FIELD OF INVENTION
[0001] THIS INVENTION relates to a screwdriver having a shaped blade tip and, more particularly,
a blade tip shaped to diminish slipping of a blade tip relative to a screw slot engaged
thereby and, accordingly, to promote a satisfactory grip between the screwdriver blade
tip and such screw slot.
BACKGROUND TO THE'INVENTION
[0002] It is a well known problem with screwdrivers that the screwdriver tip tends to slip
out of a screw slot when torque is applied thereto. This is generally in consequence
of the tapered faces of the screwdriver blade contacting the screw slot at its outer
edge and the resultant sliding action of the two surfaces. Clearly the screwdriver
surface is generally harder than the screw itself and the edge of the screw slot does,
accordingly, not assist in holding the two parts in engagement.
[0003] This problem has been recognised in the past and numerous different solutions have
been offered to the problem. The most common solution to the problem is to provide
a screwdriver blade, and in some cases a specially manufactured screw, with a reverse
taper at the end region of the screwdriver blade. This has been achieved by providing
a screwdriver blade with a dovetail or other flared shape so that the thickness of
the screwdriver blade increases towards the tip of the blade. The problem encountered
is to maintain adequate strength of the blade whilst not increasing the widths of
the tip to an extent where it will not be accommodated in a slot of a screw. Also,
such blades are difficult to manufacture.
[0004] U.S. Patent No. 3,923,088 thus proposed a screwdriver blade wherein the normally convergent
major faces of the screwdriver tip were made concave in the longitudinal direction.
This provided an end region adjacent the tip in which the faces were divergent and
a substantially thinner region at about the middle of the length of the screwdriver
blade itself. Substantial weakening of the blade resulted in view of the extremely
thin portion which was formed well inwardly of the operative end. Indeed, such is
the weakening effect that the patent itself proposed the introduction of integral
ribs extending longitudinally along the blades on each side thereof in an attempt
to overcome the weakening effect. This arrangement suffers from two disadvantages,
the one being that as the screwdriver tip requires sharpening, the thickness of the
blade diminishes until the position of minimum thickness is reached whereafter an
effect opposite to that desired would be achieved. Furthermore, sharpening would be
made difficult in some cases particularly those where the ribs are provided. Finally,
the arrangement suffers from the most serious disadvantage in that it is difficult
to manufacture a screwdriver of this nature.
[0005] U.S. Patent No. 1,055,031, on the other hand, in an effort to retain strength, provided
that the central region of the blade tip should remain of standard thickness whilst
grooves should be provided only on one or both sides of the centre in respect of each
major face of the screwdriver with the groove forming a sharp edge with the end edge
of the screwdriver. This arrangement suffers from the disadvantages of difficulty
of manufacture and problems in reproducing the grooves when sharpening of the screwdriver
is necessary. Also the thickness in any event decreased at the position of maximum
shear, ie. at the side edges of the blade. U.S. Patent No-. 1,479,506 proposed a similar
arrangement to that of U.S. Patent 3,923,088 with substantially the same disadvantages
apart from those associated with the ribs which were absent in this case.
[0006] Other patents, for example U.S. Patent 2,684,094 proposed the formation of projections
standing proud of the blade faces at the blade tip. This proposal has the major disadvantage
that sharpening is almost impossible to achieve whilst retaining the gripping features.
[0007] U.S. Patent No. 1,997,422, on the other hand, proposes a combination of what is tantamount
to a lipped screw slot (extremely difficult to manufacture) and a co-operating screwdriver.
[0008] Other proposals are set out in U.S. Patent No. 1,056,095 (similar to the invention
of 1,055,031) and U.S. Patent No. 4,105,056 which suffers again from the disadvantage
of being difficult to sharpen whilst retaining the required features. Also weakening
would be a problem in the latter case.
[0009] Other proposals have been made all of which suffer from one or more of the disadvantages
outlined above.
[0010] It is the object of this invention to provide an improved screwdriver (by which term
is included, in this specification, a screwdriver bit or the like for use in a brace,
powerdrill or other means for rotating it) in which manufacture is relatively simple,
strength is substantially retained, and sharpening is accommodated adequately.
SUMMARY OF THE INVENTION
[0011] In accordance with this invention there is provided a screwdriver having a blade
defined by two convergent, flat, major faces terminating in a straight transverse,
operative blade tip having a predetermined thickness and wherein a groove is provided
adjacent the edge of the blade tip in each of the two major faces thereof, the groove
extending across the entire blade tip width in each case and having a maximum depth
chosen to provide a minimum thickness to the blade of between 75% and 95% of said
predetermined thickness at the position of maximum depth of the grooves, said position
of maximum depth being located at a distance of from 40% to 120% of said predetermined
thickness from the edge of the blade tip, the grooves being arcuate in cross-section
and defining, with the adjacent end face of the blade tip, an acute angle in cross-section.
[0012] Further features of the invention provide for said minimum thickness of the blade
to be between 40% and 100%, preferably 80% and 90% of said predetermined thickness
of the blade tip; for the position of maximum depth of the grooves to be located at
a distance of between 50% and 80% of said predetermined thickness from the edge of
the blade tip; for the groove to be part circular in cross-section; for the radius
of curvature of said part circular grooves to be from 1,5mm to 5mm and, preferably,
from 2mm to 4mm; and for the edges of the blade tip to be located in or below the
plane of the surface of the flat major face with which it is associated.
