From hand tools to power
The first tools were stone, then copper, bronze and iron. A craftsman cut, hammered and filed each part. He used his own muscle, so work was slow, and no two parts were exactly alike.
Then people used the power of falling water (water wheel) and later steam. A shaft and belts carried the power to machines inside the workshop. Power from an engine does not get tired.
Machine tools and exact parts
A machine tool is a power-driven machine that holds the tool and the work firmly and moves them in a controlled way. The lathe turns round parts. The drilling machine makes holes. The milling machine cuts flat faces and slots. The shaper and planer cut straight surfaces.
Because the tool is guided by a rigid machine and not by a shaky hand, the size is more exact. About 250 years ago, boring machines made steam-engine cylinders round enough that pistons fitted well.
Interchangeable parts and mass production
If every part is made to the same size within a small tolerance (allowed error), then any part fits any product. Such parts are called interchangeable. A broken part can be replaced from the shop, not made again by hand.
This allowed mass production: many identical products, made in a line. A moving assembly line (made famous in the early 1900s for cars) cut the time to build one product a lot. Measuring tools and gauges (see our lesson on measurement) became very important.
Numerical control, CNC and today
In numerical control (NC), numbers punched on tape told the machine how to move. Later, a small computer replaced the tape: this is CNC (computer numerical control). The programme lists the (X, Y, Z) points the tool must visit and the speed.
CAD (computer drawing) and CAM (computer making) link the drawing to the machine. Today we also have robots, laser and water-jet cutting, and 3D printing, which builds a part by adding material instead of removing it. CNC gives the same part again and again, with a very small error.
Try it: count the cubes
In the 3D, first guess how many parts the next era will make, then tap the era and check. At home: draw a 5 cm square freehand 5 times, then draw it again 5 times with a ruler and a set square. Which set looks more alike? That is the same jump as hand work to machine work.
Key formulas and definitions
- Machine tool: a power-driven machine that holds the tool and the work and moves them in a controlled way.
- Tolerance: the allowed difference from the exact size (for example 20 mm ± 0.02 mm).
- Interchangeable part: a part made so closely to size that any one fits any product.
- NC / CNC: numerical control / computer numerical control, a machine steered by numbers.
Worked examples
1. A hand worker makes 2 parts an hour. A CNC machine makes 40 parts an hour. How many times faster is the CNC machine?
40 ÷ 2 = 20. The CNC machine is 20 times faster (using these example numbers).
2. Hand work has an error of ± 1 mm. CNC has ± 0.005 mm. How many times smaller is the CNC error?
1 ÷ 0.005 = 200. The CNC error is 200 times smaller.
3. A shop makes bolts. Why does the shop use a lathe and not a hammer and file for 10,000 bolts?
A lathe is faster and gives bolts of the same size, so every nut fits every bolt. By hand the sizes would differ and it would take far too long.
Common mistakes
- Thinking CNC means "no worker needed". A person still designs the part, writes or checks the programme, sets the tool and checks the size.
- Mixing up NC and CNC. NC used tape or fixed circuits; CNC has a computer inside that can be reprogrammed.
- Saying interchangeable parts means "all parts are exactly equal". They are equal within the tolerance, not perfectly.
- Believing 3D printing removes metal like a lathe. It adds material layer by layer.