Question 1

Understanding centerless grinding  Which statements correctly describe essential features of centerless grinding used for high-rate production?

Accepted Answer

Introduction / Context: Centerless grinding is widely used for producing high-precision cylindrical parts such as pins, rollers, and shafts. Unlike between-centers grinding, the part is supported by a work-rest blade and driven by a regulating wheel, enabling continuous feeding and excellent throughput. Given Data / Assumptions: Through-feed or infeed (plunge) centerless configurations. Work-rest blade used for support, grinding wheel for cutting, regulating wheel for rotation and feed control. Production environment where continuous processing is valued. Concept / Approach: Because the work is supported along a line generatrix on the blade, the effective support extends along its length, not just at the ends. The regulating wheel both rotates the part and controls the axial feed in through-feed mode, so no chucks or centers are required. This geometry enables continuous, high-rate production. Step-by-Step Solution: Assess support: the work-rest blade and wheel pair provide full-length line support → Statement (A) is correct.Assess process mode: through-feed allows continuous part flow → Statement (B) is correct.Assess fixturing: absence of centers/chucks, the regulating wheel drives the work → Statement (C) is correct.Hence, all statements are true → (D). Verification / Alternative check: Typical centerless lines feed bar stock continuously, with minimal setup time and excellent repeatability for diameter control. Why Other Options Are Wrong: (E) is incorrect; long and short parts can be ground, with guides and supports as needed. Common Pitfalls: Confusing centerless with cylindrical grinding between centers; misadjusting work-rest blade height causing lobing; overlooking the role of the regulating wheel in both rotation and feed. Final Answer: all of the above

Question 2

Specific cutting energy and machinability  The specific cutting energy used for comparing machinability of metals depends primarily on which material characteristics?

Accepted Answer

Introduction / Context: Specific cutting energy (energy per unit volume removed) reflects how difficult it is to shear and remove material. It serves as a practical basis for comparing machinability across alloys and heat treatments under similar cutting conditions. Given Data / Assumptions: Orthogonal cutting model insights apply to practical 3D cutting. Steady cutting conditions: tool geometry, speed, feed, and depth fixed for comparison. Concept / Approach: Specific cutting energy captures both material resistance to shear deformation and frictional losses. Friction at the tool–chip interface elevates the required energy. Microstructure controls flow stress (e.g., pearlite vs ferrite, precipitates), while work hardening raises flow stress with strain, further increasing energy demand. Step-by-Step Solution: Relate cutting energy = shear energy + friction energy.Coefficient of friction ↑ → interface friction ↑ → energy ↑ (A).Microstructure affects yield/flow stress and chip formation → energy varies (B).Strain hardening exponent n influences stress with plastic strain → energy varies (C).Therefore, all listed factors influence specific cutting energy → (D). Verification / Alternative check: Empirical data show higher specific energies for hardened or heavily work-hardening alloys; free-machining grades with sulfides reduce friction and energy. Why Other Options Are Wrong: (E) is too narrow; hardness alone does not fully predict cutting energy without considering microstructure and frictional behavior. Common Pitfalls: Comparing specific energy across tests with different tool geometries; ignoring coolant effects that alter friction; overlooking chip thickness effect on apparent energy. Final Answer: all of these

Question 3

Flat (spade) drill geometry  For a flat drill used on metals, the typical relief (clearance) angle provided at the cutting edge is in which range?

