an earth boring cutter is disclosed that will operate as a unitized assembly until cutting structure replacement is necessary . at that time the worn cutting structure is simply removed , and replaced with a new cutting element . this is accomplished with simple hand tools . the replaceable cutting structure may be constructed from steel or other abrasion resistant material , such as tungsten carbide , or any combination of such materials . the cutting structure geometry may vary to accommodate different degrees of boring difficulty . the replaceable cutting structure utilizes a tapered connection between the mating surfaces of the elements . the surfaces are properly stressed by means of axial preload threaded bolts . the torsional loads encountered during operation are carried by the induced presssures in the mating tapered surfaces and in one embodiment of the invention by dowel pins between the elements . the worn cutting element is removed by disconnecting the preload bolts and freeing the worn cutting element . referring now to the drawings and fig1 in particular , a cutter constructed in accordance with the present invention and generally designated by the reference numeral 10 is illustrated . the cutter 10 includes a rolling cutter shell 11 positioned around a bearing shell 12 with bearing shell 12 being securely locked in a saddle mount 13 . the saddle mount 13 may be connected to the rotary head of an earth boring machine or to the body of a bit for boring a large diameter hole . the bearing shell 12 is locked in position in the saddle mount 13 by a pin 14 and a retainer element 15 . the retainer element is driven through a hole in the saddle mount . the bearing shell 12 remains firmly locked in place throughout the drilling operation due to a tenon and groove arrangement disclosed in u . s . pat . no . 3 , 203 , 492 to c . l . lichte , patented aug . 31 , 1965 . a multiplicity of bearing systems including a series of ball bearings 16 , a series of inner roller bearings 17 and a series of outer roller bearings 18 promote rotation of the cutter shell 11 about the bearing shell 12 . lubricant is retained in the bearing area by two sets of seal elements . the inner set of seal elements includes a pair of annular metal seal rings 19 and 20 that are positioned near the inner end of the cutter 10 . a flexible rubber o - ring 21 is positioned between seal ring 19 and the bearing shell 12 to retain the seal ring 19 in the desired position and resiliently urge seal ring 19 against seal ring 20 . a flexible rubber o - ring 22 is positioned between the cutter shell 11 and the seal ring 20 to retain the seal ring 20 in the desired position and resiliently urge the seal ring 20 against seal ring 19 . the outer set of seal elements includes a pair of annular metal seal rings 23 and 24 that are positioned near the outer end of cutter 10 . a flexible o - ring 25 is positioned between the seal ring 24 and bearing shell 12 to retain the seal ring 24 in the desired position and resiliently urge seal ring 24 against seal ring 23 . a flexible rubber o - ring 26 is positioned between the cutter shell 11 and seal ring 23 to retain seal ring 23 in the desired position and resiliently urge seal ring 23 against seal ring 24 . the cutter 10 includes a replaceable cutter shell 26 mounted over the rolling cutter shell 11 . the replaceable cutter shell 26 will be locked on rolling cutter shell 11 until replacement is required at the end of the cutting structure &# 39 ; s service life . at that time , the replaceable cutter shell 26 may be removed by disconnecting it from the rolling cutter shell 11 . a new replaceable cutter shell may be inserted in place of the worn replaceable cutter shell 26 and the earth boring operation continued . the replaceable cutter shell 26 includes a multiplicity of carbide inserts arranged to form a series of annular rows 28 - 35 . the individual carbide inserts are designated by the reference number 27 . the external surface 36 of the rolling cutter shell 11 is relatively smooth . a shoulder is provided adjacent the external surface 36 . the inside surface 37 of the replaceable cutter shell 26 mates with the surface 36 on the rolling cutter shell 11 . a shoulder 38 of the replaceable cutter shell 26 is in substantially abutting relationship with the shoulder on rolling cutter 11 . the outer surface 36 of the rolling cutter shell 11 slopes in wedge fashion and the inner surface 37 of replaceable cutter shell 26 is of a corresponding shape . the slope of cutter shell 11 and inner surface 37 is at an angle within the range of substantially 2 ° to substantially 40 °. the bolt 39 extends through the shoulder 38 and into the end of rolling cutter shell 11 . tightening of bolt 39 induces a preload into the mating surfaces 36 and 37 . the structural details of cutter 10 having been described , the operation of the cutter 10 will now be considered with reference to fig1 . the cutter shells 11 and 26 are adapted to be mounted in the saddle 13 that is affixed to the rotary head ( not shown ) of an earth boring machine or to the body of a raise bit . the cutting inserts 27 contact the formation and form the desired borehole or tunnel . should the replaceable cutter shell 26 and cutting structure 27 thereon become worn or damaged before the other elements of the cutter 10 fail , it is desirable to replace the replaceable cutter shell 26 . in order to replace the cutter shell 26 , the retainer element 14 is removed from saddle mount 13 . the bearing shell 12 and cutter shells 11 and 26 are removed from the saddle mount 13 . the replaceable cutter shell 26 is withdrawn from the rolling cutter shell 11 . in order to remove the worn cutter shell 26 , the preload bolts are unthreaded to free the worn cutting element . some tapping with a sledge may be required . a new replaceable cutter shell is placed on the rolling cutter shell 11 . the cutter shells 11 and 26 and bearing shell 12 are again inserted in the saddle mount 13 . the pin 14 and the retainer 15 are repositioned in the saddle mount 13 to lock the cutter in place . the cutter 10 is ready for continued operation . referring now to fig2 another embodiment of a cutter constructed in accordance with the present invention is illustrated . the cutter 40 includes a rolling cutter shell 41 adapted to be positioned around a bearing shell . the bearing shell will be securely locked in a saddle mount . the saddle mount may be connected to the rotary head of an earth boring machine or to the body of a bit for boring a large diameter hole . the bearing shell remains firmly locked in place throughout the drilling operation due to a tenon and groove arrangement disclosed in u . s . pat . no . 