[0013] It has been found that a totally satisfactory non-slip, or gripping screwdriver blade
tip, is formed when the above constraints are applied thereto.
[0014] As far as Applicant has been able to determine, the best results are obtained when
the position of minimum thickness of the blade, ie. maximum depth of the grooves,
is located at different positions according to the actual thickness of the tip of
the screwdriver. Thus, for screwdrivers having the following thicknesses the preferred
location of the position of minimum thickness, or maximum depth of groove is as follows:-
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] In order that the invention may be more fully understood one embodiment thereof will
now be described with reference to the accompanying drawings in which:-
Figure 1 is an isometric view of a manually operable screwdriver,
Figure 2 is a very much enlarged isometric view of the tip region of the blade thereof,
Figure 3 is a side elevation of the tip region illustrated in Figure 2, and,
Figure 4 illustrates, in side elevation, the tip region in engagement with a slotted
screwhead.
EXPANDED DESCRIPTION WITH REFERENCE TO THE DRAWINGS
[0016] As illustrated in Figure 1, a screwdriver is basically of conventional design and
has a plastics handle 1, from which extends a co-axial shank 2, having a screwdriver
blade formation 3 at its end remote from the handle.
[0017] The screwdriver blade formation is of substantially conventional design having two
major faces 4 which converge towards the tip face 5 itself and two sides 6 which also
converge in the usual way.
[0018] By means of any suitable process, and conveniently by grinding, there is formed in
each of the major faces 4, and along the end face 5 of the blade, a groove 7 of part-circular
shape in cross-section. The groove communicates with the edge 8 of the end face 5
of the tip to define therewith an acute angle as shown most clearly in Figure 3.
[0019] Each of the grooves is specially shaped and dimensioned as provided by this invention
and, to this end, the radius of curvature, which is indicated by "R" in Figure 3,
is made to between 1,5mm and 5mm and, in particular, to a radius of 3mm. It has been
found that the radius of curvature does not have to extend outside of this range for
any normal sized screwdriver.
[0020] The position of minimum thickness of the blade, which is shown as being a distance
"L" from the end face 5 of the tip, is located at a position as indicated above according
to the thickness of the tip of the screwdriver which is indicated by the letter "X"
in Figure 3. These constraints then determine the minimum thickness indicated by letter
"T" in Figure 3.
[0021] As shown most clearly in Figure 4, a screwdriver tip shaped as above described, co-operates
with a slot 9 in the head of a screw 10 so that the edges 8 engage in the inner region
of the slot and will, when torque is applied to the screwdriver, tend to bite into
the side of the screw slot at or near the base of the slot.
[0022] It will be understood that the invention provides an extremely simple, yet adequately
strong screwdriver tip, which will not slip in use, and which has advantages over
prior art attempts at non-slip screwdrivers in that the strength is not appreciably
impaired, and the screwdriver tip can be adequately sharpened, when required. In order
to achieve sharpening, the end face can be ground flat and the groove can then be
adjusted by grinding or, alternatively, simply by means of a small circular sectioned
file where manual sharpening is effected. In view of the fact that no external projections
or the like are required, the screwdriver blade can last for a number of sharpenings,
in the usual , manner whilst operating effectively in use.
[0023] Manufacture is also simple.
1. A screwdriver having a blade defined by two convergent, flat, major faces terminating
in a straight transverse, operative blade tip having a predetermined thickness and
wherein a groove is provided adjacent the edge of the blade tip in each of the two
major faces thereof, the groove extending across the entire blade tip width in each
case and having a maximum depth chosen to provide a minimum thickness to the blade
of between 75% and 95% of said predetermined thickness at the position of maximum
depth of the grooves, said position of maximum depth being located at a distance of
from 40% to 120% of said predetermined thickness from the edge of the blade tip, the
grooves being arcuate in cross-section and defining, with the adjacent end face of
the blade tip, an acute angle in cross-section.
2. A screwdriver as claimed in Claim 1 in which the said minimum thickness of the
blade is between 80% and 90% of the said predetermined thickness of the blade tip.
3. A screwdriver as claimed in either of claims 1 or 2 in which the position of maximum
depth of the grooves is located at a distance of from 40% to 100% of said predetermined
thickness from the edge of the blade tip.
4. A screwdriver as claimed in Claim 3 in which the position of maximum depth of the
grooves is located a distance of from 50% to 80% of said predetermined thickness from
the edge of the blade tip.
5. A screwdriver as claimed in any one of the preceding claims in which the grooves
are of part circular shape in cross-section.
6. A screwdriver as claimed in Claim 5 in which the radius of curvature of said part
circular shape is from 1,5mm to 5mm.
7. A screwdriver as claimed in Claim 6 in which the said radius of curvature is from
2mm to 4mm.
8. A screw driver as claimed in Claim 1 and substantially as herein described or exemplified
with reference to the relevant accompanying drawings.