GB2182597A - Variable rate cutting tool - Google Patents

Abstract

A hand-operated cutting tool (e.g. secateurs) with variable cutting rate blade action comprises a handle 3 disc 5 rotatably mounted on one end. Disc 5 contacts the upper edge of blade 4 which is so shaped to give the required cutting rate of blade 4 at any position within the cutting stroke. Squeezing the handles 2, 3 to their closed position causes disc 5 to move blade 4 to its closed position where it co-operates with fixed blade 13 to sever the item to be cut. <IMAGE>

Description

SPECIFICATION Variable rate cutting tool This invention relates to a hand operated cutting tool (e.g. secateurs) with variable cutting rate blade action.

Secateurs are well known hand operated gardens pruners. Priorartsecateurs have included various blade shapes to give improved cutting efficiency. However, the single pivot point between the two handle/blade components, limits the variation possible to the rate of cut throughoutthe cutting stroke. Also, the rate of cut normally increases (and the cutting force decreases) through the cutting stroke. Therefore the rate of cut cannot be matched to suit the item to be severed. Prior art cutters also suffer loss of mechanical advantage as larger items are cut. This is because larger items are forced to a position between the bladesfurtheraway from the pivot. Mechanical advantage cannot be regained by extending the handles without requiring two handed operation of the cutters.

This invention features the provision of a hand-held and hand-operated cutting tool with variable cutting rate blade action. The cutting force at any position within the cutting stroke can be matched to suit the cutting resistance for a specified maximum cut. Provision is also made that partial blade closing can be effected by an initial handle closing,then after re-opening the handles and actuating a switch, blade closing can becompleted by a repeated handle closing.

The tool (see Figure 1 ) comprises stationary handle 2, movable handle3 and blade 4. Disc 5 is rotatably mounted on the outer (non-handle) end of movable handle 3. Movable handle 3 is pivotallymounted by bolt6 on stationary handle 2. Blade 4 is pivotally mounted by pin 7 on stationary handle 2. Blade 4 has a sharpened lower edge which co-operates with fixed blade 13 mounted on the outer end of stationary handle 2, to cutthe item to be severed. The upper edge of blade 4 contacts the circumferential edge of disc 5. Action to move movable handle3from an open to a closed position causes blade 4to move from its open to its cut-off position. The upper edge of blade 4 is so shaped to provide the desired cutting rate at any position within the cutting stroke.

Means is provided to bias movable handle 3 to return to its open position. Means is provided to secure movable handle 3 in its closed position when the tool is not in use. Means is provided to bias blade 4 such that the upper edge of blade 4 remains in contact with disc5. Means is provided to guide blade 4 such that it remains in planewithfixed blade 13.

Provision is made (see Figures 3-6) for an alternative (stepped) disc5', having a sector of large diameter and a sector of small diameter. Projecting tabs divide the sectors. Selecting the small circumferential edge of disc 5' to contact blade 4causes partial closing of blade 4when movable handle3 is moved from its open to its closed position. With movable handle 3 returned to its open position and large circumferential edge of disc 5' selected to contact blade 4, a repeated closing of movable handle 3 will move blade 4 to its cut-off position.

Specific embodiment ofthe invention will now be described byway of example with reference to the accompanying drawings. Corresponding reference characters indicate corresponding parts throughoutthe several views ofthe drawings.

Figure lisa side elevation view of a first embodiment (two-bladed version) of a cutter of this invention showing the cutter in the open position. Fixed blade 13 has a sharpened edge.

Figure2is a viewsimilarto Figure 1 showing the cutter in the closed position.

Figures 3-6show side elevation views of a second embodiment (repeated cut version). Figure 3 shows the cutter in the open position. Figure 4 shows the cutter in the intermediate position on completion ofthefirst cut. Figure5 shows the cutter at the start of the second cut and Figure 6 shows the cutter in the cut-off position on completion of the second cut. This second embodiment is particularly suited for operators unableto utilise thefull handle opening.

Figure 7 is a side elevation view of a third embodiment (single-bladed version) of a cutter of this invention showing the cutter in its open position. Fixed blade 13' is an anvil. Blade 4' ofthethird embodiment hasan upper edge profile designed to progressively reduce the rate of movement of blade 4' as movable handle 3 is moved from its open to an intermediate position, and as movement of movable handle 3 is continued to its close position, the rate of movement of blade 4' is progressively increased.

Figure 8 is a sketch showing the variables (moments of force, distances, angles) which may be selected to provide the desired cutting rate at any position within the cutting stroke.

Referring now to the drawings the tool may be constructed, byway of example, by cutting from sheet metal of suitable strength, duplicate copies of stationary handle 2, duplicate copies of movable handle 3, a single copy of blade4 (or4') and a single copy offixed blade 13 (or 13').

Blade 4(or 4') is pivotally mounted on pin 7 between the two copies of stationary handle 2. Fixed blade 13 (or 13') is retained between the two copies of stationary handle 2 by bolt6and bolt 14. One copy of movable handle 3 is pivotally mounted on each side of the above assembly by bolt 6. Disc5 (or 5') is rotatably mounted on pin 11 between the two movable handle components 3. Compression spring 8 and safety catch 10are mounted between stationary handle 3 and blade 4 (or4').

