GB2042948A

GB2042948A - Blade Sharpening Device

Info

Publication number: GB2042948A
Application number: GB7907772A
Authority: GB
Original assignee: Parr W G
Current assignee: Parr W G
Priority date: 1979-02-19
Filing date: 1979-03-06
Publication date: 1980-10-01
Also published as: IT7920812U1; JPS55125955A; IT7920812V0; FR2452998A1; GB2042948B; BR8000930A; DE2920648A1

Abstract

Apparatus for sharpening blades in which the blade 50 is reciprocated against guides 5, 7, 9 and sharpened by a movable, spring-mounted, flat- faced abrasive stone 23. A preferred mounting employs leaf springs 15, 17 pre-curved to restrict stone rotation relative to the guides. Means are described for setting the stone to produce an edge square to the blade. <IMAGE>

Description

SPECIFICATION Blade Sharpening Device This invention relates to a device for sharpening blades, for example the blades of certain woodworking tools having a cutting edge defined by the intersection of two ground surfaces.

Sharpeners in which the cutting edge of a tool, for instance a chisel, is reciprocated against a resiliently mounted abrasive stone have been proposed. Such devices are generally constructed so as to be able to accommodate several width of blade. When they are used with blades of less than the maximum width, problems may arise owing to the tendency of the abrasive stone to twist on its mounting, which leads to ground surfaces which are not plane. Also, unless the components of the device are manufactured to a high standard of precision, small cumulative irregularities can lead to the mounting of the abrasive stone being out of true in relation to the tool.

The invention seeks to provide a device in which the above problems are reduced or eliminated, which is economical to produce, and which is easy to use by persons unskilled in the use of an oilstone.

According to the present invention there is provided a blade sharpening device comprising a support having a plurality of substantially coplanar surface elements against which the blade may be reciprocated, guide means associated with said surface elements for assisting in preventing the blade from rotating about axes perpendicular to the plane defined by said surface elements, an abrasive surface at an angle to said plane, said abrasive surface being carried on a frame resiliently mounted on the support by two or more separated precurved leaf springs arranged to urge the frame to bring said abrasive surface towards said surface elements, to permit movement of the frame having a component perpendicular to said plane and to provide resistance to torsional movement about the axis of reciprocation of the blade.

The aforementioned tendency of the stone to twist when sharpening blades of less than the maximum width is due in part to reaction forces between blade and abrasive surface which are offset from the forces due to spring deflection: the greater the deflection, the greater the tendency to twist. If the leaf springs employed to mount the frame resiliently to the support are initially flat, then, when the spring deflection and the resulting spring forces are low, the springs will be near-flat providing high resistance to twist. As the deflection increases, the twist will be increased both by the increase in the forces exerted by the springs and by the decrease of twist resistance as the springs become more curved.It is therefore desirable to employ pre-curved leaf springs providing comparatively low twist resistance at low spring deflections and higher resistance at high deflections, the spring profile being chosen to restrict variation of twist within the working range of spring deflections.

Preferably, the pre-curvature should be chosen so that the spring profile at all stages of deflection remains sufficiently undulatory to avoid unduly high torsional stiffness and consequent unduly low twist. Preferably, the maximum available stiffness to torsional deflection of the frame relative to the support about the axis of reciprocation of the toll should be less than 30 times the torsional stiffness provided by the undeflected springs.

For convenience of design, manufacture and assembly, the springs may be made substantially symmetrical so that the two ends are interchangeable. Conveniently, such a substantially symmetrical spring may have a substantially flat central portion and end portions of substantially constant opposite curvatures. If said substantially constant curvatures are confined to the portions of spring close to the end, the torsional stiffness at certain low deflections may be too high and that at high deflections too low. If the length of the substantially flat central portion between the symmetrical constant curvature ends is around 43% of the length of spring between anchorages, the springs may become near-straight at certain deflections, providing extremely high torsional stiffness: the stiffness at low deflections will be low.

Advantageously, the substantially straight central portion of a leaf spring between symmetrical end portions of substantially constant equal and opposite curvatures comprises between 55% and 70% of the length between anchorages.

The invention also provides a method of making a sharpening device which comprises providing a support, a frame carrying an abrasive surface, and a resilient mounting linkage, loosely attaching the frame and/or the support to the linkage, aligning the abrasive surface with the support, and then fixedly attaching the mounting linkage.

