7.1 Describe the function of a screw-thread micrometer - NSC Mechanical Technology Fitting and Machining - Question 7 - 2022 - Paper 1

The Brinell hardness test involves pressing a hard steel or carbide ball into the material being tested under a specific load. The diameter of the indentation left in the material is measured, and the Brinell hardness number (BHN) is calculated with the following formula:

$ext{BHN} = \frac{2P}{\pi D (D - \sqrt{D^2 - d^2})}$

where:

• $P$ is the load in kgf,
• $D$ is the diameter of the ball in mm, and
• $d$ is the diameter of the indentation in mm.

A labelled drawing would typically include the test specimen, the indenter, and measurements indicating the indentation diameter.

1. Tensile Force: A force that attempts to stretch or elongate an object.
2. Compressive Force: A force that attempts to compress or shorten an object.

In the diagram of the metric screw thread:

• A would typically be labeled as the 'Major Diameter' of the screw;
• B would represent the 'Minor Diameter';
• C would be the 'Pitch Diameter' which is the diameter at which the width of the thread and the width of the groove are equal.

The pitch diameter of a screw thread can be calculated using the formula:

$D_p = D - \frac{P}{2}$

Where:

• $D$ is the major diameter and for an M12 screw, it's 12 mm,
• $P$ is the pitch, which is 1.75 mm.

Thus, the pitch diameter is: $D_p = 12 - \frac{1.75}{2} = 12 - 0.875 = 11.125 ext{ mm}$

Therefore, the pitch diameter of an M12 x 1.75 screw thread is 11.125 mm.