GB2082114A - Mechanical fret saw - Google Patents

Abstract

Belts 42 and 52, tensioned by adjustable guide pulley 43, connect saw blade 38 with drive lever 44 which is oscillated by eccentric bearing 56 located in slot 55 and rotated, for example, by a portable electric drill 59. With this arrangement, the need for oscillating the entire saw frame 10 is obviated, so the power necessary for driving the saw is minimised. <IMAGE>

Description

SPECIFICATION Mechanical fret saw This invention concerns a mechanical fret saw.

As hitherto proposed, mechanical fret saws generally comprise an elongate C-shaped frame providing elongate approximately horizontal arms between the free ends of which a blade is held in tension, the entire frame being adapted to oscillate to cause the blade to reciprocate up and down. Such arrangements suffer from a number of disadvantages.

Firstly, some of the power consumed by the saw is dissipated in the reciprocation of the frame which serves no more than to carry the blade. Secondly, because the blade is tensioned between the free ends of the arms of the frame, these arms and the frame itself must be relatively robust to eliminate the risk of deflection. Accordingly, provision has to be made for such a robust frame to be mounted for oscillation. Thirdly, because the entire frame oscillates about a fulcrum spaced away from the blade, the blade does not just reciprocate vertically but moves angularly also in a vertical plane. This can result in striated cut surfaces on workpieces cut on the saw.

An object of the present invention is to provide a construction of mechanical fret saw having an improved arrangement for reciprocating the saw blade which dispenses with the need for oscillating the frame and which provides for the blade to be reciprocated linearly in a simple, convenient and economical manner, so that the power required to drive the saw is less than in the prior known constructions, the frame can be constructed to be less robust than hitherto and does not have to be pivotally mounted for oscillation, and cuts produced by the blade are clean and smooth.

With this object in view, the present invention provides a mechanical fret saw comprising an elongate C-shaped frame providing a pair of substantially horizontal arms the free ends of which are provided with aligned guides respectively for complementary upper and lower blade clamps, flexible connecting means extending along the arms of the frame and connecting the blade clamps, and drive means for reciprocating the flexible connecting means and thereby correspondingly reciprocating, substantially vertically relative to the frame, a blade extending between the clamps.

The connecting means may conveniently comprise a cable or flexible belt.

A guide arrangement for the cable or belt may comprise guide pulleys in the arms of the frame, one of which guide pulleys is displaceable to vary the tension in the cable or belt and in the blade. This adjustable pulley may be mounted in a bifurcated bracket carried by an adjusting screw extending through the upper arm and provided with a knurled nut by which the position of the bracket and the pulley can be varied.

The drive means may simply comprise a lever, pivotally mounted on the frame for oscillation substantially about its mid point, and an eccentric member engaging the lever and serving, when rotated, to oscillate the lever. This eccentric member may be a disc or ball bearing accommodated in a slot in the lever and carried by a spindle disposed so that it may be rotated e.g. by means of a portable electric drill or by a pulley drive.

The invention will be described further, by way of example, with reference to the accompanying draw inns which illustrate a preferred embodiment, it being understood that the following description is illustrative, and not restrictive, of the scope of the invention. In the drawings:: Figure 1 is a part-sectional elevation illustrating the said preferred embodiment of the mechanical fret saw of the invention with one of its walls removed to reveal its internal mechanism, and a stand of the saw omitted; Figure 2 is a detached view illustrating details of the work table of the saw of Figure 1 when viewed in the direction indicated at 2 - 2 in Figure 1; Figure 3 is an enlarged view of part of an alternative form of blade showing it fitted with an adapter to enable it to be used in the saw of Figures 1 and 2; Figure 4 is a front view corresponding to Figure 3; Figure 5 is a part-sectional view corresponding to the line 5 - 5 of Figure 1, showing how the saw of Figures 1 and 2 may be pulley driven; and Figure 6 is an enlarged perspective view illustrating a modified arrangement of saw clamp, for the apparatus of Figure 1, which permits variation of the orientation of the saw blade.

The illustrated preferred embodiment of the mechanical fret saw of the invention comprises an elongate C-shaped metal frame indicated generally by the numeral 10 and comprising a pair of C-shaped wall components 11 and 12 defining a free space 13 between them and having thickened marginal portions 14, 15, 16 and 17, 18, 19, with bolt holes 20 therethrough, by which the wall components 11, 12 are clamped together. The shape of the frame 10 is, therfore, such as to provide elongate substantially horizontal upper and lower arms 21 and 22 one end of each of which is a free end and the other ends of which are connected by an upright 23, the arms 21 and 22 and the upright 23 being hollow.

