A factory may say that a seam allowance of 20 mm is needed, with an acceptable tolerance of +/- 5%.
5% = 20 × 0.05
The seam can have a tolerance of 1 mm either side of the 20 mm.
20 + 1 = 21 mm maximum
20 - 1 = 19 mm minimum
If this were the case, then the seam allowance could range from 19 mm to 21 mm.
A factory has specified that a seam allowance of 25 mm is needed, with an acceptable tolerance of +/- 8%.
What is the possible range of tolerance?
8% = 25 × 0.08
The seam can have a tolerance of 2 mm either side of the 25 mm.
25 + 2 = 27 mm maximum
25 - 2 = 23 mm minimum
The seam allowance can range from 23 mm to 27 mm.
During the manufacturing process, quality control (QC) checks are carried out - for example, to check whether:
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.
In a manufacturing flow chart, quality control checks are placed as decisions in diamonds to show where a step would need repeating if there was a mistake: