The invention relates to a hand-operated power tool, and especially, but not solely a circular saw.

A number of hand-operated circular saws are available which are operated by an electric motor. The most common type use circular saw blades greater than 125mm in diameter for cutting. This type is driven by an electric motor on the same axis as the axis of rotation of the blade. Normally there is a base plate or pressure plate, with adjustable height which is placed upon the wor piece. The cutting is done by a portion of the blade that protrudes below the base plate when the tool is manually displaced in the cutting direction. For correct operation this conventional circular saw requires to be held with two hands.

A more modern variation of the above tool uses an adjustable base plate which is sprung with respect to the main case and attached by a pivot point. This type of tool has the advantage that the base plate can be placed securely onto the workpiece before the blade starts to cut, giving improved control, also enabling cut-outs to be made.

However, problems are encountered with conventional circular saws when used on thin materials, small parts or for grooving are:

(i) The blade is offset to one edge of the tool and the tools are cumbersome with large base plates making accuracy, intricacy and manoeuvrability difficult.

(ϋ) Both hands are required to manoeuvre the tool into position and operate, which requires that in a lot of cases, the workpiece is clamped. This presents problems with small parts and can be time consuming.

(iϋ) The workpiece must be large enough and rigid enough to support the saw.

Electrically operated jig saws are known. These are generally smaller and can be operated with one hand. The cutting is done by a thin blade that protrudes below a base plate which is placed onto the workpiece. However, these also have disadvantages of which the two main disadvantages are:

(i) Although excellent when making intricate cuts, they make slow, poor quality straight cuts.

(ϋ) As the blade must protrude significantly below the workpiece it cannot be fully supported causing vibration and clamping problems especially when cutting thin materials or small parts.

Sawing attachments are also available for connection to other types of tools, such as electric drills. However, sawing attachments which are operated by an electric drill or other form of drive, normally operate at a lower speed and higher torque than specialised

tools, which results in reduced efficiency.

A wide variety of electric motor driven hand saws for model making exist. They have smaller motors and are of a simpler construction than the tools mentioned previously and are normally only suitable for cutting very thin materials.

In accordance with a first aspect of the present invention, a hand-operated power tool comprises a motor; a cutting element driven by the motor; a pressure plate adjacent the cutting element for engagement against a surface to be cut; a handle for the user, the handle being fixed in use with respect to the motor and the cutting element; and a biasing device acting on the pressure plate to bias the pressure plate from a second position to a first position with respect to the motor and the cutting element; and in which the pressure plate is disposed at an acute angle to the handle and is moveable, from the first position to the second position against the action of the biasing device, with respect to the handle at a constant angle to allow a desired amount of the cutting element to be exposed below the pressure plate.

In accordance with a second aspect of the present invention, a hand-operated power tool comprises a motor, a cutting element driven by the motor, a pressure plate adjacent the cutting element for engagement against a surface to be cut, and a handle for the user; the handle being fixed in use with respect to the motor and the cutting element, and the pressure plate being movable in use with respect to the motor and the cutting element; and in which the pressure plate forms part of a guard enclosing the cutting element and is movable against a biasing force

with respect to the handle, to allow a desired amount of the cutting element to be exposed below the pressure plate, there being no exposed cutting element in the absence of user applied pressure.

In accordance with a third aspect of the present invention, a hand-operated power tool comprises a motor, a cutting element driven by the motor, an elongate handle for the user, and a pressure plate positioned adjacent the cutting element to allow the user to urge the cutting element into predetermined engagement with a surface to be cut; the cutting element being driven for cutting in a plane parallel with the longitudinal axis of the handle and within the lateral extent of the handle.

In accordance with a fourth aspect of the present invention, a hand-operated power tool comprising a motor, a cutting element driven by the motor, a pressure plate adjacent the cutting element for engagement against a surface to be cut, and an elongate handle for the user; the handle being fixed in use with respect to the motor and the cutting element and the pressure plate being movable in use with respect to the motor and the cutting element; the pressure plate being disposed at an acute angle to the handle and movable against a biasing force with respect to the handle at a constant angle to allow a desired amount of the cutting element to be exposed below the pressure plate, there being no exposed cutting element in the absence of user applied pressure; the cutting element being driven for cutting in a plane parallel with the longitudinal axis of the handle and within the lateral extent of the handle.

