GB2336190A

GB2336190A - Timber joint and method of manufacture

Info

Publication number: GB2336190A
Application number: GB9807187A
Authority: GB
Inventors: David Edward French
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-04-04
Filing date: 1998-04-04
Publication date: 1999-10-13
Anticipated expiration: 2018-04-04
Also published as: GB2336190B; GB9807187D0

Abstract

A timber joint of the mortise and tenon type ortongue and groove type has a tapered projecting element (14) on one piece of the wood (12) matched to fit snugly in a correspondingly shaped hole in a second piece of wood. The projecting element has a radiussed or bevelled shoulder at its base. The projecting element is preferably of circular or oval cross-section perpendicular to its longitudinal axis. The same cutter (10) is preferably used to shape both the projecting element (14) and the hole.

Description

2336190 1 TIMBER JOINT AND METHOD OF MANUFACTURE The present invention

relates to a timber joint and to a method for its manufacture.

is Timber joints assembled using, for example, a mortise and tenon arrangement or a tongue and groove arrangement have been used for centuries. A tenon is a projection on a piece of wood made for insertion into a corresponding hole called a mortise in another piece of wood. A tongue is a projecting strip on a wooden board, for example, made for insertion into a corresponding groove in another wooden board. The tongue and groove may extend fully or partly along at least one edge of each board.

Traditional timber joints are prepared using hand tools, which dictate the nature of the joint formed. For example, in a traditional mortise and tenon arrangement the tenon has rectangular and parallel faces (see figure 1). Wood-working machines are now able to do the work of hand tools but they have, in general, simply duplicated the results achieved by traditional wood working methods rather than changing and improving them.

mortise and tenon joints are successfully used to construct framed buildings, primarily as a location Joint where they resist compression loads. Wood is used structurally as a 'prop and beam' material.

However in other applications, such as exterior]oinery (e.g. window casements, sashes and framed doors) or furniture, 2 mortise and tenon joints experience environments and/or undergo stresses which undermine their stability and rigidity. The joint is fundamentally weak relative to the sizes of the pieces of wood used to prepare it. The thickness of a piece 5 of wood is reduced by up to two-thirds when a tenon is cut. Similarly the thickness of a piece of wood is reduced by up to a third when a mortise is cut. Moreover, the angular shape of the mortise and tenon means stress concentrates in certain locations such as the right angled shoulder of the tenon where 10 it meets the body of the piece of wood from which it projects. This provides the joint with poor resistance to lateral forces and with poor tensile strength. To overcome these problems, pegging, diagonal bracing, wedging, adding further structural members and/or gluing are used to try to improve the lateral 15 strength of the joint.

However, it is difficult to successfully glue mortise and tenon joints because the rigid containment of the mortise prevents effecting clamping of the joint, which is required whilst the glue is setting, particularly as the glue may contract as it sets. Also, it is difficult to ensure that the glue forms a continuous layer between the mortise and tenon, especially if the tenon is a tight fit in the mortise since glue is then displaced during assembly of the joint. Moreover, gluing does little to counter the concentration of stress at the right angled shoulder of the tenon since end grain is present there and bonding thereto has little effect. Also, if the tenon is a loose fit in the mortise, there is not sufficient contact to create an adequate bond.

However, the flat, parallel surfaces of the mortise and tenon 3 lend themselves favourable to being pegged together by wooden pegs, although the function of the pegs is largely lost in the event of shrinkage of the wood.

When the tenon is cylindrical, the strength of laterally stressed conventional joints is limited to the cross-sectional strength of the tenon at the shoulder where it meets the body of the piece of wood from which it projects.

The present invention seeks to provide an improved timber joint and a method for its manufacture.

According to the present invention there is provided a timber joint comprising a first piece of wood having a body and an element projecting therefrom and a second piece of wood having a body and a hole therein, the projecting element comprising a shoulder where it meets the body of the first piece of wood; wherein the shoulder is radiused or bevelled, the projecting element has a shape which tapers away from the body of the first piece of wood, and the hole is correspondingly- shaped so that the projecting element fits snugly therein.

