(a) (i) Discuss the mechanism of material removal in ultrasonic machining. (10 marks) (ii) What are the different parameters…

(a)(i) Mechanism of material removal in USM

A tool of the required shape vibrates axially at 15–30 kHz with an amplitude of about 10–50 μm over the workpiece, with a slurry of hard abrasive (B₄C, SiC, Al₂O₃) in water filling the gap under a small static feed force.

• Hammering: on each downward stroke the tool drives abrasive grains into the work surface. The impact produces Hertzian cracks and brittle chipping, removing tiny fragments. This is the dominant mechanism.
• Free impact: grains thrown by the vibrating tool strike the surface and cause additional chipping.
• Cavitation: the pressure cycles in the slurry form and collapse bubbles, whose micro-jets and shock waves erode the surface and help circulate fresh abrasive.
• Chemical action of the fluid plays a minor role with some materials. Because removal is by brittle fracture, USM suits hard, brittle materials (glass, ceramics, carbides, gemstones), while ductile metals deform rather than chip.

(a)(ii) Parameters affecting MRR

• Amplitude of vibration: MRR increases with amplitude, since each grain penetrates deeper (Shaw model predicts a less-than-linear rise).
• Frequency: MRR rises with frequency because there are more impacts per second, but at very high frequency the attainable amplitude falls and so does MRR.
• Static feed force: MRR rises to a maximum and then falls, as excessive force crushes grains and blocks slurry flow.
• Abrasive grit size: MRR increases with grit size until it is comparable with the amplitude, then decreases because fewer grains act.
• Slurry concentration: MRR rises with concentration and levels off around 30–40% by weight, while high viscosity reduces flow.
• Work and tool materials: brittle work with high hardness ratio (work to tool) gives higher MRR, and a tough tool (mild steel, stainless steel) wears less.

(b)(i) ABC, VED and SDE classification

• ABC (value): items are ranked by annual consumption value. A items, roughly 10–20% of items, account for about 70–80% of value and get tight control and frequent review. B items are intermediate. C items, the bulk of items with about 5–10% of value, get simple control and larger orders.
• VED (criticality): Vital items stop production if not available and need high safety stock. Essential items cause loss of efficiency if short. Desirable items cause little disruption and can be substituted.
• SDE (availability): Scarce items, often imported or with long lead time, need careful planning and stock. Difficult items are available with effort or long lead time. Easy items are readily available locally. The classifications are combined (ABC-VED matrix) so that, for example, a C-vital spare is still stocked adequately.

(b)(ii) ABC classification of the products

Annual value = demand × unit cost, total = ₹5,010.40. In descending order (cumulative share): XA-101 ₹1,600 (31.9%), XA-102 ₹1,500 (61.9%), XA-108 ₹480 (71.5%), XA-109 ₹414.40 (79.7%), XA-103 ₹360 (86.9%), XA-110 ₹252 (91.9%), XA-104 ₹160 (95.1%), XA-105 ₹125 (97.6%), XA-106 ₹60 (98.8%), XA-107 ₹44 (99.7%), XA-111 ₹15 (100%).

A: XA-101, 102, 108, 109 (4 items, about 80% of value). B: XA-103, 110, 104 (3 items, about 15%). C: XA-105, 106, 107, 111 (4 items, about 5%).

(c) Steps in sourcing and influencing factors

Steps:

1. Identify the need and prepare specifications (quantity, quality, timing).
2. Make-or-buy analysis.
3. Market survey and identification of potential suppliers.
4. Pre-qualification and short-listing (capacity, quality systems, finances).
5. Request for quotation or tender and receipt of bids.
6. Technical and commercial evaluation of bids (total cost of ownership).
7. Negotiation and supplier selection.
8. Contract award and supplier onboarding.
9. Performance monitoring and relationship management (rating, audits, development).

Factors influencing the sourcing decision: cost and total cost of ownership, quality, delivery lead time and reliability, capacity and flexibility, technology and innovation capability, financial stability, location and logistics, supply risk (single source, geopolitical), government policy (local content, import duties), sustainability and ethical practices, and after-sales service.