I have a rod with a 10-24UNC thread and for which I need a metal disc tapped to screw on to that thread. I have already done this using a tap for a 10-24 NC thread which I believe is the same as the UNC one despite the U missing but the disc will not screw on apart from about a half turn before it seizes. The materials of these parts are brass and form part of a drone key switch in my Uilleann Pipes so I need to get the parts to fit properly as the instrument is valuable. Can anyone advise as to how I can get the disc to screw on properly and what the problem might be? Thanks in hopeful anticipation of some assistance.