Ob chav xov sab nraud txo kev sib koom tes tseem hu ua hexagonal txo kev sib koom tes thiab hexagonal threaded sib koom.Nws yog ib qho evolutionary version ntawm qhov sib npaug ntawm txoj kab uas hla threaded sib koom.Tam sim no, qhov loj me peb tuaj yeem muab tau yog nyob nruab nrab ntawm 1/4 nti thiab 4 nti.