Accepted Answer

Introduction / Context: Relief (clearance) angle prevents rubbing of the flank behind the cutting edge and influences cutting forces, heat generation, and edge strength. Flat (spade) drills have simpler geometry than twist drills but still require carefully chosen relief. Given Data / Assumptions: General-purpose drilling of steels and non-ferrous alloys with a flat drill. Balanced need: avoid rubbing while maintaining edge strength. Concept / Approach: Too small a relief angle causes rubbing and heat; too large weakens the edge and increases chipping risk. Practical shop recommendations position flat drill relief in a modest range sufficient to clear the flank without undermining strength—typically a few degrees. Step-by-Step Solution: Assess functional need: reduce contact behind the cutting edge.Select moderate relief to balance sharpness and strength.Adopt the standard range 3°–8° as suitable for metal cutting. Verification / Alternative check: Toolroom guides and handbooks specify small relief angles for spade/flat drills; larger angles are reserved for very soft materials or special geometries. Why Other Options Are Wrong: 20°–30° (B) is excessive for general metal drilling and weakens the edge. 60°–90° (C) and 90°–120° (D) are unrealistic for relief; these are in the range of point or included angles, not clearance. 0°–2° (E) risks severe rubbing and overheating. Common Pitfalls: Confusing point angle with relief angle; over-grinding relief leading to rapid chipping; ignoring material-specific tweaks. Final Answer: 3° to 8°

Question 4

Effect of shear angle on chip formation  In orthogonal cutting, what is the qualitative effect of a larger shear angle on the shear path length and the resulting chip thickness?

Accepted Answer

Introduction / Context: The shear angle is a fundamental variable in chip formation mechanics. It dictates how the material shears along a plane ahead of the tool, thereby controlling chip thickness, cutting force, and heat generation. Given Data / Assumptions: Orthogonal cutting with a sharp tool and continuous chip formation. Constant uncut chip thickness and rake angle for reasoning. Concept / Approach: A larger shear angle rotates the shear plane closer to the cutting edge, reducing shear plane length. Shorter shear path generally lowers the volume of material undergoing intense shear at once, producing a thinner chip and potentially reducing cutting forces. Step-by-Step Solution: Consider geometry: increasing shear angle reduces shear plane length.With the same uncut thickness, chip thickness decreases as shear angle increases (chip ratio improves).Therefore, a large shear angle → short shear path → thin chip. Verification / Alternative check: Merchant-type relations link larger positive rake (which tends to increase shear angle) with lower forces and thinner chips, consistent with the qualitative effect described. Why Other Options Are Wrong: Options (B), (C), and (D) contradict the geometric implications of increased shear angle; (E) is incorrect because shear angle directly governs chip thickness and path length. Common Pitfalls: Confusing the effect of feed (uncut chip thickness) with shear angle; assuming shear angle is fixed independently of tool geometry and friction. Final Answer: path of shear is short and chip is thin

Question 5

Choosing drill helix for aluminum machining  When drilling aluminum and other ductile, free-cutting non-ferrous alloys, which helix characteristic is preferred for the twist drill to improve chip evacuation and reduce built-up edge?

Accepted Answer

Introduction / Context: Aluminum tends to produce long, continuous chips and can form built-up edge at the tool tip. Drill flute geometry, particularly the helix angle, critically affects chip evacuation and cutting temperature. Given Data / Assumptions: Ductile, gummy materials such as aluminum and its alloys. Conventional twist drills with flutes and point geometry optimized for non-ferrous materials. Concept / Approach: A higher helix angle improves chip lifting and evacuation from the hole, decreases rubbing, and promotes a more positive effective rake along the cutting lips. This reduces the tendency for built-up edge and improves surface finish. Step-by-Step Solution: Identify material behavior: long, ductile chips need efficient evacuation.Select geometry: high helix angle increases chip flow capacity of flutes.Outcome: cooler cutting, less BUE, better hole quality → choose high helix. Verification / Alternative check: Manufacturers offer “bright-finished, high-helix aluminum drills” specifically for this purpose, confirming the selection. Why Other Options Are Wrong: Zero or low helix (A/B) hampers chip lifting and can cause clogging. (D) ignores the strong influence of helix on chip flow. Negative helix (E) is not used for standard drilling. Common Pitfalls: Using standard steel drills with low helix on aluminum, leading to chip packing and poor finish; insufficient coolant or pecking cycles in deep holes. Final Answer: high helix angle

Question 6

Thread milling capability  Which milling machine arrangement is specifically suited for producing internal or external threads of various pitches without changing lead screws?