The lower arm 22 has a stand (not shown) for the frame 10 secured to its underside by bolts (not shown) through the stand engaging into the bottom of the lower arm 22.

As shown in detail in Figure 2, the lower arm 22 has a table-mounting block 24 secured between integral upwardly-projecting tongues 33 thereof adjacent to the free end of said arm, and a work table 25 is bolted to the block 24 so as to overhang the free end of said arm 22. A blade hole 26 is provided through the table 25 adjacent said free end. The securement of the table 25 to the block 24 is by way of axially aligned screws 27 and 28 engaging into opposite ends of the block 24 and extending through respective brackets 29, 30 projecting downwards from the underside of the table 25. As can be seen in Figure 2, a further screw 31 through an arcuate slot 32 in the bracket 29 permits the angular disposition of the table 25 to be adjusted.

Formed in the ends of the two arms 21, 22 are vertical guides, in the form of opposed shoulders 34, 35, these being aligned vertically and each defining a respective recess accommodating a respective stripform J-shaped slide 36,37 formed as a respective blade clamp. These two blade clamps 36, 37 serve to mount in place a vertical fret saw blade 38 which has its ends engaged with the respective clamps 36,37 by way of transverse joins 39, and extends downwards through the blade hole 26 in the table 25. A respective cable-or belt-anchoring lug 40,41 protrudes from each slide 36,37 into a slot defined by the ends of the free space 13 of the respective arms, and a belt 42 is connected to the lug 40 of the upper slide 36, from which it extends over an upper guide pulley 43 mounted within the upper arm 21 of the frame, near the end thereof.From this upper guide pulley 43, the belt 42 extends along the upper arm 21, within the latter, and is connected with the upper end of a two armed lever 44 which is accommodated in the upright part 23 of the frame 10 and will be described in more detail later.

The upper guide pulley 43 is carried by a spindle 45 whose ends are located in respective slots 46 in the sidewall components 11, 12 of the frame 10. The spindle 45 is also engaged by a yoke 48 supported on the lower end of an adjustment bolt 49 which projects upwards through the top of the upper arm 21 and has a knurled adjusting knob 50 screw thereon. A helical spring 51 surrounds the bolt 49 within the arm 21, and abuts by its upper end against the underside of the top of the upper arm 21,and by its lower end against the yoke 48. This arrangement permits the tension in the belt 42 to be adjusted.

A second belt 52, extending around guide pulley 53, connects the lower end of the lever 44 with the lug 41 of the lower slide 37. The two belts 42 and 52, with the lever 44 therebetween, constitute flexible connecting means connecting together, for movement in unison, of the two blade clamps 36,37.

The lever 44, disposed within the upright part 23 of the frame 10, between the upper and lower arms 21, 22 is in an approximately upright disposition and is mounted on a pivot 54 extending approximately through its middle and secured in the sidewalls of the upright part 23 of the frame. In its lower part, this lever 44 is formed with a slot 55 accommodating a drive member in the form of a ball bearing 56 mounted eccentrically on a spindle 57 by its inner race 58 fitting onto the spindle 57 which is journalled in the frame and projecting as a stub (not visible) outside the frame so that a portable electric drill (whose position is indicated diagramatically at 59 in Figure 1) can be fitted to the spindle 57 to rotate the same and the ball bearing 56 which fits by its outer race 58 in the slot 55 in the lever 44.Because the ball bearing 56 is eccentrically mounted, its rotation causes oscillation, about its pivot 54, of the lever 44 to which the ends, remote from the blade clamps 36, 37 of the belts 42,52 are secured.

It will thus be understood, that with the belts 42,52 and saw blade 38 under suitable tension, as adjusted by means of the adjustable pulley 43, the lever 44, when driven by the eccentric bearing 56, acts as an oscillating beam whose oscillation is transmitted by the belts 42,52 to the blade clamps 36,37 and causes corresponding oscillation of the blade 38.

