Patent Abstract:
A drywall finishing head is provided for applying mastic material over a tape joint between adjacent wallboard surfaces. The finishing head includes a coater connected to a pneumatic applicator for delivering the mastic material. A wiper blade extends across the length of the coater with a cover underlying the wiper blade and a backer bar overlying the wiper blade. A cam assembly is rotatably mounted on a tensioning portion of the coater. Adjustment of the cam assembly is constructed and arranged to change the force of the backer boar so that a proper crown or profile of mastic material can be delivered to the tape joint through a gap between the cover and the wiper blade.

Full Description:
CROSS REFERENCE TO RELATED APPLICATION  
     This application is related to provisional application U.S. Ser. No. 60/393,764 filed Jul. 3, 2002.  
    
    
     FIELD OF THE INVENTION  
     The present invention relates generally to finishing tools or coaters for applying mastic material, drywall compound or other similar material over tape joints between adjacent pieces of wallboard. More particularly, the present invention is directed towards a coater having a tensioner assembly which provides improved control for adjustably applying the proper amount of mastic over the taped joint.  
     BACKGROUND OF THE INVENTION  
     Wallboard or drywall has become the dominant material in the production of interior building partitions. In particular, interior building partitions generally comprise a series of spaced vertical studs which are used as support for preformed wallboards which are attached to the studs by the screws, nails, adhesive or the like. Typically, a paper or fiberglass tape is applied to the joint between adjacent wallboard panels. In order to provide a continuous flat surface to the wall, it is necessary to “finish” the tape joint between adjacent panels. Generally such “finishing” entails the building up or accumulation of multiple layers of mastic material over the taped joint. During the finishing process, it is important that the proper crown or profile of mastic material is applied given the amount of shrinkage that occurs when each coat of mastic material dries. Crown control is dependent upon several variables including coat thickness and the amount of water mixed in the drywall compound.  
     Drywall heads for applying mastic compound to finish drywall joints are known in the art. Precision Taping Tools of Arthur, Ill. produces a coater (model K-520A, K-530A, K-540A) with a flat finishing head formed with a coater body having side plates and skids or arm links adapted to engage a wall during mastic application. One wall of the coater body has a mounting plate which rotatably receives a ball member connected to a hollow arm member which allows mastic material to be moved from a pneumatic applicator to the flat finishing head. A C-spring has one end secured to a clevis integrally formed on the arm member, and another end joined to a rotatable clevis on the mounting plate. The coater body also includes a bottom cover plate, a wiper blade and a generally flat backer bar. A cam lever acts against a tensioner wire having L-shaped ends which contact the backer bar at two points. Mastic material delivered through the ball member to the interior of the coater body is controllably squeezed through a gap defined between the cover and the blade. The cam and tensioner wire are used to control the crown or profile of mastic material delivered from the finishing head.   
     When using the Precision Taping Tool coater, the flat backer bar relies on the viscosity of the drywall compound, the setting of the cam tensioner wire and the deflection of the cam tensioner wire/backer bar to provide the desired mastic profile. In addition, the prior art coater uses a cam having a half-round profile with an offset pivot to provide adjustability. However, as the cam moves through its travel, it reaches a point where a small adjustment of the cam lever makes abnormally large adjustments of the backer bar/blade and vice versa. In some occasions, the prior art finishing head deflects so far as to overcrown the compound making a subsequent coat “near to impossible” without scraping or sanding the joint. Further, the prior art finishing head has its cam tension or wire contacting the backer bar at two points which sometimes allows three crowns to form instead of one. It has also been established that the prior art coater does not return to its starting position each time it is lifted off the wall. It is further noted that the prior art coater skids are subject to excessive sliding friction and wear as they are drawn over the wallboard.  
     Accordingly, it is desirable to provide a drywall finishing head having an adjustment arrangement for enabling a greater predictability of crown or profile of mastic material delivered to the tape joint. It is also desirable to provide a drywall finishing head which moves along the wallboard with less friction and wear, and which returns to a start position each time it is lifted off the wallboard.   