The resilient mounting linkage may be any of a variety of mechanisms, e.g. a leaf spring in conjunction with a pivoted arm, a pivoted arm parallelogram linkage with a separate spring, a long spring-loaded pivoted arm, a spring-loaded carriage running on guide members, or preferably two or more leaf springs, especially two or more pre-curved leaf springs as mentioned above.

The frame or the support may be loosely attached to the mounting mechanism by any suitable means. For example, a bolt may be passed through suitable holes in abutting surfaces and secured by a partially tightened nut. After alignment the nut may be fully tightened to effect fixed attachment. Advantageously however a progressively expanded blind rivet (e.g. a 'Pop' rivet) may be used, at first partially expanded, permitting deliberate alignment but holding the aligned condition against normal disturbances, and then after alignment further expanded to fix the parts together.

The use of the method of the invention enables components to be used whose manufacturing tolerances are too poor to be used in a device having no facility for alignment. This considerably reduces the cost of manufacturing the device.

Advantageously, the sharpening device is provided with simple means for indicating when the abrasive surface and the support are in a satisfactory alignment. Conveniently, said indicating means comprises two first surfaces attached to the support each with a corresponding second surface attached to the abrasive surface arranged so that when each of the first surfaces touches its corresponding second surface, the abrasive surface is satisfactorily aligned with the support.

Advantag'eously, the first surfaces are extended so that the abrasive surface can be guided towards alignment by constraining each of the second surfaces to slide along its corresponding first surface.

The abrasive surface may be of any suitable material but is advantageously the flat surface of an oilstone.

The support may be of any materials and form of construction but is advantageously of a one piece construction in a plastics material.

A blade sharpener constructed in accordance with the present invention will now be described with reference to the accompanying drawings by way of example only and is in no way limitative of the invention. This particular embodiment is intended for sharpening wood chisels. It is described as positioned for normal operation but could of course be used in other positions.

Figure 1 is a cross-section in sideview of the exemplary blade sharpener.

Figure 2 is a side elevation of that part of the said sharpener which supports the abrasive stone.

Figure 3 is a plan view of the said sharpener.

Figure 4 is a side elevation of an alternative form of stone holder.

Figure 5 is a cross-section in sideview of the leaf spring linkage assembly shown in Figure 1.

The base of the sharpener comprises a hollow one-piece moulding 1 of an abrasion resistant plastics material, open at the bottom and at one end, generally rectangular in form and symmetrical about a central, longitudinal vertical plane surface of symmetry. An outwardly extending stiffeningwflange 3 is provided with holes for screw attachment to a bench or other suitable support. At the top of the moulding is a striaght, shallow, longitudinal guide channel 4 which is shorter than the length of the blade of any chisel to be sharpened, is slightly wider than the widest of these blades and has parallel vertical side wails 5. The chisel, supported by the two coplanar horizontal platforms 7 and 9, is reciprocated against one side wall 5 of channel 4.

The blade of the mortise chisel 50 shown in the drawings has a width slightly less than half that of the channel and is reciprocated against its left hand side, viewed from beyond the chisel handle.

The inner vertical plane face of the end wall of the moulding which is perpendicular to said surface of symmetry serves as an abutment for a riveted linkage assembly 10 comprising two similar channel-section brackets 11 and 13 and two similar pre-curved leaf springs 1 5 and 17, the webs of the two brackets in the assembled linkage being parallel. Bracket 13 is attached to carriage 1 9 by two blind rivets: bracket 11 is attached to the base moulding 1 by a single, larger blind rivet 43. The mode of assembly will be described later.

The carriage 19 which lies symmetrically about said surface of symmetry is made by folding a single plane sheet metal blank and comprises a vertical rectangular web perpendicular to said surface of symmetry and riveted to bracket 13 and two plane parallel vertical flanges 21 projecting from the sides of the web and partly overlapping the base moulding. The flanges extend upwards beyond the top of the base moulding and support a sharpening stone assembly comprising a sharpening stone 23 with a plane abrasive surface 24 mounted between left and right hand stone holders 25 and 27. The assembly pivots on a transverse hexagon-headed clamping bolt 29 of which the unthreaded portion passes in turn through a shakeproof washer 31, through a hole in one of the flange extensions 21, through pivot holes in the left and right hand stone holders and through a hole in the other flange extension.The threaded end portion of the bolt passes through a tubular spacer 33 to engage a wingnut 35. The reversible sharpening stone 23 made from bonded alumina granules has two square parallel faces and narrow rectangular edges perpendicular to these faces so that it can occasionally be rotated through 90 to even out wear on the abrasive face.

