Abstract:
A bracing system for canvas stretcher frames allows for positioning of one or more braces in a stretcher bar frame, to prevent inward pulling of the stretcher bars under canvas tension. The braces are modular in design and allow convenient assembly of multiple-braced stretcher bar frames. The braces have tenons which engage with a groove extending around the inner periphery of the stretcher bar frame, and the braces themselves preferably have grooves in their edges, to receive the tenons of further braces.

Description:
REFERENCE TO PROVISIONAL APPLICATION 
     This application claims the benefit of provisional application Ser. No. 60/195,592, filed Apr. 6, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention concerns canvas stretching frames, and particularly a bracing system for relatively large canvases, to prevent pulling inward of the peripheral stretcher bars due to tension. 
     Canvas stretcher frames have previously used bracing for large canvases. However, these were limited by several factors. The braces, typically in a cross or X configuration in the frame, comprising a pair of brace bars orthogonally arranged relative to the peripheral stretcher bars, usually crossed over one another with routed out sections of each brace, in a manner such that the two crossing braces lay in the same plane. Where the ends of the braces met the stretcher bars, typically each brace had a protruding tongue or tenon at its end, which was received in a routed out recess or mortise in the stretcher bar at that location, dedicated to that purpose. 
     In conventional stretcher bar bracing systems there was no specific provision for addition of further braces beyond the two crossing braces described. In very large canvases further bracing is often needed, requiring custom-made additional braces, further routing out of components to receive bracing, etc. 
     SUMMARY OF THE INVENTION 
     The present invention provides a relatively simple and easily used bracing system for large canvas frames, wherein the bracing bars are modular in nature and can be added as needed, and wherein braces do not cross over one another. 
     The peripheral stretcher bars in the system of the invention each have a groove formed on the inner side, so as to face inwardly toward the interior of the space behind the canvas. These grooves are employed to receive bracing bars wherever needed, and in as great a number as needed, and these can include angled braces. Thus, at the back of a canvas frame of the invention, a contiguous groove runs completely around the inner periphery of the frame. 
     The braces useful for these canvas frames are relatively simple bars, but with tabs or tongues at their ends, insertable into the grooves of the stretcher bars which make up the canvas frame. In the invention, in one typical application, a single brace will extend through the entire length or width of the stretcher bar frame. The remaining bracing is accomplished between that brace, typically centrally located, and the peripheral stretcher bars which are parallel to that single brace, these additional braces being perpendicular to the center brace. In other applications, two or three or four full-length braces can be assembled to retain stretcher bars from pulling inwardly in a long dimension of a large canvas. Then, shorter sections of brace can be inserted in the perpendicular direction, completing the bracing of the entire frame. To receive the additional bracing, the braces themselves have grooves along their side edges, and these grooves receive the tabs or tongues of the additional, smaller braces that are inserted as described above. 
     In a variation of the above, the bracing can be accomplished with a series of braces all of which are shorter than the full dimension between parallel stretcher bars. A pattern or matrix of the shorter braces can be put together, if desired, all fitting together with the tab and groove connections, so as to build a bracing system that retains all four frame stretcher bars from pulling inward under tension, without any single brace extending from bar to bar. 
     By this modular set of components, the bracing of a large canvas is made much more efficient, the bracing system can be made stronger, and the modularity of the components allows a framing shop to build a multiplicity of different sizes of fully braced frames from stretcher bars and standard brace components held in inventory, or a customer can purchase components to build a braced frame as needed. This is in contrast with the conventional situation in which a large frame often has to be custom built, with custom-formed bracing. 
     It is thus among the objects of the invention to improve the construction of braced canvas frames, and to modularize a bracing system so as to allow braced frames to be put together in a modular manner without custom-made components. These and other objects, advantages and features of the invention will be apparent from the following description of a preferred embodiment, considered along with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a canvas frame having a groove on the inner side of the stretcher bars in accordance with the invention. 
     FIGS. 2A through 2T show various bracing configurations built from the system of the invention. 
     FIG. 3 is a schematic view showing another arrangement of bracing. 
     FIG. 4 is a simplified cross-sectional view, showing a single brace engaged between two stretcher bars. 
     FIG. 5 is a fragmentary view showing wedge-shaped keys which can be used to secure a brace to a stretcher bar, and which can be used to hold the frame in an expanded configuration when the frame has been expanded to re-tension a canvas. 
     FIG. 6 is a fragmentary view showing use of a corner support at a stretcher bar frame corner. 
