# Accuracy and quality control

When working with timber it is important to remember that it is a natural material that changes shape as different amounts of water are absorbed by it. It can warp and twist at different times of the year depending on how dry or wet it is. For example, if a timber door for a shed is made to be a tight fit in the dry summer months, it is likely that the door will be too tight to open when it is wet or damp in the winter.

Digital vernier caliper

When making something from timber, are often used to measure to 1/100th of 1 mm. When working with timber, a tape measure can be used to measure a correct length to 1 mm. This is acceptable due to the nature of the material and the fact that it will change shape at some point while being used.

### Example

If a length of timber is measured with a tape measure, it could be written that the timber is 1,000mm +/- 1mm. This indicates that the timber could be 999 mm or 1,001 mm.

1,000 mm + 1 mm = 1,001 mm

1,000 mm - 1 mm = 999 mm

The range of possible sizes is anywhere between 999 mm and 1,001 mm. This means that some lengths might be 2 mm bigger than others.

If the tolerance was +/- 2 mm:

1000 mm + 2 mm = 1,002 mm

1000 mm - 2 mm = 998 mm

The range of possible sizes is anywhere between 998 mm and 1,002 mm and some lengths might be 4 mm bigger than others.

Question

A 1 m length of PAR ash needs to be cut with a tolerance of +/- 1%.

What would the range of tolerance be?

1 m = 1,000 mm

1% = 1,000 ÷ 100 = 10mm

Maximum length = 1,000 mm + 10 mm = 1,010 mm

Minimum length = 1,000 mm - 10 mm = 990 mm

Range of tolerance = 1,010 mm - 990 mm = 20 mm

Note: If worked in metres (m), the range of tolerance would be 0.02m.