a screw with replaceable threads is particularly useful to releasably join the spiders of rotational molding machines . the screw comprises a master bolt that passes through a first spider , and a tip that is removably assembled to the master bolt . the tip is threaded to selectively engage and disengage a receiver on the second spider . when the tip is assembled to the master bolt , the screw is captured in the first spider . when the tip has worn through use , it can be replaced without disturbing the master bolt . similarly , when the tip seizes to the receiver , the tip and receiver are replaceable as a unit without disturbing the master bolt .