GB2174642A - Jig saw - Google Patents

Abstract

So as to reduce vibration in a motor-driven jigsaw a stationary assembly comprising shaft slide bearings (86, 88) and a retainer (90) therefor has a weight at least as great as that of the assembly which reciprocates the sawblade (96). In the preferred embodiment a motor bearing (16) and a gear bearing (46) have rectangular cross sections to simplify manufacture and assembly. The slide bearings (86, 88) slideably support the reciprocating assembly (82,84, 94,96) driven through a crosshead bearing block (82) from a rotating gear (30) and the slide bearing retainer (90) is of sheet-metal. The center of gravity of the sheet-metal slide bearing retainer (90) is displaced toward the axis of reciprocation of the reciprocating assembly (84, 94) by a pair of wings formed thereon. The crosshead bearing block (82) includes an integrally molded metal insert to reduce the wear on the sides of the crosshead slot (80). A fan (18) forces air through cooling-air inlets (20) and outlets (22) and a sawdust blower duct (24). <IMAGE>

Description

SPECIFICATION Jig saw drive mechanism BACKGROUND OF THE INVENTION The present invention relates to electric hand tools and, more particularly, to electric hand tools having a reciprocating element for driving a member such as, for example, a saw blade.

Hand-held jig saws are popular for home and industrial use for cutting wood, plastic and metal. They conventionally employ an electric motor geared down to drive a crosshead bearing block which converts the rotational motion of the electric motor to reciprocating motion of a reciprocating shaft.

One way to increase the efficiency of manufacturing labor includes utilizing clam-shell casings which part into two mating halves generally along an axis of symmetry. Individual parts are assembled into one of the two halves, preferably by drop-in assembly methods requiring a minimum number of individual fasteners and thus requiring a minimum of manufacturing labor. Once all of the parts are thus installed, the other half of the casing is affixed over the installed parts to complete the assembly.

Injection-molded plastics offer an efficient, low-cost technique for forming a clam-shell casing for a jig saw. The motor, rotary bearings and slide bearings of a jig saw generate a substantial amount of heat. All practical plastic materials are relatively poor heat conductors. In addition, low-cost engineeringgrade thermoplastic materials tend to melt and flow when subjected to temperatures which corresponding metal structures are fully capable of withstanding. Thus, in order to substitute plastic casings for metal casings, special accommodations must be provided to discharge, or otherwise accommodate, the internal heat. Furthermore, the effectiveness of conventional lubricants in porous bearings degrades with increasing temperature. Thus, if the bearing temperatures are permitted an excessive temperature rise, the life of such bearings may be degraded.

U.S. Patents 3,831,048 and 3,959,677 disclose the use of air-cooled heat sinks for limiting a bearing temperature transmission to values which can be tolerated by conventional thermoplastic casing materials.

Jig saws, since they include a reciprocating element, generally require some means for limiting or attenuating vibrations originating in the reciprocating mass. U.S. Patent 2,639,737, among several others, provides a counterbalance weight moving counter to the motion of the reciprocating shaft to overcome vibrations from this source. Most of the lower-cost jig saws of the prior art rely exclusively on their relatively massive metal casings to absorb and dampen these vibrations. When light-weight, flexible, thermoplastic casings of low specific gravity are substituted for the relatively higher specific gravity metal casings of the prior art, the reduced casing mass tends to permit increased vibrations to reach the exterior of the casing.

One technique for reducing the vibrations in a thermoplastic casing includes mounting the reciprocating shaft and its slide bearings in a relatively massive metallic casting which is affixed to the forward end of the thermoplastic casing and is covered by a cap. Such castings are expensive and require substantial manufacturing labor to assemble. The need for a cap covering the casting, as well as the labor to install it, further adds to the cost of this technique.

Electrical safety of electric hand tools requires double insulation. That is, all possible conductive paths between internal electrical components and conductive items, with which the user may come into contact, must be broken by insulating barriers. One of the paths of interest extends from the motor to the reciprocating shaft.