The projecting element may be a tongue or a tenon. The surfaces of the projecting element are preferably substantially non-angular. The projecting element is preferably symmetrical in shape and may have a circular or oval cross-section in a direction perpendicular to its longitudinal axis. Preferably the taper of the projecting element is uniform between the shoulder and the tip of the projecting element. Preferably the tip of the projecting 4 element is radiused or bevelled.

The projecting element may further comprise an abutment adjacent the shoulder, the abutment having substantially- straight sides. This abutment is for neatness and for facilitating cleaning-up operations.

The hole may be a groove or a mortise.

The tapering shape of the projecting element and the hole distributes applied stress relatively evenly over the joint; this improves the strength of the joint. Moreover the shape of the projecting element minimizes abrupt changes in crosssection thereof. This strengthens the shoulder of the projecting element.

A shape which tapers means a shape which diminishes or reduces in thickness towards one end. For the purpose of the present invention, a shape which tapers includes a shape in which its thicker end has areas which are not tapered. For example, a tapered projecting element includes a projecting element comprising a straightsided abutment adjacent the shoulder.

Preferably the joint is glued. The tapering shape of the projecting element and the hole means the joint can be assembled with minimal displacement of adhesive. It also provides a substantially even distribution of adhesive and good wetting of all contact surfaces. Moreover, the tapering shaoe minimises the presence of voids and air, maximises the area of facegrain contact and provides smooth contact surfaces, all of which are important in providing good adhesion. Good contact is therefore maintained between the surfaces as the adhesive sets, resulting in a good bond and seal. Whilst the glue is setting there is no need to apply an external force as the tapering shape of the joint provides moderate self-clamping pressure. A moderate externallyapplied pressure may be required to close the joint finally. The glue is preferably water-proof or water-resistant and is, for example, polyvinylacetate, polyurethane or a synthetic resin glue such as epoxy resin.

Alternatively the joint may be retained using a nut and bolt arrangement.

is The arrangement of the joint of the present invention results in minimal exposure of the end grain. This maximises the effective bonding area if adhesive is used, thus providing a better seal and improved durability. Minimal exposure of the end grain also means that the joint can be successfully veneered.

The joint of the present invention provides improved restraint to relative movement by expansion or contraction of the wood. The angle of taper and the dimensional relationship between the projecting element and the hole may be varied to account for factors such as material and design.

Although traditional timber joints such as a mortise and tenon joint are primarily location joints and are not used in situations where tensile strength is of paramount importance, a timber joint according to the present invention has an 6 improved ability to withstand stress and may be used where torsional and tensile stresses are likely, for example in chair joints. The comprehensive stress joint makes it suitable for use in applications. It has intrinsic rigidity the joint minimises the concentration process of removing wood during shaping is to minimise the weakness of the joint.

resistance of the a wide range of since the shape of of stress and the preferably designed In view of the superior strength of the joint, less wood is required, hence the joint can be used to lighten the construction of furniture and can also improve the elegance of the items being designed; moreover additional structural members are generally not required.

is Joinery exposed to sun and rain commonly fails due to instability of the joints when shrinkage of the wood occurs, for example. Joints of the present invention have superior stability in such conditions, particularly when glued. Since the joints of the present invention generally require less wood than traditional joints in order to resist a certain load, the joints of the present invention are less subject to timber movement: moreover, when they are glued, an effective seal results.

According to the present invention there is also provided a method of making a timber joint comprising the steps of:

a) shaping a first piece of wood using a profiled cutter to form an element projecting therefrom, the element tapering away from the remainder body of wood and comprising a shoulder where it meets the body of wood; b) shaping a second piece of wood using a profiled cutter 7 to form a hole in the wood, wherein the hole is shaped so the projecting element fits snugly therein; and c) fitting the projecting element in the hole; wherein the shoulder of the projecting element is radiused or 5 bevelled.

Preferably the profiled cutter used in step a) is also used in step b), optionally using the same settings when driving the cutter to provide an accurate fit of the 10 projecting element in the hole.

