referring now to the drawing , it will be seen that the screw 10 of the present invention which is illustrated therein has a head 12 and an elongated shank 14 . this shank 14 has an exterior thread 16 . also at its exterior , the shank has a plurality of lobes 18 , shown on an enlarged scale in fig2 and these lobes 18 are circumferentially distributed about the axis of the shank 14 and extend spirally therealong in an angular direction opposite from the thread 16 , as apparent from fig1 . as may be seen from fig2 in any plane which is normal to the shank axis , the shank has an inner core portion 20 which is of circular configuration and concentric with such axis , within the dot - dash line circle 21 . the lobes 18 project radially from and are integral with the core portion 20 , as is apparent from fig2 and thus the core portion constitutes the central shank portion within the generated circle 21 coaxial with the shank axis . when the screw is turned into a body , the screw is turned in the direction of the arrow 22 indicated in fig2 . it will be seen that in this turning direction 22 , the lobes are non - symmetrical . that is , each lobe 18 has an elongated leading leg 24 which progresses gradually away from the axis of the shank in the direction of turning 22 . also , each lobe has a trailing leg 26 which abruptly returns inwardly toward the shank axis and which is much shorter than the longer leg 24 . each leg 24 is convexly curved and merges smoothly into the trailing leg 26 which is concave only at its innermost region where it adjoins the circular core portion 20 and the leading leg of the next lobe , as is apparent from fig2 . the progression of each leading leg 24 away from the axis of the shank is continuous , as shown in fig2 from the beginning of the leading leg up to its merger with the abruptly returning trailing leg 26 . moreover , as may be seen from fig2 the thread 16 has between its crest 28 and its root 30 a uniform radial width . thus , the configuration of the lobes 18 at the crests 28 of the thread 16 matches with the configuration of the lobes along the thread root 30 . in practice , it has been found best to provide , in any plane normal to the shank axis , three lobes uniformly distributed about this axis with each lobe extending over an arc close to 120 °, as is apparent from fig2 . the above - described screw of the invention can simply and inexpensively be manufactured by using a thread roll - die , since the lobes are spiral , and the same pressure is used along the entire length of the screw , so that slippage in thread rolling is at a minimum . moreover , roundness of the finished screw is maintained since there is no interruption in the rolling of the lobes , as would be the case if the lobes simply extended axially . thus , the screw is manufactured in one simple operation of rolling , simultaneously with the threading of the screw , as contrasted with various other thread - forming screws which must be made of pre - drawn trilobular wire headed by specially made trilobular heading dies and by various other specially shaped tools which are required to accommodate the trilobular blank . screws of the present invention have been found to perform well in a standard &# 34 ; pull - out &# 34 ; test . this is because the lobes are spirally situated and a high point of a lobe is always found at the exterior of the screw around the entire 360 ° of the shank , a pull - out will engage 360 ° of the mating member . thus , much more force will be required to pull out the screw of the invention than any other type of lobular screw which has all of the lobes distributed axially along the shank . after the screw of the invention has been used to form the thread in a hole of a body into which the screw is turned , since the lobes are situated so as to &# 34 ; wipe &# 34 ; in a thread and simultaneously form spirally a thread which is tapped in smoothly around 360 ° of the shank , an especially low torque results . each lobe on the screw has the configuration described above where it gradually , yet continuously , rises out to its maximum distance from the shank axis and then abruptly drops back in toward the axis , before the gradual rise which forms the next lobe . each lobe thus progresses gradually away from the shank axis in the direction in which the screw is turned into a body . when the screw is backed out of the threaded bore , the abrupt heel formed by the shorter leg of each lobe will offer resistance to unscrewing , since the metal of the mating threaded part will tend to close in behind the heel or shorter leg of each lobe during formation of the thread . since the threads thus rise and fall according to the conformation of the lobes described above , it is possible to achieve the thread - forming operations in a much easier manner since a lower torque is required than would be the case with the use of , for example , three lobes each of which is of a symmetrical construction , as is encountered with other thread - forming screws . therefore , with the above - described screw of the invention , the starting torque is substantially lowered . it has also been found that for any given round hole into which the screw is tapped , a much fuller thread is formed . as is well known , thread - forming screws displace metal in the mating member so that the metal moves in both directions , moving to form both major and minor diameters . with the screw of the present invention , the effect is to displace metal to a greater degree , making for a smaller dimension of the major diameter in the part which is threaded . in other words , if a cylindrical plug gauge is inserted into the hole in which the thread has been formed by the screw of the invention , it will be found that a smaller plug gauge will be required to be inserted into the tapped hole than into any other tapped hole which has been thread - formed by a conventional screw . it follows , therefore , that with the screw of the invention , there is a much greater engagement with the threaded body and a much greater holding power . thus , the particular trilobular construction of the screw of the present invention allows the screw to form a thread . as is apparent from the above description , however , the thread - forming operations are greatly facilitated by the particular construction of the lobes referred to above . inasmuch as the lobes are disposed about the shank axis in a spiral , the formation of the thread in a body into which the screw is turned is performed smoothly and without any interruption , thus further facilitating the thread - forming operation . accordingly , the thread - forming screw is especially suitable to function as a tapping device and the definition thread - forming screw is intended to comprehend within its scope such tapping device having the characteristic lobular construction aforedescribed . the manner in which the spiralled lobal arrangement and thread configuration