It is conventional to employ a crosshead slot in a crosshead bearing block engaging a drive bearing which extends from a gear rotated by the motor. The crosshead bearing block, commonly known as a scotch-yoke drive system, converts the rotary motion of the gear into reciprocating motion of the reciprocating shaft. It is desirable to make the crosshead bearing block containing the crosshead slot from an insulating material to provide the desired double insulation. The interface between the bearing and the sides of the crosshead slot is the site of substantial friction, heat and potential wear. In the prior art, metal crosshead bearing blocks are commonly used. Some plastic materials such as, for example, Nylon, have been used for the crosshead bearing block to take advantage of the manufacturing economies and the lubricating properties of such materials.Such plastic materials are not capable of withstanding the wear in the sides of crosshead bearing blocks encountered in a jig saw. A technique for using a plastic crosshead bearing block, while retaining the wear resistance of metal, is desirable.

SUMMARY OF THE INVENTION It is an object of the ivention to provide a jig saw which overcomes one or more of the drawbacks of the prior art.

According to the present invention there is provided a jig saw, comprising: a casing; an electric motor in said casing; a stationary assembly in said casing supporting for reciprocation a reciprocating assembly adapted to drivingly reciprocate a saw blade; means, drivingly connected between said motor and said reciprocating assembly, for converting rotary motion of said motor to reciprocating motion of said reciprocating assembly; said stationary assembly comprising first and second slide bearings mounted on a slide bearing retainer, said first and second slide bearings guidingly supporting said reciprocating assembly; and said stationary assembly having a weight as least as great as the weight of said reciprocating assembly to reduce vibration of the jig saw due to reciprocation of said reciprocating assembly.

The weight of said stationary assembly is preferably greater than the weight of said reciprocating assembly, and may advantageously be at least three times the weight of said reciprocating assembly.

A reciprocating shaft may be mounted in the slide bearings. The slide bearings preferably include grooves engaging the edges of slots in a slide bearing retainer. The slide bearing retainer and slide bearings may drop into openings in one half of a plastic casing of the jig saw. The slots may provide angular retention of the slide bearings.

The slide bearing retainer may have a relatively massive metallic member capable of acting both as a heat sink for slide-bearing heat and as a damping mass. To improve its performance as a damping mass, the slide bearing retainer preferably includes wings bent to encircle a portion of the reciprocating shaft, thereby displacing the center of mass of the slide bearing retainer close to the axis of the reciproctingshaft.

The reciprocating assembly may include a crosshead bearing block which includes a Ushaped metallic member integrally molded in a molded plastic portion. The arms of the Ushaped metallic member advantageously line the sides of the crosshead slot of the crosshead bearing block to provide wear resistance.

Preferably, bearings for an armature shaft of the motor and a drive gear of the motion converting means have square exterior cross sections.

The reciprocating assembly preferably comprises the crosshead bearing block adapted to be attached to a reciprocating shaft and driven by an eccentric rotated by a drive gear. The molded plastic portion of the crosshead bearing block may include a plastic layer covering an inside surface of the base of the U-shaped metallic member, and means in the U-shaped metallic member for permitting plastic to flow during molding within the U-shaped metallic member to form the plastic layer.

A sawdust blower and bearing arrangement may be provided for the jig saw. A bearing may rotatably support a shaft of the motor which may include a fan. A sawdust blower duct may be integrally formed in the jig saw casing, the sawdust blower duct including an entry portion, an exit portion and at least one flow portion connecting the entry portion and the exit portion. Means may be included in the casing for directing at least a portion of air from the fan into the entry portion, whereby the at least a portion of air flows through the at least one flow portion and exits through the exit portion. A thermally conductive material may be disposed in contact with an external surface of the bearing, the thermally conductive material also being in contact with air flowing in said flow portion whereby heat from the bearing is absorbed by the air in the flow portion.

A bearing and bearing clamping system may be provided in the jig saw, this system comprising a first metallic bearing retainer in the casing, the first metallic bearing retainer including atleast a first V-shaped portion therein, a second metallic bearing retainer in the casing, the second metallic bearing retainer including at least a second V-shaped portion therein facing the first V-shaped portion, the first and second V-shaped portions forming a first substantially rectangular shape, a bearing having an external surface and a journal opening therein fittable into the first substantially rectangular shape, the external surface having a second rectangular shape substantially congruent with the first substantially rectangular shape, means for affixing the first and second metallic bearing retainers to the plastic casing, and the means for affixing including means for applying a predetermined clamping force to the external surface whereby the bearing is retained in the casing and rotation thereof is prevented.

According to a preferred aspect of the present invention, there is provided a jig saw employing clam-shell construction to permit substantially all of its assembly to be performed by drop-in methods; a motor bearing and a gear bearing having rectangular cross sections to simplify manufacture and assembly; a sheet-metal slide bearing retainer which includes slots engaging grooves in a pair of slide bearings; the slide bearings slideably supporting a reciprocating assembly driven through a crosshead bearing block from a rotating gear; the thickness of the sheetmetal slide bearing retainer being selected so that the combined weight of the slide bearing retainer and the two slide bearings is at least as great as the weight of the reciprocating assembly; the center of gravity of the sheetmetal slide bearing retainer being displaced toward the axis of the reciprocating assembly by a pair of wings formed thereon; and the crosshead bearing block including an integrally molded metal insert to reduce the wear on the sides of the crosshead slot.

The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side view, partly cut away, of a jig saw according to an embodiment of the invention.

Figure 2 is a view taken along Il-Il in Fig. 1.

Figure 3 is an exploded view of the stationary and reciprocating assemblies of Fig. 1.

Figure 4 is an end view of the crosshead bearing block of Fig. 1.

Figure 5a is a perspective view of a Ushaped metallic member of Fig. 4.

Figure 5b is a cross section taken along Vb-Vb in Fig. 3 showing an end view of the crosshead bearing block.

Figure 5c is a view in the direction Vc-Vc in Fig. 5b.

Figure 6 is a closeup view of a portion of one half of the casing of Fig. 1 with nonessential elements removed to show the sawdust blower duct.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1, there is shown, generally at 10, a jig saw according to an embodiment of the invention. Jig saw 10 includes a casing 12 of an insulating material and, preferably, of a thermoplastic material. Casing 12 is most preferably of injection-molded polypropylene containing an effective amount such as, for example, about 20 percent of a glass fiber reinforcement. A conventional electric motor 14, preferably a universal motor, is disposed within casing 12 in a position indicated in dashed lines. Electric motor 13 rotates a shaft 16 at its load end. A fan 18 is affixed to rotate with shaft 16. Since electric motor 14 is conventional, and since the selection of such a device would be well within the capability of one skilled in the art, further description of electric motor 14 is omitted.

Electric motor 14 generates a substantial amount of heat. Since the plastic material of which casing 12 is made is a poor conductor of heat, electric motor 14 and the interior of casing 12 are cooled by air drawn into casing 12 by fan 18 through a plurality of openings 20 at an idler end of electric motor 14. After passing and cooling electric motor 14, most of the cooling air is exhausted through a plurality of exhaust openings 22 adjacent fan 18.

The remainder of the air is forced through a sawdust blower duct 24, as will be explained more fully hereinafter.

Shaft 16 continues past fan 18, through a collared bearing 26, and terminates in a pinion 28. Pinion 28 meshes with teeth on a drive gear 30. Drive gear 30 is rotatable on a shaft 32.

