1. Field of the Invention
This invention relates to a resinous belt for holding a train of screws. More specifically, the present invention relates to a screw train holding belt which is used in an automatic screw driver for successively feed screws.
2. Description of the Prior Art
As often experienced by many people, manual screw driving is a laborious and time-taking job. In particular, the screw driving job can be a serious trouble when it must be done extensively for a long time. For this reason, it has become a common practice to use an automatic screw driver when the screw driving job must be done extensively.
For the convenience of description, a typical example of automatic screw driver is illustrated in FIGS. 21 through 23 of the accompanying drawings. A similar screw driver is also disclosed in U.S. Pat. No. 4,059,034 to Hornung.
As shown in FIGS. 21-23, the prior art automatic screw driver mainly comprises a screw driver main 11 and a screw feeding attachment 12 removably connected to the driver main 11. The driver main 11 carries a driver bit 13 which is rotated by a built-in motor (not shown).
The screw feeding attachment 12 has an elongate case 14 and a slider 15 slidably fitted in the case 14. The slider 15 is always urged forward by a coil spring 16 and carries a pair of indexing sprockets 17 (only one shown) for indexing a screw train holding belt 18 which is made of a soft synthetic resin. The screw train holding belt 18 carries a train or series of screws 19 and has pairs of notches 18a for engagement with the pair of indexing sprockets 17 of the screw feeding attachment 12, as shown in FIG. 24.
In use, the slider 15 is held in abutment with a suitable portion of an object B (see FIG. 22), and the driver main 11 is pressed forward. As a result, the case 14 together with the driver bit 13 in rotation advances to drive a screw 19 (in front of the driver bit 13) of the screw train holding belt 18 into the object B while allowing the slider 15 to retreat into the case 14 against the coil spring 16.
Upon finishing a screw driving operation, the driver main 11 is brough away from the object B, thereby allowing the slider 15 to return to its initial position by the storing force of the coil spring 16. Such a returning movement of the slider 14 causes the sprockets 17 to index the screw train holding belt 18 for bringing another screw 19 to a position in front of the driver bit 13.
As shown in FIGS. 24 and 25 (and as disclosed in Japanese Utility Model Publication No. 4(1992)-49367 for example), the screw train holding belt 18 has a series of screw retaining bores 20 at a constant pitch longitudinally of the belt. Each of the screw retaining bores 20 is defined by a tube 21 integral with the belt 18 for engagement with the threaded shank of the corresponding screw 19. Further, the screw retaining bore 20 is surrounded by a plurality of removal facilitating holes 22 communicating with the bore 20 via respective cuts or slits 23. The removal facilitating holes 22 and the cuts 23 cooperate to facilitate passage of the enlarged head of the screw 19 through the screw retaining bore 20 at the time of driving the screw 19.
When the head of the screw 19 has a flat bottom, the screw train holding belt 18 shown in FIGS. 24 and 25 has been found to work well because the screw head can rest stably on the flat surface of the belt to maintain the screw 19 perpendicular to the belt. However, if the screw 19 has an undercut or countersunk head which cannot stably rest on the flat belt surface, it becomes difficult for the belt to hold the screw 19 with correct orientation.
FIGS. 26 through 30 of the accompanying drawings show another screw train holding belt 18' which is already known. Such a belt is disclosed in Japanese Utility Model Publication No. 58(1983)-47299 for example.
As shown in FIGS. 26-30, the belt 18' is formed with a series of screw retaining bores 20' each defined by a tube 21' integral with the belt. The screw retaining bore 20' has a shank retaining portion 20a' (see FIG. 29) for engagement with the threaded shank 19a of the screw 19, and a conical root portion 20b' for coming into face to face contact with the countersunk head 19b.
According to the arrangement described above, since the screw 19 is supported at both of the shank 19a and the head 19b, it is possible to hold the screw 19 correctly perpendicular to the belt 18'. However, this prior art belt is still disadvantageus for the following reasons.
As shown in FIGS. 27-29, each screw retaining bore 20' of the screw train holding belt 18' is formed by a burring method which uses a punch 24 and a die 25. The punch 24 has a diametrically smaller shaft portion 24b and a diametrically larger conical portion 24b. Similarly, the die 25 has a diametrically smaller bore portion 25a and a diametrically larger conical bore portion 25b. Apparently, the screw retaining bore 20' is formed by plastic deformation of the belt material caused by pressing the punch 24 into the die 25 (see FIG. 28).
However, since the belt 18' is inherently elastic, the conical root portion 20b' tends to spring-back slightly upon drawing the punch 24 out of the die 25, as appreciated by comparing FIG. 28 with FIG. 29. Thus, it is difficult to form the conical root portion 20b' of the screw retaining bore 20' with dimensional accuracy.
When the screw 19 is fitted into the thus formed screw retaining bore 20', the conical root portion 20b' of the bore 20' must elastically deform for coming into full face to face contact with the countersunk head 19b of the screw 19. As a result, the belt 18' is stressed longitudinally thereof, which causes longitudinal warping of the belt 18', as shown in FIG. 30. Apparently, such warping hinders smooth feeding movement of the belt 18' through the screw feeding attachment 12 (FIGS. 21-23).
A similar problem also occurs when the belt 18' is used for holding screws having a countersunk head of a different countersink angle. Further, the conical root portion 20b' of the screw retaining bore 20' cannot adapt to a non-conical undercut screw head.