23-Ind-B2 Manufacturing Processes · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2016 — 98-Ind-B2 Manufacturing Processes. Closed book; candidates may use one of two calculators, the Casio or Sharp approved models. Any five of the seven questions constitute a complete paper; all questions are of equal value (20 marks each). Answers are written in point form but fully, with all calculations shown, as instructed. Complete answers to all seven questions follow.
Reference texts: Groover, Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, 6th ed. — material selection and heat treatment, polymer processing, metal-cutting theory, welding processes, and automation/numerical control.
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.
Resistance welding (RW) is a family of fusion-welding processes that join metal parts by generating heat from the electrical resistance of the workpieces at their contacting interface, combined with a mechanically applied force. Two (or more) electrodes clamp the parts together and pass a high electrical current (typically low voltage, very high amperage) through the joint for a short, controlled time; the interface, which has the highest electrical resistance in the circuit (poorest contact, thinnest cross-section relative to the surrounding parent metal), heats fastest and reaches fusion temperature while the surrounding electrodes (usually water-cooled copper) stay comparatively cool. The applied electrode force is maintained through and after current flow to forge the softened/molten interface into a solid-state or fusion bond as it cools.
Main advantages: no filler metal, flux, or shielding gas is required, which keeps consumable cost low and the process clean; very fast cycle times (a spot weld is made in a fraction of a second), well suited to high-volume automated production (e.g. automotive body assembly); the process is easily automated and robotized; and it produces minimal distortion and a relatively small heat-affected zone compared with arc welding, because the heating is localized and brief.
The general expression for the heat generated is $H=I^2Rt$, where $H$ is the heat generated (J), $I$ is the current (A), $R$ is the electrical resistance of the workpiece interface ($\Omega$), and $t$ is the time current flows (s) — the standard resistive (Joule) heating relation, sometimes written with a heat-loss factor $K<1$ as $H=I^2RtK$ to account for heat conducted away into the surrounding metal and electrodes rather than used to make the weld.
Resistance spot welding (RSW) uses pointed/rounded electrode tips to make an individual, discrete weld nugget at one location where the electrodes pinch the overlapping sheets; the electrodes are lifted and repositioned for each new weld. Resistance seam welding (RSEW) replaces the pointed electrodes with rotating wheel-shaped electrodes that roll along the joint while current is applied in a continuous or rapidly pulsed sequence, producing a series of overlapping nuggets that form a continuous, leak-tight welded seam rather than isolated spots.
Advantages of spot welding: simple, low-cost tooling and equipment; very fast per-weld cycle time; ideal for joining sheet-metal assemblies at discrete points where a continuous seam is not needed (e.g. automotive body panels), and it is the most widely automated/robotized of all welding processes. Advantages of seam welding: produces a continuous, gas- and liquid-tight joint (useful for tanks, mufflers, radiators, and other pressure-containing sheet-metal assemblies); the rolling-wheel electrodes allow long, continuous joints to be made in one pass rather than as a series of discrete operations.
Oxyfuel gas cutting (OFC) is a thermal cutting process that uses the heat of a fuel-gas/oxygen flame (commonly oxyacetylene) to preheat the steel to its kindling (ignition) temperature, then directs a high-purity oxygen jet through the same torch onto the heated metal; the oxygen chemically reacts with (oxidizes/burns) the iron to form iron oxide, and the oxygen jet's kinetic energy simultaneously blows the molten oxide (slag) out of the kerf, progressively cutting through the section as the torch is advanced. It is fundamentally a chemical (oxidation) cutting process, not a melt-and-vaporize process like laser or plasma cutting.
Process capabilities: OFC is capable of cutting relatively thick sections of plain carbon and low-alloy steel economically (from a few millimetres up to several hundred millimetres in heavy fabrication), because cutting speed and section-thickness capability are less limited by equipment power than with mechanical cutting; equipment is low-cost and portable, making it well suited to field/site work (structural steel erection, scrap/demolition cutting, pipe cutting); it can perform straight or contoured (with hand or machine-guided torches) cuts and is commonly used for shape-cutting plate as a precursor to further fabrication. Its limitations are equally important to state: it is largely restricted to metals that oxidize readily and whose oxide has a lower melting point than the base metal (essentially plain-carbon and low-alloy steels — it does not work well on stainless steel, aluminum, or copper, whose oxides are refractory or that do not sustain the exothermic reaction the way iron does); it produces a wider kerf, more heat-affected zone, and generally lower dimensional accuracy than plasma or laser cutting; and it requires skilled manual technique (or a machine guide) to hold consistent speed and torch angle for a clean edge.
