CA2086051A1 - Dowel joint system for connecting elements of a solid material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
DOWEL JOINT SYSTEM FOR CONNECTING WOODEN ELEMENTS
A system for fastening a dowel to a wooden element includes a drilling assembly for forming a blind hole in the wooden element which includes a first portion which is cylindrical and a second portion which tapers outwardly in frusto conical shape from the lower end of the first portion. A dowel having initially a cylindrical shape is inserted into the blind hole and the expanded outwardly into the frusto conical portion. The expansion can be carried out by a tapered plug which is forced into the end of the dowel, by a wedge member which is forced into slits in the end of the dowel or by expansion of the dowel itself from an initial compressed position by absorption of moisture from an adhesive. The drilling is effected by a compound drilling assembly which includes a first and second drilling element, the latter being movable axially relative to the former with the axial movement causing the latter to move outwardly along the frusto conical shape to form the second frusto conical portion after the first element drills to the required depth of the hole of the bore.

Description

DOWEL JOINT SYSTEM FOR ÇONNECTING ELEMENTS OF A SOLID
MATERIAL
BACKGROUND OF THE IN ENTION
This invention relates to a dowei joint system for connecting elements of a solid material includin~ a method for connecting a dowel ~o a wooden element, a dowel for use in the method and to a drill bit for forming a required hole for ~he system.
Various techniques have previously been proposed for connecting elements of a solid material such as wood in the manufacture of furniture and similar wooden constructions. One simple widely used teçhnique is the use of a dowel which is inserted into blind holes drilled into the wooden elements so that the dowel bridges the abutting surfaces between the two elements and extends into each of the blind holes. The dowel is generally cylindrical and is a tight friction fit within the blind holes and is supplemented by th~ provision of adhesive which is intended to hold the two elements fully connected together.
However in some cases the connection is not sufficient to accommoda~e the forces involved and breakage or loosening of the joint can occur.
SUMMARY OF THE INVENTION
It is one object of the present invention to provide an improved dowel conneotion system for connecting two elements of a solid material.
Accordiny to the first aspect of the invention there is provided a method of fastening a dowel to an element of a solid material comprisin0 .
', ' :, ' , , drilling a blind hole in the element from a surface of the elernent, the blind hole having a first portion spaced from the surface which is of diameter greater than that of a second portion thereof closer to the surface, inserting a cylindrical end of the dowel into the blind hole throu~h the surface, the cylindrical end having a diameter substantially equal to that of the second portion and causing a part of the cylindrical end to expand radially into said first portion of the hole.
According to a second aspect of the invention there is provided a dowel for use in fastening to an eiement of a solid material cornprising an elongate body having a cylindrical portion for insertion into a blind hole in the elernent, a part of the cylindrical portion at one end of the body being expandable radially outwardly so as to increase in diameter.
According to a third aspect of the invention there is provided a drilling assembly for attachment to a rotary drive member for drilling a blind hole having a first cylindrical portion and a second substantially outwardly tapered frusto-conical portion comprising a first drilling element and a second drilling element, means holding the first and second elements in fixed relationship for common rotation to form the first portion which is cylindrical and means to cause subsequent movement of the second element relative to the first element ~o cause the second element to form said second portion which is substantially frusto-conical.
The formation of the blind hole therefore with a portion within the hole which increases in diameter preferably in a frusto-conical shape enables a dowel which expands into the same shape to be locked into place and -: ' x ~

held by the physical interrelation between the materials rather than simply by the friction fit and adhesive techniques of the conventional arrangement One or more embodiments of the inven~ion will now be described in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a vertical cross sectional view through a combination drilling assembly for drilling a blind hole of the required shape for the present systern.
Figure 2 is a cross sectional view similar to that of Figure 1 showing the combined drilling assembly in a second position.
Figure 3 is a cross sectional view along the lines 3-3 of Figure 1.
Figure 4 is a cross sectional view along the lines 4-4 of Figure 1.
Figure 5 is a similar vertical cross section through a second embodiment of drilling assembly.
Figure 6 is a cross sectional view throu~h a lower part of the drilling assembly of Figure 1 showing the drilling assembly in operation drilling a hole in a wooden element.
Figure 7 is a similar view to that of Figure 6 showing a subsequent stage of the drilling process.
Figure 8 is a view similar to that of Figures 6 and 7 showing a yet further s~age of the drillin~ process.
Figure 9 is a cross sectional view through a wooden element showing a first sta~e in the insertion of a dowel for attachment to the wooden element.

