Tolerances: Why and How
What a tolerance is and why it matters: the nominal size, the upper and lower limits, the tolerance band, and how to work the limits from a plus-or-minus figure so parts fit together and can be swapped.
Justify the choices, the way you are assessed
Free interactive practice on the decisions the assessment actually asks you to justify.
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A little wrong is still right
No workshop can cut a part to the exact size every time, so designers state how much a size may vary and still be acceptable. That allowance is the tolerance, and it lets parts made separately still fit together and be swapped. This module works through nominal size, upper and lower limits, the tolerance band, and how to work them out.
Tolerance words to know
Learn these before you measure a part.
A tight band against a wide band
How much variation you allow is a trade-off.
Add and subtract from the nominal
In the exam, work a tolerance from the nominal size. Add the stated amount to the nominal to get the upper limit, and take it away to get the lower limit; the space between them is the tolerance band. A tighter band gives a better fit but costs more, because parts must be made more precisely. Any part measuring between the two limits passes, and one outside them is rejected.
Match the term
- tolerance
- nominal size
- upper limit
- lower limit
- how much a size may vary and still pass
- the ideal target size
- the largest allowed size
- the smallest allowed size
Match the measurement
- nominal 50 mm, plus or minus 2 mm
- the upper limit of that part
- the lower limit of that part
- the tolerance band of that part
- aims for 50 mm
- is 52 mm
- is 48 mm
- is 4 mm wide
What is the upper limit?
A part is 50 mm plus or minus 2 mm. What is its upper limit?
- 52 mm
- 48 mm
- 50 mm
- 2 mm
True about tolerances
Select the TWO true statements about tolerances.
- A tolerance is how much a size may vary and still be accepted
- A tighter tolerance usually costs more to make
- The lower limit is bigger than the upper limit
- A part measured outside the limits should still be used
Work out the upper limit
A part has a nominal size of 60 mm with a tolerance of plus or minus 3 mm. Add 3 to 60 to find the upper limit in mm. What is the answer?
Order the working
Put the steps of finding the limits in order, earliest first.
- Start with the nominal size
- Add the tolerance to get the upper limit
- Take the tolerance away to get the lower limit
- Check the measured part falls between the limits
Complete the rules
The amount a size may vary and still pass is the _____. The ideal target size is the _____ size. The largest allowed size is the _____ limit. The smallest allowed size is the _____ limit.
Spot what tolerances allow
Tap the TWO things a tolerance helps a factory do.
- make parts that fit together reliably
- swap one part for another of the same kind
- make every part identical to the last atom
- ignore the size of the parts completely
Accept or reject
Read each case and choose the best answer.
- A part must be 40 mm plus or minus 1 mm. A batch measures exactly 41 mm. Are they acceptable?
- Why does a tighter tolerance usually cost more?
- A part measures 45 mm, but the limits are 46 mm to 50 mm. What happens to it?
Explain tolerances
Explain what a tolerance is and how to work out the upper and lower limits.
- Explain what tolerance and nominal size mean
- Explain how to find the upper and lower limits from a plus-or-minus figure
- Explain why tolerances matter in manufacturing