     SUMMARY OF THE INVENTION  
     It is one object of the present invention to provide a drywall finishing head for applying mastic compound which may be used with a pneumatically-actuated storage body for providing a controllable flow of mastic material over a tape joint between adjacent wallboard panels.  
     It is also an object of the present invention to provide a drywall finishing head having a backer bar, a cam and a cam follower plate uniquely designed to provide the desired crown of mastic compound to the tapered wallboard joint.  
     It is another object of the present invention to provide a drywall finishing head which may be operated with arm links off the surface of the wallboards.  
     In one aspect of the invention, a drywall finishing head is provided for applying mastic material over a tape joint between adjacent wallboard surfaces. A coater body has a retaining assembly attached thereto for movably receiving a ball connected via an arm to a storage body for delivering mastic material through an opening in the wall to a mastic chamber in the coater body. A wiper blade extends substantially across a length of the coater body. A cover underlies the wiper blade and also extends substantially across the length of the coater body. A curved backer bar extends substantially across the length of the coater body and overlies the wiper blade. A cam assembly includes an elliptical cam rotatably mounted on the retaining assembly and acts against a cam tensioner plate fixed behind the cam. The cam further acts on a tensioner wire having a central portion extending behind the retaining assembly and end portions wrapped around the front of the retaining assembly. The wire contacts a cam follower plate engageable with the backer bar. Adjustment of the cam assembly is constructed and arranged to change the force on the backer bar so that a proper crown or profile of mastic material can be delivered to the tape joint through a gap between the cover and the wiper blade.   
     The coater body includes a pair of side plates to which a pair of arm links is attached, the arm links carrying a set of wheels rotatably mounted thereto and adapted to rollably engage a wallboard surface with reduced friction. A C-shaped spring has one end attached to the arm and another end attached to the ball retainer/tensioner assembly, there being an auxiliary coil spring connected between the ends of the C-shaped spring. A baffle is secured to the coater body, the baffle having a curved bottom end receiving a lower end of the wiper blade. The curved bottom end of the baffle and the cover define a restricted passage for receiving mastic material in the mastic chamber of the coater body and allowing the mastic material to pass to the gap between the cover and the wiper blade. The cam follower plate is formed with a groove having a width and a depth, the walls of the groove being engageable with the tensioner wire. The backer bar has a spring rate controlled by the width of the groove in the cam follower plate. The amount of deflection in the wiper blade is limited by the depth of the groove in the cam follower plate. The cam is moveable between a full crown setting in which the cam is out of contact with the tensioner wire and mastic material is delivered through the gap in a crown profile over the tape joint, and a full flat setting in which the cam engages the tensioner wire and mastic material is delivered through the gap in a flat profile over the tape joint.  
     The invention further contemplates a drywall finishing tool having a coater body provided with a cover, a wiper blade and a backer bar, and a cam acting against a tensioner wire to move the backer bar so that mastic material is controllably squeezed between a gap between the cover and the blade. The invention is improved wherein the cam is an elliptical cam rotatably mounted on a retaining assembly attached to the coater body and acting against a cam tensioner plate fixed behind the cam. The cam further acts on a tensioner wire having a central portion extending behind the retaining assembly and end portions wrapped around a front of the retaining assembly. The wire contacts a cam follower plate engageable with a curved backer bar.   
     Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings.  
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       The drawings illustrate the best mode presently contemplated of carrying out the invention.  
       In the drawings:  
         FIG. 1  is a perspective view of a drywall finishing head for applying mastic material to a taped wallboard joint;  
         FIG. 2  is an end view taken along line  2 — 2  of  FIG. 1  showing a cam assembly for the finishing head in a full crown setting;  
         FIG. 3  is a view similar to  FIG. 2  showing the cam assembly in a full flat setting;  
         FIG. 3A  is an elevational view of a cam follower plate used in the cam assembly;  
         FIG. 3B  is a view of the cam acting against a cam tensioner wire and the cam follower plate;  
         FIG. 4  is a partial sectional view of the drywall finishing head as applied along a wallboard and corresponding to the full crown setting; and  
         FIG. 5  is a view similar to  FIG. 4  showing the drywall finishing head in the full flat setting.  
