Limit samples – also referred to as border samples – are used as frame of reference. These parts are the
ones that can be correlated to the actually produced parts in order to make decisions.
Border samples are pre-selected parts of which have given characteristics on (or very near) to the
specification limit. These samples are most commonly used if the given characteristic is not, or hardly measurable.
Examples of use:
Limit sample for visual inspection: An aesthetic product is being checked by human eye. The customer
requirement – based on the specification - allows one visual failure with the maximum size of 0,2 mm. The human eye is incapable to measure, so
the supplier uses a limit sample that can be correlated to the checked part. If the visual defect of the actual part is bigger, than the one
on the limit sample, the part must be rejected by the technician / operator. Note: this kind of inspection is unreliable, so try to use automatic system
whenever it is possible.
Limit sample for visual inspection (Source: qMindset.com)
Limit sample for test system verification:
The length of a component is checked by a camera system, combined with a software. The engineer uses different limit samples to
teach and frequently verify the measuring capability
of the system. This method is also called "Check the checker". The specification for the length
is 50 +/- 0,5 mm. The engineers have pre-selected parts in the specification range, and also parts out of it.
Limit sample for test system verification (Source: qMindset.com)
Many limit samples are used continuously, with various frequencies. Those of used for visual inspection can wear
out earlier, that the ones used for system verification. Anyway, you need the take care of your limit samples frequently:
- Revalidate your limit sample with the needed frequency (e.g. daily in case of visual inspection, weekly in case of test system
- If it’s possible, prepare your limit samples in the way to keep them in good condition with conservation (supporting materials, glues,
- Limit samples must be marked perceptibly to avoid any part mixing with the serial production.
- If your limit sample is not a usable any more, scrap it immediately.
Important: the limit sample is not equal to the golden / master sample!