     FIG. 7 is a fragmentary view showing a tenoned corner support or angle brace as used at the joint between a brace and a stretcher bar. 
     FIG. 8 is a side view showing one form of brace of the system, used for extending from one stretcher bar to the opposite stretcher bar. 
     FIG. 9 is a view showing a different form of brace, for extending from a stretcher bar to another brace. 
     FIG. 10 is another view similar to FIGS. 8 and 9, showing in edge view a brace which extends between two braces. 
     FIG. 11 is a fragmentary view showing a corner of an assembled frame, and showing a diamond brace or angled brace between two right-angle stretcher bars. 
     FIG. 12 is a perspective view showing another aspect of the invention, in which stretcher bars have double mortise-and-tenon joints at frame corners. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows in perspective and somewhat schematically a stretcher bar frame  10  for receiving a canvas. FIG. 1 is only for purposes of illustration; the frames with which the invention is concerned are generally of somewhat different proportion, with at least one dimension which is quite long so as to require bracing to prevent the stretcher bars  12  and  14  from pulling inwardly under the tension of a stretched canvas. 
     FIG. 1 shows a groove  16  which is provided in each stretcher bar, on the inner surface such that it faces inwardly in the completed stretcher bar frame  10 , forming a contiguous groove around the inner periphery of the frame. Preferably this groove  16  is located off center on the inner surface  18  of each stretcher bar, either toward the canvas at the front of the frame or toward the back of the frame. This is to accompany a preferred embodiment of the invention which will be described below. 
     FIG. 1 also shows recesses  20  and  22  which are portions of mortises, of the mortis-and-tenon joinery preferably used at the mitered edges of the stretcher bars of the frame. 
     FIGS. 2A through 2T show various configuration of bracing according to the system of the invention, with the bracing in different patterns on frames, for purpose of example, that have essentially the same proportions. FIG. 2A, for example, shows a stretcher bar frame  10  having side and end stretcher bars  12  and  14 , and having a single, simple brace  24  spanning across the shorter dimension, so as to maintain separation between the longer stretcher bars  14  under conditions of canvas tension. 
     FIG. 2B shows a similar frame  10   b  with three simple braces  24  supporting the longer stretcher bars against pulling inwardly. 
     FIGS. 2C and 2D are similar to FIGS. 2A and 2B but show that the simple braces  24   a  can be of greater width when needed. 
     FIGS. 2E and 2F show the use of a simple brace  24   a  crossing the narrow dimension of a frame, while braces  26  in the perpendicular direction are provided for bracing the end stretcher bars  14 . The braces  26 , which can be called cross braces, do not extend the full length between the end stretcher bar  12  but extend from a stretcher bar to the simple brace  24   a.    
     FIGS. 2G and 2I are included to show that the simple brace  24   g  or  24   i  can either be positioned in the horizontal direction or the vertical direction, with the cross braces  26   g  or  26   i  being perpendicular. In each of these drawings, the braces are shown with continuous lines for the brace is continuous, and cross braces are shown abutting against the continuous braces. 
     FIGS. 2P through 2T show diamond braces or angled braces  30  near corners of the stretcher bar frames. These can be used alone, as in FIG. 2P, or in combination with other bracing as in the other drawings. FIG. 2Q shows four diamond braces  30  and a simple brace  24  extending across the narrower dimension of the frame, as does FIG.  2 R. FIGS. 2S and 2T show other patterns. 
     FIG. 3 shows another exemplary frame pattern wherein a stretcher bar frame  32  has a set of braces none of which is a “simple” brace extending the entire length between parallel stretcher bars. This differs from the arrangements shown in the Figures above. 
     FIG. 4 shows one preferred arrangement for attachment of the braces to the stretcher bars. A simple brace  24  is shown extending between two parallel stretcher bars  14 , the stretcher bars being shown in cross section. The stretcher bar grooves are shown at  16 . The grooves receive a tenon  34  which extends from each end of the simple brace  24 . FIG. 4 illustrates the reason why the stretcher bar groove  16  is located either toward the front of the frame or back of the frame, rather than centrally located along the width of the stretcher bar. The tenons  34  preferably are at one edge of the brace  24  as shown. The brace has an adjacent end  36  which does not protrude as does the tenon, and which abuts against the face of the stretcher bar  14  as shown. Between the end  36  and the tenon  34  is preferably included a groove  38 , which is known from previous canvas frame braces. The groove  38  allows a wedge-shaped key  40  to be tapped in with a hammer when the frame needs to be expanded for re-tensioning the canvas. 