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Figure 10 is a similar cross sectional view through that of Figure 9 showing a final stage in the insertion of the dowel element of Figure 9.
Figure 11 is a similar cross sectional view showiny a modified dowel arrangemen~ inserted into a drilled hole according to ~he present invention .
Figure 12 is a cross sectional view through tow abuttlng wooden elements showing the assembled dowel system in place therein.
Figure 13 is a similar cross sectional view through tow wooden elements showing a further arrangement of the assembled dowel system.
In the drawings like characters of reference indicate corresponding parts in ~he different figures.
~ETAII ED DESCRIPT!ON
A drill bit assembly as shown in Figures 1 through 4 which is arranged for drilling a blind hole in a wooden element with the blind hole having a first cylindrical portion at the surface through which the drilling takes place and a second outwardly frustoconical portion coaxial with the first portion and commencing at the inner end of the first portion.
The drilling assembly includes a first drilling element 10 and a second drilling element 1 1. The first drilling element 10 comprises an elongate rod portion 12 with a lower drill bit 13 attached to the rod portion and extending axially therefrom to a cutting edge 14 of conventional construction. As shown the drill bit 13 includes a space type cuttiny edge 14 including a central point element 15. Alternatively the drilling bit 13 can be of the helical screw type, In both cases the drill bit includes two or more ~'' '' ,, ~ .

diametrically opposed side edges 16 and 17 which extend longitudinally of the drill bit and act as guide surfaces for the second drilling element 11.
The second drilling element 11 includes an elongate sleeve portion 18 surrounding the rod portion 12 and slidable longitudinally relative thereto. At a lower end 19 of the tube 18 is provided a pair of drill bit elements 20 and 21 each of which is integrally connected with the lower edge 19 of the tubular portion and extends downwardly therefrom. Each of the cutting elements 20 and 21 is substantially arcuate as best shown in Figure 3 and extends only partly around the axis indicated at 22 so as to define open areas 23 and 24 therebe~ween. Each of the cutting elements includes a cutting edge 25 extending in a cutting direction angularly surrounding the axis 22 so that as ~he two elements 20 and 21 are rotated about the axis a cuttin~ action occurs in an annulus defined by the path of the cutting edges 25.
Each of the arcuate cutting elements 20 and 21 is connec~ed to the adjacent edge 19 of the tubular section by a spring portion 26 which allows the cutting elernents to flex outwardly. Each of the cutting elements includes a longitudinally extending ~roove 27 within which is a respective one of the edges 26 and 27 is received for co-rotation of the edges 16 and 17 and the cutting element 20 and 21 about the axis and for axial sliding movement of the cutting element 20 and 21 along the edges 16 and 17.
The first cutting element 10 is interconnected with an outer collar 28 surrounding the tubular section of the second drilling element by a transverse pin 29 which passes through the rod portion 12 and into the .
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sleeve 28 to hold those elements for common rotation about the axis 22 and for common longitudinal movement. The pin 29 passes through a pair of slots 30 in the tubular section 18 so that the pin maintains the first and second driiling elements for co-rotation but allows some longitudinal movement of the second drilling element relative to the first by an amount determined by the length of the slots 30. An end stop screw 31 is carried in the transverse plate 32 attached ~o the tubular portion 18 at right angles thereto. The end stop screw 37 engages an upper ~ace 33 of the first drilling element to halt longitudinal movement between the first and second drilling elements a~ a required adjustable position. A spring 34 extends between the underside of the plate 32 and a top face 35 of the sleeve 28.
The spring thus biases the sleeve and the first spring element in a direction downwardly as shown in Figure 1 to take up the initial position of the assembly as shown in Figure 1. A locating bail 36 is positioned in a hole 37 defined in the tubular portion 18. The ball in the initial position projects into a recess 38 in the rod portion 12 to hold the tubular portion 18 and the rod portion 12 in fixed position relative to each other for common rotational and axial movement. The ball 36 can however be received within a channel 39 forming a gradually increasing depth from an upper end to a lower end thereof so that while the ball is maintained within the opening 37 it can be released from a recess 38 by moving outwardly into the channel 39. In the upper most position of the channel as shown in Figure 1 the ball is forced into the recess 38 to hold the elements in fixed common position. In ~he position shown in Figure 2 the ball is released into the channel 39 allowing . , .