of the present invention operates to obtain fuller depth threads with minimal interference can best be seen in the fragmentary section view of fig3 . the section shown in fig3 is taken along a plane which passes through the axis of the screw . since the lobes 18 are spirally arranged about the circumference of the screw , the plane of the section of fig3 cuts the lobes on adjacent threads along the length of the screw at a different location thereon . thus , whereas one lobe 18a is cut by the section plane at its highest point , successive lobes 18b , 18c , etc ., on successively adjacent threads to the left of the one lobe 18a are cut at successively lower points along their respective gradually rising leading legs 28 ( fig2 ). the lobe immediately to the right of the highest cut lobe 18a , i . e ., lobe 18d , however , is cut by the section plane at its lowest point , so that the thread formation thereon is closer to the axis of the screw . this occurs because the trailing leg 26 ( fig2 ) of the lobes drops abruptly so that , with the lobes being spiralled , the lowest point on the lobe 18d is in axial alignment with the highest point on the lobe 18a . continuing along to the right in fig3 it will be seen that the section plane cuts each successive lobe to the right of the lobe 18d , i . e ., lobes 18e , 18f , etc ., at successively higher points up to the highest point of a lobe 18g . the same section plane , however , cuts the next adjacent lobe , 18h , at its lowest point for the reason set forth above in connection with lobe 18d . noted it will be noted from fig3 that the threads on the screw maintain their full thread height and configuration at every location along each lobe irrespective of where the lobe is cut by the section plane . because of this , it will be noted , in those situations where the highest point along one lobe is axially adjacent to the lowest point on another lobe , e . g ., between lobes 18a and 18d and between lobes 18g and 18h , a clearance space 30 exists . this clearance space accommodates the flow of metal which is at a maximum in the region of the highest point of each lobe . this permits metal to flow unobstructedly in a manner which allows the formation of threads of maximum depth without the interference which usually accompanies swage - type thread - forming operations . the formation of full depth threads permits the screw to achieve maximum thread contact surface area so that the vibration - resistance of the screw is enhanced . fig4 shows a roll die 32 used to form the lobular screw threads described above . the roll die 32 comprises an essentially rectangular block of hardened metal having a working face 34 lying in the plane of the drawing . alternate grooves 36 and ridges 38 are formed in the working face 34 and these grooves and ridges have a cross - sectional configuration corresponding in size and configuration to the cross - section of the thread to be formed . it will also be noted that the grooves 36 and the ridges 38 extend at an angle α with respect to an upper edge 39 or direction of relative movement of the die , this angle corresponding to the pitch angle of the thread to be rolled . the thread - forming grooves 36 and ridges 38 follow undulations 40 in the plane of the working face 34 of the die . these undulations rise gradually up out of the plane of the working surface in a direction moving from right to left in the drawing and then they drop abruptly back into the working surface and begin a subsequent gradual rise . the undulations 40 in each groove 36 and ridge 38 are shifted slightly to the left with respect to the undulations 40 in the groove and ridge immediately thereabove . as a result , undulations 40 are arranged in columns which form an acute angle β with respect to the upper edge 39 or direction of relative movement of the die 32 . the angle β , which corresponds to the spiral pitch of the thread lobes formed by the die , is small enough so that along any imaginary line ( e . g ., phantom line 42 ) drawn perpendicular to the upper edge 39 of the die , at any location therealong , such line will be intersected by the highest points ( e . g ., points 44 and 46 ) of the undulations in two different columns . in operation , two of the dies 32 are positioned with their working faces 34 parallel to each other and spaced apart , as shown in fig5 by an amount sufficient to accommodate a shank 48 of a blank to be threaded and formed into the screw 10 of fig1 . the dies 32 are then moved in the planes of their working faces 34 in mutually opposite directions , as indicated by the arrows a and b in fig5 so that the shank 48 is rotated clockwise , as indicated by the arrow c . during the initial portion of the thread rolling operation , the non - undulated portions of the grooves 36 and ridges 38 in the dies 32 form threads about the shank 48 in conventional manner . thereafter , however , the undulations 40 engage the shank 48 and these operate , as illustrated in fig6 to form the lobes 18 in the screw threads . because the undulations 40 are formed at an angle β with respect to the upper edge of the dies 32 , or more particularly , with respect to the path of die movement , the shank 48 is engaged at all times by the highest points of at least two undulations 40 on the dies 32 . consequently , the dies support the shank in a stable manner and permit it to roll smoothly even though the dies are , during the rolling operation , imposing lobes on the screw . in the present situation , the twisted or spiralled arrangment of the thread lobes permits them to be formed by rolling without the need for special arrangement for shifting the planes of the die element working surfaces . also , with the present invention , it is possible to roll various lobular thread configuration without need for specially shaped screw blanks . in fact , circularly cross - sectioned screw blanks may be provided ; and when these are rolled with dies according to the present invention , the blank axis remains stably supported equidistant between the two die surfaces . by maintaining the blank axis in a fixed plane equidistant from the two die surfaces , the blank , when pretapered , may be formed with a point configuration having threads which are undistorted and which permit screw starting in previously drilled but untapped holes without need for excessive axial thrust on the screw . it will be understood that while the dies shown herein are flat , the principles of the present invention are applicable to curved dies between which elements to be threaded move along an arcuate path . having described the invention with particular reference to the preferred forms thereof , it will be obvious to those skilled in the art to which the invention pertains , after understanding the invention , that various changes and modifications may be made therein without departing from the spirit and scope of the invention , as defined by the claims appended hereto .