A three-start square screw thread with a 6 mm pitch needs to be cut on a lathe. The crest diameter of the thread is 58 mm and the clearance angle 3°. Calculate the following: 10.1.1 The lead of the thread 10.1.2 The helix angle of the thread 10.1.3 The leading tool angle 10.1.4 The following tool angle 10.2 The measurements of a screw thread is given as M20 x 2.5. Calculate the required drill size to tap the internal screw thread. 10.3 Explain what is meant by the pitch of a screw thread.

NSC Mechanical Technology Fitting and Machining ISO metric V tread: A three-start square screw threa

A three-start square screw thread with a 6 mm pitch needs to be cut on a lathe - NSC Mechanical Technology Fitting and Machining - Question 10 - 2020 - Paper 1

Question 10

A three-start square screw thread with a 6 mm pitch needs to be cut on a lathe. The crest diameter of the thread is 58 mm and the clearance angle 3°. Calculate the ... show full transcript

Worked Solution & Example Answer:A three-start square screw thread with a 6 mm pitch needs to be cut on a lathe - NSC Mechanical Technology Fitting and Machining - Question 10 - 2020 - Paper 1

Step 1

The lead of the thread

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Answer

To calculate the lead of the thread, use the formula:

Lead = pitch \times number \ of \ starts = 6 \ mm \times 3 = 18 \ mm.

Step 2

The helix angle of the thread

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The pitch diameter can be calculated as follows:

Pitch \ diameter = OD - \left( \frac{P}{2} \right) = 58 \ mm - \left( \frac{6 \ mm}{2} \right) = 55 \ mm.

Next, using the pitch circumference:

Pitch \ circumference = \pi \times Pitch \ diameter = \pi \times 55 \ mm \approx 172.79 \ mm.

Now, compute the helix angle using:

Helix \ angle \ tan \theta = \frac{Lead}{Pitch \ circumference} = \frac{18}{172.79} \Rightarrow \theta \approx 5.95^{\circ} \ or \ 5^{\circ} 57'.

Step 3

The leading tool angle

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The leading tool angle is calculated as follows:

Leading \ angle = 90^{\circ} - (helix \ angle + clearance \ angle) = 90^{\circ} - (5.95^{\circ} + 3^{\circ}) = 81.05^{\circ}.

Step 4

The following tool angle

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The following tool angle can be calculated using:

Following \ angle = 90^{\circ} - (helix \ angle - clearance \ angle) = 90^{\circ} - (5.95^{\circ} - 3^{\circ}) = 92.05^{\circ}.

Step 5

Calculate the required drill size to tap the internal screw thread.

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For the M20 x 2.5 screw thread, the drill size is calculated using:

Drill \ diameter = OD - P = 20 - 2.5 = 17.5 \ mm.

Step 6

Explain what is meant by the pitch of a screw thread.

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Answer

The pitch of a screw thread is defined as the axial distance between any given point on the screw thread to a corresponding point on an adjacent thread. It effectively determines how far the thread advances in one complete rotation.

A three-start square screw thread with a 6 mm pitch needs to be cut on a lathe - NSC Mechanical Technology Fitting and Machining - Question 10 - 2020 - Paper 1

Worked Solution & Example Answer:A three-start square screw thread with a 6 mm pitch needs to be cut on a lathe - NSC Mechanical Technology Fitting and Machining - Question 10 - 2020 - Paper 1

The lead of the thread

The helix angle of the thread

The leading tool angle

The following tool angle

Calculate the required drill size to tap the internal screw thread.

Explain what is meant by the pitch of a screw thread.

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