WO2013119105A2

WO2013119105A2 - A stretcher for canvas, a stretcher with canvas, method for makig a stretcher for canvas

Info

Publication number: WO2013119105A2
Application number: PCT/NL2013/000002
Authority: WO
Inventors: Johannes Hendrikus Ninaber
Original assignee: Lizara Freezone N.V.
Priority date: 2012-02-12
Filing date: 2013-02-08
Publication date: 2013-08-15
Also published as: NL1039374C2; DE202013001193U1; WO2013119105A3; NL1040045C2; CN203468140U; NL1040045A

Abstract

A stretcher for a canvas (4) comprises a means (3) for tensioning the canvas. The stretcher comprises an inner frame (1) surrounded by an outer frame. The outer frame comprises frame sections (2) surrounding the inner rigid frame (1). Rotating a means for tensioning around an axis of rotation (D) transverse to the frame plane causes the distance between the inner frame and a frame section of the outer frame to be changed. The means for tensioning are accessible from the back side of the stretcher.

Description

A stretcher for canvas, a stretcher with canvas, method for making a stretcher for canvas

The invention relates to a stretcher for canvas having a front side for a canvas fastened on the stretcher and a means for controlling the tension in the canvas, a stretcher with canvas and a frame section for a stretcher for canvas. The invention also relates to a method for making a stretcher for canvas. Canvas is fastened to a stretcher, either to display a print or an image on the canvas, or without an image so that a painter can paint an image on the canvas. A frame is provided with a front side at, or against, which side the canvas is positioned. The canvas may be provided with an image prior to framing and tensioning or thereafter. The canvas is fastened and then tensioned. However, due to humidity or temperature changes, the tension in the canvas may become less and, as a consequence, the canvas may sag. The conventional, and still generally used, methods for fixing a sagging canvas rely on a canvas tensioning frame, whereby the tips of the four wooden or partly metal frame sections are equipped with corner joints.

One of the ways of tensioning the canvas is to make corner sections expand. In conventional frames comprising separate frame sections this is achieved by forcing the frame sections apart at the corners or, if the frame comprises separate corner sections, by forcing the corner sections to protrude further from the frame sections. To do so, conventional frames are provided with means for exerting a tensioning force on the frame sections or corner sections to make them move with respect to each other. In conventional frames tools are required and said tools must be positioned close to the canvas and the tools are used in close vicinity to the canvas. These circumstances carry a risk of inadvertently damaging the canvas. In addition, the frame sections do not bend only at the corners and a substantial number of repeated corrections may be needed to achieve the correct tensioning adjustment at positions other than at the corners.

Conventional stretchers and tensioning methods are complex and the constructions do not allow simple and reversible adjustment of tension in the canvas.

It is an objective of the invention to provide a stretcher for a canvas that allows varying of the tension in the canvas in a quick and easy manner. To this end, the stretcher according to the invention is characterized in that the stretcher comprises a first rigid inner rectangular frame surrounded by a second outer frame, comprised of frame sections, surrounding the inner frame wherein the first frame and the frame sections of the second frame are provided with matching grooves and tongues and the stretcher is provided with the means for tensioning the canvas, said means for tensioning having a axis of rotation transverse to the frame plane and wherein the means for tensioning are so shaped and positioned that rotating the means for tensioning around the axis of rotation causes a change in distance between the first frame and a frame section of the second and wherein the means for tensioning are accessible for rotating from the back side of the frame the means for tensioning around the axis of rotation.