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axial movement of the first drilling element relative to the second drillin~
element.
A rub collar 40 is attached to the lower end of the sleeve 28 and surrounds t he sleeve 28 in fixed position by a set screw 41. The rub collar 4Q includes a plurality of transverse openings 42 which allow the escape of chippings from the interior of the rub collar when the rub collar is in engagernent with the surface of the wood element being drilled. The axial location of the rub collar can be adjusted relative to the sleeve 28 by operation of the set screw 41.
In operation of the drilling assembly, the first and second drilling elements in the position shown in Figure 1 engage the surface of the wooden eiement to be drilled and area rotated by a drive member attached to the upper end of the tubular portion 18 at the cross plate 32. Downward pressure from the rotary drive member presses the edge 14 into ~he wooden element to be drilled due to pressure through the tubular portion 1~ and through the ball lock 36,37,38. The cutting edge 14 thus commences the cutting of a first portion 45 of the drilled bore as shown in Figure 6. The drilling action continues to the position shown in Figure 7 in which the cutting elements 20 and 21 enter the bore 45 formed by the cutting edge 14. The outside edge of the cutting elements 20 and 21 is arranged to be equal to the width of the cutting edge 14 so that no further cutting occurs as the cutting elements 20 and 21 follow the cutting edge 14 into the bore 45.

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This drilling continues until the rub collar 40 reaches the surface 46 of the element being drilled. This rub collar thus halts further forward movemen~ of the rub collar and ~he First drilling element 13 which is attached thereto by the pin 29.
Further pressure on the rotary drive member thus presses down upon the second drilling element 11 which then tends to move downwardly relative to the first drilling elernent and the rub collar 40. This downward movement releases the ball 36 from the recess 38 so that the tubuiar portion 18 and the whole of the second drilling element can then move downwardly. This downward movement moves the second drilling element and particularly the cutting elements 20 and 21 longitudinally of the first drilling element. This longitudinal movement causes the cutting elements 20 and 21 to move along the side edges 16 and 17 of the first drilling element cutting portion 13~ The side edges 16 and 17 diverge outwardly from the axis 22. These edges thus cause the cutting elements 20 and 21 to diverge outwardly as they slide longitudinal along the edges 16 and ~7. This outward movement causes the lowermost cutting edge 25 of the cutting elements 20 and 21 to form a Frustoconical bore section 47 at the lower end of the bore portion 45 with the frustoconical section diverging outwardly at the angle of the side edges 16 and 17.
This drilling is completed when the assembly takes up a position shown in Figure 2 in which the upper end of the rod 12 engages the lower end of the set screw 31`. This position is adjusted so that the edges 25 are positioned relative to the cutting edge 14 to provide a smooth FIat '- ' ' ' ~' ' circular end for the bore, excluding of course the pointed center section defined by the projection 15.
When the drilling is complete the release of pressure from the rotary drive member onto the tubular portion 18 causes the spring 34 to force the tubular portion upwardly relative to the first drilling element to re~urn to the position shown in Figure 1. In this position the whole of the drilling element can simply be pulled from the drilled bore including the portions 45 and 47 as shown in Figure 9.
The rub collar thus acts as a stop to accurately and adjustably set the depth of the bore. The length of the frustoconical portion 47 is fixed by the state of movement of the second element relative to the first. The length of the cyiindrical portion is adjustable by moving the rub collar relative to the sleeve and thus relative to the first element.
After drilling of the bore is complete, a dowel ~enerally indicated at 40 can be attached into the bore. The dowel is also visible in Figure 12 where the dowel is used to attach a first element 51 of wood to a second element 52 using two of the blind holes drilled end to end as indicated at 45, 47 and 45A, 47A.
The dowel 50 comprises a cylindrical tubular body having a cylindrical outer surface 53 and a hollow cylindrical bore 54. In addition the dowel is cut with a plurality of transverse slots 55 which are cut to a depth equal to the axial length of the second bore portion 47. A number of such slits can vary from a minimum of four arran~ed at 90 spacing upward, ~ - ~

