Shaping by removing material
Most wood and metal parts begin as a block, plank or rod that is bigger than the final part. Workers remove material until the right shape is left: by sawing, planing, filing, drilling and turning. Wood has grain, so you cut along or across it with care. Metal is harder, so it needs strong tools and often a coolant.
The lathe
A lathe is a machine for making round (cylindrical) parts. Its main parts:
- Chuck: grips one end of the work and spins it.
- Cutting tool: a hard, sharp piece held on a movable tool post. It does not spin.
- Bed: the strong base that keeps everything straight.
When the work spins and the tool is pushed against it, a thin layer comes off, called a chip or shaving. Moving the tool along gives a smooth cylinder. Moving it in and out changes the diameter (width). By steering the tool you can make a taper, a groove or a curved shape, such as a table leg. Wood lathes run fast and use sharp gouges. Metal lathes use hard steel or carbide tools, slower cuts and coolant.
Safety: wear goggles, tie back long hair, remove loose sleeves and jewellery, keep hands away from the spinning work, and never leave the chuck key in the chuck. Work only with a teacher.
Threads
A thread is a spiral ridge cut or rolled on a rod (a bolt, an external thread) or inside a hole (a nut, an internal thread). When you turn a nut, the spiral makes it move along the bolt and then lock tight. Two numbers describe a thread:
- Diameter: the width of the bolt.
- Pitch: the distance from one ridge to the next. A small pitch means fine threads; a large pitch means coarse threads.
On a lathe the tool moves along the spinning rod at a steady speed, cutting a spiral. Threads can also be cut by hand with a die (outside) and a tap (inside). Threads are everywhere: bottle caps, screws, jars and pipes.
Metal products
Metal products are made by a chain of steps:
- Marking out with a scriber and a ruler.
- Cutting with a hacksaw or shears.
- Bending and forming sheet metal, for example to make a box.
- Filing and drilling to smooth edges and make holes.
- Turning and threading on a lathe for round parts.
- Joining by bolts, rivets, soldering or welding.
Metals used in school workshops are mild steel, aluminium and brass. Examples of products: hinges, hooks, brackets, tool handles and bolts.
Finishing
Finishing is the last step. It does two jobs: it makes the surface smooth and good-looking, and it protects the part.
- Wood: sand with finer and finer paper along the grain, then apply varnish, oil or paint. This keeps out damp and insects.
- Metal: remove sharp edges with a file, polish, then paint, oil or coat it. This stops rust.
Always finish with the same order: rough tool, then fine tool, then coating.
Key formulas and definitions
- Lathe: work spins, tool stays (and moves along or in/out)
- Diameter after cutting = start diameter − 2 × depth of cut
- Thread pitch = distance between two ridges; number of ridges in a length = length ÷ pitch
- Order of work: marking, cutting/turning, threading, finishing
Worked examples
1. A rod is 26 mm wide. The tool cuts 3 mm deep (on each side). What is the new diameter?
Each side loses 3 mm, so the diameter drops by 2 × 3 = 6 mm. New diameter = 26 − 6 = 20 mm.
2. A bolt has a thread pitch of 1.5 mm. How many ridges are on a 30 mm threaded length?
30 ÷ 1.5 = 20 ridges.
3. Put in order: paint a steel bracket, cut it from sheet, file edges, drill holes.
Cut it from sheet, drill holes, file edges, then paint. Painting is always last.
Common mistakes
- Thinking the tool spins on a lathe. The work spins, the tool stays.
- Forgetting that cutting depth takes layer off both sides, so diameter drops by twice the depth.
- Mixing up pitch and diameter. Pitch is the gap between ridges; diameter is the width.
- Painting before filing and drilling. Finishing is the last step.