22-Mec-A4 Design and Manufacture of Machine Elements · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Examinations, December 2019 — 16-Mec-A4, Design and Manufacture of Machine Elements. Three hours, open book, any non-communicating calculator. Six questions divided into Part A (Q1–Q3, manufacturing processes) and Part B (Q4–Q6, machine-element design); candidates answer two from Part A and two from Part B, and all questions carry equal value (25 % each). All six questions are solved here so the paper can be used as a complete study resource.
Reference texts. R. G. Budynas & J. K. Nisbett, Shigley's Mechanical Engineering Design, 11th ed. (Ch. 3 stress, Ch. 5 static failure, Ch. 6–7 fatigue and shafts, Ch. 9 welded joints); S. Kalpakjian & S. Schmid, Manufacturing Engineering and Technology, 8th ed. (Ch. 15 bulk deformation, Ch. 16 sheet-metal forming); M. P. Groover, Fundamentals of Modern Manufacturing, 7th ed. (Ch. 19–20); R. C. Hibbeler, Mechanics of Materials, 10th ed.
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.
Fine blanking (also called precision or fineblanking) is a variant of blanking that produces a fully sheared, dimensionally accurate edge in one press stroke. Three things are added to a conventional blanking tool, and all three act at once. A V-shaped ring, or stinger, machined into the pressure pad is driven into the sheet just outside the cut line before the punch moves, gripping the stock and locking the metal in the shear zone against lateral flow. A counter-pressure cushion under the blank pushes upward against the punch face, so the blank is clamped between punch and cushion throughout the cut. And the punch-to-die clearance is made very small, of the order of 0.5 % of the sheet thickness per side, against the 5–10 % normal in conventional blanking.
The press is therefore a triple-action machine: the pad force and the counter force are applied first and held, then the blanking force is applied and the punch descends slowly — punch speeds are typically an order of magnitude lower than in conventional blanking — and at the bottom of the stroke the pad and cushion strip the slug and eject the part. Total press force is the sum of the shearing force, the V-ring force and the counter force, so a fine-blanking press of a given tonnage blanks a considerably smaller part than a conventional press of the same rating.
The characteristic feature is the edge quality: the cut face is smooth, straight, square to the sheet surface and burnished over essentially its full thickness. A conventionally blanked edge has four distinct zones — a rollover at the punch entry, a bright burnished band typically covering only one-third of the thickness, a rough fracture zone covering the remainder where the crack from the punch met the crack from the die, and a burr. In a fine-blanked part the fracture zone is absent: the burnished band runs from top to bottom, the rollover is small, the burr is minimal, and the edge is perpendicular to the surface rather than tapered.
The practical consequences are what make the process worth its cost. Because the edge is fully sheared and square, the part is dimensionally accurate and flat, tolerances of the order of ±0.01 mm on the blank profile and flatness far better than conventional blanking are routine, and the edge can be used directly as a functional surface — a gear tooth flank, a cam profile, a bearing bore, a splined hole — without any secondary machining, shaving, reaming or grinding. This is why fine blanking dominates the production of seat-recliner components, transmission and clutch plates, brake components, ratchets and levers, and why holes as small as roughly 50 % of the sheet thickness can be produced when conventional blanking needs the hole to exceed the thickness.
The fracture zone is eliminated by suppressing crack initiation and propagation in the shear band, and that is done by putting the material there under a large superimposed compressive hydrostatic stress. The V-ring bites into the sheet and prevents the metal outside the cut line from flowing sideways or lifting; the counter-pressure cushion prevents the blank from dishing away from the punch; and the very small clearance keeps the punch and die shear planes almost coincident, so the two cracks that would normally start at the punch corner and the die corner and run to meet each other have no offset to grow across. Between the clamped stock, the counter-pressure and the close-fitting tooling, the material in the deformation zone is effectively extruded through the gap in a state of triaxial compression rather than sheared and broken.
As in Question 1, the mechanism is that a compressive hydrostatic stress leaves the von Mises flow stress unchanged — the metal still yields and flows at the same deviatoric stress — while raising the fracture strain, so the material can accommodate the whole of the shear displacement plastically before any crack nucleates. The slow, steady punch speed supports the same end by keeping the strain rate low and avoiding the shock loading that would start a crack. Rounding or bevelling the punch and die edges slightly, rather than leaving them sharp, further reduces the local tensile stress peak that would otherwise nucleate a crack at the corner.
