Driver/Fastener Inter-engagement System

Abstract

Embodiments of the present invention provide a screw and driver system comprising a range of screws and a driver for said range. Each screw comprises a head having a driving recess in its surface for engagement by said driver. Each recess has a recess longitudinal axis. The recess of larger screws in the range has a plurality of superimposed recess-tiers of decreasing size with increasing depth from said surface, each recess-tier except the smallest having substantially parallel driving surfaces substantially parallel said recess longitudinal axis. The driving head of the driver has a driver longitudinal axis and a plurality of superimposed drive-tiers of decreasing size towards a tip of the driver, each drive-tier except the smallest having substantially parallel driving surfaces substantially parallel said longitudinal axis. The driver and recess are shaped so that, when the driver is engaged with the recess of any screw in said range, torque applied to the driver is transmitted to the screw through said driving surfaces. The smallest recess-tier of larger screws has a mouth in a floor of an adjacent recess-tier and has recess flanks that are all tapered from the mouth towards the recess longitudinal axis at a recess taper angle between 1.91 and 6.85. The smallest drive-tier of the driver has a root in a base of an adjacent drive-tier and has drive flanks that are all tapered from the root towards the driver longitudinal axis at a tier taper angle between minus 1.5 and plus 2.5 difference with respect to said recess taper angle. The diameter of the drive-tier at the root and the diameter of the recess-tier at the mouth are such that, on insertion of said driver head in the screw recess, said drive and recess flanks inter-engage to deform and stick together through frictional engagement before said base engages said floor.

Claims

1. A screw and driver system comprising a range of screws and a driver for said range, each screw comprising a head having a driving recess in its surface for engagement by said driver, wherein: a) each driving recess has a recess longitudinal axis, b) the driving recess of larger screws in the range has a plurality of superimposed recess-tiers of decreasing size with increasing depth from said surface, each recess-tier, except the smallest, having substantially parallel driven surfaces substantially parallel said recess longitudinal axis, c) a driving head of the driver has a driver longitudinal axis and a plurality of superimposed drive-tiers of decreasing size towards a tip of the driving head, each drive-tier, except the smallest, having substantially parallel driving surfaces substantially parallel said driver longitudinal axis, and d) said driver and recess are shaped so that, when the driving head is engaged with the recess of any screw in said range, torque applied to the driver is transmitted to the driven surfaces of the screw through said driving surfaces of the driver; and wherein the smallest recess-tier of larger screws has a mouth in a floor of an adjacent recess-tier and has said driven surfaces comprising recess flanks that are all tapered from the mouth towards the recess longitudinal axis at a recess taper angle between 1.91 and 6.85; the smallest drive-tier of the driver has a root in a base of an adjacent drive-tier and has said driving surfaces comprising drive flanks that are all tapered from the root towards the driver longitudinal axis at a tier taper angle between minus 1.5 and plus 2.5 difference with respect to said recess taper angle; and the diameter of the drive-tier at the root and the diameter of the recess-tier at the mouth are such that, on insertion of said driving head in the screw recess, said drive and recess flanks inter-engage to deform and stick together through frictional engagement before said base engages said floor.

2. A screw and driver system according to claim 1, wherein said recess flanks of the smallest recess-tier of larger screws are all tapered from the mouth towards the recess longitudinal axis at a recess taper angle between 2.5 and 5.5.

3. A screw and driver system according to claim 1, wherein the smallest drive-tier of the driver has drive flanks that are all tapered from the root towards the driver longitudinal axis at a tier taper angle between 0 and 1.0 less than said recess taper angle.

4. A screw and driver system according to claim 1, wherein the smallest drive-tier of the driving head has a tier taper angle of 3.72.5, and the smallest recess-tier of the screw recess has a recess taper angle of 4.32.5.

5. A screw and driver system according to claim 4, wherein the smallest drive-tier of the driving head has a tier taper angle of 3.70.9, and the smallest recess-tier of the screw recess has a recess taper angle of 4.30.9.

