Sectional Assembly of a Flanged Coupling
A sectional front view of a rigid flanged coupling is being drawn. Each flange is 20 mm thick and 140 mm outside diameter, keyed to a 40 mm shaft. Six M12 bolts are equally spaced on a 110 mm bolt circle. The cutting plane passes along the coupling axis. The candidate hatches the two shafts, the six bolts, the nuts and the key; uses hidden lines for the keyway in the shaft; and draws the internal thread of each nut as two parallel lines outside the nut outline. The runout at the end of the thread is omitted. All dimensions are correct. The instructor marks the sheet ‘conventions incorrect’ and asks for a corrected section. The candidate argues that hatching everything and showing the keyway with hidden lines gives more information and should be accepted.
1. Identify which parts in the longitudinal section should be hatched and which should not. Give the reason for each decision.
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Hatch only the material actually cut in the flanges and hubs (and any other main casing part). Do not hatch the two shafts, the six bolts, the nuts, or the key.
Standard machine-drawing convention treats solid cylindrical and standard fastening parts as unsectioned when the cutting plane passes along their axes; they are drawn in outside view even inside a section. Hatching them would make the assembly harder to read and would suggest a cut through a solid part whose interior is not being studied. The key is a solid standard element and is also left unhatched in position.
If any of these parts were cut transversely, a local or cross-section could be hatched, but not in this longitudinal section.
2. Correct the thread representation for the nut. Describe the line types and where hatching should stop. What is wrong with drawing two parallel lines outside the nut outline?
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For an internal thread shown in section, the minor diameter (the crest of the internal thread) is drawn as a continuous thick line, and the major diameter (the root) as a continuous thin line. The hatching of the nut extends to the major diameter, i.e. it stops at the thin root line, not at the crest. In the end view, the major circle is a thin circle and the minor circle is a thick circle broken for about one quarter.
Drawing two parallel lines outside the nut outline is wrong because it places the thread outside the body of the nut; that resembles an external thread or a symbolic callout, not a section through an internal thread. The thread lines must lie inside the nut envelope, and the runout at the threaded end should be shown where the thread dies out.
3. The candidate used hidden lines for the keyway in the shaft. Is that acceptable in this sectional assembly? State a better practice.
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In a sectional assembly, hidden lines are normally omitted unless they are essential to understand the part. The keyway is already represented by the key in position, and its depth and width can be dimensioned on the shaft detail. Drawing hidden lines for the keyway through the shaft clutters the section and conflicts with the convention that the shaft is not sectioned.
Better practice: show the key unhatched in the keyway in the assembly, omit hidden lines in the sectional view, and give a separate detail or local section of the shaft with the keyway dimensions. If the keyway must be shown in the assembly, use a local section rather than hidden lines.