Accuracy and quality control

When creating electrical or mechanical products, the accuracy of production is vital. If one part of the process is poorly fitted or too big, the products will not function and could become dangerous to the user. How accurate a product needs to be is described as the tolerance.

Tolerance describes how inaccurate a product can be and is measured in +/- mm.

Poorly fitting mechanical parts will perform badly and are likely to have a very small tolerance measurement. For example, chains or gears that are slightly bigger or smaller than necessary will be prone to jamming and breaking.

Printed circuit boards (PCBs) need to be so accurate that any tolerance measurements are likely to only be +/- 1 mm, to stop any materials touching and causing circuits to stop working.

Resistors used in electrical circuits have a tolerance relating to the amount of power they allow into a circuit. The smaller that tolesrance is, the less likely the circuit is to be overloaded and damaged, but the more expensive the resistor will be. The amount of tolerance a resistor has is shown with the colour of the fourth stripe down and is usually silver or gold


A resistor has a value of 1,500 Ω and has a tolerance of +/- 10 per cent.

1,500 × 0.1 = 150

Range of tolerance = 150 Ω either side of the 1,500 Ω mark.

1,500 Ω + 150 Ω = 1,350 Ω minimum

1,500 Ω - 150 Ω = 1,650 Ω maximum


A resistor with a value of 22,000 Ω has a tolerance of +/- 5 per cent. What are the minimum and maximum values for this resistor?

22,000 × 0.05 = 1,100 either side of the 22,000 Ω mark.

22,000 - 1,100 = 20,900 Ω minimum

22,000 + 1,100 = 23,100 Ω maximum

Quality control

During the manufacturing process, quality control (QC) checks are carried out. These can check whether:

  • PCBs are cut accurately
  • components are attached securely
  • casings are attached securely
  • edges on castings are trimmed and neat
Quality control (QC) is the system of checks throughout the manufacturing process to make sure each step is completed to a high standard.

Although quality control checks can increase waste with faulty products being thrown away, if a factory develops a reputation for being reliably high in quality, money is saved in the long term through products being reordered.