     FIG. 8 shows the simple brace of FIG.  4 . The narrow dimension of the brace is shown, and as noted above, the wide dimension can be in several different widths if desired, as illustrated in FIGS. 2A-2T. 
     FIG. 9 shows a different brace  26 , which can be called a cross brace as noted above. This brace is similar to the simple brace  24 , but it is not intended the full length from stretcher bar to stretcher bar, and thus has a different second end  42 . The second end  42  has a tenon  44  at its center, designed to engage with a groove in another brace. The groove is shown, for example, at  46  in the cross brace  26  of FIG. 9, and it should be assumed that all braces shown herein can have such grooves, although certain braces dedicated to not receiving any further bracing can be formed without the grooves. This groove  46  is also shown in FIG.  8 . 
     FIG. 10 shows a further type of brace  48 , which can be called a middle brace. Such a brace would be at a position such as shown at  48  or  48   a  in FIGS. 2N and 2O, for example, or at  48   a  in FIG.  2 T. Neither end of the middle brace  48  engages against a stretcher bar, but the brace merely extends between two parallel braces. Again, these braces may or may not have grooves for receiving further braces. 
     FIG. 6 shows a corner support  50  which can be used with frames according to the invention. The corner support  50 , which can be formed with hardboard, extends into the groove  16  of both the stretcher bars  14  and  16  at the corner formed between them, and screws  52  can be used to lock this corner support in position. This illustrates another useful purpose of the groove  16 , which extends around the inner periphery of the stretcher bar frame. FIG. 6 also-shows keys or wedges  40  which can be used to lock the frame size once the frame has been expanded (as shown at  54 , for example) to re-tension a canvas. These wedges are jammed not into the groove  16  but into mortises which a part of mortis-and-tenon joinery at the mitered corners of the frame, as further explained below. 
     FIG. 11 shows another type of angled brace  56  or diamond brace  30 , also shown in FIGS. 2P-2T. This illustrates the use of the groove  16  to receive this type of brace. Again, a screw  52  can be used on the stretcher bars at each end of the diamond brace  30 , to lock the brace in position. FIG. 7 shows a corner support  55  as used between a brace  24  or  26  and a stretcher bar  12  or  14 . The corner support is tenoned as shown at  58 ,  60  and  62  so that it enters the groove  46  of the brace and the groove  16  of the stretcher bar. 
     Gluing can be used between internal members of the brace system, but preferably braces are not glued to stretcher bars, since the stretcher bar frame may need to be expanded for re-tensioning of a canvas. 
     FIG. 12 shows a portion of a stretcher bar  14 , having a double mortise-and-tenon joinery configuration. Typically stretcher bars have a single mortise  66  and a single tenon  68 , positioned to be engaged with a complementarily placed tenon-and-mortise in another stretcher bar which is mitered together with the bar  14  at a right angle. In this aspect of the present invention, the end of a stretcher bar  14  has a double mortise and tenon, with an additional tenon  68  and an additional mortise  66  as shown. These cooperate with complementarily placed mortises and tenons on the stretcher bar end to be joined together with the bar  14 . These joints typically are not glued but are merely held together by friction, and the doubling of the mortise-and-tenon connection adds further stability and strength to the connection. 
     Although, as stated above, the braces of the invention, i.e. the simple braces or the cross braces which have at least one end engaging with a stretcher bar, have their tenons  34  at one edge of the brace, these could be centered in the end of the braces if desired, in the same manner as the tenon  44  seen in FIG. 9 or the tenons  49  in FIG.  10 . As noted above in the discussion of FIG. 4, the principal reason for the offset of the tenons in the main braces  24  is to accommodate the brace groove  38  which is useful when the frame is expanded for re-tensioning a canvas. However, modularity of the invention can be served by having all braces with the tenons centered. In that case, a different type of expansion wedge can be used between the simple braces and the adjoining stretcher bars, i.e. a forked wedge (not shown) having two wedge sections joined together in a generally U-shape. In this way, the main braces or simple braces described above can be interchangeable with cross braces or middle braces for better modularity in assembling braced frames of various sizes and configurations. In the case of centered tenons on all braces, the groove  16  in the stretcher bars would be moved more toward the center of the width of the bar. 
     Variations to the preferred embodiments are possible. For example, the braces and/or the stretcher bars could have ridges rather than grooves, with the ends of braces having a groove or mortise to engage with the ridge. The grooves or ridges on the bars or on the braces can be referred to as engagement tracks, and the corresponding engagement structure (tab or groove) on the brace ends can be referred to as cooperative engagement structure. 
     The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to this preferred embodiment will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.