Each of the stone holders 25 and 27 is a onepiece moulding made from a plastics material and has a plane, vertically extending outer face bearing against the inner face of one of the carriage flanges 21, an inner face, parallel to said outer face, bearing against one of said narrow rectangular edges of the stone, a narrow, shallow rib below the stone with its upper surface closely fitting against the edge portion of the lower face of the stone adjoining said narrow rectangular edge, an inwardly-projecting channel-section portion above the stone with its downwardly extending flanges closely fitting against edge portions of the upper face of the stone and with its inward boundary short of the said surface of symmetry and a pivot hole near one of the flanges, perpendicular to said outer face and placed so that the pivot bolt can pass inside the channel section portion above the stone. The exposed lower face of the stone between said narrow shallow ribs extends beyond the inner faces of the side walls 5 of channel 4 in base moulding 1.

The flanges 21 are flexible so that when wingnut 35 is tightened, the carriage, the stone holders and the stone are securely clamped together. The lower abrasive surface 24 of the clamped stone is then perpendicular to said surface of symmetry and is inclined to the horizontal. This inclination determines the sharpening bevel angle of the chisel which may be indicated by means of a scale 37 on one of the stone holders and a pointer 39 on the corresponding flange of the carriage (Fig. 2).

The leaf spring linkage is arranged to press carriage 1 9 downwards. In the absence of a chisel the bottom portion of at least one of the stone holders 25 and 27 touches one of the two symmetrical plane ramps 41 set at the end of the side walls 5 of guide channel 4 and inclined to the horizontal at an angle just exceeding the design range of sharpening angles. The lower portions of the stone holders which touch these ramps are curved in such a way that in the absence of a chisel the height of the carriage is least when the pointer is against the middle of the scale and is close to this minimum throughout the angular range of the scale. The stone holders and ramps are arranged to keep the operative face of the stone clear of platform 9.

Figure 4 shows an alternative, symmetrical moulding, with a pivot hole and a rounded lower portion at each end, which can serve as either a left-hand or a right-hand stone holder. The scale, at one end, is visible when the moulding is used as a right-hand stone holder but is concealed by the carriage flange when used as a left-hand stone holder.

For convenience of description, the sharpener has been described as largely symmetrical about a central vertical plane surface of symmetry with the side walls 5 of guide channel 4 parallel to said surface and with platform 7 and 9 and the face of stone 23 perpendicular to said surface. In practice, if bracket 11 is symmetrical about said surface of symmetry, imperfections in the linkage assembly 10, in carriage 19, stone holders 25 and 27, bolt 29 and in stone 23 and misalignments between these components will ensure that the operative face of the stone is not perpendicular to said surface of symmetry.

To allow the operative face of the stone to be set near-perpendicular to said surface of symmetry, the inner face of the end wall of base moulding 1 has a substantially plane surface, the abutting face of bracket 11 has a smaller, substantially plane surface and the bracket is joined to the moulding by a single blind rivet 43 of known type with a shank which can be progressively expanded, starting at the end remote from the head by withdrawing a preassembled internal mandrei. Rivets of this type are made by Tucker Fasteners Ltd. under the Registered Trade Mark 'Pop'. After the sharpening stone assembly and the linkage assembly have been attached to carriage 1 9, bracket 11 of the linkage assembly can be drawn close against the inner abutment face of base moulding 1 by placing and partially clenching rivet 43.This bracket is rotated until face 24 is properly aligned then locked in piace by fully clenching rivet 43.

To simplify judgement of proper alignment the shape of stone holder 27 is made a mirror image of stone holder 25 and stone 23 is a close fit in its holders. Then, provided the components are reasonably accurately manufactured and assembled, the operative face of the stone will be near-perpendicular to said plane symmetry when stone holders 25 and 27 both touch their corresponding ramps 41.

In a simplified assembly procedure, the stone assembly and linkage are fixed to the carriage preferably with the scale set at mid-range. A fairly long blind rivet 43 is placed through holes in moulding 1 and bracket 11, secured by expanding its tip portion, then fully clenched while both of the stone holders 25 and 27 are heid lightly against ramps 41. The holders slide along the ramps as bracket 11 is drawn towards the abutment face of base moulding 2. Friction between bracket 11, base 1 and rivet 43 and torsional flexibility in the linkage may cause bracket 11 to assume an incorrect position so that one of the stone holders moves away from its ramp when released. If the movement is small, the stone alignment may still be acceptable.

