GB2205274A - Sawing machines - Google Patents

Abstract

A sawing machine for fretsaw work comprises a flat table 35 with a hinged extension 36 and a fixed arm 10 extending from the back 12 of the table. The saw blade 40 is held at the end of the arm and passes downwards through a slot 41 in the table. The blade is driven by a motor 45 mounted at the back of the table. The blade drive is by means of a cable 50 mounted on an eccentric 48, 49 on the motor and coupled to the blade by a chain 61 passing over a cogwheel 60; the bottom end of the blade is similarly driven by a cable 50' mounted on an opposed eccentric 48', 49' and coupled to a chain 61' passing over a cogwheel 60'. The cogwheel 60 is adjustable to adjust the blade tension. A spring may be included in the cable loop or one of cogwheels 60, 60' may be spring mounted. Safety interlock switch means may be provided to prevent motor 45 being switched on if an access plate 63 is removed or if the table extension 36 is not in its raised position. A pair of front legs (20, Fig 1) are fastened to a beam 19, and a rear leg (16) is fastened to a beam 15. The chains and cogwheels may be replaced by belts and pulleys. <IMAGE>

Description

Saw iri5 Ma c tu The present invention relates to sawing machines, and more specifically to fretsaw machines.

Fretsaw machines of the type to which the present invention relates comprise a horizontal worktable with a fretsaw blade passing vertically through a small hole towards the front of the worktable. An arm projects forwardly above the worktable from the back of the worktable, leaving room between it and the worktable for the workpiece being sawn to be moved about on the worktable. The blade has its upper end held at the end of the arm, and also has its lower end held below the worktable. Means are provided for maintaining the blade under appropriate tension and for reciprocating it so that it will saw the workpiece as the workpiece is moved against the blade.

The blade is driven by means of an electric motor mounted beneath or behind the worktable. In one known machine, the motor has an eccentric or cam which drives the arm above the worktable, this arm acting as e lever which is pivoted near the back of the worktable and has the blade attached to its front end.

There is a second similar lever beneath the worktable with the lower end of the blade attached to its forward end. The two arms are coupled together at the back of the worktable by means of an adjustable bolt. The second arm is therefore driven in synchronism with the first by the bolt, and the adjustment of the bolt also determines the tension in the blade. In another known construction, the blade is positively driven from the motor by means of an eccentric or cam at one end, and its other end is attached to a fixed mounting via a spring which maintains its tension (with means being provided for adjusting the tension).

These known machines suffer from several disadvantages. In the first, the arm which drives the blade is subject to considerable stresses and therefore has to be relatively sturdy, but this means that it has considerable inertia; the drive power therefore has to be high and the whole machine is subject to vibration. Also, it is not easy to scale the design for different arm lengths, since the blade travel depends on the arm length and thus a change of arm length requires a change of the angle through which the arm oscillates. Further, the blade does not follow a strict straight-line path, but rather moves in an arc.

With the second, the blade tension obviously varies through the cycle of reciprocation of the blade, as the spring expands and contracts.

The main object of the present invention is to provide a fretsaw machine in which these disadvantages are alleviated or overcome.

Accordingly the present invention provides a sawing machine comprising a horizontal worktable, a fixed arm projecting forward over the worktable, means for holding a saw blade vertically from the forward end of the arm to pass through the worktable, and a cable drive passing along the arm from a drive motor to drive the upper end of the blade. Preferably there is a corresponding cable drive below the worktable to drive the lower end of the blade in the opposite phase. The cable drives may be connected to the blade via chains passing over cogwheels or belts passing over pulley wheels, though it can also be driven via cranked levers. The motor is preferably mounted at the rear of the machine and drives the cable drives via a pair of oppositely offset bearings.

The invention also relates to the mounting of the machine on legs.

A further feature of the invention concerns the way in which the blade is held.

Further features of the present invention will become apparent from the following description of a fretsaw machine in accordance with the invention, and various possible modifications thereof, given by way of example and with reference to the drawings, in which: Fig. 1 is a simplified and diagrammatic perspective view of the machine with the work table removed; Fig. 2 is a longitudinal vertical section through the machine; Fig. 3 is an enlarged view of a crosspiece for holding a blade; and Figs. 4 and 5 are simplified diagrammatic views of modifications of the machine.

Referring to Fig. 1, the machine has an upper beam or arm 10 and a lower beam or arm 11 both welded to a vertical rear plate 12. The arm 11 has two cross beams 13 and 14 welded to it; the work table (not shown in Fig. 1) is mounted on these cross beams. A short length of beam 15 is welded lengthwise underneath the rear end of arm 11. A rear leg 16 is welded to a short length of U-section beam 17 which fits around the beam 15, and the leg 16 can be attached to the beam 15 by means of a nut and bolt (not shown) passing through matching holes 18A and 18B in the beams 15 and 17. A short length of beam 19 is welded crosswise underneath the front end of arm 11.Two front legs 20 have a strut 21 welded between them and are welded to a short length of Usection beam 22 which fits around the beam 19, and the legs 20 can be attached to the beam 19 by means of a nut and bolt (not shown) passing through matching holes 23A and 23B in the beams 19 and 22. All the beams apart from the Usection beams 17 and 22 may conveniently be box beams.

