What is a machine shop?
A machine shop is a workshop where metal is shaped into useful parts. Think of it as a kitchen for metal. The raw material is a bar, a sheet or a lump of melted metal. The result is a bolt, a gear, a pipe frame or a motor housing.
The shop works in two moves. First we get the rough shape quickly (casting, forging, bending, welding). Then we get the exact size slowly (machining and hand finishing). Rough first, exact last, because it is cheaper to start close to the final shape.
Casting and forging
Casting. Metal is melted in a furnace and poured into a mould, a hollow with the shape of the part. In sand casting the mould is made by packing sand around a pattern (a copy of the part). The pattern is made a little bigger than the part, because metal shrinks as it cools. Casting is good for complex shapes such as engine blocks, pump bodies and bells.
Forging. A metal bar is heated until it is soft (red hot) and then hammered or pressed into shape. The metal is not melted and no metal is lost. Forged parts, like spanners and crankshafts, are strong because the grain of the metal flows along the shape. Village blacksmiths do hand forging; factories use power hammers and presses.
Both jobs are hot and dangerous. Wear gloves, apron, face shield and leather shoes.
Sheet-metal work
A sheet is thin metal, usually less than 3 mm thick. Sheet-metal work is a cold process. The main steps are:
- Marking out the shape with a scriber and ruler.
- Cutting with snips or a shearing machine.
- Bending on a bending machine or over an edge, to make boxes, ducts and trays.
- Joining with rivets, folded seams or spot welding.
Tip: when you bend a sheet, the outside stretches and the inside squeezes. So we add a small bend allowance to the flat length. Tins, lockers, fans and car bodies all start as flat sheet.
Welding and cutting
Welding joins metal by melting the joint. Common kinds:
- Arc welding: an electric spark between a rod and the plate makes heat above 3000 °C. The rod melts into the joint.
- Gas welding: a flame of acetylene burning in oxygen melts the edges.
- Spot welding: two sheets are squeezed between copper tips and a strong current melts a small spot.
Cutting metal with heat uses an oxy-fuel flame or a plasma torch, which burn or melt a thin line through the plate.
Safety: the arc is as bright as the sun. Always use a welding helmet with a dark glass, leather gloves and an apron, and never look at the arc with bare eyes. Keep the area dry and free of fuel. Keep a fire extinguisher close.
Machining: removing metal for exact size
Machining uses a hard cutting tool to shave off metal. The main machine tools are:
- Lathe: the job spins and a fixed tool moves along it. It makes round parts, such as shafts and bolts (turning, facing, drilling, threading).
- Milling machine: the cutter spins and the job moves under it. It makes flat faces, slots and gears.
- Drilling machine: a spinning drill bit makes round holes.
- Shaper and grinder: a shaper cuts flat surfaces; a grinder gives a very smooth, very accurate finish.
Three numbers control a cut: the cutting speed (how fast the surface moves past the tool), the feed (how far the tool moves for each turn) and the depth of cut (how thick a layer is removed). Coolant oil keeps tool and job from overheating. Never wear loose clothes or gloves near a spinning job, tie back long hair, and never touch the chips with a bare hand.
Hand finishing and assembly
Machines leave sharp edges and tiny marks. A fitter finishes the part by hand:
- Filing removes sharp edges and a little metal. Scraping and emery cloth make surfaces smooth.
- Sawing and chiselling cut off extra metal.
- Tapping and threading cut internal and external screw threads by hand.
- Measuring with a steel rule, a vernier caliper or a micrometer checks that the size is right.
Assembly puts parts together: bolts and nuts, rivets, keys, press fits or welds. A good assembly turns smoothly, has no wobble and uses the correct torque on every bolt.
Try it: the paper-and-clay shop
You can feel every job at home with safe things. Cast: press a coin into clay, remove it, and fill the hollow with hot candle wax (ask an adult). Forge: squeeze a ball of clay flat. Is the clay lost? Bend: fold a piece of card along a line. Weld: stick two paper strips with glue along the edge. Machine: spin a clay rod and scrape it with a stick. In the 3D above, first guess what the slider will do, then move it and check.
Key formulas and definitions
- Cutting speed v = π × d × N / 1000 (m/min), with d in mm and N in turns per minute (rpm)
- Machining time t = L / (f × N), with L = length of cut (mm), f = feed (mm per turn)
- Diameter after one pass = old diameter − 2 × depth of cut
- Pattern size ≈ casting size × (1 + shrinkage); shrinkage of about 1 to 2 % for many metals
- Volume stays the same in forging: length × area before = length × area after
Worked examples
1. A round bar of diameter 40 mm is turned on a lathe with a depth of cut of 2 mm. What is its diameter after one pass?
The tool removes 2 mm from each side. New diameter = 40 − 2 × 2 = 36 mm.
2. Find the cutting speed when a 40 mm bar spins at 500 rpm. Use π = 3.14.
v = π d N / 1000 = 3.14 × 40 × 500 / 1000 = 62.8 m/min.
3. A casting must be 200 mm long. Metal shrinks by 2 % while cooling. How long should the pattern be?
Pattern ≈ 200 × 1.02 = 204 mm. The extra 4 mm will shrink away.
4. A shaft is 120 mm long. A lathe tool feeds 0.2 mm per turn while the job spins at 600 rpm. How long is one pass?
Time = L / (f × N) = 120 / (0.2 × 600) = 120 / 120 = 1 minute.
5. A hot bar 100 mm long and 20 mm in diameter is forged until it is 25 mm in diameter. Find the new length (volume is constant).
Length is proportional to 1/d². New length = 100 × (20/25)² = 100 × 0.64 = 64 mm.
6. A bar of 50 mm diameter must be turned down to 42 mm in passes of 2 mm depth of cut. How many passes are needed?
Each pass reduces the diameter by 4 mm. We need to remove 50 − 42 = 8 mm, so 8 / 4 = 2 passes.
Common mistakes
- Thinking forging melts the metal. It only makes it soft and red hot; casting is the process that melts it.
- Making the pattern the same size as the casting. Metal shrinks when it cools, so the pattern must be slightly larger.
- Using diameter reduction = depth of cut. A lathe cuts on both sides of the axis, so the diameter drops by twice the depth of cut.
- Looking at the welding arc without a helmet, or wearing gloves and loose sleeves near a spinning machine. These are the most common shop injuries.