Accuracy and quality control

Unlike metal, materials such as timber, paper and board change shape easily depending on temperature or the amount of moisture they are holding. Metal can change shape and expand in the heat, but it is not prone to the same amount of movement as timber is. It is often the case that engineers can work to a fine tolerance when using metal.

Digital micrometers can be used to measure the width of a material and digital vernier calipers can be used to measure the outside width, inside dimensions and depth of holes. Both tools measure to 1/100 of 1 millimetre (mm) and can be read quickly because of the digital screen.

Someone holding a digital micrometre with a lathe in the background.

Digital micrometer

Quality control takes place during the manufacture of any product, but, since metal parts are engineered to a fine tolerance, there are specific quality control tools to ensure that metal parts have been made correctly - one such tool is called a ‘go-no-go gauge’. The ‘go-no-go gauge’ has a ‘go’ side and a ‘no-go’ side - when testing the product one side must pass and one side must fail.

A go-no-go gauge with a green 'go' side and a red 'nogo' side, used for quality control.

Example

It is common to hear engineers say they can work to a tolerance of ‘one thou’, meaning 1/1,000th of an inch.

1 inch = 25.4 mm

25.4 ÷ 1,000 = 0.0254 mm, so:

‘one thou’ = 0.03 mm (to 2 decimal places)

If an engineer was asked to mill a 50-mm slot in a block of aluminium to the tolerance above, it would be possible to check whether the slot was correct by using a ‘go-no-go gauge’:

50 mm - 0.03 mm = 49.97 mm

This side of the gauge must be able to slide into the milled slot.

50 mm + 0.03 mm = 50.03 mm

This side of the gauge must not be able to slide into the milled slot.

Question

If 1 m lengths of steel bar were cut +/- 5%, what would the range of tolerance be?

1 m = 1,000 mm

5% = 1,000 × 0.5 = 50 mm

Range of tolerance = 50 mm either side of the 1 m mark:

50 mm + 1,000 mm = 1,050 mm maximum

1,000 mm - 50 mm = 950 mm minimum

= 100 mm range