10.1 Explain the following terms as applicable to screw thread terminology: 10.1.1 Lead 10.1.2 Helix angle 10.2 A two-start square threaded bar needs to be manufactured - NSC Mechanical Technology Fitting and Machining - Question 10 - 2022 - Paper 1

The helix angle is the angle between the tangent to the thread helix and the axis of the screw. It influences the strength and durability of the screw, affecting how well it engages with the nut.

The pitch of the thread can be calculated using the formula: ext{Pitch} = rac{ ext{Lead}}{ ext{Number of Starts}} For a two-start thread with a lead of 42 mm, the pitch is: ext{Pitch} = rac{42 ext{ mm}}{2} = 21 ext{ mm}

The pitch diameter can be estimated using the formula: ext{Pitch Diameter} = ext{Crest Diameter} - rac{ ext{Pitch}}{2} Substituting the values provides: ext{Pitch Diameter} = 90 ext{ mm} - rac{21 ext{ mm}}{2} = 79.5 ext{ mm}

The helix angle can be found using: an( ext{Helix Angle}) = rac{ ext{Pitch}}{ ext{π} imes ext{Crest Diameter}} With a pitch of 21 mm and crest diameter of 90 mm, calculate: an( ext{Helix Angle}) = rac{21 ext{ mm}}{ ext{π} imes 90 ext{ mm}} Thus, the helix angle can be determined through inverse tangent function.

The leading angle can be calculated using: ext{Leading Angle} = an^{-1} igg( rac{ ext{Pitch}}{ ext{Crest Diameter} imes ext{cos(Clearance Angle)}} igg). Substituting values with the clearance angle at 3° gives the leading angle.

The following angle is similar to the leading angle but takes into account the clearance angle. It can also be calculated using a similar formula as for the leading angle, adjusted for the direction of movement.