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 Screw cutting: 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

The lead of a screw thread is calculated as the product of the pitch and the number of starts. For a three-start square thread with a 6 mm pitch:

ext{Lead} = ext{Pitch} imes ext{No. of Starts} = 6 ext{ mm} imes 3 = 18 ext{ mm}

Step 2

The helix angle of the thread

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To find the helix angle, we can use the pitch diameter and the pitch circumference. First, the pitch diameter is determined:

ext{Pitch Diameter} = ext{Crest Diameter} - rac{ ext{Pitch}}{2} = 58 ext{ mm} - rac{6 ext{ mm}}{2} = 55 ext{ mm}

Next, calculate the pitch circumference:

ext{Pitch Circumference} = ext{π} imes ext{Pitch Diameter} = ext{π} imes 55 ext{ mm} ext{ (approximately 172.79 mm)}

Finally, the helix angle is calculated using:

ext{Helix Angle} an( heta) = rac{ ext{Lead}}{ ext{Pitch Circumference}} = rac{18}{172.79}

This yields:

heta ≈ 5.95° ext{ or } 5°57'

Step 3

The leading tool angle

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The leading tool angle can be calculated using:

ext{Leading Angle} = 90° - ( ext{Helix Angle} + ext{Clearance Angle}) = 90° - (5.95° + 3°) = 81.05°

Step 4

The following tool angle

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The following tool angle is calculated similarly:

ext{Following Angle} = 90° - ( ext{Helix Angle} - ext{Clearance Angle}) = 90° - (5.95° - 3°) = 92.05°

Step 5

Calculate the required drill size to tap the internal screw thread

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Answer

For a screw thread measurement given as M20 x 2.5:

The drill diameter can be calculated as:

ext{Drill Diameter} = ext{OD} - ext{P} = 20 - 2.5 = 17.5 ext{ 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 measured from any given point on the screw thread to a corresponding point on an adjacent thread. It essentially indicates the distance between the crests of the thread.

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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