A motor (not visible in Fig. 1) is mounted on the rear of the vertical plate 12, under a motor cover 26. The two arms 10 and 11 have housings 24 and 25 forming enlarged forward ends of the arms and containing parts of the blade drive mechanism, driven by means of wires (not shown in Fig. 1) passing from the motor through apertures in the plate 12 and along inside the arms 10 and 11.

The blade (not shown) passes vertically between the two housings 24 and 25.

Fig. 2 is a front-to-rear central section through the machine. The worktable is formed of two portions; a fixed portion 35 mounted on the beams 13 and 14, and a movable portion 36 mounted on a cross beam 37 with a hinge element 38 welded to it and hinged to the housing 25 by a hinge rod 39. The movable portion 36 can thus be rotated forwards and downwards to allow ready access to the housing 25; when it is hinged up to the position shown, it is held in position by means of case clips (not shown) mounted on the meeting underneath edges of the fixed and movable portions 35 and 36 of the work table. The forward hinging of the table portion 36 also allows the blade 40 to be fixed in place in the machine.The blade 40 is stretched between the two housings 24 and 25 and passes vertically through the rear end of a slot 41 in the work table; this slot extends to the front edge of the fixed portion 35 to allow the blade to be fixed in place easily.

A drive motor 45 is mounted on the rear side of the vertical plate 12.

This motor has a vertical shaft 46, which has fixed to its top end a disc 47 with an eccentric stub axle 48 attached to it. A bearing 49 is mounted on the stub axle 48 and has a wire 50 attached to it. The lower end of the shaft 46 has B similar disc 47', stub axle 48', bearing 49' and wire 50' associated with it.

The two stubs 48 and 48' are 1800 out of phase, so that as the shaft 46 rotates, so the two bearings 49 and 49' move forward and backward (ie side to side as seen in Fig. 2) with opposite phases, so causing the wires 50 and 50' to move forwards and backwards correspondingly.

The housing 24 at the front end of the arm 10 contains a block 55 mounted to slide vertically in the housing. The block 55 consists of two side plates (one of which, 56, is shown, the other not being shown as it is in front of the section plane) separated by spacers 57 and 58. An adjustment screw 59 passes through the top of the housing 24 and engages with a threaded hole in spacer 57, so that the block 55 can be moved upwards to apply tension to the blade 40.

The block 55 has rotatably mounted therein a cogwheel 60, over which passes a chain 61 of bicycle chain type. One end of the chain 61 is attached to the front end of wire 50, and the other end terminates in a hook 62. The front of the housing 24 is closed by a plate 63 which is slidable upwards to allow access to the interior of the housing.

The mechanism in the housing 25 at the front end of the lower arm 11 is similar but simpler. The wire 50' is attached to one end of a chain 61' which has a hook 62' attached to its other end. The chain 61' passes around a cogwheel 60', which is rotatably mounted directly in the housing 25; there is no vertical adjustment means provided in- the housing 25. The hinge element 38 forms the closure means for the housing 25. Safety interlock switch means (not shown) may be provided to prevent the motor 45 being switched on if the plate 53 is raised or removed or the movable portion 36 of the work table is not raised and locked to the fixed portion 35.

The blade 40 has two crosspieces fixed to its ends, so that it can be held by the two hooks 62 and 62'. Fig. 3 shows one of these crosspieces on an enlarged scale. It consists of a short length of internally threaded tube 65 with a hole 66 into which the end of the blade is inserted. A nut 67 is screwed into the tube from one end, and a corresponding nut (not shown) from the other; the two nuts are tightened so as to grip the end of the blade (not shown) between them in the middle of the tube 65. The nuts 67 may have slotted or hexagonal nut heads, or may have hexagonal recesses as shown so that they can be tightened by Allen keys.

To use the machine, the blade is attached to its two crosspieces, and then attached between the two hooks 62 and 62'. For this, the plate 53 is raised or removed and the movable table portion 36 disengaged from the fixed table portion 35 and lowered, and the block 55 lowered by the screw 59. Once the blade is in position, the plate 63 is replaced (if it was removed) and lowered, and the movable table portion 36 raised and locked in the horizontal position. The block 55 is then raised by screw 59 to set the tension in the blade 40 to the desired level. The motor can then be switched on by a switch (not shown).