Referring momentarily to Fig. 2, a plastic web 34, integrally molded with casing 12, supports a lower bearing retainer 36 having first and second upward-opening V-shaped portions 38 and 40. A rectangular bearing portion 42, preferably having a square cross section, of collared bearing 26 is disposed in V-shaped portion 38. Shaft 16 is rotatable in a journal opening 44 centrally located in rectangular bearing portion 42. Similarly, a rectangular bearing 46, disposed in V-shaped portion 40, includes a journal opening 48 centrally disposed therein rotatably supporting shaft 32. An upper bearing retainer 50 includes downward-opening V-shaped portions 52 and 54 engaging the tops of rectangular bearing portion 42 and rectangular bearing 46, respectively.The rectangular, preferably square, shapes of the openings formed by Vshaped portions 38 and 40 and V-shaped portions 52 and 54 are substantially congruent with the external shapes of rectangular bearing portion 42 and rectangular bearing 46.

Lower bearing retainer 36 includes mounting tabs 56 and 58. A locating tab 60, extending upward from plastic web 34, provides axial guidance for positioning mounting tab 56. A locating boss 62, integrally formed with plastic web 34, extends upward through a locating slot (not shown) in lower bearing retainer 36 and further extends through a corresponding locating slot 64 in a mounting tab 66 of upper bearing retainer 50 (best seen in Fig. 1).

A mounting tab 68 on upper bearing retainer 50 is disposed atop mounting tab 56. A first mounting screw 70, passing through aligned holes (not shown) in mounting tabs 56 and 68, is tightened into plastic web 34. Similarly, a second mounting screw 72, passing through aligned holes (not shown) in mounting tabs 58 and 66, is tightened into plastic web 34.

A predetermined amount of springback is provided in upper bearing retainer 50 so that, before mounting screws 70 and 72 are tightened, a slight gap exists between facing surfaces of mounting tabs 56 and 68 and between mounting tabs 58 and 66. The width of the gap is selected so that, when mounting screws 70 and 72 are tightened, a predetermined clamping force is exerted on rectangular bearing portion 42 and rectangular bearing 46.

Although cylindrical outer surfaces could be used in place of rectangular bearing portion 42 and rectangular bearing 46, if such cylindrical surfaces are used, some means such as, for example, a pin or a tab, must be provided to prevent them from rotating. As a consequence, only a single correct angular orientation is provided for assembly. The rectangular cross sections, together with the mating shapes of lower bearing retainer 36 and upper bearing retainer 50, avoid the need for additional means to prevent rotation. In addition, a square cross section provides four equally correct angular orientations in which rectangular bearing portion 42 and rectangular bearing 46 may be installed. This facilitates manual or robotic assembly.

It would be clear to one skilled in the art that shapes other than rectangular and square may be employed on the external surfaces of rectangular bearing portion 42 and rectangular bearing 46. For example, any regular polygonal external surface such as, for example, triangular, pentagonal and hexagonal may be employed with corresponding results. Suitable modification would be required in the shapes of lower bearing retainer 36 and upper bearing retainer 50 to accommodate such changed external shapes, but one skilled in the art would be fully enabled to make such modifications in the light of the present disclosure without further description herein. It will be further recognized that an external shape having more than four sides provides a corresponding increase in the number of correct angular assembly orientations.In some applications, this may make preferable the use of regular polygonal external shapes having more than four surfaces.

Rectangular bearing portion 42 and rectangular bearing 46 may be made of any convenient material. In the preferred embodiment, rectangular bearing portion 42 is a conventional sintered, self-lubricating bearing to withstand the high speeds of shaft 16 encountered in this location. Rectangular bearing 46 is preferably also may be of sintered, selflubricating material.

Referring now also to Fig. 1, an alignment tab 74, integrally molded with casing 12, provides a reference for axial positioning of rectangular bearing 46. That is, rectangular bearing 46 is positioned in V-shaped portion 40 and urged into contact with alignment tab 74 during tightening of mounting screws 70 and 72 to establish the desired final axial position of rectangular bearing 46. The desired axial position of collared bearing 26 is established by moving the collar of collared bearing 26 into contact with the edges of lower bearing retainer 36 and upper bearing retainer 50 before tightening mounting screws 70 and 72.