Accepted Answer

Introduction / Context: Threads can be produced by turning, tapping, die-threading, or milling. Thread milling allows flexible production of internal and external threads, including non-standard pitches, using helical tool paths and appropriate kinematics. Given Data / Assumptions: Availability of a milling setup capable of synchronized circular and axial motions. Use of a thread milling cutter or a helical path generator (planetary motion). Concept / Approach: A planetary milling machine (or thread milling attachment) imparts a compound motion that generates helical threads. It decouples pitch generation from the machine lead screw, enabling a wide range of pitches by changing gearing or CNC parameters, not the physical screw. Step-by-Step Solution: Identify requirement: produce internal/external threads of different pitches.Match method: thread (planetary) milling achieves helixes by coordinated motions.Conclude: planetary milling is the correct choice. Verification / Alternative check: Modern CNCs perform thread milling with circular interpolation; the classical planetary attachment achieves similar kinematics mechanically. Why Other Options Are Wrong: Pantograph and profiling machines copy contours; they do not generate helical threads. Piano-miller is for planing large flat surfaces. Planer with single-point tool (E) generates linear cuts, not helixes without complex gearing. Common Pitfalls: Confusing hobbing (gear generation) with thread milling; assuming only taps/dies can produce internal threads in milling contexts. Final Answer: planetary milling machine (thread milling)

Question 7

Cutting speed selection for drilling  For drilling operations, when the work material is softer (e.g., aluminum or brass) compared to harder steels, the recommended cutting speed is generally:

Accepted Answer

Introduction / Context: Proper cutting speed selection in drilling influences tool life, hole quality, and productivity. Material properties such as hardness and thermal conductivity determine allowable surface speeds for twist drills. Given Data / Assumptions: Softer, ductile non-ferrous materials versus hard steels. Standard HSS or carbide drills with appropriate geometry and coolant. Concept / Approach: In general, softer and more thermally conductive materials permit higher cutting speeds without exceeding tool temperature limits. Harder materials require lower speeds to control heat and wear. Manufacturers provide speed charts reflecting this trend. Step-by-Step Solution: Compare materials: aluminum/brass vs hardened steels.Assess speed guidance: aluminum can be drilled at much higher surface speeds.Therefore, for softer materials, select high cutting speeds. Verification / Alternative check: Typical charts: HSS on aluminum allows far higher m/min than on alloy steel; carbide widens the gap further. Why Other Options Are Wrong: (A) ignores material differences; (B) contradicts standard practice; (D) and (E) are incorrect because speed is a primary parameter alongside feed. Common Pitfalls: Using steel drilling speeds on aluminum leading to built-up edge due to too low speed and excessive feed; neglecting coolant and chip evacuation. Final Answer: high

Question 8

Gear finishing methods  Among the following operations, which one is classified as a gear finishing process aimed at improving surface finish and slight profile accuracy after generation?

Accepted Answer

Introduction / Context: Gear manufacturing often proceeds from roughing/generation to finishing. Generation methods produce the tooth geometry efficiently, while finishing methods enhance surface quality, noise performance, and service life. Given Data / Assumptions: Common gear processes: milling, hobbing, shaping (generation) versus finishing such as shaving, honing, lapping, and burnishing. Objective: identify which listed process is a finishing step. Concept / Approach: Burnishing is a cold-working finishing method using hardened rollers or gears to plastically smooth asperities, improving finish and sometimes slightly correcting minor deviations. It does not remove material like hobbing or shaping. Step-by-Step Solution: Classify options: shaping, milling, and hobbing → generating methods.Burnishing → finishing by plastic deformation, not chip removal.Therefore, the finishing process among the choices is burnishing. Verification / Alternative check: Gear finishing catalogues list burnishing alongside shaving, honing, and lapping for noise reduction and improved pitting resistance. Why Other Options Are Wrong: Shaping, milling, and hobbing are used to generate tooth form; they are not finishing operations. Broaching (E) is a generating/forming method for internal gears, not finishing. Common Pitfalls: Assuming any post-cutting pass is “finishing”; confusing gear shaving (material removal) with burnishing (cold work only). Final Answer: burnishing

Question 9

Feed directions on a centre lathe  With reference to the lathe axis, in which directions can the cutting tool be fed on a conventional centre lathe during turning operations?

