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CAM_Part_2
| Question | Answer |
|---|---|
| Q: Explain how strain-rate sensitivity and thermal softening of a material combine to affect chip formation during high-speed machining. Give the implications for cutting speed selection. | A: Strain-rate sensitivity increases flow stress with higher strain rates (hardening), while thermal softening reduces flow stress as temperature rises. In high-speed machining, high strain rates in the shear zone tend to raise strength (promote serrated/ |
| Q: For two alloys with identical tensile strength, alloy A has higher thermal conductivity than alloy B. How does that influence tool selection and cutting parameters? | A: Higher thermal conductivity (A) removes heat from the cutting zone faster, reducing local temperature and thermal softening; it generally leads to lower tool wear due to heat spreading into the workpiece but may increase cutting forces (less softening) |
| Q: Define machinability. List three quantifiable indicators and explain trade-offs when optimizing for them. | A: Machinability: relative ease of removing material by cutting, considering tool life, surface finish, power, and chip control. Indicators: (1) Tool life (time until wear criterion), (2) Cutting force/power required, (3) Surface roughness/quality. Trade- |
| Q: Explain how microstructure (grain size and phase distribution) affects tool wear mechanisms in turning of steels. | A: Fine grains generally yield more uniform plastic deformation and can improve toughness; coarse grains cause localized harder/softer regions leading to abrasive wear and micro-chipping. Phase distribution: hard carbides or brittle phases increase abrasi |
| Q: How do residual stresses introduced by prior cold forming influence distortion during subsequent machining? Provide mitigation strategies. | A: Residual tensile/compressive stresses cause elastic spring-back when material is removed, leading to distortion or dimensional errors. Cold-formed parts often have locked-in stresses that relax during machining. Mitigation: apply stress-relief heat tre |
| Q: Compare and contrast the roles of hardness and toughness in selecting tool material for machining hardened steels (HRC > 55). | A: Hardness resists abrasive wear and plastic deformation (essential for HRC steels). Toughness prevents brittle fracture and chipping under impact or interrupted cuts. For HRC>55, tool materials must be extremely hard (PCD often unsuitable for steel due |
| Q: Describe the effect of workpiece thermal expansion on tolerance control during finish turning and how to account for it in an oral exam answer. | A: Temperature rise during cutting causes local thermal expansion altering measured dimensions and potentially violating tight tolerances. During finish turning, the heated zone may expand then relax leading to undersize after cooling. Account by: machini |
| Q: Why are some aluminum alloys (e.g., 2xxx series) harder to machine than certain steels despite lower hardness? Explain. | A: 2xxx Al alloys have high strength due to Cu-containing precipitates and may form ductile, sticky chips that adhere to tools (BUE). Low thermal conductivity and chemical affinity can worsen built-up edge. Steels may form brittle chips easier to evacuate |
| Q: Provide a decision tree (high-level steps) to select cutting tool material/coating given: workpiece material, cutting speed, and presence of interrupted cuts. | A: 1) Identify workpiece group (ferrous/hardened/non-ferrous). 2) If hardened (>45-50 HRC): choose CBN. 3) If ferrous but not hardened and continuous cuts: choose carbide with TiAlN/TiN coating for higher speeds. 4) If interrupted cuts or shock: favor tou |
| Q: How does anisotropy in composite materials complicate conventional machining theory? | A: Anisotropy means directional mechanical properties; chip formation varies with fiber orientation, causing heterogenous cutting forces, delamination, fiber pull-out, and rapid tool wear. Classical shear-plane models fail because deformation localizes ar |
| Q: Explain why thermal conductivity and specific heat are critical for selecting cutting speed in high-temperature alloys (e.g., Inconel). | A: Low thermal conductivity and specific heat concentrate heat in the shear zone, raising tool temperature quickly, accelerating diffusion wear and loss of hardness. Thus cutting speeds must be limited; use sharp tools, coolant or high-performance coating |
| Q: Derive and explain the significance of the cutting ratio r = h / h_c (undeformed chip thickness / deformed chip thickness) and how to estimate shear angle phi. | A: r = h / h_c = cos(phi) / (sin(phi - alpha) * cos(alpha)), derived from geometry of shear plane and tool rake alpha. Significance: r < 1 indicates compression and thickening of chip; relates to material strain and cutting forces. Solve Ernst-Merchant: p |