Since the axial position of mounting tab 56 is established by locating tab 60 and mounting screw 70, a predetermined axial positioning of collared bearing 26 is obtained.

Referring now specifically to Fig. 1, a drive bearing 76 affixed to a forward face 78 of drive gear 30 engages a crosshead slot 80 in a crosshead bearing block 82. Crosshead bearing block 82 is rigidly affixed to a recipro cating shaft 84 which is slideably disposed in an upper slide bearing 86 and a lower slide bearing 88. Upper slide bearing 86 and lower slide bearing 88 are maintained in alignment by a slide bearing retainer 90. An external portion 92 of reciprocating shaft 84, extend ing outward from casing 12, terminates in a blade clamp 94 for securing a saw blade 96 thereto.

It will be recognized that reciprocating shaft 84, together with crosshead bearing block 82, blade clamp 94 and saw blade 96, forms a reciprocating assembly which is concertedly driven in linear motion in response to the rotary motion of drive gear 30. Upper slide bearing 86, lower slide bearing 88 and slide bearing retainer 90 form a stationary assembly whose mass is effective to absorb or attenuate the vibrations set up by the reciprocating assembly. For maximum reduction of vibration, the stationary assembly should be as massive as possible. From the standpoint of ease of use, however, it is desirable that the overall weight of jig saw 10 be as low as possible.

We have discovered that a satisfactory reduction in vibration transmitted to casing 12 is achieved if the mass of the stationary assembly is about the same as, or greater than, the mass of the reciprocating assembly. In the preferred embodiment, the combined weight of reciprocating shaft 84 and blade clamp 94 is about 27.4 grams. The weight of slide bearing retainer 90 is about 74.8 grams and the weights of upper slide bearing 86 and lower slide bearing 88 are each about 16.4 grams. Thus, allowing about 4.4 grams for saw blade 96, the weight of the stationary assembly exceeds three times times the weight of the reciprocating assembly. In order to achieve the desired weight for slide bearing retainer 90, a metal thickness is chosen which may exceed the thickness required for the other functions which slide bearing retainer 90 performs.If specific self-resonance frequencies become a problem in the stationary and/or reciprocating assemblies, the weight of slide bearing retainer 90 may be varied by changing its thickness to fine-tune the stationary assembly by shifting a troublesome self-resonance frequency to a frequency region where it is less likely to become excited by stimuli in jig saw 10.

Refe#rring now to Fig. 3, there is shown an exploded view of the reciprocating and stationary assemblies. Slide bearing retainer 90 includes a planar plate 98 having wings 100 and 102 at the transverse edges thereof.

Wings 100 and 102 partially encircle reciprocating shaft 84. A bearing alignment slot 104 in one end of planar plate 98 includes parallel edges 106 and 108. Upper slide bearing 86 includes a pair of grooves 110 and 112 which engage edges 106 and 108, respectively. An abutment surface 114 at an inner end of bear ing alignment slot 104 limits the travel of up per slide bearing 86 toward the center of side bearing retainer 90. Similarly, a bearing alignment slot 116 at an opposing end of pla nar plate 98 includes parallel edges (of which, only edge 118 is seen in Fig. 3) and an abut ment surface 120. A groove 122 in lower slide bearing 88 engages edge 118. A second groove in lower slide bearing 88 (hidden in Fig. 3) engages the second edge (also hidden) of bearing alignment slot 116. The inward motion of lower slide bearing 88 into bearing alignment slot 116 is limited by abutment surface 120.

Upper slide bearing 86 includes a rectangu lar bearing slot 124 dimensioned to provide a sliding fit to reciprocating shaft 84. Similarly, lower slide bearing 88 includes a rectangular bearing slot 126, also dimensioned to provide a sliding fit to reciprocating shaft 84.

Crosshead bearing block 82 is affixed to reciprocating shaft 84 in any convenient manner.