The means for tensioning is accessible from the back side of the frame and has an axis of rotation transverse to the frame plane. Adjustment of the means for tensioning is easy, compared to means such as, for instance, screws, which have to be turned in the frame plane. It is not needed to perform actions in the vicinity of the canvas. The first, inner frame forms a rectangle. On this rectangle the frame sections of the second outer frame are provided. The first frame forms a rigid skeleton on which the frame sections of the second frame are provided. The frame sections of the second frame use grooves and tongues in a direction parallel to the frame sections provided on the first inner rigid frame. The matching grooves and tongues provide for a robust connection between the first frame and a frame section of the second frame to withstand forces exerted by the tensioned canvas while allowing the frame section of the second frame to be moved with respect to the first frame. The stretcher according to the invention also provides for a sturdy construction even in absence of the means for tensioning. By turning a means for tensioning, the distance between a frame section of the second frame and the inner frame is changed, for instance by moving the frame section away from the first frame. This allows an easy way of adjusting the tension on the canvas. The frame sections form a flexible frame all the way around the canvas, i.e. surrounding the first inner frame, allowing tension in the canvas to be adjusted all around the canvas by the means for tensioning.

It is noted that US patent US 4,635,700 discloses a construction for a frame for a canvas comprising an inner rigid frame provided at top and bottom side with top and bottom members. Adjustable springs or spring and bolt mechanisms allow putting force on the top and bottom members to adjust the distance at top and/or bottom between the rigid frame and the top and bottom members. Adjustable springs are also fastened at left and right sides of the rigid frame and said springs are attached to rings provided in the canvas. By tightening the springs the canvas can be tensioned. The construction disclosed in US 4,635,700 does not comprise a flexible frame around the inner frame. It does not comprise a means for tensioning other than springs and spring and bolt mechanisms which do not have an axis of rotation transverse to the frame plane. The parts do not comprise matching grooves and tongues. The use of two separate spring constructions, one of which connected to rings in the canvas, as well as the use of top and bottom members only, is described in US 4,635,700 as being essential.

Furthermore it is noted that the European Patent application EP 0277 107 discloses a construction wherein, in the corners of a simple frame formed by frame sections that have straight edges under an angle of 45 degrees with the direction of the frame sections, a rotatable disc is provided between the frame section. Turning the disc will provide a change in the distance between the frame parts, forcing the corners apart. EP 0 277 107 describes the presence of the disc in the corners as essential. The construction described in EP 0 277 107 does not comprise an inner frame surrounded by a flexible outer frame. The construction does not comprise matching grooves and tongues. The frame parts comprise grooves in which the rotating disc is provided. However, the frame parts themselves do not have matching grooves and tongues. Without the rotating disc the construction falls apart. Preferably the means for tensioning are formed such that said means have a first contact surface in contact with the inner frame and with a second contact surface in contact with the frame section of the second frame, whereby the distance between the first and second contact surfaces varies with the angle of rotation. The variation in distance between the contact surfaces changes the distance between the first frame and the frame section of the second frame.

Preferably the means for tensioning comprise a non-rotational symmetric portion (3B) with respect to the axis of rotation comprising the first contact surface, where the nonsymmetrical portion (3B) has an outline (O) in the shape of a spiral, preferably extending over more than 90 degrees, and most preferably substantially over 270 degrees. Such design provides for good control.

The method for making a stretcher according to the invention comprises the following steps: - providing a first and second longitudinal profile comprising matching groove and tongues and aligning the groove and tongues

- providing a hole in one of the profiles

- providing a means for tensioning aligned with the hole

- inserting the means for tensioning in the hole

- pushing the first and second profile together to form an assembly

- cutting the assembly at an angle

This provides, in a manner that is fast and accurate, a number of assemblies. A number of assemblies can be sold as a kit for manufacturing one or more stretchers, or subsequently immediately manufactured into a stretcher by attaching four assemblies at the corners, whereby the inner profiles are connected to each other and the outer profiles are not. Preferably a strip comprising the means for tensioning is provided and aligned with the hole. Providing a strip of means for tensioning allows for a fast and accurate way of aligning the means for tensioning with the hole.

These, and further, aspects of this invention are described and illustrated below with references to the figures.

Herein

Figure 1 a back side (rear) view of a stretcher according to the invention.

Figure 2 a stretcher comprising a first and second frame according to the invention.

Figure 3 a detail of a stretcher according to the invention.

Figures 4A to 4C further examples of a stretcher according to the invention.

Figures 5 to 7 several examples of possible positions for a means for tensioning.

Figure 8 an embodiment in which frame section 2 is hollow.

