22-Mec-A4 Design and Manufacture of Machine Elements · December 2016

Question 3 of 6

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

Notes on this paper

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.

S. Kalpakjian and S. R. Schmid, Manufacturing Engineering and Technology, 8th ed. — Part A (sheet-metal forming, casting, machine-tool selection).
M. P. Groover, Fundamentals of Modern Manufacturing: Materials, Processes and Systems, 7th ed. — fine blanking, die casting, turning-machine classes.
R. G. Budynas and J. K. Nisbett, Shigley's Mechanical Engineering Design, 11th ed. — Part B (welded joints Ch. 9, shafts Ch. 7, bolted and riveted joints Ch. 8).
R. L. Norton, Machine Design: An Integrated Approach, 6th ed. — corroborating weld and riveted-joint treatments.

Check — weld allowable stress basis (Q4). The paper gives an electrode ultimate strength (E60, S_u = 60 ksi) and a safety factor of 3.0, but not the strength theory. This solution uses the distortion-energy shear strength of the deposited metal, S_su = 0.577 S_u, divided by the given factor — the treatment used throughout Shigley Ch. 9. The alternative code route (AISC allowable 0.30 S_u, which already embeds its own reserve) is worked out at the end of Q4 and gives a smaller leg. State whichever basis you adopt; the marker is looking for the weld-as-a-line method, not the code table.

Question 3 (25 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.

Machine-tool selection for turned parts is decided almost entirely by production quantity, because quantity determines how much setup, tooling and programming cost can be amortized. Slenderness adds a second, independent constraint (workpiece support against deflection and chatter), and a transverse feature adds a third (whether the machine can index and drive a rotating tool, or whether a second setup is cheaper). Each part below is answered on that basis.

(a) Turning, drilling, boring, parting off — 10 000/month. This is roughly 500 parts per working day, which is production, not job work, and the four operations are all coaxial. The economical choice is a single-spindle automatic bar (screw) machine or a CNC turning centre with a bar feeder and a turret; at the upper end of this rate a multiple-spindle automatic bar machine becomes cheaper still, because four to eight spindles index simultaneously so the cycle time equals the longest single operation rather than their sum. The bar feed makes the process continuous, and parting off releases the finished part automatically. The high tooling and setup cost is spread over 10 000 parts and becomes negligible per part.
(b) The same part — 10/month. Setup and programming now dominate: at ten parts, an automatic's setup would cost more than the whole batch. Use a conventional engine lathe (toolroom lathe) with a tailstock and standard chucks, or a small CNC lathe already on hand if the shop is programming-fluent, run from bar or from cut blanks with manual tool changes. General-purpose machine, general-purpose tooling, minimum setup — the classic low-quantity answer.
(c) Very slender, high precision, turning and parting off — 1000/month. A slender bar deflects away from the tool and chatters, so the cutting point must be supported. The purpose-built answer is the Swiss-type automatic lathe (sliding-headstock screw machine): the bar is fed through a close-fitting guide bushing and the tools cut immediately at the face of that bushing, so the unsupported overhang is essentially zero regardless of part length. The headstock slides to generate the longitudinal feed. This is the standard machine for watch parts, connector pins, bone screws and similar slender precision work, and 1000/month comfortably justifies it.
(d) The same slender part — 10/month. Back to a general-purpose machine, but the support problem does not go away. Use a precision engine or toolroom lathe fitted with a follower rest (travelling steady) or a steady rest, light depths of cut and sharp, high-rake tooling. A collet chuck rather than a jaw chuck preserves concentricity for the precision requirement.
(e) As (a), plus a transverse hole — 10 000/month. At this quantity a second setup for one hole is unacceptable, so the part must be finished complete in one machine. Use a CNC turning centre with live (driven) tooling and a C-axis, which indexes the spindle to an angular position and drives a rotating drill in the turret; or, in the automatic-screw-machine world, a multiple-spindle automatic with a cross-drilling attachment. Either way the transverse hole is drilled in-cycle, with no re-fixturing and therefore no loss of positional accuracy relative to the turned features.
(f) As (b), plus a transverse hole — 10/month. Now the economics reverse. Live tooling is not worth buying for ten parts. Turn the part complete on the engine lathe, then move it to a drill press or a vertical mill with a simple V-block or a drill jig for the transverse hole. Two setups on machines the shop already owns beat one setup on a machine it would have to acquire.

Recommended machine tool by part characteristic and quantity
Case	Governing constraint	Recommended machine tool
(a) 10 000/month, coaxial features	quantity	Single- or multiple-spindle automatic bar machine / CNC turning centre with bar feed
(b) 10/month, same part	setup cost	Engine (toolroom) lathe
(c) 1000/month, slender, precise	workpiece support + quantity	Swiss-type sliding-headstock automatic with guide bushing
(d) 10/month, slender, precise	workpiece support, low quantity	Precision engine lathe with follower / steady rest and collet
(e) 10 000/month + transverse hole	avoid a second setup	CNC turning centre with live tooling and C-axis (or multi-spindle automatic with cross-drilling attachment)
(f) 10/month + transverse hole	setup cost	Engine lathe, then drill press with a jig or V-block

Topic: Machine-tool selection and the quantity–flexibility trade-off in turning.

Key relations: Unit cost equals the setup and tooling cost divided by lot size plus the run time multiplied by the machine rate. General-purpose machines have low setup cost and high cycle time; special-purpose automatics invert both. The crossover for turned parts sits in the low hundreds of pieces per lot, which is why every "10 000/month" case in this question resolves to an automatic and every "10/month" case to an engine lathe.

Why this works: Slenderness and transverse features do not change that economic logic; they change which machine within each quantity band. Slenderness demands support at the cutting point — a guide bushing at high quantity, a follower rest at low quantity. A transverse feature demands either a rotating tool on the lathe (worth buying at high quantity) or a second setup (cheaper at low quantity).

Common pitfall: Recommending a CNC turning centre for every case. CNC is flexible and fast to program, but for ten parts a manual engine lathe is still usually cheapest, and for 10 000 simple parts a multiple-spindle automatic beats CNC on cycle time because its spindles work in parallel.

Source: Kalpakjian & Schmid, Manufacturing Engineering and Technology, 8th ed., Ch. 23 (Machining Processes: turning machines, automatic bar machines, Swiss-type, turning centres) and Ch. 40 (Process capabilities and economics); Groover, 7th ed., Sec. 22.2.

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