Basic Machining: Lathe and Drilling Machine For students.

A lathe is a machine that spins a workpiece while a fixed cutting tool shaves metal off it, producing round parts like shafts, bolts and pulleys. The drilling machine does the complementary job: it spins a drill bit to cut accurate round holes in a stationary workpiece.

Together they are the foundation of every machine shop: nearly every mechanical device contains turned and drilled parts, and their speeds and feeds are the first step to real machining.

Key Concepts

• Lathe parts: the headstock holds the spindle and chuck; the tailstock supports long work; the carriage slides the tool; the lead screw drives thread cutting.
• Lathe operations: turning (reducing diameter), facing (flattening the end), taper turning, thread cutting, knurling (diamond grip pattern), drilling and boring on the lathe axis.
• Drilling machine parts: the spindle, drill chuck, work table and feed handle; larger shops use radial drilling machines for heavy work.
• Drilling operations: drilling, reaming (exact size), boring (enlarging), counterboring, countersinking and tapping (internal threads).
• Cutting speed (v): surface speed of the work in metres per minute. Feed (f): tool advance per revolution in mm/rev. Depth of cut (d): thickness of the layer removed in mm.
• Tool material: high-speed steel (HSS) for general work, carbide for production.

Formulas

• Spindle speed: N = (1000 × v) / (π × D) revolutions per minute, where v is cutting speed in m/min and D is the workpiece (or drill) diameter in mm.
• Material removal rate (turning): MRR = v × f × d, with v in mm/min, f in mm/rev and d in mm, giving mm³/min. Assumes a single-point tool and constant parameters.
• Drilling time: time = (hole depth + drill point allowance) / (f × N) minutes, where f × N is the feed rate in mm/min.

Worked Example

A 40 mm diameter mild steel bar must be turned at a cutting speed of 30 m/min, with feed 0.2 mm/rev and depth of cut 2 mm. Find the spindle speed and material removal rate.

1. Spindle speed: N = (1000 × 30) / (π × 40) = 30000 / 125.66 = 238.7, so set 239 rpm (lathes offer stepped speeds — pick the nearest).
2. Convert cutting speed to mm/min: 30 m/min = 30000 mm/min.
3. Material removal rate: MRR = 30000 × 0.2 × 2 = 12000 mm³/min — about 12 cubic centimetres of steel removed every minute.
4. Drilling a 12 mm hole 30 mm deep at 600 rpm, 0.15 mm/rev: feed rate = 90 mm/min, time = 30/90 = 20 seconds. Blue chips or squealing mean the speed is too high — reduce rpm and add cutting fluid.

Common Misconceptions

• "Faster spindle speed always means faster work." Beyond the right speed the tool overheats and dulls. Correct speed with proper feed removes more metal per hour than reckless rpm.
• "Drilling and reaming are the same." A drill makes the rough hole; a reamer follows with a tiny cut (0.2–0.3 mm) to bring it to exact size and fine finish. Skipping the reamer leaves an oversized, rough hole.
• "The chuck grips anything safely." Long bars must be supported by the tailstock centre, or centrifugal force whips them into a dangerous bend at speed.

Real-World Applications

• Shafts: rotating shafts in motors and gearboxes are turned on lathes.
• Threaded fasteners: bolts, studs and lead screws are turned and thread-cut.
• Engine components: pistons, valves and crankshaft journals need lathe accuracy.

The lathe spins the work against the tool; the drilling machine spins the tool into the work — both live or die by speed, feed and depth of cut. Setting rpm with N = 1000v/(πD) makes parts right the first time.