Accepted Answer

Introduction / Context: Lathe tool feeds determine the surface generated on the work. Understanding the available feed axes clarifies which operations a centre lathe can perform efficiently, such as facing, turning, and taper turning. Given Data / Assumptions: Conventional engine/centre lathe with carriage, cross slide, and compound rest. Work held between centers or in a chuck, rotating about the spindle axis. Concept / Approach: Two principal feed motions exist: longitudinal feed (parallel to spindle axis) for cylindrical turning and shouldering, and cross feed (perpendicular to spindle axis) for facing and diameter changes. The compound rest can be set to an angle for short tapers, but the primary feed axes remain longitudinal and cross. Step-by-Step Solution: Identify slideways: carriage moves longitudinally; cross slide moves radially (cross).Operations: turning uses longitudinal; facing uses cross; tapers use compound or offset tailstock.Hence, the tool can be fed in both cross and longitudinal directions. Verification / Alternative check: Lathe controls include separate feed clutches/leads for longitudinal and cross feeds, confirming both are standard. Why Other Options Are Wrong: (A) and (B) are incomplete. (D) is incorrect because slideways constrain motion to defined axes. (E) the compound rest provides angled feeds for limited travel but does not replace the two principal feed axes. Common Pitfalls: Overusing the compound for long tapers leading to chatter; ignoring appropriate feed directions for the intended surface. Final Answer: both cross and longitudinal direction

Question 10

Manufacturing process: For which applications is broaching typically used in practice?

Accepted Answer

Introduction / Context: Broaching is a high-productivity machining process where a multi-tooth tool (the broach) is pushed or pulled past the work to remove metal in a single pass. It is widely used for internal profiles such as keyways and splines, as well as for external flats and contours. This question checks your understanding of the scope of parts and surfaces that can be produced by broaching. Given Data / Assumptions: The broach has progressively taller teeth that perform roughing to finishing in one stroke. Fixtures and guides are available to control alignment for internal or external features. Production context favors repeatability and speed over flexible toolpaths. Concept / Approach: Because the broach's tooth geometry is pre-formed, any shape that can be guided through the work envelope within force limits can be reproduced—internal (holes, splines, polygons), external (flats, slots, contours), and specialized (rifling, turbine root forms). Broaching excels when the same geometry is repeated in medium to high volumes. Step-by-Step Solution: Identify typical internal broaching: keyways, hex/square holes, involute splines.Identify typical external broaching: flats, fir-tree and dovetail forms, serrations.Conclude that both internal and external surfaces—as well as round/irregular holes—are feasible with the right broach and setup. Verification / Alternative check: Industry case studies and tooling catalogs show dedicated internal broaches (push/pull) and surface broaches (external) for a wide range of geometries, confirming the comprehensive capability of the process. Why Other Options Are Wrong: External flat and contoured surfaces: true, but not the only use. Round or irregular shaped holes: true, but not the only use. Internal and external surfaces: true, but still not exhaustive. Only keyways inside hubs: overly restrictive; broaching is applicable far beyond keyways. Common Pitfalls: Believing broaching is only for internal keyways; ignoring the need for precise fixturing and the high initial cost of custom broaches which is justified by production quantities. Final Answer: all of these

Question 11

Electro-discharge machining (EDM): In the EDM setup, is the tool connected as the cathode (negative terminal)?