The saw is used, of course, in exactly the same way as a conventional mechanical fret saw. Because only the lever 44, the belts 42, 52, the saw clamps 36, 37 and the saw 38 are reciprocated, and the frame 10 is stationary, the saw requires much less powerthan the conventional saws, and a much greater proportion of its driving power goes towards actual cutting.

The drive is, of course, very simple, so it is inexpensive to produce, and requires very little maintenance. Because the frame 10 has only to maintain the blade 38 under tension, it does not have to be of massive construction; moreover it can easily be made with arms 21,22 of sufficient length to accommodate very large workpieces, without involving uneconomically heavy components.

The invention is not confined to the precise details of the foregoing example and variations may be made thereto. For instance, the structure of the saw need not be of plates as described. It could, for example, be an openwork frame, e.g. of tubes, providing the necessary guides for the belts and any suitable means may be provided for reciprocating the same for producing corresponding reciprocation of the blade clamps.

If relatively short saw blades 70 (Figures 3 and 4) are desired to be used, these may, as shown, be provided with adapter strips 71 with transverse pegs 72 for engaging with the blade clamps and slidable jaws 73 for gripping the respective blade end.

As shown in Figure 5, the saw may incorporate a bracket 60, secured to the frame by angle bolts 61 and supporting a beaming structure 62 in which the spindle 57 is journalled. This spindle has a pulley 63 mounted thereon, whereby the saw may be driven by a V-belt (not shown).

In the modification shown in Figure 6, each blade clamp 36,37 is formed with a respective block 90,91 formed with aligned openings 92,93 for respective shanks 94, 95 with which the respective ends of the blade 38 engage. Each such block 90, 91 has a respective radial peg 96,97 selectively engageable in respective slot or recesses 98,99 and 100 in the blocks 90,91 so that the blade can be oriented with its teeth to one side or the other of the saw or as shown in Figure 1.

In the described embodiment, other means may be provided for maintaining the tension of the belts.

Other forms of flexible connecting means, such as a cable or chain, may be provided in the place of the belts and such connecting means need not be in two parts as shown but may comprise a single length of material extending between the two clamps 36,37 and secured to the lever 44 or its equivalent. The mechanism may, if desired, incorporate means for enabling the amplitude of reciprocation of the blade 38 to be varied, e.g. by varying the effective throw of the lever 44. This can be achieved, for instance by providing for the pivot 54 of the lever 44 to be adjustable, e.g. along a slot in the lever.

Claims (11)

1. A mechanical fret saw comprising an elongate C-shaped frame providing a pair of substantially horizontal arms the free ends of which are provided with aligned guides respectively for upper and lower blade clamps, flexible connecting means extending along the arms of the frame and connecting the blade clamps, and drive means for reciprocating the flexible connecting means and thereby correspondingly reciprocating, substantially vertically relative to the frame, a blade extending between the clamps.

2. A fret saw as claimed in claim 1 wherein the connecting means comprises a cable or flexible belt.

3. A fret saw as claimed in claim 1 or 2 wherein a guide arrangement for the flexible connecting means comprises guide pulleys, in the arms of the frame, one of which guide pulleys is displaceable to vary the tension in the connecting means and in the blade.

4. A fret saw as claimed in claim 3 wherein the adjustable pulley is mounted in a bifurcated bracket carried by an adjusting screw extending through the upper arm and provided with a knurled nut by which the position of the bracket and the pulley can be varied.

5. A fret saw as claimed in any preceding claim wherein the drive means comprises a lever, pivotally mounted on the frame for oscillation substantially about its mid point, and an eccentric member engaging the lever and serving, when rotated, to oscillate the lever.

6. A fret saw as claimed in claim 5 wherein the eccentric member is a disc or ball bearing accommodated in a slot in the lever and carried by a rotatable spindle.

7. A fret saw as claimed in claim 6 wherein the spindle is adapted to be rotated by means of a portable electric drill.

8. A fret saw as claimed in claim 6 wherein the spindle is adapted to be rotated by a pulley drive.

9. A fret saw as claimed in any preceding claim wherein the frame comprises two C-shaped wall components clamped together and defining between them a free space accommodating the flexible connecting means and the drive means.

10. A mechanical fret saw substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 2 of the accompanying drawings.

11. A mechanical fret saw as claimed in claim 10 but modified substantially as hereinbefore described with reference to and as illustrated in Figures 3 and 4 and/or in Figure 5 of the accompanying drawings.