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

Up-Cut Milling

In up-cut milling, the cutter rotates against the direction of the workpiece feed. This process begins cutting at the bottom of the workpiece surface, gradually lifting the workpiece upward. Key points:

• The cutter moves upward against the workpiece feed.
• Typically generates a smoother finish but can lead to tool wear.

Down-Cut Milling

In contrast, down-cut milling has the cutter rotating in the same direction as the workpiece feed. The cutting occurs from the top surface downwards. Key points:

• The cutter moves downwards with the workpiece feed.
• Can lead to better chip removal and less vibration.

The width of the key can be calculated with the formula:
$Width = \frac{D}{4}$
Where D is the diameter of the shaft. Given D = 102 mm:

$Width = \frac{102}{4} = 25.5 \, mm$

The thickness of the key can be determined via:
$Thickness = \frac{D}{6}$
Where D = 102 mm:

$Thickness = \frac{102}{6} = 17 \, mm$

The length of the key is generally standardized; for this problem, it can be taken to be the diameter of the shaft. Therefore, the length is:

$Length = D = 102 \, mm$

The thickness at the smaller end, considering the taper, can be approximated based on the dimensions provided and typical manufacturing standards. Given no specific taper ratio, the small end thickness can often be considered as half the thickness of the key:

$Small \, End \, Thickness = \frac{Thickness}{2} = \frac{17}{2} = 8.5 \, mm$. This is a rough estimate and should be adjusted based on actual taper specifications.

For a spur gear with 60 teeth and a module of 3:

5.4.1 Addendum

Addendum = Module * 1 = 3 * 1 = 3 mm

5.4.2 Dedendum

Dedendum = Module * 1.25 = 3 * 1.25 = 3.75 mm

5.4.3 Clearance

Clearance = Dedendum - Addendum = 3.75 - 3 = 0.75 mm

5.4.4 Pitch Circle Diameter

PCD = Module * Number of Teeth = 3 * 60 = 180 mm

5.4.5 Outside Diameter

OD = PCD + 2 * Addendum = 180 + 2 * 3 = 186 mm

5.4.6 Cutting Depth

Cutting depth varies depending on the operation—assume a practical depth of:

• 2.157 mm or deeper as specified by required design.

5.4.7 Circular Pitch

Circular pitch is calculated as: $Circular \, Pitch = \frac{\pi * Pitch \, Circle \, Diameter}{60} = \frac{\pi * 180}{60} = 9.43 \, mm$.