the pulp - burr illustrated in fig1 c comprises a tool 1 of conventional shape , i . e . comprised of a conical or cylindrical wire forming a helix which is itself conical or cylindrical . this wire 1 has an extension 2 also wound in the form of a cylindrical helix , housed in a cylindrical bore or caviity 3 in a mandrel 4 , the bore extending axially of the mandrel . as shown in fig1 c the part 2 is supported against one side of the bore 3 by its central turns , whereas its end turns are supported against the opposite side of the bore 3 . this support takes place with a pressure such that the frictional forces are sufficient to connect the tool 1 and the mandrel 4 for the purpose of rotating the tool with the mandrel under normal operating conditions . this pressure on the wall of the bore 3 is obtained in a particularly simple manner . first of all , a cylindrical helix 2 is formed as illustrated in fig1 a , of diameter less than the internal diameter φd ( fig5 ) of the bore 3 , the length of the helix 2 being slightly less than the length of the housing . a permanent deformation is then imparted to the helix 2 by forming a bend through a certain angle α ( fig1 b ), so that at least part of the helix 2 is bent at that angle relative to the axis of the tool . it is then sufficient to introduce the part 2 into the bore 3 of the mandrel , the inlet of which initially has the inner diameter of the housing , as shown in dot dash line at 5 &# 39 ; and in fig5 . then the bore is partly closed by crimping in order to form a rim 5 retaining the part 2 in the housing . at the time of the introduction of the part 2 into the bore 3 , the bent part 2 is straightened under stress thus ensuring friction as described above . the value of the angle α determines the frictional force necessary for the correct operation of the instrument . the turns of the part 2 may be stretched apart as illustrated in fig1 a to 1c or be close together side by side as illustrated in fig4 a and 4b . the pressure of the part 2 on the walls of the bore may be obtained by other means . for example , in fig2 the helical extension has substantially the shape of an ellipsoid or barrel of which the maximum diameter d1 of the envelope defined by the ellipsoid is greater than the internal diameter of the bore 3 . in the variation illustrated in fig3 the helical extension defines an envelope having substantially the shape of a hyperboloid with one nappe , a maximum diameter d2 of this envelope being greater than the internal diameter of the bore 3 . these tools are force - fitted in the bore of the mandrel which is closed by crimping , as described previously , or by any other means . instead of providing radial friction , it is possible to use axial friction such as in the embodiment in fig6 in which the wire 1 is also extended by a cylindrical helical part 7 , the diameter of which is less than the internal diameter of the bore 3 . the length of the part 7 is greater than the length of the bore , so that the part 7 is compressed axially between the blind end or bottom of the bore and an edge 8 of the bore . the edge 8 is formed by crimping . the first and last turns of the part 7 frictionally engage the edge 8 and the bottom of the bore . instead of being cylindrical , the part 7 may have a variable diameter ( fig7 ) or may be of conical shape ( fig8 ), the length l1 still being greater than the length l of the bore . a closed bore 3 may be obtained by means other than crimping . for example , in fig9 the bore is formed in a tubular extension 9 of the mandrel , and is closed by a cover 10 , in the form of a sleeve comprising metal or synthetic material driven onto the tube 9 . in the embodiment in fig1 , the bore 11 is formed by a metal sleeve 12 provided with an orifice 13 for the passage of the tool , the sleeve being driven onto a reduced diameter cylindrical part 14 of the mandrel . these two methods of assembly shown in fig9 and 10 may also be used when the tools are as shown in fig1 a , 1b , 2 , 3 , 4a and 4b , 7 or 8 . however , the methods are particularly advantageous in the case of axial compression , since they make it possible to obtain the desired compression automatically . fig1 shows an embodiment of a pulp - burr in which the rear part of the helical wire 1 is embedded in an auxiliary part 15 of cylindrical shape preferably but not necessarily formed of plastics material . the part 15 is provided with three annular flanges 15a , 15b and 15c pressing , with a certain elastic deformation of the flanges , against the wall of a cylindrical bore 16 in the mandrel 4 . the bore 16 is closed by crimping 19 to form a housing retaining the auxiliary part 15 axially , which parts acts as a friction member for the rotary connection between