5.1 A spur gear with 118 teeth must be cut on a universal milling machine - NSC Mechanical Technology Welding and Metalwork - Question 5 - 2017 - Paper 1

Up-Cut Milling

In up-cut milling, the cutter rotates against the direction of the workpiece feed. As the cutter engages with the material, it starts from zero depth and increases to full depth. This process tends to push the workpiece upwards, resulting in less stability.

![Up-Cut Milling Diagram]

Down-Cut Milling

In down-cut milling, the cutter rotates in the same direction as the workpiece feed. The cutter begins at full depth and decreases to zero depth as it advances. This method gives a better surface finish and ensures higher stability by pulling the workpiece down.

![Down-Cut Milling Diagram]

5.3.1 The width of the key:

Using the formula:

$$\text{Width} = \frac{D}{4} = \frac{102}{4} = 25.5 \text{ mm}$$

5.3.2 The thickness of the key:

Using the formula:

$$\text{Thickness} = \frac{D}{6} = \frac{102}{6} \approx 17 \text{ mm}$$

5.3.3 The length of the key:

Using the formula:

$$\text{Length} = D \times 1.5 = 102 \times 1.5 = 153 \text{ mm}$$

5.3.4 The thickness of the key at the smaller end:

Using the formula:

$$\text{Thickness at smaller end} = \frac{T \times L}{100} = \frac{17 \times 153}{100} \approx 15.47 \text{ mm}$$

5.4.1 Addendum:

The addendum can be calculated as:

$$\text{Addendum} = m = 3 ext{ mm}$$

5.4.2 Dedendum:

The dedendum is calculated as:

$$\text{Dedendum} = m \times 1.25 = 3 \times 1.25 = 3.75 ext{ mm}$$

5.4.3 Clearance:

The clearance is typically:

$$\text{Clearance} = 0.25 \times m = 0.25 \times 3 = 0.75 ext{ mm}$$

5.4.4 Pitch circle diameter:

The pitch circle diameter (PCD) is calculated as:

$$\text{PCD} = n \times m = 60 \times 3 = 180 ext{ mm}$$

5.4.5 Outside diameter:

The outside diameter (OD) is:

$$\text{OD} = PCD + 2 \times m = 180 + 2 \times 3 = 186 ext{ mm}$$

5.4.6 Cutting depth:

The cutting depth is:

$$\text{Cutting depth} = 2.157 \text{ mm} ext{ or } 2.25 ext{ mm}$$

5.4.7 Circular pitch:

The circular pitch (CP) can be calculated as:

$$\text{CP} = m \times \pi = 3 \times \pi \approx 9.43 ext{ mm}$$