lo bearing in mind the requirernent of course to leave a significant arnount of material forming the dowel at the end portion containing the slits.
A plug member 56 is provided with an upper end of the plug member just inserted into the end of the bore 54 at the outer end of the 5ii'tS
55. The plug 56 is frusto conical in shape following the angie of the frustoconical end portion 47 of the bore. The end plug 56 is just inserted sufficiently into the bore to have frictional engagement therewith so that it is carried by ~he dowel into the bore to a position shown in Figure 9 in which ~he end face of the frusto conical plug member 56 engages the lower face of the bore 47. From this position axial movement of the dowel 50 forces the dowel into the bore and at the same time forces the plug 56 into the bore 54 of the dowel. As the dowel is li~ by the transverse slit 55, the dowel can be forced open at the slit so as to expand radially to take up the position shown in Figure 10 in which the end portion of the dowel which is slit is expanded to follow the frusto conical shape defined by the inside surface of the bore portion 47 and the outside surface of the plug 56. As shown in Figure 12, the slits 55 thus widen. The dowel is formed of material which allows the flexing necessary at the lower end of the slits without cracking of the material. Prior to insertion the bore can receive an adhesive material to assist in engagement of the dowel with the bore.
However this expansion within the bore provides a physical engagement between the bore and the dowel which prevent the dowel frorn being pulled longitudinally out of the bore without physical damage to one oF the parts.

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In an alternative arrangement of the dowel (not shown), the plu~
can be of cylindrical shape and the inside bore of the dowel can be formed of frustoconical shape tapering invvardly toward the end face of the dowel so that the insertion of the cylindrical plug from the upper end of the dowel downwardly toward the base of the bore forces the end of the dowel outwardly into the frusto conical bore portion 47.
In a further alternative arrangement shown in Figure 13 the dowel is formed without a hollow internal bore but is instead simply formed with transverse slits again preferably cut at right angles. These slits are then forced apart by a cross shaped wedge member 60 which is forced axially into the slits so as to drive the end portions of the dowel outwardly by the physical presence of the cross shaped edge member. In a yet further arrangement shown in Figure 11, the dowel body indicated at 62 is formed of a compressed material. That compressed material is initially of a shape so as to follow the portions 45 and 47 of the bore. Thus the dowel is initially compressed at its end section to take up a cylindrical shape having a diameter equal to the diameter of the bore 45. The material is ~urther of a type which on contact with moisture from an adhesive ma~erial 48 reexpands to its initial position to take up the expanded position to fill the bore portion 47.
An alternative arran~ement for attaching to wooden elements 63 and 64 is shown in Figure 13. In this arrangement a blind hole includin~ the portions 45 and 47 is forrned in the element 63. However the element 64 has a hole 65 drilled 7rom an outermost surface 66 through to a surface 67 .
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abutting with the surface 46 of the element 63. This hole is drilled so that it follows the general shape of the blind hole 45, 47 but in this case the hole 65 is not blind but breaks out on both surfaces 66 and 67. The hoie 65 thus includes a first portion 68 which is cylindrical and a second portion 69 whioh is frusto conical tapering inwardly from the surface 66. This action can be carried out by a device substantially in the form of a countersink which can be attached to the rub collar 40 so as to follow the first drilling element 13 into the bore portion 68 with a counter sink type drilling element forming the tapered bore 65.
From this arrangement a dowel 70 can be inserted which has an expanding portion 71 at its inner most end of the type as previously described and a tapered portion 72 at its outermost end which does not need to expand but is instead of fixed frustoconical shape so as to engage into the portion 69 of the bore 65. The dowel thus forms the two frustoconical portions necessary to hold the elements 63 and 64 in abutting position with the necessity only for a single expanding portion 71.
Turning now to Figure 5 there is shown an alternative arrangement of drilling assembly which is of a similar construction to that of Figures 1 through 4. Thus the drilling assembly of Figure 5 comprises a primary drilling element 80, a second~ry drilling element 81 and a rub collar 82 which are coupled together for corotation. in this case the prirnary drilling element 80 is in the form of a tubular member which has a pair of diametrically opposed .openings 83 and 84 adjacent a cutting end 85 thereof. The openings 83 and 84 break out on the side wall of the tubular :~ , ~g~

