5.1 Neatly sketch and label an isometric V-screw thread to indicate the following screw thread terminology on: 5.1.1 Crest, major or outside diameter 5.1.2 Included screw-thread angle 5.1.3 Screw-thread pitch 5.1.4 Screw-thread crest 5.1.5 Screw-thread flanks 5.2 Identify the milling processes indicated in FIGURES 5.2.1 and 5.2.2 below - NSC Mechanical Technology Automotive - Question 5 - 2017 - Paper 1

The included screw-thread angle is the angle formed between the two sides of the thread. For a standard V-thread, this angle is typically 60 degrees. In the sketch, it can be shown as the angle at the apex of the V-shape.

Screw-thread pitch is the distance between corresponding points on adjacent threads, measured parallel to the axis. This can be illustrated in the sketch by indicating the distance along the axis between two adjacent crests.

The screw-thread crest is the top surface of the thread. In the isometric sketch, this is displayed as the highest horizontal line connecting the two outermost points of the thread profile.

The screw-thread flanks are the flat surfaces that connect the crest and the root of the thread. In the sketch, these can be represented as the angled lines descending from the crest towards the root.

The process shown in FIGURE 5.2.1 is identified as up-cut milling, where the cutter rotates against the direction of the feed, resulting in a climb cut.

The process in FIGURE 5.2.2 is identified as down-cut milling, where the cutter rotates in the same direction as the feed, resulting in a conventional milling operation.

Sector arms B and H are used for calibration and to set the dividing head to specific angles for precise cutting. They help in selecting the ratio of the worm gear to the index plate.

The index plate G has accurate spaced holes that allow for different settings of the dividing head for facilitating indexing, thus enabling precise angular positioning.

Index pin A is used for locking the selected hole on the index plate during operation to maintain the precision of the angular positioning.

The ratio between worm gear E and gear D is 40:1. This means that for every turn of the worm gear, the gear D will turn 1/40th of a rotation.

The pitch-circle diameter (PCD) is calculated using the formula:

PCD = m × T

Where:

• m = module (3)
• T = number of teeth (94)

Thus, PCD = 3 x 94 = 282 mm.

The outside diameter (OD) can be calculated using:

OD = PCD + 2m

So,

• OD = 282 + 2 × 3 = 288 mm.

The dedendum (b) is calculated as:

Dedendum = m - 1.25 For the module of 3:

• Dedendum = 3 - 1.25 = 1.75 mm.

The cutting depth can be calculated as:

Cutting depth = Dedendum + m For a module of 3:

• Cutting depth = 1.75 + 3 = 4.75 mm.