Figure 9 various embodiments showing grooves and tongues

Figure 10 a further embodiment

Figure 11 an embodiment of figure 10 in more detail

Figure 12 a preferred embodiment of a means for tensioning.

Figure 13 a sectional view of a part of a means for tensioning Figure 14A a sectional view of a part of a further embodiment for a means for tensioning.

Figure 14B a detail of a part of a means for tensioning

Figure 14C in graphical form, the relationship between the angle of rotation and the distance between first frame and a frame section of the second frame.

Figure 14D a variation on the shape shown in figure 14 A.

Figures 14E and 14F further example of spiral outlines.

Figure 15 the part of figure 14A in more detail.

Figures 16 to 19 embodiments of a means for tensioning.

Figure 20 illustrates steps in a method according to the invention

The figures are examples only, comparable components are in general indicated by means of the same number.

Figure 1 shows a back side view of a stretcher according to the invention, i.e. a view on the back side of a stretcher according to the invention, the front side being the side at which, in use, the canvas is fastened. The stretcher comprises a first, inner frame 1. Frame 1 may be composed of more than one part. Around frame 1, thus around the edges, frame sections of an outer frame 2 are provided. The stretcher is provided with a means 3 for moving outwards, thus away from inner rigid frame 1, the frame sections 2. Rotated in one direction the frame sections 2 move outwards, When the means for tensioning 3 are rotated in the opposite direction, the tension in the canvas moves frame section 2 towards inner frame 1. This provides flexibility to the outer frame.

Figure 2 shows a stretcher according to the invention in cross section. V denotes the front side of the stretcher and A the back side. In this exemplary embodiment stretcher 3 is provided with a hole in which means for tensioning 3 are present. Said means for tensioning is accessible from the back side of the stretcher. Rotating means for tensioning 3 around an axis of rotation D transverse to the frame plane, said plane being parallel to the canvas, moves frame section 2 with respect to inner frame 1. Canvas 4 is tensioned and attached to the flexible frame 2 as shown in the top part of figure 2, or on frame 1 as shown in the bottom part of figure 2, but preferably on frame 2 only. Attachment to the flexible frame 2 is preferred. Attachment to frame 1 will cause movement of canvas 4 over the edges of frame 2 and gives a risk of wear. If the tension in the canvas has changed, means for tensioning 3 can be turned to increase or decrease the distance between frame section 2 and inner frame 1. If there is a risk that the tension becomes too high, one may turn the means in an opposite direction to reduce the tension in the canvas. Frame sections 2 and frame 1 are provided with matching grooves and tongues. The matching grooves and tongues provide for a sturdy construction between first inner frame and second, flexible outer frame, while still allowing movement of frame sections 2 to and from inner frame 1. The construction is sturdy and independent of whether or not the means for tensioning are provided. It is to be noted that in the above mentioned prior art documents the construction falls apart without the presence of the rotatable disk or the spring and bolt mechanisms. Canvas 4 may be attached to the stretcher by means of staples or by any other suitable means, for instance by using a clamping or locking construction.

Figure 3 shows a detail of a stretcher according to the invention. Means for tensioning 3 can be rotated around axis of rotation D. In this embodiment the means for tensioning is provided in a hole in the first frame 1. The edge of tongue 6 in frame section 2 touches an, in this example, serrated surface of means for tensioning 3. Said surface has such a form that the distance between said surface and axis of rotation D, along a line transverse to the axis of rotation D and towards frame section 2, is a function of the angle of rotation of means for tensioning 3 in the hole in inner rigid frame 1. Rotation of means for tensioning 3 changes the distance between axis of rotation D and the surface or point at which tongue 6 of frame section 2 touches means for tensioning 3. Rotation causes frame section 2 to be pushed away from inner frame 1, or, if rotation is in the opposite direction, the tension in canvas 4 pushes frame section 2 back towards inner frame 1. In this embodiment, the means for tensioning is formed such that it first touches with a surface of inner frame 1 ; in this embodiment the contact surface touches the inner wall of the hole in inner frame 1. This contact surface is stationary. The means for tensioning comprises a second contact surface being formed by the contact surfaces of tongue 6 of frame section 2 and means for tensioning 3. The distance between the first and second surfaces, and thereby the distance between inner frame 1 and frames section 2 of outer frame, changes as a function of the angle of rotation of means for tensioning 3 in the hole.