| Q: In the orthogonal cutting model, explain how secondary shear zone and tool–chip friction influence tool wear and surface finish; include mitigation tactics. | A: Secondary shear is at tool–chip interface causing additional deformation and heat; tool–chip friction increases temperature and abrasive/adhesive wear, leading to built-up edge and flank wear. Effects: higher friction raises cutting forces, worsens sur |
| Q: Explain the mechanisms of crater wear and flank wear, and how cutting parameters (speed, feed, depth) influence each. | A: Crater wear (on rake face) arises from chemical diffusion, oxidation, and abrasion by chip sliding; increases with temperature and contact time (higher speed). Flank wear (on flank face) results from abrasive rubbing against finished surface—higher wit |
| Q: Define Taylor's tool life equation and explain how you would experimentally determine constants for a new tool–material pair. | A: Taylor: V*T^n = C, where V is cutting speed, T tool life, n exponent, C constant. Experiment: run turning tests at various speeds with fixed feed and depth until tool life criterion (e.g., flank wear VB=0.3mm) is reached, measure T, plot log(V) vs log( |
| Q: Scenario: You observe serrated/chatter-like segmented chips on a nickel superalloy at moderate speeds. Diagnose likely causes and propose countermeasures. | A: Serrated chips indicate cyclic shear localization due to thermal softening and strain-rate sensitivity; may be exacerbated by low thermal conductivity and high cutting speeds. Chatter-like patterns could be tool vibration too. Countermeasures: adjust s |
| Q: Discuss the heat flow paths in the cutting zone and the consequences for tool material selection. | A: Heat splits into chip (~60%), tool (~30%), workpiece (~10%) depending on material and process. If chip takes most heat, tool sees less but chip temperature affects chip flow and adhesion. Tool selection requires high hot hardness for when tool retains |
| Q: How does built-up edge form and how does it affect dimensional accuracy and tool life? Give practical remedies. | A: BUE forms by adhesion of workpiece material to the tool due to pressure and moderate temperatures, creating a layer that periodically forms and tears off, altering effective geometry. Effects: fluctuating dimensions, poor surface finish, increased forc |
| Q: Explain the concept of minimum chip thickness in micro-machining and its effects on surface roughness and forces. | A: Minimum chip thickness is the fraction of tool edge radius below which material deforms elastically/plastically (ploughing) rather than shearing, so cutting becomes inefficient. Below this thickness, forces increase disproportionally, surface roughness |
| Q: Provide a strategy to reduce cutting forces while maintaining material removal rate in a roughing operation. | A: Split MRR across higher spindle speed with smaller axial depth but larger radial engagement (increase number of passes at smaller DOC while keeping feed per tooth), use multi-insert cutters to raise engagement without increasing force per tooth, use cl |
| Q: Describe how you would experimentally measure cutting forces during a turning test and use them to infer shear stress in the shear plane. | A: Use a dynamometer to record tangential and feed forces (Ft, Ff). Compute resultant cutting force Fc = sqrt(Ft^2 + Ff^2). From orthogonal cutting relations, shear force Fs = Fc * cos(alpha) / cos(phi - alpha) (or use transformation equations). Knowing c |
| Q: Scenario: During turning of stainless steel you notice excessive surface tensile residual stress causing early fatigue failure in the part. What process parameters and strategies can you change to induce compressive residual stress on the surface? | A: Increase feed and use sharper tools to raise compressive plastic deformation; apply burnishing or roller finishing; use larger negative rake or compressive honing operations; use cryogenic cooling or lower cutting temperature to limit tensile stress du |
| Q: Explain how tool geometry (nose radius, lead angle, rake, and clearance) affects: surface roughness, cutting forces, and tool life in turning. | A: Nose radius: larger radius improves surface finish (blends peaks) but increases radial cutting force and power and raises ploughing at small feeds; lead angle spreads cut over larger circumference, reducing depth per unit length and smoothing forces; r |
| Q: Derive the relationship between material removal rate (MRR), spindle speed, feed, and depth of cut in turning, and explain how to prioritize parameters when constrained by power. | A: MRR = cross-sectional area * cutting speed = (pi * D_avg * f * a_p * N)??? Wait correct: For turning, MRR = cutting speed * cross-section area / cutting edge? Better: MRR = (pi * D_mean * N) * f * a_p where D_mean = average diameter, N spindle rev/s, f |
| Q: Scenario: A long slender shaft is to be turned—list clamping and process strategies to minimize chatter and deflection while achieving required tolerances. | A: Use steady/rest supports (tailstock, live center), reduce overhang length, use rigid tooling and short tool overhang, decrease depth of cut and increase number of passes, use lower feed and higher spindle speed to avoid resonance, choose sharp tool and |