In step a), the profiled cutter may be traversed about a predetermined longitudinal axis of the projecting element being shaped. The profiled cutter may also rotate about its 15 own longitudinal axis in step a) and/or step b).

The profiled cutter may provide an abutment adjacent the shoulder on the projecting element, the abutment having substantially-straight sides.

In a preferred embodiment, the first and second pieces of wood have equal amounts of wood removed during shaping to maintain a balance of strength between them.

The shaping of the projecting element and the hole may be carried out using machines such as plunge or machine routers or CNC machines: such machines generally reduce the time taken to produce a joint of the present invention in comparison to the time taken to produce and strengthen a 30 traditional timber joint. This reduces labour costs.

8 The projecting element may be a stub-tenon, which is ideal for fitting edging strips to plywood, blockboard or particle board, for example, particularly in view of the fact that no clamping is required, since the joint has self5 clamping properties.

When the projecting element is a tongue and the hole is a groove the present invention provides items of safer construction than items assembled using traditional tongue and groove joints. Moreover the tongue and groove arrangement of the present invention has a wider range of applications than traditional tongue and groove arrangements.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which:

Figure 1 is a perspective view of a prior art tenon;

Figure 2a is a perspective view of a tenon being cut; Figure 2b is a perspective view of a tenon being cut; Figure 2c is a cut away perspective view of a mortise being cut, Figure 2d is a perspective view of a mortise being cut; Figure 3a is a perspective view of an apparatus used in cutting a piece of wood; Figure 3b is a perspective view of one end of a template clamped by guide bars; Figure 4 is a perspective view of an alternative apparatus used in cutting a piece of wood; Figures 5a to 5d are plan views of a cutting cycle for a tenon; Figure 6 is a front view of a profiled cutter; 9 Figure 7 is an equal areas diagram for a mortise and tenon joint, the tenon having a radiused shoulder and a radiused tip; Figure 8 is a cross-sectional view of a tapering tenon having a radiused shoulder and a radiused tip; Figure 9 is a cross-sectional view of a tenon having a bevelled shoulder and a bevelled tip; Figure 10 is a cross-sectional view of a tapering tenon having a radiused shoulder and a radiused tip; Figure lla is a cross-sectional view of a mortise and tenon joint, the tenon having a radiused shoulder and a radiused tip; Figure lib is a cross sectional view of a mortise and tenon joint along line A-A of figure lia; Figure llc is a cross-sectional view of a mortise and tenon joint along line B-B of figures lia and lib; Figure 12 is a cut-away perspective view of a tongue and groove edging or clamping joint having bevelled surfaces and a straight-sided abutment; Figure Figure Figure tenon Figure tenon joint; Figure 15a is a perspective view of a mortise and a tenon; Figure 15b is a cut-away perspective view of a mortise and tenon joint; and Figure 16 is a cut-away perspective view of a mortise and tenon joint having a nut and bolt fixing.

13a is a perspective view of a mortise; 13b is a perspective view of a tenon; 13c is a cut-away perspective view of a mortise and joint; 14 is a cut-away perspective view of a mortise and Referring to figure 2a or figure 2b, a tenon 14 is cut by traversing a rotating profiled cutter 10 around one end of a piece of wood 12.

Referring to figure 2c or figure 2d, a mortise 16 is cut by guiding a rotating profiled cutter into the face of a piece of wood 18.

A hand held plunge router f itted with a guide ring is used in conjunction with a guidance jig held in a bench vice to cut a mortise or a tenon. Referring to figure 3a, the frame 2 of the guidance jig supports slidable tables 4 and parallel guide bars 6 closed down to the guide ring diameter for mortise cutting and opened up to accept templates 7 (see figure 3b) for tenon cutting. This apparatus is laterally adjustable for positioning the mortise or tenon relative to the material thickness, with the jig incorporating adjustable stops to control the dimensions of the mortise or tenon. For tenon cutting, a piece of wood to be cut is positioned and clamped against an adjustable vertical fence 8 (shown separated from the jig in figure 3a).