The change in distance is indicated in figure 3 by the double arrow Δ. An additional advantage is that the outer circumference of the stretcher can be adjusted so that one and the same stretcher may be used for more than one canvas size by rotating means 3 while also being able to accommodate variations in tension. Preferably the variation in distance Δ is larger than 4 mm, preferably between 5 and 8 mm. Such a large freedom in variation of dimension makes it also possible, when using more than one means for tensioning per side of the stretcher, to properly tension a not perfectly rectangular canvas. This is a particular advantage when an old canvas has to be mounted on a stretcher. Figures 4A to 4C show a further example of a stretcher according to the invention. In figure 4A an example is shown in which the means for tensioning 3 is provided in between the first inner frame 1 and the flexible outer frame 2. The first frame 1 is, in tongue 5 of frame 1, provided with apertures 7. In said apertures means for tensioning 3 are set. Said means for tensioning comprise in this example, a substantially circular middle part, to turn means 3 in aperture 7, and an upper and optionally also lower part, shaped in an approximate drop shape. Figure 4A shows the various parts separate from each other, Figure 4B shows them in an assembled form, in a condition where the distance between parts 1 and 2 is minimal. Turning means for tensioning 3 will cause a change in the distance between parts 1 and 2.

Of the two embodiments shown in figures 3 and 4, the embodiment of figure 3 is preferred. The moving contact surfaces are located inside the stretcher and thereby protected.

In figure 5 the means for tensioning 3 is located in a hole in the first inner frame 1. If a hole is made in the first frame 1 or in a frame section of outer frame 2, it is preferably formed as a blind hole. The means for tensioning 3 provides for the relative movements of frame sections 2 and inner frame 1, and said means for tensioning is preferably provided in first inner frame 1 , but could also be provided between inner frame 1 and flexible outer frame 2, or mainly in flexible outer frame 2. Various possible positions for the means for tensioning 3 are shown in Figure 5. The means for tensioning 3 may be provided mainly in inner frame 1, as shown in figure 3, which is preferred, or inbetween inner frame 1 and the outer frame 2, as shown in figures 4A to 4C or mainly in the outer frame. Figure 6 shows various positions for means for tensioning. The frame sections 2 form a flexible frame around the inner rigid frame 1. The distance between the frame sections 2 and inner rigid frame 1 is easily adjustable at various points along the frame 1. Preferably a rectangular stretcher comprises at the long sides two or more, and as in the figure three, means for tensioning. Along each side the means for tensioning are preferably provided symmetrically with respect to a vertical or horizontal symmetry axis of the stretcher.

Figure 7 illustrates a means for tensioning that extends mainly in the outer frame, the top part of Figure 7 showing a means for tensioning positioned in a blind hole.

Figure 8 shows an embodiment where frame 1 is hollow. Of course, frame section 2 may also be hollow. Hollow parts may be made by means of extrusion of, or from, plate steel.

Figure 9 show various embodiments for using grooves and tongues. Figure 10 shows a further embodiment. In the embodiment of Figure 10 there is some play between the grooves and tongues. Rotating the means for tensioning 3 will cause a small change in the angel between frame sections 2 and inner frame 1.

Figure 11 illustrates the embodiment in more detail. The means for tensioning is, in this embodiment, provided with a larger head on the front side, i.e. the side closest to the canvas 4, than on the back side. Rotating the means for tensioning 3 causes frame section 2 to tilt with respect to inner frame 1. The front side of frame section 2 is pushed further away from inner frame 1 than the back side of frame section 2. Therefore rotating the means for tensioning causes a difference between the front and back side of frame section 2 related to the change in distance between frame section 2 and inner frame 1. This causes a tilt. This is advantageous since the tension in the canvas tends to tilt the frame section 2 in the opposite direction, and any increase in tension in the canvas increases this tilt caused by the tension in the canvas. By making the means for tensioning asymmetric with respect to the front and back sides of the stretcher, the two effects can be counterbalanced, leading to greater control and less wear and tear.

