23-Ind-B2 Manufacturing Processes · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2014 — 98-Ind-B2 Manufacturing Processes. Closed book; Casio or Sharp approved calculators only. 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, casting, metal-cutting theory and machinability, polymer processing, welding, grinding/finishing, and automation/numerical control; Montgomery, Introduction to Statistical Quality Control, 8th ed. — statistical process control and X-charts.
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.
Given. Hole diameter $D = 2.5$ in.; part thickness (hole depth) $t = 4.5$ in.; drill point angle $\theta = 118^\circ$; drill speed $N = 240$ rpm; feed $f = 0.25$ in./rev; soft cast iron, high-speed-steel (HSS) drill.
Find. The cutting time $T_m$ (min) for the drill-press operation.
Approach. Compute the feed rate from speed and feed per revolution, add the drill-point approach allowance to the hole depth to get the total axial travel of the drill tip, then divide travel by feed rate.
| Quantity | Value |
|---|---|
| Feed rate, $f_r$ | 60 in./min |
| Approach allowance, $A$ | 0.751 in. |
| Total axial travel | 5.251 in. |
| Cutting time, $T_m$ | 0.0875 min (≈5.25 s) |
Cutting fluids act on the machining process through two related mechanisms. The first is lubrication: the fluid forms a boundary film at the chip-tool and tool-workpiece interfaces that reduces friction and adhesion, which lowers the cutting forces and power required, reduces built-up-edge formation, and improves the resulting surface finish; lubrication is the dominant mechanism at low cutting speeds, where the tool and chip are in contact long enough for the fluid to penetrate and act. The second is cooling: the fluid removes heat by convection from the cutting zone, the chip, the tool, and the workpiece, which slows tool wear (most wear mechanisms accelerate sharply with temperature), controls thermal expansion of the workpiece so dimensions stay in tolerance, and prevents thermal softening/damage to the tool's cutting edge; cooling becomes the dominant mechanism at high cutting speeds, where flow rate and thermal conductivity matter more than boundary-film lubrication because there is too little dwell time for a lubricating film to form. A secondary but important action is chip removal — flushing chips away from the cutting zone, particularly in drilling and other hole-making operations where chips must travel back out along the flutes.
Workpiece material. Cooling reduces thermal distortion and keeps machined dimensions within tolerance on precision work; however, water-based fluids can cause staining or corrosion on ferrous workpieces if the rust-inhibitor package is inadequate or the fluid is not properly maintained, and some fluids attack or craze certain plastics and elastomers, so fluid chemistry must be matched to the work material. Rapid, uneven cooling can also alter the surface residual-stress state left by machining.
Machine tools. Cutting fluid helps stabilize machine temperature on precision equipment (reducing thermal-growth error), but it can corrode unprotected machine surfaces, dilute or wash away way lubricants, attack seals and paint if the additive package is aggressive, and — if it leaks into electrical enclosures or bearing housings — cause premature component failure; machines running with flood coolant need filtration/skimming systems to keep tramp oil and swarf from recirculating and degrading the fluid and the machine's guideways.
Biological/health. Prolonged skin contact with cutting fluids is a leading cause of occupational dermatitis; fine mist or aerosol generated at high speeds can be inhaled, causing respiratory irritation; and fluid sumps are a favourable environment for bacterial and fungal growth, producing odour, biological hazards, and fluid degradation if not treated with biocides and regularly filtered/changed. These risks drive the use of enclosures, mist extraction, PPE (gloves, eye protection), and scheduled fluid maintenance.
External environment. Spent cutting fluid, particularly oil-based and oil-in-water emulsion fluids, is regulated hazardous waste and cannot be discharged to drain; it requires collection, treatment/reclamation (oil-water separation, filtration) and licensed disposal, and any leak or spill risks soil/groundwater contamination. These costs and risks are a major driver behind minimum-quantity lubrication (MQL) and dry-machining strategies, which sharply reduce fluid consumption and the associated disposal burden.
