10.1 A two-start square threaded bar needs to be manufactured - NSC Mechanical Technology Fitting and Machining - Question 10 - 2021 - Paper 1

The helix angle can be derived from the formula:

$\tan(\theta) = \frac{P}{\pi D_m}$

Where:

• $D_m$ = Mean diameter = 75 mm
• $P$ = Lead of the thread = 40 mm

Substituting the values:

$\tan(\theta) = \frac{40}{\pi(75)}$

Calculating:

The leading tool angle can be calculated using the formula:

$\alpha = 90° - \text{helix angle} - \text{clearance angle}$

Substituting the values:

• Helix angle ≈ 9.63°
• Clearance angle = 3°

$\alpha = 90° - 9.63° - 3° = 77.37°$

So, the leading tool angle is approximately 77.37°.

The following tool angle can be derived similarly:

$\beta = 90° - \text{helix angle} + \text{clearance angle}$

Substituting the values:

$\beta = 90° - 9.63° + 3° = 96.63°$

Thus, the following tool angle is approximately 96.63°.

A: Crest B: Flank C: Root D: Thread Pitch

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