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     Referring now to the drawings,  FIG. 1  illustrates a drywall finishing head  10  embodying the present invention. The drywall finishing head or coater  10  has a generally rectangular coater body  12  with a pair of side plates  14  fixed by fasteners  16 . A pair of generally triangular shaped shoes  18  are joined to the side plates  14  by fasteners  20 . Extending downwardly from the side plates  14  is a pair of arm links  22  which are connected by fasteners  24 . The lowermost ends of the links  22  are equipped with a pair of inwardly directed, rotatable wheels  26  which are adapted to rollably engage a wallboard  56  and reduce friction as the head  10  is drawn downward along the wallboard  56 . The coater  10  also includes a bent cover  28  which overlies a wiper blade  30  and is removably joined to the coater body  12  by thumbscrews  32 . The cover  28  may be removed as desired to enable cleaning of the finishing head  10 . Attached to the other side of the coater body  12  is a retaining assembly  34 .   
     Referring now to  FIG. 4 , the retaining assembly  34  includes a spherical cavity or ball socket  36  designed for movably receiving a ball assembly  38 . The ball assembly  38  consists of an apertured ball member  40  designed to fit into the socket  36 , a curved arm member or goose neck  42  and a coupling member  44 . The goose neck  42  has a hollow interior that allows mastic material to be moved from a pneumatically-actuated storage body  46  to the finishing head  10 . The goose neck  42  is curved so the head  10  may be parallel to the wallboard while a user is holding the storage body  46  at an angle to the wallboard  56  and head  10 . The coupling member  44  is designed to attach the finishing head  10  to the storage body  46 .  
     A C-shaped knee spring  48  has one end pivotally connected to a fixed clevis  50  extending from the goose neck  42 , and another end pivotally formed to a fixed clevis  52  integrally formed on the retaining assembly  34 . An auxiliary coil spring  54  is connected between the ends of the knee spring  48 . During operation of the storage body  46  and finishing head  10 , the knee spring  48  biases the head into a known orientation as illustrated in  FIG. 4 . Thus, when the drywall head  10  is removed from the wallboard  56 , the head  10  returns to the position shown in  FIG. 4 . The auxiliary spring  54  supplements the knee spring  48  to allow finishers the ability to operate the coater  10  with the wheels  26  off the wallboard  56 . This is necessary, for example, when coating to the edge of a window opening or when the wheels  26  would contact and make tracks in another nearby wet joint. Referring now to  FIGS. 2 ,  3  and  4 , the coater  10  further includes an inner baffle  58  which is J-shaped in cross section ( FIG. 4 ) and has a top end secured to the coater body  12  by fasteners  60 . A bottom end of the blade  30  is retained in the curved bottom of the inner baffle  58 . A spacer and shim assembly  62  to provide a precise relationship between the retaining assembly  34  and a backer bar/blade assembly to be described is interposed between the coater body  12  and the retaining assembly  34  and surrounds the ball member  40 .   
     A cam assembly for the finishing head  10  will now be described. A nut  64  has a shaft  65  which is screw threaded into the ball retainer/tensioner assembly  34  and provides a mounting surface for an elliptically shaped cam  66  having a cam lever  68 . A cam follower plate  70  is formed with a groove  72  as shown in  FIGS. 3A and 3B . The cam  66  acts against a cam tensioner wire  74  having a central section  76  which runs behind the retaining assembly  34  and lies adjacent the top of the cam follower plate  70  as seen in  FIG. 3B . Cam tensioner wire  74  acts against the cam follower plate  70 . The ends  78  of the cam tensioner wire  74  are bent around the ends of the retaining assembly  34 . Underlying the central portion  76  of the cam tensioner wire  74  is a cam tension plate  80  which may be fixed to the nut shaft  65 . The tensioner wire  74  contacts the cam follower plate  70  which, in turn, acts against a curved backer bar  82  which with the wiper blade  30  extends across substantially the entire length of the coater body  12 .  
