Quality assurance and quality control, tolerance, standards and the use of jigs, fixtures and templates in volume production.

Quality assurance (QA) is the whole system of planned procedures put in place across the organisation to ensure a product will meet its required quality (for example documented processes, staff training, supplier checks, and certification such as ISO 9001). It is preventative and applies before and during production.

Quality control (QC) is the checking and inspection of the product itself at stages of manufacture to confirm it meets the specification (for example measuring dimensions with gauges, testing samples, rejecting faulty parts). It is detection-based.

A clean way to put it: QA builds quality into the system (prevent faults); QC inspects the output (detect faults).

Jigs and templates maintain consistency in batch production by guiding tools or marking out so every part is made identically without re-measuring: a jig holds the work and guides the cutting/drilling tool to the same position each time; a template is a pattern drawn or cut around to repeat a shape exactly. Both reduce variation, speed up production and lower the skill needed, so the batch stays within tolerance.

Markers reward the QA = system/prevention vs QC = inspection/detection contrast and a correct explanation of how jigs/templates give repeatable, consistent parts.

Tolerance is the permitted variation in a dimension: the difference between the maximum and minimum acceptable sizes, allowing for the fact that no process is perfect.

For 20.0 mm with an upper limit of +0.00 and a lower limit of -0.05:

• Maximum acceptable size = 20.0 + 0.00 = 20.00 mm.
• Minimum acceptable size = 20.0 - 0.05 = 19.95 mm.
• The tolerance = 20.00 - 19.95 = 0.05 mm.

Any shaft measured between 19.95 mm and 20.00 mm passes; outside that range it is rejected. Markers want the two limits computed correctly and a clear definition of tolerance as the allowed range of variation.