Figure 5 shows the linkage assembly 10 removed from the sharpener. Springs 1 5 and 1 7 are made from 0.315 mm thick spring steel; brackets 11 and 13 from 1.63 mm thick mild steel. The central 65% of the portion of each leaf spring between the brackets is straight: the end portions between brackets have equal and opposite constant curvatures. As bracket 1 3 is displaced upwards relative to bracket 11 , the curved portions of the springs become straighter, increasing their lateral stiffness and the torsional stiffness of the linkage, but the initially straight portions become curved tendingto reduce torsional stiffness.The torsional stiffness increases with relative displacement until the relative displacement is about 70% of the initial offset BB' and then falls, but remains fairly high up to the maximum working displacement which is equal to BB. At the displacement giving maximum torsional stiffness, the end portions of the springs retain part of their original curvature and the central portion is curved so that the torsional stiffness of the linkage is well below that obtainable with flat springs.

The line of action of the download on the sharpening stone assembly due to the leaf springs and to the weight of carriage 1 9 and its attachments will be close to said central vertical surface of symmetry. When stone holders 25 and 27 have been lifted clear of the base by an offset blade such as 50 (Fig. 3), this download and the upward reaction of the chisel against face 24 will form a couple tending to twist stone 23 and carriage 1 9 against the torsional stiffness of linkage 10.

To set the sharpening angle, wingnut 35 is slackened, scale 17 set to the required angle and the wingnut retightened. One setting can serve for many sharpenings. The chisel is held, touching the stone, with the flat of the blade against platforms 7 and 9 and one side of the blade against one of the side walls 5 of guide channel 4 and is reciprocated backwards and forwards. As it moves forwards, carriage 1 9 is displaced upwards allowing the blade to slide across the abrasive surface; as it moves backwards, the resilient linkage keeps the stone pressed against the blade abrading it further. Most modern chisels have accurately ground flats and edges so that the reciprocatory motion will be substantially linear.

The recess between platforms 7 and 9 ensures a controlled reciprocatory motion if the chisel blade is slightly curved.

Claims

1. A blade sharpening device comprising a support having a plurality of substantially coplanar surface elements against which the blade may be reciprocated, guide means associated with said surface elements for assisting in preventing the blade from rotating about axes perpendicular to the plane defined by said surface elements, a substantially flat abrasive surface at an angle to said plane, said abrasive surface being carried on a frame resiliently mounted on the support by two or more separated pre-curved leaf springs arranged to urge the frame to bring said abrasive surface towards said surface elements, to permit movement of the frame having a component perpendicular to said plane and to provide resistance to rotation of the frame relative to the support about the axis of reciprocation of the blade.

2. A blade sharpening device according to Claim 1 wherein said pre-curved leaf springs are of substantially constant thickness.

3. A blade sharpening device according to Claim 2 wherein said pre-curved leaf springs have an undulatory profile in all working positions of said frame relative to said support.

4. A blade sharpening device according to Claim 3 wherein said precurved leaf springs restrict variation of the rotation of said frame relative to said support during the operation of the sharpener.

5. A blade sharpening device according to Claim 4 wherein said leaf springs are preformed to have a substantially flat central portion.

6. A blade sharpening device according to Claim 5 wherein the length of said substantially flat portion is between 55% and 70% of the length of said leaf spring between anchorages.

7. A method for manufacturing a blade sharpening device, said device comprising a support provided with guide surfaces against which the blade may be reciprocated, an abrasive surface mounted on a frame and resilient linkage means substantially rigidly attached to said support and to said frame, wherein said linkage means is first rotatably connected to said support and is subsequently substantially rigidly attached to said support.

8. A method for manufacturing a blade sharpening device, said device comprising a support provided with guide surfaces against which the blade may be reciprocated, an abrasive surface mounted on a frame and resilient linkage means substantially rigidly attached to said support and to said frame, wherein said linkage is first rotatably connected to said frame and is subsequently substantially rigidly attached to said frame.

9. A method according to Claims 7 and 8 wherein said substantially rigid attachment employs co-operating screw threaded members.

10. A method according to Claims 7 and 8 wherein said substantially rigid attachment employs one or more rivets.

11. A method according to Claim 7 wherein a liquid synthetic resinous material solidified in engagement with said linkage means and with said support.

1 2. Blade sharpening devices substantially as hereiVefore described.