In the preferred embodiment, crosshead bearing block 82 is molded in a shape which embraces reciprocating shaft 84 and permits it to be slid into position from the upper end of reciprocating shaft 84. The position of crosshead bearing block 82 is then fixed along reciprocating shaft 84 by one or more roll pins 128 driven through appropriately spaced holes in reciprocating shaft 84.

External portion 92 of reciprocating shaft 84 terminates in a wide portion 130 as part of blade clamp 94. Wide portion 130 is too wide to permit lower slide bearing 88 to be installed or removed over it. Thus, lower slide bearing 88 is installed on reciprocating shaft 84 before crosshead bearing block 82 is affixed to reciprocating shaft 84 and is thereafter captured in place to form a subassembly.

To complete the assembly, groove 122 and its hidden counterpart, are slid as far as they will go into bearing alignment slot 116 until stopped by abutment surface 120. The upper end of reciprocating shaft 84 is inserted into rectangular bearing slot 124 and upper slide bearing 86 is also slid as far as it will go into bearing alignment slot 104 until stopped by abutment surface 114.

Referring now also to Fig. 1, the entire subassembly described in the preceding paragraph is dropped into molded-in cavities provided in casing 12 with crosshead slot 80 engaging drive bearing 76. Upper slide bearing 86 is retained in its axial position by contact with a retaining web 132 molded into casing 12. Similarly, lower slide bearing 88 is retained in its axial position by contact with an inner surface of casing 12. A plurality of slots 134 (only one of which is shown) are molded into the mating halves of casing 12 to engage and retain planar plate 98 of slide bearing retainer 90. In this manner, the drop-in subassembly, comprising both the reciprocating and stationary subassemblies, is fixed with respect to casing 12 both in displacement and angle.

Besides enabling drop-in assembly, this embodiment of the invention also permits slide bearing retainer 90 to be an inexpensive sheet-metal part of simple shape rather than one of the complex and expensive cast or molded parts encountered in the prior art.

Wings 100 and 102 serve dual functions.

First, slide bearing retainer 90 is a thermal mass into which heat generated by friction in upper slide bearing 86 and lower slide bearing 88 is dumped, thereby limiting the temperature rise in upper slide bearing 86 and lower slide bearing 88. In addition to increasing the thermal mass, wings 100 and 102 also increase the surface area available to reject heat by convection and radiation.

In their second function, wings 100 and 102 improve the manner in which slide bearing retainer 90 reduces vibration. In the ideal, the center of gravity of a damping mass should lie along the axis of the reciprocating assembly. Wings 100 and 102 shift the center of gravity of slide bearing retainer 90 toward the axis of reciprocating shaft 84 and thus contribute to increasing the effectiveness of slide bearing retainer 90 in its vibrationdamping function. In addition, wings 100 and 102 add a substantial portion to the total weight of slide bearing retainer 90 without requiring excessive dimensions or thickness in planar plate 98.

Referring now to Figs. 4 and 5a, crosshead bearing block 82 includes a U-shaped metallic member 136 integrally molded into a molded plastic portion 138 to provide metal bearing surfaces on the sides of crosshead slot 80.

As best shown in Fig. 4, a base 140 of Ushaped metallic member 136 is embedded in molded plastic portion 138 with a plastic layer 142 formed above it during the molding process. In order to encourage the plastic to flow into U-shaped metallic member 136 to form plastic layer 142, one or more holes 144 are disposed in base 140. In addition, undercuts 146 and 148 are provided at each end of Ushaped metallic member 136 adjacent base 140 to permit the plastic to flow inward from the ends of U-shaped metallic member 136. In combination, holes 144 and undercuts 146 and 148 provide secure mechanical keying between U-shaped metallic member 136 and molded plastic portion 138.