To cut a tenon, a rotating profiled cutter is inserted into the piece of wood to a predetermined depth. The cutter is traversed using a guidance jig, keeping the guide ring in contact with the guide bars and the templates.

Referring to figure 4, a tenon is cut from a piece of wood using an alternative apparatus in conjunction with a machine router. The apparatus has a base 20 provided with stops 22 supporting a rotating base plate 24 which has an aperture therein to allow a profiled router cutter to pass therethrough. Mounted on the base plate is a traverse slide 26 with adjustable stops and a captive latch. The slide is operated by a rack and pinion or a lever. Mounted on the traverse slide is a work holder 28 which is slidable at 45' for adjusting the size of the tenon or mortise to be cut. Means 30 for clamping the wood is provided on the work holder.

The apparatus is mounted on a router table and the axis of a router cutter is aligned with the vertical axis of the base plate 24. A piece of wood 32 is loaded and clamped into the work holder, which is traversed about an axis X off-set from the longitudinal axis Y of the cutter, as illustrated in figures Sa to 5d. In figure 5a, the work holder rotates 180' to a stop. In figure 5b, the work holder is moved linearly to a stop. In figure 5c, the work holder rotates 1800 to a stop. In figure 5d, the work holder moves linearly to a stop. For the purpose of illustration the profile of the cut wood is thicker than that of the uncut wood in figures Sa to 5d.

The stops are arresting captive stops.

A mortise is cut by using machines equipped for slot mortising, using a profiled cutter.

In a further arrangement for cutting a mortise and a tenon, a machine having two horizontal traverse slides and vertical motion is used, together with an adjustable pantograph for radially traversing the cutter. In use, a piece of wood is located and clamped onto a horizontal work table against a fence. The fence is positioned parallel to the longitudinal traverse for mortise cutting and is 12 positioned at right angles to the longitudinal traverse for tenon cutting, when the radial traverse is also used. Alternatively the fence is positioned at intermediate angles or other locations. Additionally, traverse stops can be used and preset for either cutting operation, to ensure the equality of the width of the tenon and the length of the mortise.

Alternatively, a three-axis CNC machine may be used.

is After cutting a tenon or mortise using any of these arrangements, some trimming may be required where the projecting element or hole joins the remaining body of wood.

In these arrangements, the cutting operation is performed manually or automatically. In the latter case, it is preferably controlled by computer.

Referring to figure 6, a profiled cutter 10 has two cutting edges 34 which are symmetrical about the longitudinal axis of the cutter. Each edge also has 180' rotational symmetry about a point 36 midway along each edge. A tenon 14 and a mortise 16 cut using this cutter have equal volume, since equal volumes of waste are removed when cutting the mortise and tenon; this is apparent from figure 7.

To cut a tongue or a groove in a board, a profiled cutter is traversed perpendicularly to its longitudinal axis along one edge of the board.

Referring to figures 8 to 10, a tapering tenon having a radiused or bevelled shoulder has a length TL which is one and a half times the width N of a cutter, a fillet radius R which is 0.2 N and a taper angle of approximately 100 between the shoulder and tip of the tenon: width N optionally corresponds to the thickness of the wood (rail) being shaped. These tenon dimensions are preferred for heavy joinery. A tapering tenon having a radiused shoulder and a straight-sided abutment SF has a length TL which is equal to the width N of a cutter, a fillet radius R of 0.125 N and an abutment width SF of 0.0833 N. These tenon dimensions are preferred for light joinery and cabinet making. A tapering stub-tenon (not shown) having a radiused or a bevelled shoulder has a length of 0.75 N, a fillet radius of 0.25 N and a tapering angle of approximately 100 between the shoulder and tip of the tenon is With reference to figures 11 to 15, a variety of mortise and tenon joints are possible, which are optionally glued.

Alternatively, the joints are secured using a nut 40 and bolt 42 arrangement (see figure 16).

