7.1 Determine the depth micrometer reading shown in FIGURE 7.1 - NSC Mechanical Technology Fitting and Machining - Question 7 - 2021 - Paper 1

1. Select the Load: Choose the desired load for the hardness test based on the specimen's material and size.

2. Prepare the Test Surface: Ensure the surface of the steel sample is polished and clean for accurate measurement.

3. Apply the Test Device: Position the Brinell hardness tester so that the test ball comes into contact with the specimen surface.

4. Set the Loading Time: Apply the chosen load steadily. Hold the load for approximately 15 to 30 seconds to allow effective indentation.

5. Measure the Indentation: After removing the load, measure the diameter of the indentation left on the steel surface using a microscope or appropriate measuring tool.

6. Calculate the Hardness Value: Use the Brinell hardness formula:

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

   where:

   • $P$ is the applied load,
   • $D$ is the diameter of the ball,
   • $d$ is the diameter of the indentation.

1. Ultimate Tensile Strength: The maximum stress that a material can withstand while being stretched or pulled before necking occurs.

2. Yield Strength: The stress at which a material begins to deform plastically. Beyond this point, permanent deformation occurs.

3. Elongation: The percentage increase in length of the material before fracture, indicating ductility.

The instrument shown in FIGURE 7.4 is a micrometer screw gauge.

The function of the micrometer screw gauge is to measure small dimensions with high accuracy, typically used to measure the thickness or diameter of objects.