6. A screw and driver system according to claim 1, wherein: the driver has a smallest-drive-fillet at the root between the smallest drive-tier and the base of said adjacent drive-tier, wherein the radius of the smallest-drive-fillet is between 0.1 and 0.5 mm, the screw has a smallest-recess-chamfer at the mouth between the smallest drive-recess and the floor of said adjacent recess-tier, wherein the radius of the smallest-recess-chamfer is between 0.1 and 0.5 mm; and the diameter of the smallest drive-tier of the driving head where the smallest-drive-fillet begins, is larger than the diameter of the smallest drive-recess where the smallest-screw-chamfer ends, by between 0.04 and 0.1 mm.

7. A screw and driver system according to claim 1, wherein: the drive flanks of the smallest drive-tier engage with the driven flanks of the smallest drive-recess when the driver is mated with the screw, before the smallest-drive-fillet on the driver impacts the smallest-screw-chamfer in the screw, and before the base of the adjacent drive-tier of the driver impacts the floor of the adjacent recess-tier of the screw.

8. A screw and driver system according to claim 1, comprising at least three sizes of screw, being: a largest size screw comprising three recess-tiers, being said smallest recess-tier formed in the floor of said adjacent recess-tier and a largest recess-tier, in whose floor said adjacent recess-tier is formed; a middle size screw comprising two recess-tiers, being said smallest recess-tier formed in the floor of said adjacent recess-tier; and a small size screw comprising one recess-tier, being said smallest recess-tier formed in the surface of said head of the screw; wherein the driver comprises at least three said drive-tiers arranged to fit the corresponding recess-tiers of each screw size.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Embodiments of the invention will now be described by way of example only, with reference to the accompanying figures, in which:

[0040] FIGS. 1a to d show respectively an end view, a side view, a second orthogonal side view and detail B from FIG. 1c, of a two-tier punch for forming a two recess-tier screw in accordance with the present invention;

[0041] FIGS. 2a to c show respectively an end view, a side view, and a second orthogonal side view, of a driver in accordance with the present invention;

[0042] FIGS. 3a to c show respectively an end view, a side view, and a second orthogonal side view, of a three-tier punch for forming a three recess-tier screw in accordance with the present invention;

[0043] FIGS. 4a and b show respectively a driver in side view and a screw in side section in accordance with another embodiment of the present invention; and

[0044] FIG. 5 is an enlarged view (not to scale) of engagement between a screw recess and driver in accordance with the present invention.

DETAILED DESCRIPTION

[0045] With reference initially to FIGS. 4a and 4b, a screw 10 has a driving recess 14 in its head 12 which opens at the top surface 13 of the head 12. The recess 14 comprises three super-imposed hexagonal recesses 16a,b,c, each of reducing dimension. It is possible to have fewer recesses, or more.

[0046] A driver 30 for the screw 10 comprises a shaft 32 and a driving head 34 comprising three tiers 36a,b,c (or more if there are more recess-tiers in the largest screws). The cross sections of the driving tiers 36a,b,c correspond with the cross sections of the recesses 16a,b,c of the screw 10 and reduce in cross-sectional diameter towards a tip of the driving head 34. Accordingly, when the driver head 34 is inserted into the recess 14, the screw 10 is seated on the end of the driver 30 and can be driven, by rotation of the driver, and screwed into a workpiece (not shown).

[0047] Each of the largest and middle recess-tiers 16a and 16b of the recess 14 have parallel sides, defining driven surfaces, which sides are parallel a recess longitudinal axis X of the recess. Likewise, each of the largest and middle drive-tiers 36a and 36b of the drive head 34 have parallel sides, defining driving surfaces, which sides are also parallel a driver longitudinal axis Y of the driver 30. The dimensions of the drive-tiers and recess-tiers are closely matched, whereby torque applied to the driver is transmitted to the recess through the abutting driving surfaces or flanks of each drive-tier against the corresponding driven surfaces or flanks of the screw recess. Because these sides are parallel to the axes of rotation X, Y of the driver and recess, there is no tendency for cam-out.