Figure 12 illustrates a preferred embodiment of means for tensioning 3. The means for tensioning comprise a circular symmetrical portion 3 A and a portion having a circumference O which is not rotationally symmetric with respect to axis of rotation. This portion lies between the first inner frame 1 and a frame section 2 of the flexible outer frame. Rotating means for tensioning 3 increases the distance between frame section 2 and the first inner frame.

The outline of part 3B is preferably in the form of a spiral over an angle of 270 degrees and a straight line connecting the beginning and end point of the spiral form. The means 3 is provided with a triangular notch H. The notch may be rectangular or another multi-angular notch. In the notch H a tool with a corresponding head may be placed for rotating the means 3.

Figure 13 illustrates schematically a preferred embodiment of means for tensioning 3. The outline of drop shaped part 3B of means for tensioning 3 is partly in the form of an approximate spiral, formed by segments between the points E, A, B and C, wherein the tangents in the points C and E are transverse to each other, meaning that the spiral covers an angle of 270 degrees. The outline O also comprises^ straight segment between the points C and E. The length of this segment in indicated in Figure 13 by the letter R for Radius. In this embodiment the axis of rotation substantially coincides with the point C of the spiral form. Preferably the spiral is a logarithmic spiral. For a logarithmic spiral the torsion needed to turn the means of tensioning is relatively stable and small.

Figure 14A illustrates schematically a further preferred embodiment of means for tensioning 3. The outline of drop shaped part 3B of means for tensioning 3 is partly in the form of an approximate spiral, formed by segments between the points Kl, K2 etc. The points Kl, K2 etc lie on a spiral S. The tangent to said spiral at points Kl and K7 are perpendicular to each other, which means that the spiral covers 270 degrees. A straight segment connects points K8 and Kl. The length of said straight segment is indicated in Figure 13 by R for Radius, since in the figure the axis of rotation D coincided with the centre of the spiral S. This axis of rotation (see Figure 3) stays stationary with respect to inner frame 1, in cases where the means 3 is provided in a hole in inner frame 1. If means 3 is provided in a hole in frame section 2, see Figure 7, the axis of rotation stays stationary with respect to frame section 2. Figure 14B gives some detail. The angle β between lines D-Kl and K1-K2 is smaller than 90 degrees. The same holds for the angles between lines D-Ki and Ki-Ki+1. This is preferred, an angle β smaller than 90 degrees has the advantage that it takes some effort to turn back the means for tensioning.

In formula: If p<90 than

Distance axis of rotation D and edge 1 < length segment D-Ki

In order to turn backwards (i.e. lower the tension on the canvas) the canvas should first be put under increased tension since the distance between the axis of rotation and the edge is smaller than segment D-Ki. This may seem not a good idea, but, since it takes effort to turn the means for tensioning backwards, it also prevents tension in the canvas from forcing the means of tensioning to turn backwards.

Preferably the angle a lies between 30 and 60 degrees.