member spaced just back from the cutting edge 85. The primary eiement 80 has spiral grooves (not shown) ~o facilitate waste removal from the bore hole.
Within the hollow centre of the primary element 80 is mounted the second drilling element 81 which includes a rod 86 and a pair of springsteel cutter elements 87 and 88 which project from the rod 86 now carried thereby but extend outwardly through the openings 83 and 84 respectively. Each cutter element has a cutting edge extending axially along the element and the position shown in Figure 5 ~he cutting edges are retracted within the openings 83 and 84.
Within the end of the primary bit is provided a triangular section including edges 89 and 90 upon which the underside of the cutting members 87, 88 sit. It will be appreciated therefore tha~ as the secondary element 81 is moved forwardly the underside of the cutting rnembers 86 and 87 ride outwardly aiong the surfaces 89 and 90.
The rub collar 82 is connected by a spring 91 to an end plate 92 carrying a cross pin 93 which connects the end plate 92 and the primary drilling element 80. A stop member 94 on the primary drilling element allows longitudinal movement of the rub collar 82 up to a predetermined position at which it hits the stop member 94. A first ball lock 95 including a ball 96 and a groove 97 is located between the primary drilling element and the rub collar to hold the secondary drilling element in fixed position during an initial drilling step. Once the rub collar is moved axially toward the stop ~ ' ' ~ ~ ' ' , .
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94 the ball lock 95 is released allowing the secondary drilling element to move longitudinally. A second ball lock is indicated at 98.
In operation the drilling element enters the material until the rub collar contacts the upper surface. The rub collar is pushed upwards along the drilling element until it contacts the collar stop 94. In the process this also releases the secondary drillin~ element ball lock 95 and compresses the return spring 91. When engaged against the stop 94, the rub collar prevents the primary bearing element 80 from entering the material any further. Further force on ~he drilling assembly therefore presses a secondary drilling element downwardly to travel along the inside of the primary drilling elernent where it is forced apart at the cutting edge and by the tapered section at a cutting end of the primary drilling element thus causing the edges of the secondary drilling element to cut a gradual taper or undercut in the existing hole. The primary drilling element acts as a pilot as the whole assembly turns in unison.
In an alternative arrangement (not shown) the dowel can be shaped initially ~o include an outwardly tapered end position shaped to follow the shape of the lower portion of the bore. The end portion is then drilled and has transverse slots allowin~ it to be compressed to pass throu~h the cylindrical portion of the bore. This allows the dowel to be effeGtively unstressed in the finished condition. A plug can be used as previously described.
In a further alternative arrangement ~not shown~ the drill bit includes a split spade portion which is normally cylindrical to drill an initial - , ;
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cyiindrical hoie. The split spade portion can then be driven radially outwardly by a substantially triangular wedge member sliding longitudinally of the bi~ so as to take up the required frusto-conical shape for the second portion of the hole. This movement is effected while the bit remains at constant depth so that side edges of the spade portion effect the cutting action.

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Claims

(1) A method of fastening a dowel to an element of a solid material comprising drilling a blind hole in the element from a surface of the element, the blind hole having a first portion spaced from the surface which is of diameter greater than that of a second portion thereof closer to the surface, inserting an end of the dowel into the blind hole through the surface, the cylindrical end of the dowel having a diameter as it passes through the second portion substantially equal to that of the second portion and causing a part of the end of the dowel to expand radially into said first portion of the hole.
(2) The method according to Claim 1 wherein the second portion of the hole is arranged at the surface.
(3) The method according to Claim 1 wherein the second portion of the hole is cylindrical.
(4) The method according to Claim 1 wherein the first portion of the hole is frusto-conical so as to diverge outwardly from an end of the second portion to a blind end of the hole.

