Manufacturing Practices (BT-106) - Important Questions

Explain the mechanism of metal cutting and chip formation in machining processes. Discuss the zones in the primary shear region, secondary deformation zone and the tertiary (tool–workpiece) zone, and explain how material properties and cutting conditions affect chip morphology.

Differentiate between orthogonal and oblique cutting. Draw the force diagram for orthogonal cutting and derive the relationships between cutting force, thrust force and resultant force. Explain how cutting forces are related to power and specific cutting energy.

State Taylor's tool life equation and explain each parameter. How is Taylor's equation used to determine the cutting speed for a desired tool life? Show the equation in the standard form $$V\,T^{n}=C$$ and explain how to compute $V$ for a specified $T$.

Derive the expressions for spindle speed $N$ (in rpm) and machining time $T$ (in minutes) for a turning operation. Use cutting speed $V$ (m/min), work diameter $D$ (mm), length of cut $L$ (mm) and feed per revolution $f$ (mm/rev). Show the standard formulae $$N=\frac{1000\,V}{\pi\,D}$$ and $$T=\frac{L}{f\,N}\,.$$

Define Material Removal Rate (MRR). Derive expressions for MRR in turning, milling and drilling processes. State the variables involved and explain qualitatively how feed, depth of cut and cutting speed influence MRR.

Describe the main types of milling operations (peripheral, face and slot milling). Derive or state the formula to calculate machining time for a milling pass mentioning spindle speed, cutter diameter, number of teeth and feed per tooth. Indicate how to compute time for a given length of cut.

Discuss the commonly used cutting tool materials (HSS, cemented carbide, ceramics, CBN and diamond). For each material list its main properties, typical applications and limitations. Explain selection criteria for tool material for high-speed turning of steel.

Explain the types of tool wear (flank wear, crater wear, notch wear, built-up edge, chipping) and the mechanisms leading to each. Describe methods to reduce tool wear and extend tool life, including changes in cutting parameters, tool geometry and use of coatings/coolants.

Describe the grinding process. Explain the characteristics of grinding wheels (abrasive type, grain size, grade, structure and bond). How are wheel specifications selected for cylindrical grinding of hardened steel to achieve required surface finish?

What are the functions of cutting fluids in machining? List types of cutting fluids and explain selection criteria for a machining operation where improved tool life and surface finish are desired.

Discuss the factors affecting surface roughness in turning. For a single-point turning operation with feed per revolution $f$ and nose radius $r_{e}$, state the approximate relation between arithmetic average roughness $R_{a}$ and $f,\;r_{e}$. Use the commonly used approximation $$R_{a}\approx\frac{f^{2}}{32\,r_{e}}\,,$$ and explain the assumptions behind this relation.

Derive the expression for the optimum cutting speed that minimizes the total cost per part using Taylor's tool life equation $V\,T^{n}=C$ and a simple cost model where machining cost per part includes cost of tool life (tool cost per unit time) and machine operating cost (per unit time). Show the steps to obtain the optimum $V_{opt}$ in terms of the model parameters and discuss how $V_{opt}$ depends on tool cost, machine cost and the exponent $n$.

For a drilling operation, derive expressions for spindle speed $N$ (rpm) and drilling time $T$ (minutes) required to drill a hole of depth $L$ (mm) in a workpiece using a drill of diameter $D$ (mm) with cutting speed $V$ (m/min) and feed per revolution $f$ (mm/rev). Use the standard relations and state any assumptions.