Preferably the spiral has approximately the form of a logarithmic spiral. This provides the best control. The larger the angle of rotation that is possible, the more control one has over the distances. Simple non spiral forms, such as ovals and ellipses allow for not more than 90 degrees angle of rotation. Such forms have a horizontal and vertical axis of symmetry, and that gives a repeat rotational frequency of 90 degrees. An angle of rotation of approximately 270 degrees, using a spiral shaped outline, as in preferred embodiments, provides a much larger degree of control. Figure 14C illustrates the difference in control. The drawn line sketches the distance between the edge of frame 1 and the axis of rotation as a function of the angle of rotation of means for tensioning 3, whereby the angle for the smallest difference has been set to 0. The dotted line sketches the same but using an ellipse of oval shaped outline. For both the dotted line and for the drawn line, the tangent to the line gives a measure for the amount of forces needed to make the means 3 rotate. If the line is very flat, one has to turn over a large angle before there is any appreciable difference. If the line is very steep, a large force is needed. The two lines illustrate that, using a means for tensioning as shown in Figure 14A, i.e. one with a contour in the shape of a spiral of 270 degrees, the degree of control is significantly increased. For an ellipse or oval there are position whereby turning the means for tensioning has almost no effect, and other positions where a small change in angle of rotation has a very large effect. This makes it difficult to control the tension. Using a spiral, illustrated by the drawn line in Figure 14C, the tangent to the drawn line does not change to nearly the same degree as that to the dotted line. This leads to better control. Also, if the line is very steep, a small change in angle of rotation creates a large change in the tension. Since every action has a reaction, this also means that there is a risk that the means for tensioning will automatically turn back to a smaller angle of rotation under the action of the tension in the canvas, unless some sort of locking or latching means is provided. One type of such means would be to have a large amount of friction. A large amount of friction increases the power required to rotate the means for tensioning and also results in increased wear and tear and a decrease in control.

Using a spiral form has the advantage that the maximum forces and the torsion are reduced compared to situation where the angle of rotation is 90 degree or less, because the change in distance is distributed over a rotation by 270 degrees, instead of over less than 90 degrees, and also because the tangent over the spiral is more or less the same and that has a smoothing effect. For the preferred embodiment of a logarithmic spiral form the angle is the same. Using an ellipse or oval shape, for which the angle of rotation is limited to 90 degrees, the tangent changes dramatically, with the result that it is difficult to control the distance setting. In Figure 14A, an outline is shown wherein between the ends of the lines K1-K2, the outline is formed in straight sections. This further reduces the risk of back turning of the means for tensioning. Forming straight sections forms a type of serration, at the points of intersection of the straight sections, in the spiral form.

In conclusion: A spiral form in the means for tensioning is a form showing a outline in a curve wherein the distance between a central point, the axis of rotation, and the contact surfaces or touching points at the curve increases as function of the angle of rotation. This allows a gradual increase in distance between the inner frame and the flexible outer frame. Using a spiral over more than 90 degrees, preferably more than 180 degrees, most preferably over an angle of approximately 270 degrees, an increase of a factor 5 to 8 in accuracy and control is obtainable, compared to using an oval or an ellipse shape.

Figure 14D shows a variation on the embodiment shown in Figure 14B.

The first two straight segments have been made curved. Since the distances are determined by the points K1, K2 etc, the distance between the axis of rotation and the line connecting K1 and K2 in the figure indicated by the dotted line are the same. Therefore the angle β is the same. The curved segment however, does have an effect of preventing even more back toning of the means for tensioning.

Figure 14E and 14F give various spiral shaped outlines with respect to the axis of rotation. Figure 14F, although it looks like a rectangle, nevertheless has a spiral outline, since the outline is taken with respect to the axis of rotation. The distances between the axis of rotation D and the segment K1-K2 and further segments are ordered as follows:

Distance between axis of rotation D and line segment K1 to K2

<

distance between axis of rotation D and line segment K2-K3

<

distance between axis of rotation D and line segment K3-K4

<

distance between axis of rotation D and line segment K4-K5

This is an extreme example of a spiral shaped outline. This outline provides only 4 different distances and within the framework of the invention, the number of distances that can be chosen is preferably larger than 4, preferably 6 or more.

Figure 15 shows an example for a radius R of 8 mm. In this example a part of the outline O is serrated. The serrations have the effect that back turning of the means for tensioning resulting from the tension in the canvas is avoided or at least reduced. Preferably, the x coordinate of point B(X1), the point where the tangent to the spiral form is transverse to the tangent at the starting point of the spiral, i.e. when the spiral has turned 90 degrees, is smaller or equal to R/2. The Y- coordinate of point A (Yl), the point where the tangent runs parallel to the tangent at the starting point, i.e. when the spiral has turned 180 degrees, is preferably equal to, or smaller than, R and larger than 2*X1. In the example R is 8 mm, X1 3.2 mm and Y1 7.7 mm.