[0048] The smallest recess-tier 16c, and the smallest drive-tier 36c, do not have parallel flanks, however, and are instead tapered. The taper can be straight, or as shown in FIGS. 4a and 4b, it may be curved. In any event, they are matched, and by closely matching the tapers of the smallest-recess-tier, as well as matching the dimensions of the other recess-tiers with the corresponding drive-tiers of the driver, the largest and middle drive-tiers can be fully engaged and yet the tapers of the smallest tiers of the recess and driver sufficiently snugly inter-engaged so that the slight pinching between them is enough to prevent the screw from being dislodged by inversion of the driver with respect to the recess. Furthermore, because the contact between the flanks of the smallest drive-tier and smallest recess-tier is through their respective driving/driven surfaces, rather than between a line contact, the connection between driver and recess is more stable and less likely to wobble when the screw is engaged with a workpiece.

[0049] Turning to FIGS. 3a to 3c, a punch 40 has a recess forming head 42 comprising three hexagonal punch-tiers 46a,b,c, and is for forming the recess 14 of the screw shown in FIG. 4b.

[0050] Turning to FIGS. 1a to 1d, a punch 40 has a recess forming head 42 comprising two hexagonal punch-tiers 46b,c, and is for forming the recess of a screw not shown in the drawings but having only two recess tiers, which tiers are arranged to correspond precisely with the recess-tiers 16b,16c of the screw shown in FIG. 4b.

[0051] However, the screw recess formed by the punches 40,40 differ from the screw shown in FIG. 4b in that the smallest punch-tier 46c of the punches 40,40 has straight flanks 44.

[0052] It will be understood that, in a cold-forming process to form a screw recess, by driving a punch into the blank head of a screw, the metal of the screw head becomes temporarily liquid under the extreme pressure and impact of the punch, and flows around the shape of the punch. The recess so-formed adopts almost exactly the shape and dimensions of the punch, whereby the shape and dimensions of the punch essentially mirror precisely the dimensions of the recess formed.

[0053] In FIGS. 1a to 1d, details of the smallest punch-tier 46c are shown, especially in FIG. 1d. This corresponds to the shape and dimensions of the smallest recess-tier 16c of a screw 10 to be formed (although not precisely as illustrated in FIG. 4b because that screw has a curvingly-tapered smallest recess 16c).

[0054] Thus, references to features of a screw's recess, particularly its smallest recess-tier, are frequently made hereinafter by reference to FIG. 1, and to the punch 40 that forms them, (or indeed, the punch 40 in FIGS. 3a to c, which only differs in having a third, largest punch-tier 46a).

[0055] As mentioned, the smallest recess-tier 16c, formed by the punch 40, 40, is tapered at a recess taper angle inwardly towards the recess longitudinal axis X. may be 4.35. The punch has an adjacent-tier base 48 from which the punch-tier 46c extends. If there is a perfect angle between the flank 44 and the base 48, the diameter D.sub.nom of the tier at the base 48 is 2.56 mm. However, a fillet 50.sub.R inevitably remains (in the course of manufacture of the punch 30, 30) between the flank and base and this punch-fillet 50.sub.R has a radius R.sub.R, which may be between 0.15 mm and 0.2 mm. The punch-fillet provides a corresponding chamfer 50c in the mouth of the smallest tier-recess of the screw (see FIG. 5). Likewise, the base 48 forms floor 118 of the adjacent recess-tier 16b of the screw 10 (see FIG. 4b).

[0056] The flank 44 therefore actually begins between 0.15 mm and 0.2 mm from the base 48 and, here, may have a diameter D.sub.act of 2.535 mm. The depth H.sub.R of the flank 44 of the smallest tier-recess 46c of the screw may be 1.17 mm.

[0057] Turning to the driver, FIGS. 2a to 2c show a driver 30 to cooperate with the recesses 14 of screws formed by the punches 40,40. The smallest drive-tier 36c has a height H.sub.D of 1.09 mm, but it also has an inevitable drive-fillet 38 of radius R.sub.D at its root between the smallest drive-tier 36c and the base 39 of adjacent drive-tier 36b. R.sub.D is likewise restricted to between 0.15 mm and 0.2 mm. Immediately adjacent the drive-fillet 38, the diameter D.sub.D of the smallest drive-tier is 2.615 mm. The flanks 34 of the smallest drive-tier 36c have a tier taper angle , which may be 3.7.

