22-Mec-A4 Design and Manufacture of Machine Elements · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Examinations, December 2016 — 07-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) is qualitative manufacturing-process theory, Part B (Q4–Q6) is quantitative machine-element design. The candidate answers two from Part A and two from Part B; four questions of equal value (25 % each) constitute a complete paper. All six are solved here, because the set is a study resource rather than an exam script.
Reference texts.
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.
(a) Yes — but not by conventional blanking. An ordinary blanking die produces an edge with four distinct zones: rollover, a burnished band, a rough fracture zone, and a burr. The burnished band typically occupies only 10–30 % of the stock thickness, and the fracture zone below it is both rough and tapered, so the tooth flank is neither square to the sheet nor smooth. Since the question demands flanks parallel to the gear axis (that is, perpendicular to the sheet surface over the full thickness) and a smooth finish, the answer is that the proposition is feasible only if the shearing operation is converted to one of essentially pure plastic shear. That process is fine blanking, and it is precisely the process by which small flat gears, cam plates, ratchets and similar thin profiled parts are mass-produced.
(b) The fine-blanking die and its elements. Fine blanking differs from conventional blanking in three specific pieces of hardware and one process parameter, all of which appear in the sketch below.
(c) Why it works. The V-ring, the counterpunch back pressure and the tiny clearance together superimpose a large compressive hydrostatic (mean) stress on the narrow band of material being sheared. Two facts of plasticity then do the work. First, the von Mises flow stress is essentially independent of hydrostatic pressure, so the material still yields and shears at the same equivalent stress — the process is no harder to run. Second, the fracture strain is strongly increased by compressive hydrostatic stress, because voids and microcracks are closed rather than opened. The shear zone can therefore accumulate enough plastic strain to sever the sheet before any crack nucleates. The result is a 100 % burnished edge, square to the sheet through the entire thickness, with a surface finish of roughly 0.4–1.6 µm Ra and dimensional tolerances an order of magnitude tighter than conventional blanking. That is exactly the "tooth surfaces parallel to the axis, smooth finish" requirement.
(a) Explanation — snap-through (breakthrough) shock. While the punch penetrates, the press frame, ram, tooling and drive train are elastically strained by the cutting force, and they store elastic strain energy in proportion to that force. A hard, low-ductility steel shears only a short distance before brittle fracture runs across the remaining ligament; the cutting force then collapses from its peak to essentially zero in a fraction of a millisecond. The stored elastic energy has nowhere to go except into rapid acceleration of the ram and reverse loading of the frame and gibs. That sudden elastic recoil is the "snap"; the bang is the acoustic pulse it radiates. In press terminology this is snap-through, and the reversed load it produces is reverse tonnage — a well-known cause of cracked frames, spalled bearings, loosened tie rods and premature punch failure. Hard, thick stock and a flat punch face acting on the whole profile at once are exactly the conditions that maximize it, because the whole cut is completed simultaneously.
(b) How to minimize it without changing the material. Every effective remedy attacks the same variable: the rate at which the cutting force falls. Grinding shear (bevel) onto the punch or die face, typically one to two stock thicknesses of shear height, staggers the cut so that only part of the profile is being severed at any instant; the peak force drops and, more importantly, the force decays progressively instead of collapsing. Stepping the punch lengths in a multi-punch die achieves the same staggering. Beyond the tooling, one can increase the punch–die clearance slightly (crack initiation is encouraged earlier and more gradually), fit hydraulic or nitrogen shock absorbers / snap-through dampers to the die set, run the job on a hydraulic press whose fluid column absorbs the release rather than a flywheel-driven mechanical press, and reduce ram speed. Machine-side measures — a heavier die cushion, keeping the tonnage well below press capacity, and derating for reverse tonnage — complete the list.
(a) Bending without orange peel. Specify a fine grain size — a high ASTM grain-size number, typically ASTM 7 or finer. Orange peel is the roughening of a free surface during plastic straining, caused by individual grains deforming by different amounts and rotating out of the surface plane according to their crystallographic orientation. The height of the resulting relief scales with the grain diameter, so refining the grain reduces the surface roughening below visibility. Fine grain also raises the yield strength through the Hall–Petch relation, which is a bonus rather than a penalty here.
(b) Bending to zero radius. Specify maximum ductility as measured by the tensile reduction of area — Kalpakjian's correlation gives the minimum bend radius in stock thicknesses as a function of tensile reduction of area r, and a value of r approaching 50 % is what drives the minimum radius to zero. Support that with: low inclusion and second-phase content and a clean, well-deoxidized, low-sulphur steel (inclusions are crack starters on the outer fibre); an annealed, low-work-hardening temper rather than a hard-rolled one; and orientation of the bend line across (transverse to) the rolling direction, since the elongated inclusion stringers then lie along the bend axis instead of across it. Smooth, deburred, unscratched sheared edges are also part of the specification, because a zero-radius bend puts the outer fibre at its strain limit and any edge notch will start the split.
(c) Greatest resistance to permanent deformation in service. Specify the highest yield strength the other requirements will tolerate. Permanent (plastic) set begins when the service stress reaches the yield strength, so yield — not ultimate strength, not hardness, and definitely not stiffness — is the governing property. In practice that means a high-strength low-alloy or dual-phase grade, or a work-hardened temper, and it directly conflicts with requirement (b): the same cold work that raises the yield strength consumes the ductility that zero-radius bending needs. The standard resolution is to form the part in the soft condition and raise the yield strength afterwards, by bake-hardening, by a strain-ageing grade, or by heat treatment.