Arms 150 and 152 of U-shaped metallic member 136 fully line the sides of crosshead slot 80 with metal which bears the full load imposed by contact with drive bearing 76 (Fig. 1). Thus, molded plastic portion 138 is not required to withstand friction. As a result, crosshead bearing block offers substantially improved endurance than is possible without U-shaped metallic member 136. In the preferred embodiment, molded plastic portion 138 is molded of a nylon plastic containing a proportion of glass fiber.

Referring now to Figs. 5b and 5c, molded plastic portion 138 of crosshead bearing block 82 includes a C-shaped opening 153 in each end thereof fittable upon reciprocating shaft 84. In order to simplify the molding of molded plastic portion 138, C-shaped opening 153 is formed by first and second arms 155 and 157, respectively. A recessed face 159, below C-shaped opening 153 in each end of molded plastic portion 138, includes a stop boss 161 which limits the penetration of roll pin 128 beyond reciprocating shaft 84. A support surface 163 contacts and supports the underside of reciprocating shaft 84 between C-shaped openings 153. A web 165 at each extremity of support surface 163 reduces the tendency of reciprocating shaft 84 to bind in molded plastic portion 138. Two gussets 167 stiffen each web 165.

As best seen in Fig. 5c, first arm 155 and second arm 157 are cantilevered outward beyond the planes defined by their respective recessed faces 159. One skilled in the art will immediately recognize that this arrangement permits molding molded plastic portion 138 using dies which close along a single axis defined by a line from top to bottom of Fig. 5b.

As a consequence, the complexity of a die requiring motion of its parts normal to the above-defined axis is avoided.

Referring now to Fig. 6, a close-up view of sawdust blower duct 24 is shown from which upper bearing retainer 50, collared bearing 26, rectangular bearing 46 and mounting screws 70 and 72 are omitted, and lower bearing retainer 36 is shown in dashed line, to reveal the elements beneath them. The airflow through sawdust blower duct 24, besides clearing the vicinity of the active area of saw blade 96 (Fig. 1), also is effective to cool collared bearing 26 and rectangular bearing 46. It will be recalled that casing 12 is of clamshell design in which corresponding elements in the two halves thereof mate with each other for support and sealing. All molded plastic webs upon which lower bearing retainer 36 lies are shaped for substantially airtight contact therewith.The mating half of casing 12 (not shown) contains substantially identical webs sealingly interfacing with upper bearing retainer 50 in the assembled jig saw 10.

Sawdust blower duct 24 is defined by an entry portion 154, a flow portion 156 and an exit portion 158. Entry portion 154 includes an entry web 160 partly surrounding a perimeter of fan 18 and effective for directing a flow of air into entry portion 154. One side of flow portion 156 is defined by plastic web 34 and by a parallel plastic web 34'. The second side of flow portion 156 is defined by webs 162 and 164. Lower bearing retainer 36 seal ingly contacts web 164, plastic web 34 and plastic web 34' to constrain air flow from entry portion 154 to exit portion 158. A semi circular cutout 166 sealingly fits the perimeter of the collar of collared bearing 26 both to support such collar and to retain the collared end of collared bearing 26 in a position where it is exposed to a flow of air moving there past on its way to entering fan 18 near its axis.This tends to keep the temperature rise experienced by collared bearing 26 within lim its which can be tolerated by the plastic ma terial of which casing 12 is made. In addition, air flowing in entry portion 154 contacts and cools lower bearing retainer 36 which, in turn, absorbs heat from rectangular bearing portion 42 of collared bearing 26 and from rectangu lar bearing 46. Corresponding air flow in the unillustrated half of casing 12 cools upper bearing retainer 50 for removal of additional heat from the rectangular bearing portion 42 and rectangular bearing 46.

Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of the invention as defined in the ap

Claims (19)

pended claims. CLAIMS

1. A jig saw, comprising: a casing; an electric motor in said casing; a stationary assembly in said casing supporting for reciprocation a reciprocating assembly adapted to drivingly reciprocate a saw blade; means, drivingly connected between said motor and said reciprocating assembly, for converting rotary motion of said motor to reciprocating motion of said reciprocating assembly; said stationary assembly comprising first and second slide bearings mounted on a slide bearing retainer, said first and second slide bearings guidingly supporting said reciprocating assembly; and said stationary assembly having a weight as least as great as the weight of said reciprocating assembly to reduce vibration of the jig saw due to reciprocation of said reciprocating assembly.

2. The jig saw of Claim 1, in which the weight of said stationary assembly is greater than the weight of said reciprocating assembly.

3. The jig saw of Claim 2, in which the weight of said stationary assembly is at least three times the weight of said reciprocating assembly.

4. The jig saw of Claim 1, 2 or 3, in which said slide bearing retainer includes means for displacing the center of gravity of said stationary assembly towards an axis of reciprocation of said reciprocating assembly.

5. The jig saw of Claim 4, in which said displacing means comprises first and second wings on a planar plate of said slide bearing retainer, said first and second wings being disposed to surround a portion of said reci procating assembly.

6. The jig saw of Claim 5, in which said planar plate includes first and second bearing alignment slots, and said first and second slide bearings each includes first and second grooves engaging a respective bearing alignment slot whereby said first and second slide bearings are positioned and retained.

7. The jig saw of any preceding claim, in which said slide bearing retainer comprises a thermal mass and a surface area effective for limiting a temperature rise of said stationary assembly during operation of said jig saw.

8. The jig saw of Claim 7, in which at least a portion of said thermal mass and said area is associated with portions of said stationary assembly which extend on opposite sides of said reciprocating assembly.

9. The jig saw of any preceding claim, in which said reciprocating assembly includes a crosshead bearing block attached to and for reciprocation of a reciprocating shaft, and said converting means comprises a drive gear having an eccentric drivingly engaged in said crosshead bearing block.

10. The jig saw of Claim 9, in which said crosshead bearing block comprises: a molded plastic portion; a U-shaped metallic member having a base and first and second arms integrally molded into said molded plastic portion; said U-shaped metallic member forming a crosshead slot with said first and second arms forming metallic sides for engagement with said eccentric; said molded plastic portion including a plastic layer covering an inside surface of said base; and means in said U-shaped metallic member for permitting plastic to flow during molding within said U-shaped metallic member to form said plastic layer.

11. The jig saw of Claim 10, in which said means for permitting plastic to flow comprises at least one hole through said base.

12. The jig saw of Claim 10 or 11, in which said means for permitting plastic to flow comprises a first undercut at a first end of said U-shaped metallic member, said first undercut including a first end of said base and portions of first ends of said first and second arms contiguous to said base, and a second undercut at a second end of said U-shaped metallic member, said second undercut including a second end of said base and portions of second ends of said first and second arms contiguous to said base.

13. The jig saw of any one of Claims 10, 11 or 12, in which said molded plastic portion includes first and second C-shaped slots encircling said reciprocating shaft, said first and second C-shaped slots each being formed by first and second arms integrally formed with said molded plastic portion.

14. The jig saw of Claim 13, in which said first and second arms are cantilevered beyond an adjacent surface of said molded plastic portion whereby said first and second arms may be molded with a mold which requires motion along a single axis.

15. The jig saw of Claim 14, in which said first and second arms include a gap therebetween and said adjacent surface includes a stop boss generally aligned with said gap.

16. The jig saw of any preceding claim, in which said electric motor has a cooling fan, and further comprising an air discharge duct from said fan disposed to discharge cooling air over at least a portion of said stationary assembly.

17. The jig saw of any preceding claim, in which said converting means comprises a drive bearing which drivingly engages in said reciprocating assembly for reciprocating the latter.

18. The jig saw of Claim 1, in which said reciprocating assembly includes a crosshead substantially as hereinbefore described with reference to Figs. 3, 4, 5a, 5b and 5c of the accompanying drawings.

19. A jig saw substantially as hereinbefore described with reference to the accompanying drawings.