Accepted Answer

Introduction / Context: Electro-discharge machining removes material by a series of controlled electrical discharges (sparks) between a tool electrode and the workpiece in a dielectric fluid. Polarity choice influences tool wear and stability. This item tests the basic polarity convention used in die-sinking EDM. Given Data / Assumptions: Conventional die-sinking EDM with hydrocarbon or deionized water dielectric. Workpiece is conductive and is connected to the positive terminal. Tool is typically copper/graphite connected to the negative terminal. Concept / Approach: In most die-sinking EDM applications, the tool is the cathode (negative), and the workpiece is the anode (positive). This polarity reduces tool erosion relative to workpiece removal and provides stable spark characteristics. Alternate polarities can be used for special purposes, but the usual practice is tool negative. Step-by-Step Solution: Identify standard polarity: tool negative (cathode), work positive (anode).Reason: reduces tool wear and improves machining efficiency on the workpiece.Conclude that the statement is correct under typical conditions. Verification / Alternative check: EDM manuals and application notes consistently recommend tool negative for common die-sinking operations with steel workpieces to minimize electrode wear. Why Other Options Are Wrong: Wire EDM uses continuously fed wire and different power supplies, but the question targets the common die-sinking convention; dielectric choice does not reverse the basic rule in standard practice. Common Pitfalls: Assuming polarity is arbitrary; overlooking that changing polarity can intentionally increase tool wear for special burnishing or dressing operations but is not the default. Final Answer: Correct

Question 12

Machining cast iron: Is it generally acceptable to machine grey cast iron without cutting fluid (dry machining)?

Accepted Answer

Introduction / Context: Grey cast iron contains free graphite, which acts as a solid lubricant during machining. Many shops machine cast iron dry to avoid slurry, protect machines, and maintain good surface integrity. This question checks whether you know the common practice regarding coolant use on cast iron. Given Data / Assumptions: Work material: grey cast iron with graphite flakes. Conventional turning/milling parameters. Adequate dust extraction or housekeeping is arranged since dry machining releases fine dust. Concept / Approach: The graphite in cast iron provides inherent lubrication, lowering the need for cutting fluids. Using coolant can mix with dust and create abrasive slurry that accelerates wear on machine ways. Therefore, dry machining is common and acceptable, especially with proper chip/dust management and appropriate tool materials (HSS or carbide). Step-by-Step Solution: Identify material property → graphite provides lubricity.Assess coolant effects → potential for slurry and mess; limited benefit.Conclude → dry machining of cast iron is standard practice in many operations. Verification / Alternative check: Machining guides recommend dry cutting for most CI operations; mist or air blast may be used for cooling and chip evacuation without liquid flood. Why Other Options Are Wrong: Restricting to carbide only or low speed only is unnecessary; both HSS and carbide can be used dry within appropriate cutting parameters; external constraints like dust control are the key considerations. Common Pitfalls: Using water-based coolants that create abrasive sludge; neglecting dust control which can affect operator health and machine components. Final Answer: Correct

Question 13

Electro-chemical machining (ECM): What is the typical inter-electrode gap maintained between tool and work?

Accepted Answer

Introduction / Context: Electro-chemical machining removes material by anodic dissolution, with the workpiece as anode and the tool as cathode in a flowing electrolyte. Maintaining a proper inter-electrode gap is vital for stable current density, accurate shape replication, and removal of reaction products (sludge and gas). Given Data / Assumptions: Typical aqueous electrolyte (e.g., NaCl, NaNO3) with high flow rate. Moderate current densities and standard tool feed controls. Production ECM, not micro-ECM or macro-roughing extremes. Concept / Approach: Practical ECM gaps are small to achieve good accuracy yet large enough to prevent arcing and ensure debris flushing. A commonly cited nominal gap is about 0.25 mm for many setups. Larger gaps reduce accuracy and voltage efficiency; smaller gaps risk shorting and clogging without very precise control and high flushing velocity. Step-by-Step Solution: Consider flushing and current density → gap must allow electrolyte circulation.Balance accuracy and stability → ≈0.25 mm provides a practical compromise.Select 0.25 mm as the typical ECM gap. Verification / Alternative check: Process handbooks list 0.2–0.5 mm as common; 0.25 mm is a frequently used nominal value in general-purpose ECM tooling. Why Other Options Are Wrong: 0.10 mm: possible in specialized micro-ECM, but risky in general practice. 0.40–0.75 mm: workable but tends to reduce accuracy and efficiency. 1.50 mm: excessively large for normal ECM, leading to poor control. Common Pitfalls: Ignoring electrolyte cleanliness and flow; forgetting that gap varies dynamically with feed and voltage; using too small a gap causing shorts. Final Answer: 0.25 mm

Question 14

Cylindrical grinding methods: What best describes transverse (traverse) grinding on a cylindrical grinder?

