Tolerances
Timbers and metals
When engineering something from timber or metal, digital vernier caliperA digital device for measuring with accuracy. 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 toleranceThe amount by which a measurement can vary without affecting the ability of the product to be manufactured accurately. is acceptable due to it being a natural material that may warp or twist depending on how dry or wet it is.
Example
If a length of timber is measured with a tape measure, it could be written that the timber is 1,000 mm +/- 1 mm. 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 planed all round (PAR)The process of using a plane to smooth both the edges and sides of a plank. 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 = 10 mm
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
(If worked in metres, the range of tolerance would be 0.02 m)
Textiles
Tolerance in textiles is the amount of acceptable variation from the specified measurement from which pattern pieces can be cut out, componentA part that when put together makes a product. added or seams sewn.
seam allowanceThe amount of material between the edge of a fabric and the seam line. is the amount of material between the edge of the fabric and the seam, and it is tested to check it meets the tolerance stated in the manufacturing specificationDocument containing clear and detailed instructions for the manufacture of a product..
Example
A factory may say that a seam allowance of 20 mm is needed, with an acceptable tolerance of +/- 5%.
5% = 20 × 0.05
= 1
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.
Question
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
= 2
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.
Systems
resistorAn electrical component that restricts the flow of electrical charge. Fixed-value resistors do not change their resistance, but with variable resistors it is possible to vary the resistance. limit the flow of current around a circuit and can prevent damage to components.
Resistors used in electrical circuits have a tolerance of how much power they allow into a circuit - the smaller that tolerance is, the less likely the circuit will be overloaded and damaged, but the more expensive the resistor will be.
When reading the value of a resistor, it must be held with the gold or silver band to the right - this is the tolerance band. The E12 series resistors have a silver fourth band and the E24 series resistors have a gold forth band.
E12 resistors have a tolerance of 10 per cent, meaning that the value of the resistor could be smaller or larger by 10 per cent, eg a 300 Ω E12 resistor could range from 270 Ω to 330 Ω.
E24 resistors have a tolerance of 5 per cent, meaning that the value of the resistor could be smaller or larger by 5 per cent, eg a 300 Ω E24 resistor could range from 285 Ω to 315 Ω.
