BTEB Machinist cheat sheet

Cutting speed formula: V = πDN/1000, where V is in m/min, D is workpiece or cutter diameter in mm, and N is spindle speed in rpm.

Feed rate for milling: f = fz × z × N, where fz is feed per tooth in mm, z is the number of teeth, and N is spindle speed in rpm.

Machining time for turning: T = L/(f × N), where L is the total tool travel in mm including approach and overtravel, f is feed in mm/rev, and N is rpm.

Chip thickness ratio r = t1/t2 (uncut chip thickness divided by actual chip thickness); r is always less than 1 because chip shortening accompanies chip thickening.

Safety colour code: red marks fire-fighting equipment and prohibition, yellow marks caution and physical hazards, blue marks mandatory actions, green marks safe conditions and first aid.

Machine guards must remain interlocked; a guard removed for maintenance is replaced and tested before the machine is re-energised and returned to production.

Personal protective equipment for lathe and mill work: safety glasses with side shields, close-fitting clothing, no gloves near rotating spindles, and closed-toe safety footwear.

Housekeeping rule: chips are cleared with a brush or hook, never by hand or compressed air near moving parts, and oily rags go into a lidded metal container.

Fire classes: A for solid combustibles, B for flammable liquids, C for electrical equipment, D for combustible metals; water is never used on class B, C, or D fires.

Hacksaw blade selection: 14 teeth per inch for thick sections and soft metals, 18 tpi for general work, 24 to 32 tpi for thin tube, sheet, and hard alloys.

At least three teeth must be in contact with the work at all times; otherwise teeth straddle the edge and strip.

Filing rule: pressure is applied on the forward stroke only; the return stroke is light to preserve the teeth and keep the file flat.

Cross-cut files cut on both strokes and are used for rapid stock removal; single-cut files cut on one stroke and give a smoother finish.

A file card (wire brush) cleans pinning from file teeth; files are never tapped hard against the bench because they crack and shatter.

Tap drill size for metric threads: drill diameter ≈ major diameter − pitch (e.g. M10 × 1.5 uses a 8.5 mm drill).

Tapping sequence: taper tap first, then plug tap, then bottoming tap for blind holes; lubricant is applied and the tap is reversed a quarter turn to break chips.

Scraping removes high spots revealed by marking compound on a surface plate; a typical bearing check aims for 20 to 25 contact spots per 25 mm square.

Chisel and punch heads are dressed to remove mushrooming; a mushroomed head is a flying-fragment hazard and the tool is reground or discarded.

Least count of a vernier calliper = main scale division − vernier scale division; for a 0.02 mm calliper, 49 main divisions equal 50 vernier divisions.

Vernier reading = main scale value at the zero of the vernier + (vernier coincidence number × least count).

Micrometer least count = pitch of screw / number of divisions on thimble; a 0.5 mm pitch screw with a 50-division thimble reads 0.01 mm.

Micrometer reading = barrel reading + (thimble division × least count); a zero error is added or subtracted according to its sign.

A dial test indicator reads relative displacement only; a dial gauge with 0.01 mm graduation and 10 mm range is used for runout and alignment checks.

Limits and fits: clearance fit has the hole minimum above the shaft maximum; interference fit has the shaft minimum above the hole maximum; transition fit overlaps both.

Hole-basis system is preferred in production because holes are made with fixed-size reamers and drills while shafts are turned to size.

Surface roughness Ra is the arithmetic mean deviation of the profile; a typical turned finish is Ra 1.6 to 3.2 µm, ground finish 0.4 to 0.8 µm.

Go and No-Go gauges: the Go gauge must enter or assemble with the full length of the feature; the No-Go gauge must not enter more than two turns on a thread.

Lathe operations: facing, plain turning, taper turning, knurling, threading, drilling, boring, parting off, and form turning.

Taper turning by swivelling the compound rest suits short steep tapers; the setting angle α = tan⁻¹((D − d)/(2L)).

Tailstock set-over for a taper: S = L × (D − d)/(2l), where L is the total workpiece length and l is the taper length.

Taper turning with a taper attachment suits long shallow tapers and avoids tailstock offset that causes centre misalignment.

Thread cutting requires the half-nut to engage the leadscrew at the same spindle position each pass, using the thread-chasing dial or by reversing the spindle.

Thread depth for a metric 60° thread: depth ≈ 0.6134 × pitch; the minor diameter ≈ major diameter − 1.2268 × pitch.

Four-jaw independent chuck is used for irregular and heavy work and for accurate setting of eccentric parts; a three-jaw self-centring chuck repeats within about 0.05 mm.

Parting-off rules: low spindle speed, plenty of coolant, tool exactly on centre, and a rigid setup to avoid chatter and tool breakage.

Milling: climb milling gives a better finish but requires a backlash-free table feed; conventional milling is safer on older machines with backlash.

Daily checks before start-up: lubricant levels, coolant level and concentration, air pressure, guard condition, and emergency-stop function.

Planned preventive maintenance intervals follow the manufacturer's schedule; bearings and slides are lubricated per the recommended grade and interval, not by guesswork.

Coolant concentration is checked with a refractometer; too lean causes corrosion and tool wear, too rich causes skin irritation and foaming.

Spindle runout is checked with a dial test indicator on a test mandrel; excessive runout indicates worn bearings or a damaged taper.

Lathe bed wear is checked by measuring the saddle drop with a dial gauge along the bed; the wear is greatest near the headstock where cutting is heaviest.

Leadscrew and half-nut wear shows as inaccurate thread pitch; backlash is measured with a dial indicator and adjusted per the machine manual.

Belt tension is set so the belt deflects a small amount under thumb pressure at mid-span; overtight belts load bearings and undertight belts slip.

Lockout/tagout: isolate the machine, lock the isolator, tag it, and verify zero energy before any maintenance inside a guarded area.

Vibration and abnormal noise are early failure indicators for bearings; record them and investigate before a breakdown stops production.

Plain carbon steel classification: low carbon up to about 0.3% C, medium carbon 0.3 to 0.6% C, high carbon 0.6 to 1.4% C.

Cast iron contains 2 to 4% carbon; grey cast iron has flake graphite and good damping, white cast iron has cementite and is hard and brittle.

Cutting speed order, slowest to fastest: high-carbon and alloy steels, cast iron, free-machining mild steel, aluminium, brass.

Carbide tools run at roughly three to five times the cutting speed of high-speed steel on the same material and withstand higher temperatures.

Hardening: heat to the austenitising temperature for the steel, quench in the specified medium (water, oil, or air), then temper to relieve stress and set toughness.

Case hardening adds a hard wear-resistant skin to a tough low-carbon core by carburising, nitriding, or induction hardening.

Allowance for a machined surface: rough machining leaves about 0.5 to 1 mm per side, finish machining 0.1 to 0.3 mm per side.

Workshop drawing convention: first-angle projection places the view from above below the front view; third-angle places it above.

Standard fits are designated by hole and shaft tolerance letters and grades, for example H7/g6 for a close running fit and H7/p6 for an interference fit.