Accepted Answer

Introduction / Context: Cylindrical grinding can be done by plunge (in-feed) or traverse (transverse) methods. Understanding which kinematics suit long parts or specific surface finish requirements is essential in process planning for shafts, rollers, and bearing seats. Given Data / Assumptions: Conventional cylindrical grinder with rotating work between centers or in a chuck. Wheel has finite face width. Radial infeed plus axial traverse of the work relative to the wheel. Concept / Approach: In traverse grinding, the workpiece is reciprocated axially past the wheel while the wheel feeds in radially in small steps. This is ideal for surfaces longer than the wheel face, ensuring uniform removal and good finish over the entire length. Plunge grinding, by contrast, is preferred for short surfaces equal to or narrower than the wheel face width. Step-by-Step Solution: Identify need to grind a long cylindrical surface.Choose traverse grinding → axial reciprocation of the work.Feed radially in small increments to reach size and finish. Verification / Alternative check: Process charts classify traverse grinding for L > wheel face width; plunge grinding for L ≤ wheel face width. Why Other Options Are Wrong: Options describing fixed work for long parts or “shorter than wheel face” confuse plunge vs traverse definitions. Wheel reciprocation with stationary work is not the standard cylindrical grinding method. Common Pitfalls: Attempting plunge grinding on long parts causing taper or poor finish; insufficient overlap ratio during traverses leading to feed marks. Final Answer: the work is reciprocated as the wheel feeds to produce cylinders longer than the width of wheel face

Question 15

Grinding wheels: The process of restoring sharp cutting action by removing dulled grains and bond is called…

Accepted Answer

Introduction / Context: Grinding wheel maintenance involves two distinct operations: dressing and truing. Confusing these can lead to poor surface finish, high grinding forces, and thermal damage to the workpiece. This question focuses on the operation that renews the wheel's cutting ability. Given Data / Assumptions: Dressed with diamond dresser, rotary dresser, or stick stones depending on bond. Wheel has become glazed or loaded with chips. Geometry (runout) is still acceptable or will be corrected separately if needed. Concept / Approach: Dressing removes dulled abrasive grains and bond to expose fresh, sharp cutting edges and open up the wheel structure. Truing is the geometric correction of the wheel to make it run true (corrects eccentricity and shape). Often both are done in sequence: true first, then dress, but the purposes remain different. Step-by-Step Solution: Identify symptom → wheel is glazed/loaded, cutting poorly.Apply dressing → remove top layer to expose fresh abrasives and chip pockets.Resume grinding with restored sharpness and lower forces. Verification / Alternative check: Wheel manufacturer recommendations clearly separate truing (geometry) from dressing (sharpness/porosity restoration). Why Other Options Are Wrong: Truing: corrects shape/runout, not primarily cutting action. Facing/clearing/loading: nonstandard or incorrect terms in this context. Common Pitfalls: Attempting to improve cutting by only truing; failing to redress leads to burn and chatter; over-dressing wastes wheel life. Final Answer: dressing

Question 16

Sources of surface roughness in metal cutting: Which factors commonly contribute to roughness on a machined surface?

