Shell reamers (Figure H-118) are finishing reamers. They are more economically produced,especially in larger sizes,than solid reamers because a much smaller amount of tool material is used in making them. Two slots in the shank end of the reamer fit over matching driving lugs on the shell reamer or box (Figure H-119). The hole in the shell reamer has a slight taper (1/8 in./ft) in it to assure exact alignment with the shell reamer arbor. Shell reamers are made with straight or helical flutes and are commonly produced in sizes from 3/4 to 2 1/2 in diameter. Shell reamer arbors come with matching straight or tapered shanks and are made in designated sizes from number 4 to 9.