It will be realized that the oscillating parts are relatively small and have little inertia. The machine is therefore inherently free from substantial vibration. (The discs 47 and 47' counterbalance each other, but this still leaves a slight imbalance because they are separated by the length of the motor shaft 46; if desired, they can be counterbalanced individually.) The movement of the blade 40 is accurately vertical, because of the pulley action of the cogwheels 60 and 60'.The tension in the blade 40 is kept very close to constant throughout each cycle of the blade, because the geometry is such that the total distance between the bearings 49 and 49', along the wires 50 and 50', the chains 61 and 61', and the blade 40 is very accurately constant. (The tension is mainly determined by the stretching of the whole lengths of the two wires 50 and 50', which gives excellent regulation of the tension.) Further, if the blade 40 should break, the tension is automatically lost, and the broken ends of the blade will simply hang or protrude loosely; thus the workpiece is unlikely to be damaged and injury to the operator is highly unlikely.

The cogwheel and chain combination might be replaced by a right-angled crank 70, as shown in Fig. 4, pivoted at the right angle 71, coupling the wire 50 to the hook 62 (and similarly for the lower mounting of the blade). This arrangement might be less susceptible to wear and damage from small chips from the workpiece if that were metal. Alternatively, the chains and cogwheels could be replaced by flexible belting and pulley wheels. There would however be no advantage in replacing the wires 50 and 50' by rigid rods; though these would act as wires, they would have the disadvantages that their inertia would be higher and they would also push the blade, so that if the blade broke its ends would still be driven.

The cables 50 and 50' should ideally undergo only straight-line motion along their length. In fact, the ends attached to the bearings 49 and 49' also undergo a side-to-side motion. This results in the total length around the loop formed by the cables 50 and 50', chains 61 and 61', and sawblade 40 varying slightly. If the stroke of the sawblade is short and the length of the cables 50 and 50' is long, this variation may be insignificant. But if the stroke is large, and the arms 50 and 50' are short or the cables 50 and 50' are guided through small holes at the base ends of the arms, then it may be desirable to compensate for this variation. This can be done by providing a coupling with a controlled slack between the motor 45 and the bearings 49 and 49'.

Fig. 5 shows in exploded form one way of doing this. The disc 47 is split into two discs 47A, mounted on the motor shaft 46, and 47B, carrying the stub 48.

Disc 47A has a plate 80 fixed on it, and disc 47B has a slot 81 receiving the plate 80. The plate 80 has a slight taper at its ends, while the slot 81 has parallel sides. The disc 47B thus rotates with the disc 47A but has a slight slack in its rotation, corresponding to the taper of the plate 80, which can rotate through a slight angle in the slot 81. The stub 48 is located to allow this slack to enter the cable system at the appropriate part of the saw cycle.

Disc 47' would obviously be modified similarly.

It may also be desirable to provide resilience in the cable loop, either by including a tension spring in the loop or by spring mounting cogwheel 60 or 60', preferably cogwheel 60' because cogwheel 60 has to be adjustable and mounted in a more confined space.

Claims (13)

Claims

1 A sawing machine comprising a horizontal worktable, a fixed arm projecting forward over the worktable, means for holding a saw blade vertically from the forward end of the arm to pass through the worktable, and a cable drive passing along the arm from a drive motor to drive the upper end of the blade.

2 A sawing machine according to claim 1 including a corresponding cable drive below the worktable to drive the lower end of the saw blade in the opposite phase.

3 A sawing machine according to either previous claim wherein the or each cable drive is connected to the saw blade via a belt or chain passing over a wheel.

3 A sawing machine according to either of claims 1 and 2 wherein the or each cable drive is connected to the saw blade via a cranked lever.

4 A sawing machine according to any previous claim wherein the motor is mounted at the rear of the machine and drives the or each cable drive via a respective offset bearing.

5 A sawing machine according to claim 4 wherein a small slack is provided between the motor and the or each offset bearing.

6 A sawiung machine according to any previous claim wherein the worktable has a hinged front portion and the saw blade passes through a slot extending backwards from the front edge of the fixed portion.

7 A sawing machine according to claim 6 wherein the motor is powered via a safety switch responsive to the position of the hinged front portion of the worktable.

8 A sawing machine according to claim 2 or any claim appendant thereto, wherein the lower cable drive is housed in a lower arm.

9 A sawing machine according to claim 8, wherein the worktable is mounted on a pair of cross arms mounted on the lower arm.

10 A sawing machine according to either of claims 8 and 9, wherein the lower arm has front and rear legs are demountably mounted on it.

11 A sawing machine according to any previous claim, wherein the or each arm is a box beam.

12 A sawing machine according to any previous claim, wherein each end of the saw blade is held in a respective threaded tube having a transverse hole for receiving the end of the sawblade and a pair of screws screwed in from its ends to grip the end of the sawblade between them.

13 A sawing machine substantially as herein described with reference to the drawings.