Figure 16 shows a variation on the means for tensioning of Figure 12. In this embodiment the top and bottom parts 3B are rotational non-symmetric; the middle part 3 A is rotationally symmetric with respect to axis of rotation of the means 3 for tensioning. Parts 3 A is indicated by dotted lines.

Figure 17 shows, at the right hand side of the figure, several different shapes and forms for part 3B. Preferably part 3B comprises an outline O in the shape of a spiral.

Preferably the outline is serrated. Part 3B may be provided, in embodiments, with a flexible portion 3C or a notch 3D. In the outline of the notch 3D, a part is in the form of a spiral as shown in Figure 13. The difference with Figure 13 is that a notch is provided. This provides additional flexibility to the means for tensioning and has a smoothing effect on the tension on the canvas. Figures 18 and 19 show further embodiments of a means for tensioning for a stretcher according to the invention. In Figure 18 portion 3B is provided with a notch 3D, in Figure 19 portion 3 A is also provided with a notch to add flexibility to this portion.

Figure 20 illustrates a method according to the invention.

In the method the following method steps are provided. A first and second longitudinal profile 21 and 22 comprising matching groove and tongues are aligned along the groove and tongues. A strip 24 comprising a series of means of tensioning 23 is provided. Providing a strip has the advantage that the position and orientation of the means for tensioning can be easily controlled. In principle, the means for tensioning could be provided in an aligned manner by other means, for instance by providing a means for tensioning and then orienting and aligning said means 23 by mechanical manipulation. However, providing a strip allows for a fast and easy way of providing the means for tensioning in a controlled and oriented manner. A hole is provided in one of the profiles. In this example the hole is drilled from the bottom side at drilling station 26. The hole could also be drilled from the top side. If a blind hole is provided, the hole has to be drilled from the same side as the side at which the means for tensioning 23 is provided. In Figure 20 the means for tensioning are provided from above at station 26. The interconnecting bands in the strip between the means for tensioning 23 are cut while pushing the means into the hole. This pushing could also be done from below. The means for tensioning is so oriented that the means is provided with an angle of rotation corresponding to a minimal distance between the first profile 21, which will become part of the first, inner frame and the second profile 22, which will become part of the second outer frame. In station 27 the first and second profiles are pushed together. The first and second profile now form an assembly. In station 26 the assembly is cut at an angle, usually at an angle of 45 degrees. This provides, in a manner that is fast and accurate, a number of assemblies.

The assemblies can be sold as a kit for manufacturing a stretcher or subsequently immediately manufactured into a stretcher by attaching a number, usually four, assemblies at the corners, whereby the inner profiles are connected to each other and not the other profiles. For a rectangular stretcher the cutting angle will usually be 45 degrees. The method of the invention also allows the making of stretchers of non rectangular shape, for instance an octagonal shape, by simply changing the angle at which the assemblies are cut.

Briefly summarised, the invention can be described as follows: A stretcher for a canvas (4) comprises a means (3) for tensioning the canvas. The stretcher comprises an inner frame (1) surrounded by an outer frame. The outer frame comprises a number of frame sections (2) surrounding the inner rigid frame (1). Rotating a means for tensioning around an axis of rotation (D) transverse to the frame plane causes the distance between the inner frame and a frame section of the outer frame to be changed. The means for tensioning are accessible from the back side of the stretcher.

Obviously, the invention allows for numerous variations that are not limited to the examples given above. Words are used to describe certain parts, elements or features;

denoting the same or similar parts, elements or features by a synonym does not change the content of such a part, element or feature. A frame section may for instance also be called a frame part, frame portion, frame element etc. Grooves and tongues may also be called grooves and ridges; the back side may also be called the rear side, the front side may be called the image side, pushing may be called forcing, inserting may be called positioning in or pushing in, contact surface may be called touching surface etc. etc.

One of more parts of a frame may be made of an extruded material, one or more parts may be hollow; parts may be made of metal, wood, plastic or any combination thereof.