Accepted Answer

Introduction / Context: Surface roughness after machining arises from a combination of geometric, material, and dynamic effects. Understanding these contributors is essential for selecting parameters and tooling to meet finish specifications without excessive cost. Given Data / Assumptions: Conventional turning/milling with single-point or multi-point tools. Stable machine setup but typical shop conditions. Tool geometry and speed/feed/depth of cut are adjustable. Concept / Approach: Tool feed leaves a theoretical scallop height (feed marks). Built-up edge (BUE) forms at low-to-moderate speeds and can intermittently break off, leaving surface tears. Machine/tool vibration (chatter) superimposes a waviness pattern that elevates roughness. All three mechanisms can operate simultaneously. Step-by-Step Solution: Roughness from geometry → reduce feed and increase nose radius to lower scallop height.Roughness from BUE → increase cutting speed, use sharp tools and proper cutting fluid to suppress adhesion.Roughness from vibration → improve rigidity, adjust speed/feed, and balance tooling to avoid chatter. Verification / Alternative check: Surface metrology often reveals distinct signatures (periodic feed marks, chatter wavelength). Corrective actions confirm the contributing mechanisms. Why Other Options Are Wrong: Limiting the cause to only one factor ignores the multifactorial nature of machining finish. Therefore, the comprehensive choice is best. Common Pitfalls: Chasing finish by coolant alone while ignoring feed or rigidity; using excessive nose radius that induces chatter; neglecting tool wear. Final Answer: all of these

Question 17

Cutting tool materials: How does grain size influence tool life in sintered carbides and similar tool materials?

Accepted Answer

Introduction / Context: Powder-metallurgy tool materials (e.g., tungsten carbide with cobalt binder) derive properties from grain size, binder content, and additives. Grain refinement typically increases hardness and edge strength, strongly affecting wear resistance and tool life in many applications. Given Data / Assumptions: Comparing fine-grained vs coarse-grained grades of carbide or cermet at comparable binder fractions. Cutting conditions where abrasive and diffusion wear dominate rather than chipping from heavy interruption. Proper tool geometry and coolant use. Concept / Approach: Smaller carbide grains increase hardness and inhibit crack initiation and propagation along grain boundaries, improving flank and crater wear resistance. Very fine grains also allow a keener cutting edge, reducing BUE tendency. While ultra-fine grains can reduce toughness for heavy interrupted cuts, for many continuous or light-interrupted operations a finer grain size yields longer tool life. Step-by-Step Solution: Relate grain size to hardness and wear → finer grains → higher hardness.Relate microstructure to edge integrity → finer grains support sharp edges.Conclude → smaller grain size generally enhances tool life under common conditions. Verification / Alternative check: Manufacturer grade charts show fine and ultrafine carbide grades targeted at finishing and medium cuts with superior wear life versus coarse-grained grades intended for roughing toughness. Why Other Options Are Wrong: Claiming no influence ignores well-documented microstructure–property relations; saying coarse grains are always best contradicts practice except for severe impact loading cases. Common Pitfalls: Using ultra-fine grades for heavy interrupted cuts where toughness is critical; overlooking binder content which also affects life; misattributing life solely to coatings while substrate grain size remains important. Final Answer: smaller

Question 18

Lathe speeds: A lathe with four steps on the cone pulley and with back-gears engaged provides how many distinct spindle speeds?

Accepted Answer

Introduction / Context: Older engine lathes use stepped cone pulleys for speed selection. Back-gears provide a secondary, slower set of speeds by introducing a gear reduction between the cone pulley and spindle. Understanding this arrangement helps operators choose suitable cutting speeds when variable-frequency drives are not present. Given Data / Assumptions: Four steps on the cone pulley (4 belt positions). Back-gear mechanism available for speed reduction. No additional gearbox is considered. Concept / Approach: Each belt step gives one direct (back-gear disengaged) speed. Engaging back-gears multiplies each step by a fixed reduction, creating a corresponding indirect (reduced) speed. Therefore, a 4-step cone with back-gears yields 4 direct plus 4 indirect speeds, i.e., 8 distinct spindle speeds. Step-by-Step Solution: Count belt steps → 4.Without back-gear → 4 direct speeds.With back-gear → each of the 4 steps has a reduced partner → total 8 speeds. Verification / Alternative check: Lathe manuals and nameplates show two speed columns for each belt position: direct and back-gear, confirming the 4 + 4 arrangement. Why Other Options Are Wrong: Only direct or only indirect speeds ignore the dual-mode nature; eight indirect speeds is incorrect because half are direct, half are indirect. Common Pitfalls: Trying to change belt position with back-gears engaged; forgetting to properly disengage the bull gear pin when using back-gears, which can lock the spindle. Final Answer: four direct and four indirect speeds

Question 19

Twist drill geometry: What is the usual (standard) helix angle of a general-purpose twist drill?