Preferably, the power tool is an electrically operated power tool. The tool may be powered by batteries.

Typically, the power tool includes a depth stop adjustable by the user to set the limit of travel of the pressure plate with respect to the handle.

Typically, the tool is of the type known as a circular or rotary saw, and the cutting element is a circular cutting disc.

Preferably, the pressure plate cooperates with a cutting element guard to surround the cutting element such that, when no pressure is applied against said biasing force, the cutting element is fully enclosed.

Preferably, the motor is mounted within the handle.

Preferably, the handle is at an oblique angle to the surface which the saw is cutting, and typically, may be at an acute angle. Typically, the acute angle is substantially 45° to the surface, in use. Preferably, the handle extends in a direction away from the pressure plate and opposite to the cutting direction of the power tool.

Preferably, a plane defined by the handle and at 90° to the surface is substantially parallel to the cutting plane of the cutting element.

Preferably, the guard includes indication means to indicate the length of cutting element exposed when the guard is retracted. The indication means may be provided by indicia on a member which is stationary with respect to the cutting element, the indicia cooperating with the guard to indicate the depth of

extension of the cutting element below the pressure plate.

Alternatively, or in addition, where the cutting element is circular, the guard may indicate the depth of cutting element exposed when the guard is in the retracted position. Typically, indicia are provided on a member which is stationary with respect to the cutting element, the indicia cooperating with the guard to indicate the chord length of that part of the cutting element which is exposed below the pressure plate.

Preferably, the exterior of the guard is provided with means indicating the plane of the cutting element.

Typically, the pressure plate moves relative to the handle on guides which are disposed on the centreline of the handle or symmetrically with respect thereto.

Preferably, the power tool includes a guide attachment selectively mountable on the pressure plate, the guide attachment having an upper surface for abutment against the pressure plate and a lower surface in the form of a "V", the apex of the "V" forming a line which is transverse to the cutting direction of the tool for engagement with a pipe or other cylindrical body.

Typically, the cutting element, the handle, and a control switch for the motor are so positioned relative to each other as to enable the tool to be used with one hand.

Preferably, different sizes of cutting elements may be used with the power tool.

Preferably, all aspects of the invention are combined into a hand-held electrically operated power tool, which is preferably a circular saw.

An example of a hand-operated power tool in accordance with the invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 shows an external right side view of a circular saw; Figure 2 shows an external front view of the circular saw; Figure 3 shows a sectional right side view of the circular saw; Figure 4 shows a sectional front view of the circular saw; Figure 5 shows a side view of a left gearbox casing for the saw; Figure 6 shows a front view of the left gearbox casing; Figure 7 shows a front view of a right gearbox casing for the saw; Figure 8 shows a side view of the right gearbox casing; Figure 9 shows a sectional view of a driving gear for the saw; Figure 10 shows a blade shaft with gear and bearings for the saw; Figure 11 shows shim washers for the saw; Figure 12 shows driven gear for the saw; Figure 13 shows the blade or cutter shaft for the saw; Figure 14 shows the blade or cutter retaining nut for the saw; Figure 15 shows an internal side view of a left hand main case moulding for the saw; Figure 16 shows an external front view of the left hand main case moulding;

Figure 17 shows an external front view of a right hand main case moulding for the saw; Figure 18 shows an internal side view of the right hand main case moulding; Figure 19 shows a side view of a right hand guard moulding for the saw; Figure 20 shows a left guard moulding for the saw viewed from the front; Figure 21 shows the right guard moulding viewed from the front; Figure 22 shows a plan view of the left hand guard moulding; Figure 23 shows a plan view of the right hand guard moulding; Figure 24 shows a side view of a base plate for the saw; Figure 25 shows a plan view of the base plate; Figure 26 is a partial side view of the saw showing a guide attachment attached to the saw; and, Figure 27 is a partial front view of the saw showing the guide attachment.