To assemble a stool or table having joints in accordance with the present invention, a frame is prepared by coating tapering tenons with glue, placing the tenons carefully into correspondingly-shaped mortises, closing the joints using clamp pressure or protected hammer impact, wiping away surplus glue and leaving the glue to set. The table or stool top is then fixed to the frame.

14

Claims

CLAIMS:

1. A timber joint comprising a first piece of wood having a body and an element projecting therefrom and a second piece of wood having a body and a hole therein, wherein the projecting element has a radiussed or bevelled shoulder where it meets the body of the first piece of wood, the projecting element has a shape which tapers away from the body of the first piece of wood, and the hole is correspondingly shaped so that the projecting element fits snugly therein.

2. A timber joint as claimed in claim 1, in which the projecting element is a tenon and the hole is a mortise.

3. A timber joint as claimed in claim 1, in which the 15 projecting element is a tongue and the hole is a groove.

4. A timber joint as claimed in claim 1, 2 or 3, in which the surfaces of the projecting element are substantially non-angular.

5. A timber joint as claimed in claim 1 or 2, in which the projecting element is symmetrical in shape about a longitudinal axis and has a circular or oval cross-section in a direction perpendicular to its longitudinal axis.

6. A timber joint as claimed in any preceding claim, in which the taper of the projecting element is uniform between the shoulder and the tip of the projecting element.

7. A timber joint as claimed in any preceding claim, in which the tip of the projecting element is radiussed.

is

8. A timber joint as claimed in any of claims 1 to 6, in which the tip of the projecting element is bevelled.

9. A timber joint as claimed in any preceding claim, in which the projecting element includes a straight-sided abutment adjacent to the shoulder.

10. A timber joint as claimed in claim 2, in which the projecting tenon has a length which is one and a half times the width of the cutter used to shape the tenon, the shoulder has a radius which is 0. 2 times the cutter width, and the angle of taper between the shoulder and the tip of the tenon is approximately 10'.

is

11. A timber joint as claimed in claim 9 when dependent on claim 2, in which the projecting tenon has a length which is equal to the width of the cutter used to shape the tenon, the shoulder has a radius which is 0. 125 times the cutter width, the width of the abutment is 0.0833 times the cutter width, and the angle of taper between the shoulder and the tip of the tenon is approximately 100.

12. A timber joint as claimed in claim 2, in which the projecting element is a tapering stub-tenon having a length which is 0.75 times the width of the cutter used to shape the tenon, the shoulder has a radius which is 0.25 times the cutter width, and the angle of taper between the shoulder and the tip of the tenon is approximately 10'.

13. A method of making a timber joint which comprises 16 the steps of a) shaping a f irst piece of wood using a profiled cutter to form an element projecting therefrom, the element tapering away from the remainder body of wood and comprising a radiussed or bevelled shoulder where it meets the body of wood; b) shaping a second piece of wood using a prof iled cutter to form a hole in the wood, the hole being shaped so that the projecting element fits snugly therein; and c) fitting the projecting element into the hole.

14. A method as claimed in claim 13, in which the same profiled cutter is used in step a) and in step b).

15. A method as claimed in claim 13 or 14, in which the cutter is traversed about a longitudinal axis of the 20 projecting element being shaped.

16. A method as claimed in claim 15, in which the cutter is also rotated about its own longitudinal axis in step a) and/or in step b).

17. A method as claimed in any of claims 13 to 16, in which the first and second pieces of wood have substantially equal amounts of wood removed during the shaping process.

18. A method as claimed in any of claims 13 to 17, in which the cutter has two cutting edges which are symmetrical 17 about the longitudinal axis of the cutter.

19. A method as claimed in claim 18, in which each cutting edge has 1800 rotational symmetry about a point midway 5 along each respective edge.

20. A method as claimed in any of claims 13 to 19, which includes glueing the two pieces of the joint together.

21. A timber joint substantially as hereinbefore described with reference to any one of the Figures of the accompanying drawings.

22. A method of making a timber joint substantially as 15 hereinbefore described with reference to any one of the Figures of the accompanying drawings.