22-Mec-A4 Design and Manufacture of Machine Elements · May 2014

Question 2 of 8: Fine blanking

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format. National Examination, 07-Mec-A4 Design and Manufacture of Machine Elements, May 2014 — 3 hours, open book, any non-communicating calculator permitted. Eight questions on six pages, divided into Part A (manufacturing processes, Q1–Q4) and Part B (machine-element design, Q5–Q8). The rubric asks for three from Part A and two from Part B, five questions constituting a complete paper, all of equal value (20 % each). All eight questions are solved here, because this document is a study resource rather than an examination script.

Reference texts.

Kalpakjian & Schmid, Manufacturing Engineering and Technology, 7th ed. — sand casting and casting defects (Ch. 10–12), sheet-metal shearing and fine blanking (Ch. 16), bulk deformation and hydrostatic extrusion (Ch. 15), fusion welding and weld defects (Ch. 30–31).
Budynas & Nisbett, Shigley's Mechanical Engineering Design, 10th/11th ed. — eccentrically loaded shear joints (Ch. 8), statically indeterminate beams and deflection superposition (Ch. 4), welded joints in torsion (Ch. 9).
Hibbeler, Mechanics of Materials, 10th ed. — combined loading, transverse shear and stress transformation / Mohr's circle (Ch. 7–9).
AWS D1.1 Structural Welding Code — Steel and CSA W59 — preheat, hydrogen control and minimum fillet-weld sizes (Canadian practice).
ASM Handbook Vol. 15 Casting and Vol. 6 Welding, Brazing and Soldering — hot tearing, solidification cracking and riser/chill practice.

Check: Part B is entirely figure-driven. Every number below was read from the printed figures (Figures A, B, C and S7). Two readings are worth stating explicitly so a grader can substitute a different interpretation without redoing the method: (i) in Figure A the rivet group is five rivets in the top row plus one rivet 200 mm below, the lower rivet lying on the same vertical line as the third top rivet; (ii) in Figure B the 67 500 N horizontal force acts on the centroidal axis of the section, so it produces pure tension and no additional bending.

PART A — Manufacturing Processes

Question 2: Fine blanking (20 marks)

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 2.1 — Fine-blanking tool: three independently controlled forces (punch, V-ring pressure pad, counter cushion) plus an unusually small die clearance.

(a) Description of the process

Fine blanking — also called fineblanking or, in the German literature, Feinschneiden — is a precision variant of blanking carried out on a triple-action press. Three forces act on the strip at the same time. Before the punch moves, the upper pressure pad descends and its V-shaped impingement ring (the "stinger"), which stands proud of the pad face immediately outside the part outline, is pressed into the surface of the stock; simultaneously the lower pressure cushion is raised against the underside of the blank through the die opening, clamping the blank between punch face and cushion. Only then does the blanking punch descend, and it does so slowly — typically 5–15 mm/s rather than the several hundred mm/s of a conventional press — into a die whose clearance is on the order of 0.5 % of the stock thickness per side, roughly a tenth of the 5–8 % used in ordinary blanking. After the part is severed, the cushion ejects it and the pad strips the skeleton. Stock is normally a soft, high-formability, spheroidize-annealed low- or medium-carbon steel, and the tooling is lubricated generously because the process is deliberately friction- and pressure-intensive.

(b) The main feature of fine-blanked parts

The distinguishing feature is the edge. A conventionally blanked edge is a four-zone edge — rollover, a bright burnished band covering only about a third of the thickness, a rough matte fracture zone covering the remainder, and a burr — and it is neither square nor smooth enough to be used as a functional surface. A fine-blanked edge is fully burnished over essentially 100 % of the sheet thickness: it is smooth (typically \(R_a\) of 1–2.5 µm), square to the sheet face within a fraction of a degree, has almost no rollover and a negligible burr, and shows no fracture zone at all. Dimensional accuracy across the part follows: tolerances of roughly IT7–IT9 (order of ±0.01–0.05 mm), flatness far better than conventional blanks because the strip is clamped throughout, and edges good enough to serve directly as gear-tooth flanks, cam profiles, clutch plates, seat-recliner components and lock parts — that is, the part comes off the press finished, with the secondary shaving, machining or grinding operation eliminated.

(c) How the feature is achieved

A clean edge is obtained by preventing the crack that normally does most of the cutting. In conventional blanking, cracks nucleate at the punch and die edges once the material has been rolled over and burnished a little way, run toward one another through a region that is in a tensile stress state, and meet to complete the separation by fracture; the fracture zone on the edge is simply the record of that crack. Fine blanking suppresses crack nucleation by putting the deforming metal into a state of triaxial compression and by giving the crack no room to run:

The V-ring bites into the strip just outside the cut line and locks the material radially, so the metal in the shear zone cannot be drawn inward; the hydrostatic pressure in the shear zone rises sharply, and a high compressive hydrostatic stress raises the fracture strain of the material dramatically without changing its flow stress.
The counter pressure cushion supports the blank face-to-face against the punch, so the sheet cannot bend or dish; the shear zone stays a pure shear zone instead of a bending-plus-shear zone, which removes the rollover and keeps the edge square.
The very small clearance (≈ 0.5 % \(t\)) confines the plastic deformation to a narrow band directly between the two cutting edges, so the deformation is nearly pure shear and there is no tensile ligament in which a crack could open.
The slow, controlled punch speed keeps the strain rate low and lets the material flow plastically rather than respond in a brittle manner, and it keeps the process isothermal enough that adiabatic shear banding does not initiate a crack.

The combined effect is that the material is separated entirely by plastic shear, extruded rather than broken, so the whole thickness of the edge carries the burnished appearance. The price is a punch force some 1.5–2 times the conventional blanking force plus the ring and cushion forces, tooling that must be far more rigid and accurately guided, and a press with three independently controllable rams — which is why fine blanking is justified by high production volumes of parts that would otherwise need machining.

Topic. Shearing mechanics; the role of hydrostatic stress in ductile fracture; fine blanking as a precision shearing process.

Key relations. Conventional blanking force \(F = 0.7\,S_{ut}\,t\,L\) (with \(L\) the sheared perimeter); clearance \(c = a\,t\sqrt{\,}\) with \(a \approx 0.05\)–0.08 for conventional blanking versus \(a \approx 0.005\) for fine blanking; ductile fracture strain increases with compressive stress triaxiality \(\sigma_m/\bar{\sigma}\) (the Bridgman effect).

Why this works. Ductile fracture proceeds by void nucleation, growth and coalescence, and void growth rate depends exponentially on the mean (hydrostatic) stress. Superimposing compression on the shear zone therefore does not change how much the metal can be sheared before it flows — the flow stress obeys von Mises and is insensitive to hydrostatic pressure — but it postpones the onset of fracture almost indefinitely. Every element of the fine-blanking tool exists to raise that compressive component: the V-ring supplies radial restraint, the counter cushion supplies axial restraint, and the tight clearance geometrically confines the plastic zone so that neither restraint can leak away.

Common pitfall. Writing that fine blanking works "because the clearance is small" and stopping there. Small clearance alone, on an ordinary press without the V-ring and the counter pressure, produces secondary shear and a doubly burnished but still fractured edge. The second common error is to claim the process eliminates the burr entirely; a small burr always remains on the die side, and the V-ring leaves a permanent impression in the skeleton that must be allowed for in strip layout.

Source. Kalpakjian & Schmid, Manufacturing Engineering and Technology, 7th ed., §16.2–16.3 (Shearing; Fine blanking).

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