(5) The method according to Claim 1 wherein the part of the cylindrical end of the dowel includes a longitudinal bore and a plurality of transverse slits and wherein the part of the cylindrical end is expanded by a plug moved axially relative to the part, either the plug or bore inside the part being tapered to expand the part into a frusto-conical shape.
(6) The method according to Claim 1 wherein the part of the cylindrical end includes a plurality of transverse slits and wherein the end is -., :
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(7) The method according to Claim 1 wherein the part of the cylindrical end is compressed from an initial expanded state to a cylindrical shape by compression of the material forming the part and wherein the part is expanded in response to the presence of moisture from an adhesive material.
(8) The method according to Claim 1 wherein the dowel is fastened to a second wooden element by drilling a second blind hole in the second wooden element from a surface of the second wooden element, the second blind hole having a first portion spaced from the surface which is of diameter greater than that of a second portion thereof closer to the surface, inserting a cylindrical end of the dowel into the second blind hole through the surface, the cylindrical end having a diameter substantially equal to that of the second portion and causing a part of the cylindrical end to expand radially into said first portion of the second blind hole, the second wooden element having the surface thereof abutting the surface of said wooden element.
(9) The method according to Claim 1 including attaching the dowel to a second wooden element by drilling a second hole in the second wooden element from a first surface of the second wooden element to a second surface of the second wooden element, the second hole having a first portion portion spaced from the first surface and adjacent the second surface which is of a diameter greater than that of a second portion thereof adjacent , . . . .
, the first surface of the second wooden element, and inserting a cylindrical end of the dowel through the second wooden element from the second surface thereof through the first surface thereof into said wooden element.
(10) The method according to Claim 1 wherein the blind hole is drilled by rotating a drilling assembly having a first drilling element and a second drilling element, the drilling assembly being rotated to form the second portion which is cylindrical and subsequently to move the second element relative to the first element to cause the second element to form said first portion which is substantially frusto-conical.
(11) The method according to Claim 10 wherein the second element is moved axially of the first element, the axial movement causing outward movement of the second element in a radial direction to form said first portion.
(12) The method according to Claim 11 wherein the first element includes side surfaces thereof engaging the second element, the side surfaces being tapered outwardly so that axial movement of the second element causes the second element to be forced outwardly by engagement with the side surfaces of the first element.
(13) The method according to Claim 11 including a stop element for engaging the surface, the first element and the stop element being interconnected such that engagement of the stop with the surface holds the first element against forward motion and thus defines a depth of the bore, the second element being moved axially forwardly from a first retracted position to a forward position during which movement the first portion of the bore is formed.
(14) The method according to Claim 13 including locking together the first and second elements for common movement, and releasing the locking means in response to engagement of the stop element with the surface causing rearward movement of the stop element.
(15) A dowel for use in fastening to a wooden element comprising an elongate body having a cylindrical portion of insertion into a blind hole in the wooden element, a part of the cylindrical portion at one end of the body being expandable radially outwardly so as to increase in diameter.
(16) The dowel according to Claim 15 including a hollow bore longitudinally of the body at the part and a plurality of transverse slits in the body at the part and a plug member movable axially of the part with either the bore or the plug member having a tapered surface so as to expand the part outwardly into a frusto-conical shape.
(17) The dowel according to Claim 15 wherein the part of the cylindrical end includes a plurality of transverse slits and wedge means arranged such that the end is expanded radially by insertion therein of said wedge means moved axially into the transverse slits.
(18) The dowel according to Claim 15 wherein the part of the cylindrical end is compressed from an initial expanded state to a cylindrical shape by compression of the material forming the part and wherein the part is expanded in response to the presence of moisture from an adhesive material.
(19) A drilling assembly for attachment to a rotary drive member for drilling a blind hole having a first cylindrical portion and a second substantially outwardly tapered frusto-conical portion comprising a first drilling element and a second drilling element, means holding the first and second elements in fixed relationship for common rotation to form the first portion which is cylindrical and means to cause subsequent movement of the second element relative to the first element to cause the second element to form said second portion which is substantially frusto-conical.
(20) The assembly according to Claim 19 wherein the second element is movable axially of the first element from a first retracted position forwardly to a position aligned with an end of the first element, the axial movement causing outward movement of the second element relative to the first element.
(21 ) The assembly according to Claim 20 wherein the first element includes side surfaces thereof engaging the second element, the side surfaces being tapered outwardly so that axial movement of the second element causes the second element to be forced outwardly by engagement with the side surfaces of the first element.
(22) The assembly according to Claim 20 including a stop element for engaging the surface, the first element and the stop element being interconnected such that engagement of the stop element with the surface holds the first element against forward motion and thus defines a depth of the bore, the second element being free to move axially forwardly from a first retracted position to a forward position during which movement the first portion of the bore is formed.
(23) The assembly according to Claim 22 including means locking together the first and second elements for common movement, said locking means being arranged to be released in response to engagement of the stop element with the surface causing rearward movement of the stop element.