Question

Draw to scale 1:1, the standard profile of KNUCKLE THREAD taking enlarged pitch as 50 mm. Give standard dimensions. (8 marks)

Hint:

To draw the thread continuously without errors, use alternating compass centers on the top and bottom lines spaced exactly $P/2 = 25\text{ mm}$ apart.

Answer 1

Step 1: Understanding Knuckle Thread Geometry and Proportions:
Knuckle thread is a highly robust screw thread form with fully rounded, semi-circular crests and roots. It is used in heavy-duty applications where threads are subject to rough handling, sand, or debris (such as railway carriage couplers, glass bottle necks, and plastic container caps). The standard proportions of a Knuckle Thread in terms of its pitch ($P$) are:
• Pitch ($P$): Given as $50\text{ mm}$ (enlarged scale for drafting).
• Depth of thread ($h$): $h = 0.5 \times P = 0.5 \times 50 = \mathbf{25\text{ mm}}$
• Radius of crest and root ($r$): $r = 0.25 \times P = 0.25 \times 50 = \mathbf{12.5\text{ mm}}$
• Straight flank height: The semi-circles of the crest and root merge seamlessly into each other, forming a continuous wavy curve.

Step 2: Step-by-Step Drafting Instructions:
• Draw a light horizontal line representing the mean diameter line of the thread.
• Draw two parallel horizontal lines at a distance of $h = 25\text{ mm}$ apart (one for the crest line and one for the root line).
• Along these horizontal lines, mark pitch intervals of $P/2 = 25\text{ mm}$ representing the centers of the alternating crest and root arcs.
• Using a compass set to a radius of $r = 12.5\text{ mm}$, draw alternating upward semi-circles (crests) and downward semi-circles (roots) centered on your marked points.
• Connect the semi-circular crests and roots smoothly to form a continuous wavy profile.
• Draw the thickness of the metal body and add standard cross-hatching at $45^{\circ}$ inside the sectioned material.
• Dimension the pitch ($P = 50\text{ mm}$), depth of thread ($0.5P = 25\text{ mm}$), and radius ($0.25P = 12.5\text{ mm}$) clearly.