Q: How does needle detection work on a zipper production line?
A needle detector passes the finished tape or the sewn assembly through a tunnel with a magnetic field and senses disturbances caused by ferrous metal. A broken needle tip, a blade fragment or a stray pin changes the field and triggers a reject, usually with a belt stop and an alarm. The machine is set to a sensitivity expressed as the diameter of a ferrous sphere it will detect.
Detection is only reliable if the product passes through the aperture in a controlled way. Keep the belt clean, avoid folding thick assemblies, set the speed within the manufacturer's limit, and check the machine with test cards at the start of each shift and after any adjustment. Non-ferrous metals such as aluminium or brass are much harder to detect; if they are a risk, a different technology is needed.
Acceptance is not the reading of the machine but the outcome of the check: the reject mechanism must fire on the test piece, and the rejected item must be quarantined and examined. Record the sensitivity, the test time and the operator, and treat a failed check as a stop-work condition until the machine is verified.