10.1 Draw a cross-sectional view of a square screw thread - NSC Mechanical Technology Fitting and Machining - Question 10 - 2023 - Paper 1

The helix angle (θ) can be calculated using the formula:

$\tan(θ) = \frac{Lead}{\pi \times D_p}$

Substituting the known values:

• Lead = 25 mm
• Pitch diameter (D_p) = 63.75 mm

Thus:

$\tan(θ) = \frac{25}{\pi \times 63.75} \approx 0.124827406$

To find θ:

$θ = \tan^{-1}(0.124827406) \approx 7.12° \text{ or } 7.7°$

The leading angle can be calculated as:

$Leading \ angle = 90° - (Helix \ angle + Clearance \ angle)$

Substituting:

• Helix angle = 7.12°
• Clearance angle = 3°

Thus:

$Leading \ angle = 90° - (7.12° + 3°) = 90° - 10.12° = 79.88° \text{ or } 79°53'$

The following angle can be calculated as:

$Following \ angle = 90° + (Helix \ angle - Clearance \ angle)$

Substituting:

• Helix angle = 7.12°
• Clearance angle = 3°

Thus:

$Following \ angle = 90° + (7.12° - 3°) = 90° + 4.12° = 94.12° \text{ or } 94°7'$

A - Helix angle

B - Pitch / Lead

C - Root