Accepted Answer

Introduction / Context: The helix angle of a twist drill affects chip evacuation, cutting forces, and guidance. General-purpose drills are designed with a compromise angle to suit a wide range of materials and machine conditions. This question asks for the commonly used standard value. Given Data / Assumptions: Uncoated HSS drill for general workshop use. No special aluminum or stainless-specific geometry. Standard point angle around 118° assumed. Concept / Approach: General-purpose twist drills typically use a helix angle near 30°. Lower helix (~20–25°) favors harder materials with short chips, while higher helix (~35–45°) is chosen for aluminum and ductile materials to aid chip evacuation. The standard compromise for broad applicability remains about 30°. Step-by-Step Solution: Identify common shop standard → 30° helix.Match to broad material range requirement.Select 30° as the usual helix angle. Verification / Alternative check: Catalogs from major drill manufacturers list “general-purpose” series with helix close to 30°, with variants for aluminum (higher helix) and hard steels (lower helix). Why Other Options Are Wrong: 20° or 45° are specialized ranges; 60° is not used as a helix angle for twist drills; 12° is far below practical values for chip transport in drills. Common Pitfalls: Confusing helix angle with point angle; assuming one helix fits all materials without considering chip evacuation needs. Final Answer: 30°

Question 20

Grinding practice – effect of glazing on performance In production grinding, “glazing” occurs when the abrasive grains become dull and the wheel surface turns shiny, causing rubbing instead of cutting. How does glazing influence the cutting capacity…

Accepted Answer

Introduction / Context: Glazing is a common phenomenon in grinding where the wheel surface becomes smooth and shiny because worn abrasive grains are not shed. Instead of sharp cutting points penetrating the work, glazed wheels rub and plough. Understanding its effect on cutting capacity is essential for choosing dressing frequency, wheel grade, and operating parameters. Given Data / Assumptions: A vitrified or resinoid-bonded grinding wheel is in steady operation. Abrasive grains have dulled and are no longer fracturing or releasing. No significant change in machine power or coolant aside from the glazing condition. Concept / Approach: Cutting capacity depends on the number of active, sharp cutting points and their ability to penetrate the work. A glazed wheel presents rounded, blunt grains that slide on the surface, producing frictional heat. The lack of self-sharpening reduces chip formation and increases rubbing. Therefore, glazing must reduce the net material removal rate at the same feed and depth of cut. Step-by-Step Solution: Identify state: grains are dull and bond is holding them too firmly.Consequence: fewer effective cutting points, more rubbing and ploughing.Effect on process: higher grinding power and temperature for the same feed.Result: cutting capacity (MRR for given inputs) decreases; surface burn risk rises. Verification / Alternative check: Practical shop remedy for glazing is dressing the wheel to expose new sharp edges. A freshly dressed wheel immediately restores cutting action and lowers power draw at the same infeed, confirming that glazing had reduced cutting capacity. Why Other Options Are Wrong: “Increases” contradicts the loss of sharp points. “No effect” ignores the measurable rise in power and temperature. “Initially increases, then decreases” does not match glazing behavior; that describes certain wear-in phases, not glazing. “Reverses force direction” is unrelated to wheel sharpness. Common Pitfalls: Confusing glazing with loading (chip packing). Both reduce performance, but glazing stems from dull grains; loading stems from swarf clogging. Remedies differ in dressing aggressiveness and coolant strategy. Final Answer: Decreases cutting capacity

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