| Q: Explain the differences between rough turning, finishing turning, and hard turning in terms of cutting parameters and expected surface integrity. | A: Rough turning: high feed and depth, low speed to maximize MRR—surface finish rough, subsurface damage limited as finishing follows. Finishing: low feed and small DOC, high speed for smooth finish and dimensional accuracy—control temps and forces for su |
| Q: How do you calculate cutting power for turning and how is it related to torque and spindle speed? | A: Cutting power Pc = Fc * Vc / 60 (if Fc in N and Vc in m/min convert). Alternatively Pc = 2 * pi * torque * N (with torque in N·m and N in rev/s). Torque T = Fc * D / 2 / (2*pi*N) rearranged accordingly. Ensure units consistent: Pc (W) = 2 * pi * T (N·m |
| Q: Describe methods to measure and compensate axis misalignment (runout) on a lathe to guarantee tight cylindrical tolerances. | A: Measure runout with dial indicators or spindle probes, use test bar and measure at multiple points, apply spindle/truing adjustments, use live or dead centers precision, employ chucking procedures (soft jaws, regrinding jaws), and apply correction offs |
| Q: Scenario: You have a part requiring internal deep boring to tight tolerance. Describe tooling, stability, and process parameter choices to reduce taper and vibration. | A: Use integral boring bars with minimal overhang, support with tailstock or steady rest, use bar with internal coolant supply, slow spindle speed to reduce chatter for long bars, reduce axial depth per pass with multiple light passes, use vibration-dampi |
| Q: What is tool nose radius compensation in CNC turning and why is it critical for achieving dimensional accuracy? | A: Compensation adjusts tool path to account for finite nose radius so the effective cutting path equals intended profile. Without compensation, the tool’s geometry produces offsets in dimensions and surface form. CNC uses G-codes to apply radius compensa |
| Q: Explain the roles of insert geometry (chipbreakers, top land, honed edge) when turning ductile materials. | A: Chipbreakers help curl and break continuous chips into manageable pieces, reducing entanglement and heat; top land adds strength to cutting edge reducing cratering and edge collapse; honed edge increases edge strength for interrupted cuts and harder ma |
| Q: Scenario: During rough turning with indexable inserts you notice catastrophic chipping at insert corners when cutting cast iron. Diagnose and recommend remedies. | A: Cast iron has abrasive graphite and hard inclusions causing edge chipping. Remedies: choose tougher insert grade with rounded edge to resist chipping, select negative rake inserts to increase wedge strength, reduce depth of cut or use multiple lighter |
| Q: Explain how radial depth of cut (ae) and axial depth of cut (ap) differently affect forces in peripheral milling and face milling. | A: Radial depth (ae) changes width of cut and affects instantaneous engagement and tangential forces—large ae increases cutting force nearly linearly and affects chip thinning. Axial depth (ap) changes engaged tooth immersion along axis (stacking of profi |
| Q: Derive the relation for specific cutting energy in milling and discuss how tool diameter and number of teeth influence it. | A: Specific cutting energy U = Pc / MRR. Pc depends on cutting forces summing over teeth; increasing tool diameter increases cutting speed at given spindle RPM, increasing power but also reducing specific energy if cutting conditions improve chip formatio |
| Q: Scenario: You observe chatter in face milling thin-walled aluminum parts. Suggest a sequence of diagnostic checks and corrective actions. | A: Diagnostics: check tool-holder and spindle runout, verify fixture rigidity and part clamping, measure natural frequency (modal analysis), inspect cutter geometry and balance, examine cutting parameters (ae, ap, fz) for stability lobes. Corrections: inc |
| Q: Explain climb vs conventional milling: advantages, drawbacks, and when to prefer one in CNC operations. | A: Climb milling (down milling) direction causes cutter to engage at full chip thickness and exit thinning, producing better surface finish, less tool deflection, and longer tool life but draws the work into the cutter (requires rigid fixturing). Conventi |
| Q: How does feed per tooth (fz) affect surface finish, tool life, and tool deflection in high-speed milling? Provide recommended strategies for optimization. | A: Higher fz increases chip thickness leading to higher forces, worse finish, and higher tool deflection; very low fz may produce rubbing and increased tool wear (ploughing). Optimize by choosing fz for chip-breaking and material removal efficiency given |