the mandrel 4 and tool 1 . in the modification in fig1 a and 12b the radial friction is ensured by a single flange 20 provided with slots 21 . in fig1 a and 13b , the auxiliary part is formed by a cylinder 22 having two annular grooves in which are fitted two split rings 23 and 24 of which the outer diameter is greater than the internal diameter of the bore 16 . in fig1 a and 14b , the auxiliary part is formed by a cylinder 25 provided with four flexible longitudinal ribs 26 , 27 , 28 and 29 . in fig1 , the auxiliary part 30 is formed by a cylindrical block having a frustoconical end . this block is of resilient material and the diameter of the block is slightly greater than the internal diameter of the bore 16 . in the modifications in fig1 to 19 , the auxiliary part is comprised of a simple cylinder 31 having a diameter less than the internal diameter of the bore 16 . axially directed pressure against one of the ends of the bore is provided by a coil spring 32 which is under compression in the bottom of the bore ( fig1 ), or at the open end of the bore ( fig1 ), or by a curved washer 33 which is independent of or integral with the part 31 ( fig1 ), or by a resilient buffer 34 of rubber or other material compressed at the bottom of the bore ( fig1 ). instead of being closed by crimping as illustrated in fig1 to 19 , the bore may be closed by any other means , in particular those illustrated in fig9 and 10 . in the embodiment illustrated in fig2 , the tool 1 is embedded in an auxiliary part 35 comprising a tubular extension 35a which is conical at its inside and longitudinally or otherwise split . the extension 35a surrounds a conical extension 36 of the mandrel of similar shape to the interior of the part 35a . the auxiliary part 35 has a rib 35b engaging a groove 37 in the mandrel to retain the auxiliary part . the part 36 has a frustoconical end 36a facilitating the forcefitting of the part 35 on the mandrel . the part 35 may be of metal or synthetic material . fig2 shows a modification in which the auxiliary part is provided with an end - piece 38 which is split longitudinally and introduced by elastic deformation into a bore 39 in the mandrel 4 . fig2 shows a modification in which the auxiliary part comprises a cylindrical end - piece 40 force - fitted in a bore in the mandrel formed with one or more slots 41 extending longitudinally of the mandrel to facilitate radial elastic deformation of the housing . in fig2 the auxiliary part comprises a tubular extension which is not split and in which is introduced an end - piece 43 of the mandrel formed with a slot 44 facilitating radial elastic deformation of the end - piece 43 upon its introduction under stress into the part 42 . the invention is not limited in its application to pulp - burrs , but extends to all instruments for dental canals , for example to instruments intended for drilling dental canals , whether driven manually or mechanically . the driving connection between the tool and mandrel may be produced , for example , by one of the methods illustrated in fig1 to 23 or by other methods as illustrated , for example , in fig2 and 25 . in fig2 , the tool proper , for example a reamer ( not shown ), has a shank 45 formed with a stop 46 produced by compression . one end of spring 47 presses against the stop and the other end presses against a bent or turned in edge 48 of a bore 49 in the mandrel 50 . a rear end of the tool is thus pressed against the bottom of the bore 49 . the inner diameter of the spring 47 is greater than the diameter of the shank 45 . in the embodiment illustrated in fig2 , a shank 51 of a tool , for example a reamer , is fixed in the end of a spring 52 in a manner preventing relative rotation between the shank and the part of the spring fixed thereto . in the unstressed state the spring is bent at angle α to the axis of the shank and it is straightened under stress when the spring is introduced into the bore 53 in the mandrel 54 . the frictional rotary drive takes place in the same manner as in the example shown in fig1 a to 1c .
an instrument for use in the treatment of dental canals . the instrument includes a tool such as a pulp - burr or reamer connected by a friction coupling or slip - clutch to a mandrel driven by power to rotate the tool . should the tool jam in a dental canal , the clutch allows the mandrel to rotate by slipping relatively to the tool thus avoiding application of excessive torque to the tool which might break it . the slip - clutch can be a helical spring fast with one end of the tool and fitted in a cylindrical bore in and co - axial with the mandrel , the spring being stressed so some of its turns press against inner surfaces of the bore to provide the friction coupling between the spring and bore .