Preferably laminated wooded parts are used. Preferably the means for tensioning are made of a material that is harder than the material of at least one of the parts that are pushed apart by the means of tensioning. Preferably the inner frame is stiffer than the frame sections of the outer frame;

Claims

Claims:

1. A stretcher for canvas (4) having a front side (V) for a canvas fastened on the stretcher and a means (3) for controlling the tension in the canvas wherein the stretcher comprises a first rigid inner rectangular frame (1) surrounded by a second outer frame, comprised of frame sections (2) surrounding the inner frame (1) and wherein the first frame and the frame sections of the second frame are provided with matching grooves and tongues (5, 6) and the stretcher is provided with means for tensioning (3) the canvas (4), said means for tensioning having a axis of rotation (D) transverse to the frame plane wherein the means for tensioning (3) are so shaped and positioned that rotating the means for tensioning (3) around the axis of rotation (D) causes a change in distance between the first frame and a frame section of the second frame and wherein the means for tensioning are accessible from the back side of the frame for rotating the means for tensioning around the axis of rotation.

2. A stretcher according to claim 1, wherein the means for tensioning (3) are formed such that said means have a first contact surface in contact with the inner frame or the frame section of the outer frame and a second contact surface in contact with a frame section of the second frame or the inner frame, and whereby the distance between the first and second contact surfaces varies with the angle of rotation of the means for tensioning.

3. A stretcher according to claim 2, wherein the means for tensioning comprise a non rotationally symmetric portion (3B) with respect to the axis of rotation comprising the first contact surface.

4. A stretcher according to claim 3, wherein the means for tensioning comprise a top and bottom part (3 A) which are rotationally symmetric, and a middle part comprising the non-rotationally symmetric portion (3B).

5. A stretcher according to claim 3 or 4, wherein the non-symmetrical portion (3B) has an outline (O) in the shape of a spiral.

6. A stretcher according to claim 5, wherein the outline has the shape of a logarithmic spiral.

7. A stretcher according to claim 5 or 6, wherein the outline has a spiral form extending over an angle of more than 90 degrees.

8. A stretcher according to claim 7, wherein the spiral form extends over approximately 270 degrees.

9. A stretcher according to any of the claims 4, 5, 6, 7 or 8, wherein the central point of the spiral (S) substantially coincides with the axis of rotation (D) of the means for tensioning (3).

10. A stretcher as claimed in claims 5 to 9 wherein the spiral shape (S) is approximately formed by straight segments (K1-K2, K2-K3,..).

11. A stretcher as claimed in claim 9 and 10, wherein the angle between the straight segments is less than 90 degrees.

12. A stretcher according to any of the claims 2 to 11, wherein the first and/or second contact surfaces are serrated.

13. A stretcher according to any of the preceding claims, whereby the means for tensioning is provided in the first frame.

14. A stretcher according to any of the preceding claims whereby in the first frame (1) and/or in the frame section (2) of the second frame holes are provided for means for tensioning (3)

15. A stretcher according to claim 14, wherein the holes are blind holes.

16. A stretcher according to any of the preceding claims, wherein the means for tensioning (3) are asymmetric with respect to a front and back side of the stretcher for causing a difference in the distance between a frame section and the inner frame.

17. A stretcher according to any of the preceding claims wherein the stretcher is provided with a canvas and wherein the canvas is attached to the frame sections of the second frame and not to the first frame.

18 A method for making a stretcher according to any one of the preceding claims comprising the following steps:

providing a first (21) and second longitudinal (22) profile comprising matching grooves and tongues and aligning the grooves and tongues

providing (26) a hole in one of the profiles

providing a means for tensioning (23)

aligning the means for tensioning with the hole

inserting (25) the means for tensioning (3) in the hole

- pushing (26) the first and second profile together to form an assembly

cutting (27) the assembly at an angle.

19. A method for making a stretcher according to claim 18, wherein a strip (24) comprising a means for tensioning (23) is provided and the strip is aligned with respect to the hole.

20. A method according to claim 18 wherein four assemblies are interconnected at the inner profiles only to form a stretcher.