheel
Note Not an exhaustive list.
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Some Gear Terminology
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Gear Data and Equations

• Pressure Angle Normally 20º
• Pitch Circle Diameter (PCD) Module x No. Teeth
• A circle representing the two gears as tangent
  cylinders.
• Addendum Module
• The part of the tooth above the Pitch Circle.
• Clearance 0.25 x Module (1/4 of Module)
• The gap between the tip and the root of two
  meshing teeth.
• Dedendum Module Clearance
• The part of the tooth below the Pitch Circle.

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Gear Data and Equations

• Circular Pitch Module x 3.14 (Pi)
• The distance from a point on one tooth to a
  similar on the next.
• Tooth Thickness Circular Pitch / 2
• Fillet Radius (approx) Circular Pitch / 7

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Spur Gear Calculation Drawing

• The method used here to draw a typical spur gear
  is called Unwins 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.

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Spur Gear Calculation Drawing 1
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Spur Gear Calculation Drawing 2
a circle
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Spur Gear Calculation Drawing

• Now its your Turn -gt
• Draw a Spur gear with the following details
• Module 2mm
• Number of Teeth 25

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Dimensioning to BS308 Standards

• 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

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Dimensioning to BS308 Standards

• 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.

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Dimensioning Overview
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Dimensioning Overview

• Dimensions should be placed near the middle, and
  above the dimension line.
• Larger Dimensions should be placed outside
  smaller dimensions.

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Dimensioning Methods

• Parallel Dimensioning
• Consists of a number of dimension lines
  originating from a datum feature.
• Chain Dimensioning
• Should only be used where the accumulation of
  tolerances does not endanger the function of the
  part.

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Diameter Dimensioning

• Diameter dimensions of a circle or cylinder is
  shown by the symbol Ø in front of the No.

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Radius Dimensioning

• 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.

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Dimensioning Features

• Dimensioning of equally-spaced features may be
  simplified by using methids similar to below

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Dimensioning Holes etc.

• 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.

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Dimensioning Holes etc.
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Dimensioning Tapered Features
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Dimensioning Screw Threads

• 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 tolerance6H is suitable for INTERNAL
  thread tolerance.6g is suitable for EXTERNAL
  thread tolerance.E.g. M10 x 1 6g

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Dimensioning External Threads
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Dimensioning Internal Threads

• Note the hatch lines and line weights.

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Dimensioning Threads
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Typical Dimensioned Drawing