Maharashtra HSC Mechanical Maintenance (Bifocal) flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Representative fraction (RF)

Ratio of drawing length to actual length. RF below one is a reducing scale, above one is an enlarging scale, and exactly one is full size.

First-angle projection

Object lies between observer and plane, so the top view is drawn below the front view and the left-side view appears on the right.

Cutting speed formula

V equals pi times D times N divided by 1000, giving metres per minute when diameter is in millimetres and spindle speed in revolutions per minute.

Taper turning tailstock offset

Offset equals total job length times diameter difference, divided by twice the tapered length. Used for small-angle tapers between centres.

Austenitising temperature

Heating steel above its upper critical temperature so the structure becomes austenite, the necessary first step before hardening or normalising.

Tempering purpose

Reheating hardened steel below the lower critical temperature relieves quenching stress and reduces brittleness, trading some hardness for toughness.

Case carburising

Low-carbon steel is heated in a carbon-rich medium so carbon diffuses into the surface, giving a hard wear-resistant skin over a tough core.

Grey cast iron

Iron with flake graphite, giving excellent damping and machinability but low tensile strength and almost no ductility under shock load.

Vernier least count

Difference between one main-scale division and one vernier-scale division; a 0.02 mm caliper has 49 main divisions matching 50 vernier divisions.

Micrometer least count

Screw pitch divided by the number of thimble divisions; a 0.5 mm pitch with fifty divisions gives a least count of 0.01 mm.

Slip gauge wringing

Joining gauge blocks by pressing cleaned surfaces together so they adhere by molecular attraction, allowing combinations that build an accurate dimension.

Chisel wedge angle

Cutting angle chosen by material: about seventy degrees for hard steel, sixty for mild steel, forty for brass, thirty for wood.

File grade

Indicates tooth coarseness: rough, bastard, second cut and smooth. Coarser grades remove metal fast, smooth grades give fine finish.

Tap drill size

For metric threads, subtract the pitch from the major diameter; an M10 by 1.5 thread needs an 8.5 mm tapping drill.

Scraping and blueing

Marking compound transferred from a surface plate shows high spots, which are scraped away until contact spots are evenly distributed.

Reaming purpose

Enlarges a drilled hole slightly to produce accurate diameter, straightness and smooth finish; the drill is left undersize for the reamer.

Down (climb) milling

Cutter rotation matches feed direction, so the chip starts thick and thins. Gives better finish but needs a backlash eliminator.

Wheel grade in grinding

Grade describes bond strength, not abrasive hardness. A soft wheel releases dull grains easily and is used on hard workpiece material.

Radial drilling machine

Drill head slides along a radial arm that can swing and raise, so heavy workpieces stay clamped while several hole positions are reached.

Predictive maintenance

Maintenance triggered by measured condition such as vibration, temperature or oil analysis, so intervention happens before functional failure.

Misalignment vibration sign

Dominant vibration at twice running speed, often with strong axial component at one times speed, indicates coupling or shaft misalignment.

Cavitation symptom

Noise like gravel in the pump with reduced flow and head, caused by vapour bubbles collapsing when available suction head is too low.

Lockout tagout

Isolate the energy source, apply a personal lock and tag, release stored energy, then verify zero energy before maintenance work starts.

Hydrodynamic lubrication

A full fluid film separates the surfaces so no asperity contact occurs; load capacity depends mainly on viscosity, speed and clearance.

Bearing number 6205

Six indicates a deep-groove ball bearing, two a light series, and the last two digits multiply by five to give a twenty-five millimetre bore.

Tapered roller bearing

Rollers and races are conical, so the bearing carries combined radial and axial load; usually mounted in opposed pairs with adjustable clearance.

Gear ratio

Driven gear teeth divided by driving gear teeth, which also equals driver speed divided by driven speed in a simple gear pair.

Fire class B

Fire involving flammable liquids such as oil, petrol or paint. Extinguish with foam, carbon dioxide or dry powder, never with a water jet.

Five S housekeeping

Sort, set in order, shine, standardise and sustain. A workplace discipline that removes waste and makes abnormal conditions visible.

Megger test

Insulation resistance test applying 500 or 1000 volts DC between winding and earth, read in megohms to detect insulation deterioration.

Star-delta starting

Motor starts in star so each winding receives reduced voltage; starting torque and current fall to one third of direct-on-line values.

Break-even point

Fixed cost divided by selling price minus variable cost per unit. It is the output at which total revenue equals total cost.

Payback period

Initial investment divided by annual net saving from the equipment. A short payback means the capital is recovered quickly.