GB2551120A

GB2551120A - An easy to access cam expanding corner joint

Info

Publication number: GB2551120A
Application number: GB1609662.0A
Authority: GB
Inventors: Wayne Gill Philip
Original assignee: WUNDERBARS Ltd
Current assignee: WUNDERBARS Ltd
Priority date: 2016-06-02
Filing date: 2016-06-02
Publication date: 2017-12-13
Anticipated expiration: 2036-06-02
Also published as: GB2551120B; GB201609662D0

Abstract

A stretcher frame joint has stretcher bars with mitred ends 1, each featuring a tapered groove 2 for engaging with a wedge-shaped planar tongue plate 3 with an aperture 4. The aperture retains a cam 5 when the joint is assembled by way of undercut 6 so that the cam sits in an accessible position within the frame. A hole 7 in the cam may accept a key to rotate the cam causing the frame to expand and stretch the canvas attached thereto. The kite-shaped or cuneate tongue plate being pulled further out of the slots or grooves forces the stretcher bars apart.

Description

An easy to access cam expanding corner joint

This invention relates to the expanding joint for a canvas stretching frame.

Artists, and more recently photographers, often produce their work on stretched canvases where canvas is wrapped around and attached, usually by staples or tacks, to an expandable wooden frame known as a stretcher. The frame is made of four members known as stretcher bars. The corners of the frame consist of specially cut mortice and tenon joints into which one or two wedges can be driven forcing the joint apart; this in-turn increases the outside dimensions of the frame, thus stretching the canvas. However, elaborate tooling and machinery is required for cutting these joints, plus the wood has to be of sufficient width to accept both joint and wedge. Furthermore, knocking the wedges in can be difficult as they lie in a plane parallel to the rear of the canvas, judging applied tension is difficult and over time wedges can become loose and fall out. To overcome these problems, the present invention proposes an expanding corner joint that does not require such complicated machining or the need for such a wide section of wood, and also provide an easily accessible means of expanding the joint accurately where components cannot fall out.

Preferably the stretcher bar ends are cut to a mitre with a groove cut horizontally at 90 degrees to the mitred face, that is shallower on the inner edge and deeper on the outer edge so that it accepts a 'kite' shaped tapered tongue plate, that is wider at the outer edge of the frame and narrower on the inner edge and therefore forming the joint between the stretcher bar members. The tongue plate incorporates a keyhole shaped slot to accept a cam that by its design can be rotated to apply pressure on the inner edges of the stretcher bar members without falling out. When the cam is rotated, the stretcher bar members are pushed along the widening taper of the tongue plate to open the joint. In a preferred embodiment the cam can be turned by a key that is operated at right angles to the rear of the canvas.

The invention will now be described solely by way of example and with reference to the accompanying drawings in which:

Figure 1 shows the components of the corner joint.

Figure 2 shows the tongue plate with the cam in place.

Figure 3 shows the corner joint assembled, the stretcher bars fitted on the tongue plate with the cam in place.

Figure 4 shows the assembled joint with canvas attached to the frame and the turn key entry into the accommodating hole is indicated.

Figure 5 a and b shows the opening of the joint when the cam is rotated.

Figure 6 shows an alternative where two tongue plates or a double tongue plate is used.

In figure 1, the components making the corner joint are shown; the two mitred ends of the stretcher bars 1 with the horizontally cut tapering grooves 2, the tapered tongue plate 3 with the keyhole shaped slot 4 and the cam 5, that is undercut in the midsection 6 and offset to the main cylindrical body, so that it will pass into the larger part of the slot, then slide into the narrower section. A hole 7 passes through this cam to accept a turning key.

Figure 2 shows the tongue plate 3 with the cam 5. The off centre undercut section 6 slides into the narrow section of the keyhole slot 4.

Figure 3 shows the assembled corner joint, the cam 5 is fitted to the tongue plate 3 so that the fall of the cam is facing the corner created by the two abutting stretcher bar members 1 when fitted on the tongue plate creating a right angle. In the preferred embodiment stretcher bar members and cam should all be in contact, with the edges of the tapered sides of the tongue plate being fully seated in the tapered grooves 2, and the undercut section of the cam 6 seated at the narrower section of the keyhole slot 4. When all four corners of the frame are assembled in this way canvas can be wrapped around and attached in the conventional manner.

Figure 4 shows the assembled corner with canvas attached. The turnkey 8 fits into hole 7 and is of a shape which prevents slippage between the two parts as they rotate together.

Figure 5 shows how the joint is opened when rotating the cam 5 in a clockwise direction, if only partial tension is required, as can be seen in 5a, then adjacent corners are turned in an anticlockwise direction and the opposite corner is turned clockwise - this allows for a more even stretching of the canvas and keeps the frame square due to the fact that one stretcher bar member is moved more than the other by the rotation of the cam. If full tension is required the cam is turned 180 degrees as shown in 5b, so as the full rise of the cam is facing the corner, opening the joint evenly and to the maximum amount.

An alternative embodiment using two or more tongue plates or a double tongue plate allows for parallel opening of a joint in a much deeper stretcher bar section.

Figure 6a shows an embodiment utilising two tongue plates spaced apart to locate in appropriately spaced grooves in the stretcher bar members; a cam 9 with two eccentric undercuts can hold the tongue plates at the correct spacing so when rotated applies force to both tongue plates simultaneously.

Figure 6b shows the two tongue plates and the cam 9 with two eccentric undercuts.

Figure 6c shows a solid piece double tongue plate with cam.

Claims

1. An expanding corner joint comprising of mitred stretcher bar members with tapering grooves and a correspondingly tapered tongue plate that incorporates a rotating cam that fits into the afore mentioned grooves, so as to position the cam on the inner edges of the stretcher bar members, where by revolving the cam pushes the stretcher bar members along the widening taper to open the joint.

2. An expanding corner joint according to claim 1 in which components are held in place by the assembly of the parts, and by doing so remove the risk of loosing any of those parts.

3. An expanding corner joint according to any of the preceding claims in which expansion is made by the rotation of a key that locates into the cam and is operated at right-angles to the rear of the canvas.

4. An expanding corner joint according to any of the preceding claims in which two or more tongue plates and cam are used.

5. An expanding corner joint according to any of the preceding claims in which a single piece double tongue plate and cam is used.