3 , 203 , 492 to c . l . lichte , patented aug . 31 , 1965 . a multiplicity of bearing systems promote rotation of the cutter shell 41 about the bearing shell . lubricant is retained in the bearing area by seal elements . the cutter 40 includes a replaceable cutter shell 42 mounted over the rolling cutter shell 41 . the replaceable cutter shell 42 will be locked on rolling cutter shell 41 until replacement is required at the end of the cutting structure &# 39 ; s service life . at that time , the replaceable cutter shell 42 may be removed by disconnecting it from the rolling cutter shell 41 . a new replaceable cutter shell may be inserted in place of the worn replaceable cutter shell 42 and the earth boring operation continued . the replaceable cutter shell 42 includes an annular disc element 43 . the external surface of the rolling cutter shell 41 is relatively smooth . a shoulder 44 is provided adjacent the external surface . the inside surface of the replaceable cutter shell 42 mates with the surface on the rolling cutter shell 41 . a shoulder 45 of the replaceable cutter opposes the shoulder 44 on rolling cutter 41 . the outer surface of the rolling cutter shell 41 slopes in wedge fashion and the inner surface of replaceable cutter shell 42 is of a corresponding shape . the bolts 46 extend through the shoulder 44 and into the end of rolling cutter shell 41 . tightening of bolts 46 induces a preload into the mating surfaces of the rolling cutter shell 41 and replaceable cutter shell 42 . the structural details of cutter 40 having been described , the operation of the cutter 40 will now be considered with reference to fig2 . the cutter shell 41 is adapted to be mounted in a saddle that is affixed to a rotary head of an earth boring machine or to the body of a raise bit . the cutting disc 43 contacts the formation and forms the desired borehole or tunnel . should the replaceable cutter shell 42 and cutting structure 43 thereon become worn or damaged before the other elements of the cutter 40 fail , it is desirable to replace the replaceable cutter shell 42 . in order to replace the cutter shell 42 , the rolling cutter shell 41 is removed from saddle mount . the replaceable cutter shell 42 is withdrawn from the rolling cutter shell 41 . in order to remove the worn cutter shell 42 , the preload bolts 46 are unthreaded to free the worn cutting element . some tapping with a sledge may be required . a new replaceable cutter shell is placed on the rolling cutter shell 41 . the cutter shells 41 and 42 are again inserted in the saddle mount . the cutter 40 is ready for continued operation . referring now to fig3 another embodiment of a cutter constructed in accordance with the present invention is illustrated . the cutter 47 includes a rolling cutter shell 48 adapted to be positioned around a bearing shell . the bearing shell will be securely locked in a saddle mount . the saddle mount may be connected to the rotary head of an earth boring machine or to the body of a bit for boring a large diameter hole . the bearing shell remains firmly locked in place throughout the drilling operation due to a tenon and groove arrangement disclosed in u . s . pat . no . 3 , 203 , 492 to c . l . lichte , patented aug . 31 , 1965 . a multiplicity of bearing systems promote rotation of the cutter shell 48 about the bearing shell . lubricant is retained in the bearing area by seal elements . the cutter 47 includes a replaceable cutter shell 49 mounted over the rolling cutter shell 48 . the replaceable cutter shell 49 will be locked on rolling cutter shell 48 until replacement is required at the end of the cutting structure &# 39 ; s service life . at that time , the replaceable cutter shell 49 may be removed by disconnecting it from the rolling cutter shell 48 . a new replaceable cutter shell may be inserted in place of the worn replaceable cutter shell 49 and the earth boring operation continued . the replaceable cutter shell 49 includes a pair of annular disc elements 50 and 51 . the external surface of the rolling cutter shell 48 is relatively smooth . a shoulder 52 is provided adjacent the external surface . the inside surface of the replaceable cutter shell 49 mates with the surface on the rolling cutter shell 48 . a shoulder 53 of the replaceable cutter opposes the shoulder 52 on rolling cutter shell 48 . the outer surface of the rolling cutter shell 48 slopes in wedge fashion and the inner surface of replaceable cutter shell 49 is of a corresponding shape . a plurality of bolts 54 extend through the shoulder 53 and into the end of rolling cutter shell 48 . tightening of bolts 54 induces a preload into the mating surfaces of the rolling cutter shell 48 and replaceable cutter shell 49 . in addition , a plurality of dowel pins 55 extend through the shoulders 53 and into the end of rolling cutter shell 48 . the dowel pins 55 assist in carrying the torsional loads encountered during drilling . the structural details of cutter 47 having been described , the operation of the cutter 47 will now be considered with reference to fig3 . the cutter shell 48 is adapted to be mounted in a saddle that is affixed to a rotary head of an earth boring machine or to the body of a raise bit . the cutting disc elements 50 and 51 contact the formation and form the desired borehole or tunnel . should the replaceable cutter shell 49 and cutting structure 50 and 51 thereon become worn or damaged before the other elements of the cutter 47 fail , it is desirable to replace the replaceable cutter shell 49 . in order to replace the cutter shell 49 , the rolling cutter shell 48 is removed from saddle mount . the replaceable cutter shell 49 is withdrawn from the rolling cutter shell 48 . in order to remove the worn cutter shell 49 , the preload bolts 54 are unthreaded to free the worn cutting element . some tapping with a sledge may be required . a new replaceable cutter shell is placed on the rolling cutter shell 48 . the cutter shells 48 and 49 are again inserted in the saddle mount . the cutter 47 is ready for continued operation .