[0058] The reason for the difference in recess taper angle and tier taper angle is because any manufacture of screw and driver is subject to variance. FIG. 5 (which is not to scale) shows the interaction between the smallest drive-tier 36c of driver 30 and smallest recess-tier 16c of screw 10. In Table 1 below are representative dimensions achievable across a substantial, cost-effective, manufacturing range.

TABLE-US-00001 TABLE 1 Recess Driver Minimum Nominal Maximum Minimum Nominal Maximum Angle 1.91 4.35 6.85 1.2 3.7 6.2 (, ) Diameter mm 2.46 2.535 2.83 2.58 2.615 2.87 (D.sub.act, D.sub.D) Radius mm 0.1 0.5 0.1 0.5 (R.sub.R, R.sub.D) Depth/Height 1.05 1.17 1.25 1.01 1.09 1.2 mm (H.sub.R, H.sub.D)

[0059] Table 2 below illustrates another possible embodiment of dimension possibilities.

TABLE-US-00002 TABLE 2 Recess Driver Minimum Nominal Maximum Minimum Nominal Maximum Angle 3.5 4.35 4.5 3.4 3.7 4.0 (, ) Diameter mm 2.52 2.535 2.55 2.60 2.615 2.63 (D.sub.act, D.sub.D) Radius mm 0.15 0.2 0.1 0.2 (R.sub.R, R.sub.D) Depth/Height 1.10 1.17 1.25 1.05 1.09 1.15 mm (H.sub.R, H.sub.D)

[0060] Within the limits of these ranges, especially those in Table 2, on most occasions, a driver 30 with its smallest drive-tier 36c will interact with the smallest tier-recess 16c with a first contact point CP separated from chamfer 50c at the mouth of the smallest tier-recess 16c. Assuming that the drive-fillet 38 and chamfer 50c substantially match, this leaves a gap G of between 0.04 and 0.1 mm between floor 118 of the adjacent recess-tier 16b of the screw 10 and base 48 of adjacent drive-tier 36b of driver 30. Furthermore, the contact point CP will be such as to leave separation F above it (between the contact point CP and chamfer/fillet 50c/38), while below, there is compression of the driver and expansion of the recess. However, the angle F of contact will be so minimal that the compression/expansion between the recess and driver will always be small and spread over a significant area, but generally always separated from the mouth of the recess. Thus a more stable connection is achieved.

[0061] It should of course be appreciated that the smallest tiers of the driver and recess are hexagonal in section, like the other tiers (although other polygonal sections are possible) and in the smallest screws only the single smallest-tier recess may be provided, driven only by the smallest drive-tier of the driver. Thus, because the flanks of this tier are not parallel the longitudinal axis of the driver/screw combination, and indeed both the recess and driver are tapered, there is a cam-out tendency once torque is applied between driver and screw. That is, there is an axial component of the reaction force between screw and driver on application of torque, which component tends to separate the screw and driver. However, with the taper angle being less than 5, that component of force is minor. In small screws, with only the single smallest-tier recess, the proportion of axial force compared with torque applied may be greater and there is less inherent resistance to screw/driver separation (through frictional engagement of parallel flanks in multi-tiered screws). Nevertheless, there is little difficulty in a user resisting such cam-out forces with small screws.

[0062] Furthermore, it should also be understood that use of the term diameter as used herein, with respect to hexagonal, or any polygonal, section, is (unless the context otherwise makes clear) a reference to the dimension perpendicularly across the sides of the section (flat to flat dimension). Generally, diameter is simply a measure of the size of the section in question but it could, for example, just as meaningfully, be the distances across the corners of an hexagonal section.

[0063] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

[0064] Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0065] The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.

[0066] Each of claims 3, 4 and 6 to 8 of the appended claims may be dependent on any one or more of the claims which precede them.