Eme 2101 61 introduction 64 3 draw the fillet arc

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3. Draw the fillet arc using a visible line, Figure 6.4.
Figure 6.4: Construction of the fillet arc

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6.1.4 To draw the teeth of a standard gear by approximate arc method
1. Lay off the pitch circle, the root circle and the addendum circle
2. Starting with the pitch point, divide the pitch circle into distances equal to the circular
thickness of the tooth
3. Draw the pressure line through the pitch point
4. Draw the base circle, tangential to the pressure line.
5. With the compass set to a radius equal to the distance from the pitch point to the end of
the pressure line on the base circle (radius of curvature), strike arcs through the division
points on the pitch circle, with the center on the base circle.
6. Darken the arcs for the tops of the teeth and bottom of the spaces and add tooth fillets to
complete the gear teeth as shown in figure 6.5.
Figure 6.5: Gear teeth drawing by approximate arc method

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Figure 6.6: Meshing pair of gears

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Chapter 7
Screw Threads
7.1 Introduction
A screw thread is a helical groove which is cut, rolled, or sometimes cast on a cylinder or cylindrical
hole. The threads are parallel to each other with those on the cylinder (screw) being external,
and those in the hole (nut) being internal. Tapered threads are formed on a cone or in a conical
hole. The form of the groove varies with different threads depending on the use. Threads may be
right- or left-handed and single or multi-start. Screw threads have three main applications:
• Assembly or fastening parts together.
• To adjust the position of parts.
• Power transmission purposes.
7.1.1 Screw Thread Terms
o
D
i
D
Figure 7.1: Screw thread terminology
Figure 7.1 shows the primary terms useful in defining screw threads. These terms are:
• External thread (screw) ; A thread on the external surface of a cylinder.
• Internal thread (nut) ; A thread on the internal surface of a cylinder.
• Major Diameter ; The largest diameter of a screw thread.

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• Minor Diameter ; The smallest diameter of a screw thread.
• Pitch diameter ; The diameter of an imaginary cylinder, the surface of which cuts the
thread forms where the width of the thread and groove are equal.
• Crest ; The edge or surface that joins the sides of adjacent thread forms and is farthest from
the cylinder axis
• Root ; The edge or surface that joins the sides of adjacent thread forms and coincides with
the cylinder from which the thread projects.
• Flanks ; The straight sides which connect the crest and the root.
• Pitch ; The distance between corresponding points on adjacent thread forms measured par-
allel to the axis
• Lead ; The distance a threaded part moves axially with respect to a fixed mating part, in
one complete revolution.
• Form ; The profile (cross-section) of the thread.
7.1.2 Thread Forms
”Thread Form” describes the shape of the thread if one cut a thread form in half along its axis
and then looked at the thread configurations. Different thread forms have different uses:
i. Metric Thread The ISO metric screw threads are the worlds most commonly used type of
general purpose screw threads. The design principles of this thread form are defined in the
standards ISO 68-1.