22-Mec-A4 Design and Manufacture of Machine Elements · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Examinations, December 2018 — 16-Mec-A4 Design and Manufacture of Machine Elements. Three hours, open book, any non-communicating calculator. Six questions in two parts: Part A (Q1–Q3, manufacturing processes) and Part B (Q4–Q6, machine-element analysis). The rubric asks for two questions from each part — four questions constitute a complete paper, each worth 25 %. All six are solved here, since the set is intended as a study resource.
Reference texts.
Check: every boxed result in this paper, which also carries the independent closure checks noted in each question (moment closure on Q4, volume conservation on Q2, gap compatibility on Q6).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
[Figure not reproduced: Fig. S4 redrawn: the moulding cavity surrounds a suspended sand core; metal enters at the right through the pouring channel and passes along the parting plane into the cavity, with a feeder standing over the heavy section. See the official exam paper.]
The identifications follow the flow of metal through the mould and the way the two flask halves are assembled.
| Label | Feature | What it does |
|---|---|---|
| A | Sand core | Bonded-sand body that occupies the space which must become the internal cavity of the casting. |
| B | Chaplet | Small metal support set between the core and the mould face to hold the core in position. |
| C | Riser (feeder) | Blind reservoir of liquid metal standing over the heavy section of the casting. |
| D | Sprue (with pouring cup at the top) | Tapered vertical downrunner through which metal is poured into the mould. |
| E | Parting line / parting plane | The interface at which cope and drag separate, and along which the runner is cut. |
| F | Drag | The lower half of the flask and the moulding sand it contains (the cope is the matching upper half). |
| G | Gate (ingate) | The short passage that admits metal from the runner into the mould cavity. |
Purpose of C, the riser. Nearly all metals contract on solidifying — steel by roughly 3 %, cast iron and the aluminium alloys by 3–6 % of volume — so a casting that is allowed to freeze in isolation will pull a shrinkage cavity into its last-freezing region, which is usually the thickest section. The riser is a deliberately oversized appendage placed over that section to act as a reservoir: while the casting freezes, liquid is drawn out of the riser to make up the contraction, and the void that would otherwise appear in the part appears instead in the riser, which is cut off and remelted. For this to work the riser must freeze after the section it feeds. Chvorinov's rule, $t_s = C_m\,(V/A)^{n}$ with $n \approx 2$, says freezing time scales with the square of the modulus $V/A$, so the riser is sized with a modulus about 1.2 times that of the section being fed, and its neck is kept short and fat so the feeding path does not freeze off first. A riser also vents the mould and floats slag and dross out of the cavity, and the metallostatic head it provides presses the solidifying skin against the mould face.
Purpose of B, the chaplet. A core is located by its core prints, but a core suspended in the cavity, as this one is, is subjected to a buoyant force because the liquid metal is far denser than bonded sand. For a fully submerged core the net upward force is $F_b = (\rho_m - \rho_c)\,g\,V_c$, and for a steel or iron casting the metal is roughly four times the density of the core, so the core tries to float. If the prints alone cannot carry that load the core lifts or sags, the wall thickness on one side is thinned and on the other thickened, and the casting is scrapped for wall-thickness variation or a core break-out. A chaplet is a small metal support — a stem, a double-headed stud or a spring-leg stamping — wedged between the core and the mould surface to carry the buoyant load and hold the core on centre. Chaplets are made of the same or a compatible alloy so they fuse into the casting: they must be clean, dry and correctly proportioned, because an oversized chaplet will not melt in and leaves a cold shut or a leak path, while an undersized one crushes into the sand and loses the core position it was fitted to protect.
