The thread mill used for threading was not as per the requirement.|The shank length of the thread mill used for threading  was 21mm and the part length was 20.32 |there was no relief provided in the tool and the threading operation was done for 18mm on the part and the remaining thread was done manually by tapping method |while completing the thread manually the tap selected was wrong i:e Instead of  .1640-32UNJC-3B  , .1640-32UNJC-3A was used for manual tapping 