When the terms left or right are used throughout this description it is assumed that the tool is being viewed from the front as in figure 2. Some hidden detail has been omitted from certain drawings for clarity.

A hand-operated circular saw comprises a main case consisting of two plastic mouldings 50,51. Figures 15 and 16 show a left hand moulding 50. The moulding includes motor cooling vents 5 and hollow moulding extensions 36, through which fasteners for the case pass. Ridges 37 are used to mount the motor in position. Figures 18 and 19 show a right hand case 51. A hole 38 in the moulding 51 is used to assist in blade changing as described later.

Figure 1 shows an external right view of the circular saw. Figure 2 shows an external front view. Case fixing holes 4 are shown through which case fasteners pass. The case fastners may be machine screws or self tapping screws. A rubber grommet 3 is provided though which a power supply cable passes. The saw also includes a motor on/off switch 1, a cutting depth adjuster 2 and a base plate (or pressure plate) 7 that is held in contact with the workpiece when in use. A handle 14 formed by mouldings 50, 51 is angled backwards with respect to the base plate 7. This enables a user to apply downward pressure and forward pressure for feed while holding the saw by the handle 14.

A blade guard 6 encloses the saw blade (or cutting element) 23 so that the saw blade does not protrude from the main case when not in contact with the workpiece. The guard 6 is sprung with respect to the main case which is described later.

A depth scale 10 is shown dotted as it is actually marked on the main left hand main case moulding 50. As the guard 6 is depressed the top edge of the guard, shown dotted, moves up the scale until it reaches a point set by the depth adjuster 2. The actual depth of the blade 25 that has penetrated a workpiece can now be read from the scale 10.

Vertical lines 9 are marked on one side of the guard 6. Arcs 8 are marked on the main case. One arc 8 is provided for each size of blade or cutter 23 to be made available for the saw. Each arc 8 has the same radius and centre point as the corresponding blade 23. As the guard 6 is depressed the arcs will meet with one pair of vertical lines at a time, starting with the

innermost lines. This provides indication of the front and rear intersection points of the blade or cutter 23 and a workpiece. This feature is particularly useful when making cut-outs as it allows the operator to see the length of cut.

Referring as well to figures 19 to 23. Figure 19 shows a right hand guard moulding 52. Figure 20 shows a front view of a left hand guard moulding 53. Figure 21 shows a front view of the right hand moulding 52. Figures 22 and 23 show plan views of the mouldings 53, 52 respectively. A tab 12 is provided on the moulding 52 which is used to cover the hole 38 in use. Hole 38 is necessary to allow access to blade shaft 20 and blade retaining nut 24. When mouldings 52, 53 are joined together, they form the guard 6. Tabs 11 are provided on the guard 6 and are pressed outwards to enable the guard 6 to be removed for blade changing. A pointer 13 is in line with the blade 23 enabling the user to follow a marking out line on a workpiece.

Figure 24 is a side view of the base plate 7. This part is made from steel. A groove 41 accepts a bottom ridge 54 of the guard mouldings 52, 53 for attachment. The edges of the base plate 7 are parallel to the blade or cutter 23 allowing the tool to be guided by a straight edge. Figure 25 shows a plan view of the base plate 7. A slot 39 is provided through which the blade 23 passes and holes 40 are provided into which guard guides 25 (see figure 3) are brazed or pressed. The guard guides 25 are made from ground steel and pass through phosphor bronze bearings 26. The plunge action of the guard is achieved by the use of a spring 27 which acts against the rearmost guard guide 25. The threaded depth adjuster 2 acts on the foremost guard guide 25.

The bearings 26 are held in place inside a gearbox casing 16 which comprises two sections 55, 56. These can be seen in greater detail in figures 5 to 8. They are cast in aluminium alloy.

