milling tool is generally depicted at 1 the tool 1 comprises an elongate body 2 having a first upper 3 and second lower 4 sleeve and is run into a wellbore not shown which is lined by casing and liner mounted on a work string 7 the upper 3 sleeve of the tool 1 acts as a centraliser to maintain the tool 1 or work string 2 in a central position within the wellbore whilst the lower sleeve 4 is a milling sleeve typically the milling sleeve is comprised of carbide inserts which are impregnated into a steel sleeve which are hard enough to mill or grind the liner top in the wellbore on rotation of the work string the milling sleeve 4 smooths the entrance to the liner top polished bore receptacle the tool 1 also comprises a floating component 5 between the upper 3 and lower 4 sleeve the floating component is a lantern which supports a plurality of scraper blades 6 the scraper blades 6 scrape the casing which is near to and directly above the polished bore receptacle it will be appreciated from fig1 that the scraper blades are mounted in close proximity to where milling of the liner top takes place fig2 a and 2b show a section of the locking system which holds the milling sleeve 4 to the tool 1 in more detail the locking system consists of three components namely a hex head grub screw 8 a lock stud 9 and ptfe plug 10 the lock stud is cylindrical and flat milled on one side to mount the milling sleeve 4 on the elongate body 2 the lock stud 9 and grub screw 8 are assembled together flush and inserted into corresponding holes milled in the elongate body 2.