If the frame has dirt in it, the tracing can be off and the lens can be cut off-size or with gaps. Clean frames before tracing, especially old frames when only a lens is being replaced. Thinner metal frames might bend from the pressure of the stylus, resulting in gaps and inaccuracies in lens size, and lab managers advise to be careful with these frames so watch the trace to be sure they do not bend or flex when being traced. Tracers are susceptible to dirt, dust and debris so keep the lid down or the tracer covered when not in use. Calibration should be checked regularly, consult your manual or manufacturer for their recommended schedule. Remember, first time fit is the target i.e., little or no touch-up is required.