Stationary handle 2 and movable handle 3 are fitted with hand grips (not shown). Locable screw 12 is threaded into movable handle 3to allow adjustment of blade4 (or4') position at cut-off.

The shape ofthe upper edge of blade 4(or4') may be determined to give the desired cutting rate (and therefore cutting force) at any position within the cutting stroke by selecting and calculating variables within thefollowingformula (see Figure8).

N =1 + Tcos(C-A) M ScosA where M is the applied moment offorce at bolt6 due to force applied to move handles two their closed position.

N isthe eqdivalentcutting momentofforce at pin 7 causing blade4 (or4') to move towards its cut-off position.

S is the centreline distance between bolt 6 and pin 11.

T is the centreline distance between bolt 6 and pin 7.

A is the angle between the tangent-line at contact of blade 4 (4') and discS (or 5'), and the line connecting the centres of bolt 6 and pin 11.

Cisthe handle opening angle between the lineconnecting bolt6 and pin 11 and the line connecting bolt6 and pin 7.

Byway of example, when the ratio of T/S is 1/2 and angle C is varied from a maximum of 145to a minimum of 90", the ratio of cutting moment to applied moment (N/M) can be varied as follows by selecting values of angle A: C =145" A= 65 N/M = 1.2 C = 135" A = 75" N/M = 2.0 C = 120" A = 75 N/M = 2.4 C = 105" A = 80" N/M = 3.6 C= 90" A = 80" N/M = 3.8 These values correspond to those selected to generate the shape of the upper edge of blade 4forthefirst embodiment of this invention (two-bladed version).This demonstrates that the ratio of cutting momentto applied moment (N/M) can be varied by a factor of more than 3 over the cutting stroke.

The approximate values of C, A and N/M corresponding to those selected forthe third embodiment ofthis invention (single-bladed version) areas follows:- C = 145" A= 65" N/M = 1.2 C = 135" A = 80" N/M = 3.0 C = 120" A = 82" N/M = 3.8 C = 105" A = 75" N/M = 2.7 C = 90" A = 40" N/M = 1.4 In both the above examples a high value of the ratio of cutting momentto applied moment (N/M) cor responds to a low ratio of blade movement to handle movement. Similarly, a low value of N/M corresponds to a high ratio of blade movement to handle movement.

Claims (11)

1. A hand-held and hand-operated cutting tool with variable cutting rate blade action, comprising a mov able handle pivotally mounted on a stationary handle. A blade having a sharpened lower cutting edge is pivotally mounted on the stationary handle. A disc is rotatably mounted on the non-handle end of the mov able handle. The upper edge of the blade contacts the circumferential edge of the disc. Action to movethe movable handle from an open position to a closed position causes the blade to move from its open to its closed position, where the blade co-operates with a fixed blade on the non-handle end of the stationary handle to severthe item to be cut. The upper edge of the blade is so shaped to give the desired cutting rate at any position within the cutting stroke. Means is provided to bias the movable handle to return to its open position.Means is provided to secure the movable handle in its closed position when the tool is not in use.

Means is provided to bias the blade such that its upper edge remains in contact with the disc. Means is provided to guidethe blade such that it remains in plane with the fixed blade.

2. A cutting tool as setforth in Claim 1 wherein the fixed blade has a sharpened edge.

3. A cutting tool as setforth in Claim 2 wherein the upper edge of the blade is so shaped that the ratio of angular movement of the blade to the angular movement ofthe movable handle progressively decreases as the movable handle is moved from its open to its closed position.

4. A cutting tool as set forth in Claim 1 having a spring for biasing the movement of the movable handle to return to its open position.

5. A cutting tool as setforth in Claim 1 having a spring for biasing the movement of the blade such thatthe balde remains in contactwith the disc.

6. A cutting tool as set forth in Claim 1 having a means of adjusting the closed position of the movable handle to suit the co-operation of the blade and the fixed blade when the blade is in its closed position.

7. A cutting tool as set forth in Claim 1 wherein the fixed blade is an anvil.

8. Acutting tool as set forth in Claim 7 wherein the upper edge of the blade is so shaped that the ratio of angular movement of the blade to the angular movement of the movable handle progressively decreases as the movable handle is moved from its open to an intermediate position and as movement of the movable handle is continued to the closed position the said ratio progessively increases.

9. A cutting tool as set forth in Claim 3 wherein the disc is stepped such that one sector of the disc has a large diameter and one sector has a small diameter.

10. A cutting tool as set forth in Claim 9 such that with the small diametric edge of the disc selected to contact the upper edge of the blade, moving the movable handle from its open to its closed position will cause the blade to move from its open to an intermediate position. Upon returning the movable handletothe open position and selecting the large diametric edge of the disc to contact the upper edge of the blade, movement of the movable handle to the closed position will cause the blade to move from an intermediate to its closed position.

11. A cutting tool substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.