| Q: Scenario: You must rough a pocket in stainless steel with minimal tool changes. Propose an optimal cutter type, toolpath strategy, and cutting parameters rationale. | A: Use indexable high-feed or face-mill with carbide inserts rated for stainless; toolpath: trochoidal/slotting to keep engagement constant and reduce radial load, using helical entry and adaptive clearing. Parameters: moderate speed (avoid overheating), |
| Q: Explain how cutter runout affects tool life and part accuracy in multi-flute endmills and how to detect/mitigate it. | A: Runout causes uneven chip load among flutes: overloaded flute chips, others rub, increasing wear and heat, leading to premature failure and poor finish. Detect via test cuts and measuring eccentricity with indicators, or use laser spindle probes. Mitig |
| Q: Describe the use and benefits of trochoidal milling for hard-to-machine materials. | A: Trochoidal milling (constant engagement, small radial step with circular interpolation) reduces instantaneous chip thickness and radial load, enabling higher feed per tooth and longer tool life, lowers heat concentration, and reduces deflection and cha |
| Q: How do you evaluate required spindle power for a face milling operation? List data and equations. | A: Required power Pc = (Fc * Vc) / 60, where Fc estimated from specific cutting force k_c * A_c (A_c = ae * ap), or use empirical Fc = k_c * ae * ap. Need k_c (material), ae, ap, Vc (cutting speed), feed per tooth and number of teeth to compute chip thick |
| Q: Scenario: In finishing a Ti alloy impeller vane, you need to minimize subsurface plastic deformation. Which milling techniques and tooling choices reduce surface damage? | A: Use sharp, small-inclination rake tools with positive geometry, high cutting speed with low feed to minimize force per area, use coolant and climb milling to reduce rubbing, use single-crystal diamond or coated carbide (if compatible) to reduce adhesio |
| Q: Explain the main differences in cutting mechanics between drilling and orthogonal cutting, and how they affect heat generation and tool design. | A: Drilling is essentially a rotating cutting with two cutting lips and chisel edge; material removal involves primary shearing at lips and intense compressive deformation at chisel edge. Heat concentrates near the chisel and flutes; evacuation of chips i |
| Q: How does point angle and helix angle influence drill performance in steel vs aluminum? | A: Steel: larger point angle (~118–140°) increases cutting lip strength and reduces chisel edge action, controlling burrs; helix angle moderate to low to avoid chip clogging and limit heat. Aluminum: smaller point angle and high helix aid chip evacuation |
| Q: Scenario: While deep-hole drilling a brass component, you experience poor chip evacuation causing seizing. Propose tooling and process changes. | A: Use through-coolant drills or peck drilling to break chips, increase helix angle and smooth flute finish for evacuation, reduce feed per rev to create smaller chips, use lower rpm to avoid welding, apply lubricant specifically for brass, and consider s |
| Q: Define thrust force and torque in drilling and explain how feed rate, point geometry, and material properties influence them. | A: Thrust is axial force to push drill into material (dominated by chisel edge compression); torque is rotational moment resisting cutting at lips. Increasing feed increases thrust and torque; sharper point reduces thrust; harder or tougher materials incr |
| Q: Describe the mechanisms of drill bit breakage in interrupted drilling and how to avoid them. | A: Breakage due to cyclic bending stresses from sudden engagement/disengagement, torsional shock from jammed chips, and fatigue from repeated loads. Avoid by using peck drilling, support with backing plates, select robust drills with larger core/web, use |
| Q: Explain the role of chisel edge in conventional twist drills and how web thinning changes cutting performance. | A: Chisel edge is nearly a crushing element at center causing high thrust and poor shearing; web thinning reduces chisel thickness converting chisel action into cutting lips, lowering thrust, improving hole quality, and reducing heat at center. Excessive |
| Q: Scenario: A high accuracy hole (tolerance H7) is required in stainless steel. Describe the sequence of machining operations from pilot to final, including parameter rationale. | A: Pilot drill small diameter with peck to clear chips; ream or boring for precision: use carbide reamer or boring bar depending on size; finish with reamer to H7 at low feed and stable speed with coolant; ensure rigid fixturing, use floating reamer holde |
| Q: How does entry/exit burr formation differ among ductile and brittle materials and what drilling practices reduce burrs? | A: Ductile materials form large burrs due to plastic flow at exit; reduce by using backing support, slow feed at breakout, sharp tool, optimized point angle, peck drilling to control chip formation, or using deburring tools. Brittle materials may crack—us |
Created by:
Filotì