Referring back to figures 3 and 4. A 400W motor 15 is located inside the handle section 14 of the mouldings 50, 51. A shaft extension 28 is secured to a motor shaft 57 by set screws 29. A phosphor bronze thrust bearing 17 and a needle roller bearing 18 mount shaft extension 28 in the casing 16.

Attached to the end of shaft 28 is a cross axis helical driving gear 21 which drives a cross axis helical driven gear 22 with approximately twice the number of teeth as the gear 21. Figure 12 shows that half the bore of gear 22 is threaded, the thread is left hand to prevent loosening during operation. The other half of the bore is smooth and accurately machined so as to provide location onto driven shaft 20 (see figure 13). Surfaces 30, 31 support ball bearings 19. Two shim washers 32 space the ball bearings 19 from gear 22. Figure 10 shows the driven shaft 20 with the gear 22, bearing 19 and shim washers 32 assembled onto it. The outer surfaces of the bearing 19 locate into the gearbox casing 16. A ridge 57 on the right hand gearbox casing 56, figure 7, prevents axial movement of the shaft 20 in a right hand direction by preventing movement of fixed outer ring of the right hand bearing 19. Axial movement in the left direction is prevented by the outer case.

A locating surface 63 is provided on the shaft 20 for locating the saw blade or cutter 23. The blade retaining nut 24 is attached to the driven shaft 20 via a left hand thread.

The blade 23 is changed by removing the guard 6, as described earlier, and inserting a purpose made spanner into two small holes 58 in the retaining nut 24 (see Figure 14) . A key is then inserted through a hole in the spanner into a hexagon or splined indentation 35 in the end of shaft 20, to lock shaft 20. By turning the key in the opposite direction to the spanner the retaining nut can be removed and the blade replaced through a slot in the bottom of the right hand case moulding 51.

In figures 3 and 4 it can be seen that blade 23 is enclosed in a compartment sided by internal webs of the outer case mouldings 50, 51 and by the right gearbox casing 56. This helps to prevent dust entering the motor 15.

The blade 23 would not normally be shown in such sectional drawing such as figure 3. However, it has been included to permit easier interpretation of the drawing.

The saw blade or cutting disk 23 rotates in the counter run direction to the feed direction in order to pull the workpiece into the base when cutting, as is standard practice with electrically operated circular saws.

An alternative arrangement could have a motor with a shaft in line with the blade shaft and offset, driven by gears or belt and with a separate handle at the same angle as in the example previously described. The guard, blade indication and plunge action would remain identical.

The driving motor could be in a separate casing

allowing it to be detached and used for driving other tools.

It would also be possible to provide for attachment of "V" shaped block 42 (see Figures 26 and 27) to the bottom of the base plate 7. This would permit the saw to be more easily used for cutting circular or polygonal cross-sectional objects.

Advantages of the invention over existing hand held circular saws are:

Increased accuracy, intricacy, and manoeuvrability when cutting thin or flexible materials, grooving or making cut-outs. This allows materials normally beyond the scope of such tools to be cut e.g. small parts, veneers, sheet metals, sheet plastics.

As one hand is free to hold the workpiece less clamping is required.

Safety is improved due to the small enclosed blade.

Some advantages over existing hand held jig saws are:

Increased speed and efficiency with an improved finish.

The workpiece can be fully supported allowing materials normally beyond the scope of such tools to be cut e.g. small parts, veneers, thin sheet metals and plastic.

The ability to cut grooves.

Safety is improved.

Some advantages over existing model making tools are:

Increased power allowing thicker and tougher materials to be cut, speed of cutting to be increased.

More control and ease of use.

The main features which bring about these improvements are:

One hand operation.

The small size incorporating a motor with power usually reserved for larger tools.

The size, shape and angle of the handle.

The small size of the base plate.

The sides of the base plate are parallel and close to the blade allowing a hand held guide to be followed.

The plunge action that does not effect the angle of the handle.

The blade when viewed on edge, as in figure 2, is close to the centre line of the tool.

An additional advantage is that a vee-block 42 may be mounted to the base plate as an accessory for cutting tubes.