     In operation, the coater  10  is placed over a taped joint at the topmost end thereof. As best seen in  FIG. 4 , mastic compound is delivered from the storage body  46  through the goose neck  42  and aperture  84  in the ball member  40  into a mastic chamber  86  in the coater body  12 . Mastic material moves in the direction of the Arrow A and travels through the opening between the bottom of the inner baffle  58  and the bend  88  in the cover  28 . Mastic material continues to travel in the direction of Arrow B until it is dispersed from the gap between the cover  28  and the wiper blade  30 . The head  10  is moved over the tape joint from top to bottom delivering the desired crown. Most joints require multiple coats so that progressively larger coater heads  10  are used to finish the tape joint.  
       FIG. 4  shows the finishing head  10  in a full crown setting as shown in  FIG. 2  wherein the cam lever  68  is pivoted in the direction of Arrow C. The elliptical cam  66  is out of contact with the tensioning wire  74  and the cam follower plate  70  is retracted as depicted by Arrow D in  FIG. 4  so that the backer bar  82  and  wiper blade  30  are bowed. Mastic material is delivered through the gap in a convex profile over the tape joint.  
       FIG. 5  shows the finishing head  10  in a full flat setting as shown in  FIG. 3  wherein the cam lever  68  is pivoted in the direction of Arrow E. As the cam lever  68  is shifted, the elliptical cam  66  engages and deflects the tensioner wire  74  ( FIG. 3B ) thereby pushing the cam follower plate  70  in the direction of Arrow F ( FIG. 5 ). Pushing the cam follower plate  70  causes the backer bar  82  to move against the wiper blade  30 . As a result, the amount of mastic material delivered to the gap is reduced to a flat profile over the tape joint. Obviously, the cam lever  68  may be positioned over 180 degrees to determine the amount of crown (profile) to be delivered from the finishing head  10 .  
     It has been discovered that forming the backer bar  82  with a curved surface enhances control over the desired crown (profile) of the mastic material whereas the prior art flat backer bar relies on the viscosity of the mastic compound, the setting of the cam tensioner and the deflection of the cam tensioner. In addition, elliptical profile of the cam  66  of the present invention provides a constant change of crown as compared with the prior art half rounded cam with offset pivot wherein a small adjustment of the cam lever makes abnormally large adjustments of the backer bar and blade. In the present design, the spring rate of the backer bar  82  is controlled by the width of the groove  72  in the cam follower plate  70 . A wider groove  72  provides a lower (softer) spring rate than a narrow one. The prior art finishing head does not provide for this. The amount of blade deflection is limited by the depth of the groove  72  in the cam follower plate. This provides control over the profile of the compound yet allows the blade to deflect slightly if it hits something solid without lifting the outer edges of the coater off the wall. The prior art system would deflect so far as to overcrown the compound making a subsequent coat “near to impossible” without scraping or sanding the joint. In the finishing head  10  described herein, the bottom of the cam follower plate  70  provides springable support for the backer bar  82 . The prior art device has the cam tensioner wire contacting the backer bar at two points which can result in formation of three crowns instead of one. As mentioned previously, the mounting of the knee spring  48  provides a return position for the coater head  10 . The addition of the auxiliary spring  54  provides finishers with the ability to operate the coater  10  with the wheels  26  off the surface of the wallboard  56 . In the present head  10 , the U-shaped tensioner wire  74  increases the force applied to the backer bar  82  so that the proper crown could be achieved on each of the coats. Crown control is necessary because of the amount of shrinkage that occurs when each coat dries. Further, the cam adjuster is shimable to allow for setting the relaxed convex travel of the blade  30  whereas the only adjustment in the prior art head is to bend the tensioner spring.   
     While the invention has been described with reference to a preferred embodiment, those skilled in the art will appreciate that certain substitutions, alterations and omissions may be made without departing from the spirit thereof. Accordingly, the foregoing description is meant to be exemplary only, and should not be deemed limitative on the scope of the invention as set forth with the following claims.

Technology Classification (CPC): 4