GB2527156A

GB2527156A - Joint assembly

Info

Publication number: GB2527156A
Application number: GB1420541.3A
Authority: GB
Inventors: Robert Iain Emms
Original assignee: BAE Systems PLC
Current assignee: BAE Systems PLC
Priority date: 2014-11-19
Filing date: 2014-11-19
Publication date: 2015-12-16
Anticipated expiration: 2034-11-19
Also published as: GB2527156B; GB201420541D0

Abstract

A joint assembly, for example an aircraft skin construction, comprises a first object (2) having a first bore a second object (4) having a second bore. A fastener comprising an elongate shank (32) extends through the first bore and the second bore. A retaining element (24) retains the fastener against the first object (2). At least part of the retaining element (24) is located within a first counterbore (44), the first counterbore (44) being formed in the formed in the first object (2) coaxially with the first bore. Also, at least part of the retaining element (24) is located within a second counterbore (52), the second counterbore (52) being formed in the formed in the second object (4) coaxially with the second bore.

Description

JOINT ASSEMBLY

FIELD OF THE INVENTION

The present invention relates to joint assemblies. In particular, the S present invention relates to joint assemblies comprising two objects fastened together using a fastener that is retaining against one of the objects by a retaining element, for example, a retaining ring.

BACKGROUND

Aircraft typically comprise a plurality of panels which are attached, using fasteners, to an aircraft substructure. The aircraft panels are typically moulded panels made of carbon fibre composite (CFC). The aircraft panels form an external skin of the aircraft. The aircraft substructure is typically made of metal, such as aluminium.

Figure 1 is a schematic illustration (not to scale) used to illustrate how an aircraft panel 2 may be attached to an aircraft substructure 4.

In this example, the aircraft panel 2 is made of CFC material. The aircraft panel forms an outer skin of an aircraft. Also, the aircraft substructure 4 is made of aluminium or a different appropriate metal. The aircraft substructure 4 is an underlying load bearing structure, such as an airframe of the aircraft, to which the aircraft panel 2 is to be attached.

The aircraft panel 2 has a layer of packer material (hereinafter referred to as the first packer layer" 6) attached to its lower surface, for example, by an adhesive. The first packer layer 6 includes so-called "packer landings' (hereinafter referred to as the "first packer landings" and indicted in the Figures by the reference symbol 6'. The first packer landings 6' are located along the edges of the first packer layer 6. The first packer landings 6' are tapered edges of the first packer layer 6 that taper to nothing at their distal edges.

The aircraft substructure 4 has a layer of packer material (hereinafter referred to as the "second packer layer 8) attached to its upper surface, for example, by an adhesive. The second packer layer 8 includes packer landings (hereinafter referred to as the "second packer landings" and indicted in the Figures by the reference symbol 8'. The second packer landings 8' are located along the edges of the second packer layer 8. The second packer landings 8' are tapered edges of the second packer layer 8 that taper to nothing at their distal edges.

In this example, the packer layers 6, 8 are made of a composite material such as a fibre-reinforced polymer, i.e. a polymer matrix reinforced with fibres such as carbon or glass fibres. The packer material is a different material to those from which the aircraft panel 2 and aircraft substructure 4 are made.

The second packer layer 8 provides a sacrificial layer that is selectively machined such that its upper surface conforms to a desired inner mould line (IML) for the aircraft. Use of the sacrificial second packer layer 8 provides that machining of the aircraft substructure 4 may be avoided.

Also, the first packer layer 6 provides a sacrificial layer that is selectively machined such that, when the aircraft panel 2 and the aircraft substructure 4 are fastened together as described in more detail later below with reference to Figure 3, the upper surface of the aircraft panel 2 conforms to a desired outer mould line (OML) for the aircraft. Use of the sacrificial first packer layer 6 provides that machining of the aircraft panel 2 may be avoided.

A first bore 10 passes transversely through the aircraft panel 2 and the first packer layer 6 from the upper surface of the aircraft panel 2 to the lower surface of the first packer layer 6. The first bore comprises a first countersunk portion 12, a second countersunk portion 14, and an intermediate portion 16 that connects together the first countersunk portion 12 and the second countersunk portion 14. The first countersunk portion 12 is contiguous with the upper surface of the aircraft panel 2. The second countersunk portion 14 is contiguous with the lower surface of the first packer layer 6.

In this example, the first countersunk portion 12 is a countersink, i.e. a conical hole that is coaxial with the first bore 10.

In this example, the second countersunk portion 14 is a counterbore, i.e. a substantially cylindrical flat-bottomed hole or bore that enlarges another coaxial hole or bore.

A second bore 18 passes transversely through the aircraft substructure 4 and the second packer layer 8 from the upper surface of the second packer layer 8 to the lower surface of the aircraft substructure 4.

Figure 2 is a schematic illustration (not to scale) showing an example conventional fastener assembly 20 used to fasten together the aircraft panel 2 and the aircraft substructure 4.

In this example, the fastener assembly 20 comprises a bolt 22, a retaining ring 24, and a nut 26. The fastener assembly 20 is made of metal.

The bolt 22 is a threaded fastener which comprises a head 30 connected to an elongate threaded shank 32. The annular retaining ring 24 (or retaining clip) comprises a hole through which the shank 32 of the bolt 22 may be positioned. The nut 26 comprises a threaded hole for receiving the threaded portion of the shank 32 of the bolt 22. The nut 26 may be screwed onto the threaded portion of the bolt 22 in a conventional fashion.

Figure 3 is a schematic illustration (not to scale) showing a conventional bolted joint in which the aircraft panel 2 and the aircraft substructure 4 are fastened together by the fastener assembly 20.

In this example, the aircraft panel 2 and the aircraft substructure 4 are fastened together as follows.

Firstly, the bolt 22 is positioned through the first bore 10 such that the head 30 of the bolt 22 is located within the first countersunk portion 12 and such that the shank 32 of the bolt 22 passes through the intermediate portion 16 and second countersunk portion 14 and extends from the lower surface of the first packer layer 6.

Secondly, the bolt 22 is fixedly attached to the aircraft panel 2 by fixing the retaining ring 24 to the bolt 22 such that the retaining ring 24 is wholly contained within the second countersunk bore 14 within the first packer layer 6.

This may be performed by, for example, by screwing the retaining ring 24 along the threaded portion of the shank 32 of the bolt 22.

Thirdly, the aircraft panel 2 and the first packer layer 6 (with the bolt 22 and retaining ring 24 fixed thereto) is moved in the direction indicated by solid arrows in Figure 1 such that the lower surface of the first packer layer 6 contacts with the upper surface of the second packer layer 8, and such that the threaded portion of the shank 32 of the bolt 22 passes through the second bore 18. Thus, the first bore 10 is axially aligned with the second bore 18.

Lastly, the nut 26 is screwed onto the distal end of the shank 32 of the bolt 22 until the nut 26 contacts the aircraft substructure 4. Thus, the aircraft panel 2, the first packer layer 6, the second packer layer 8, and the aircraft substructure 4 are clamped between the head 30 of the bolt 22 and the nut 22.

In this example, the retaining ring 24 provides that, when the aircraft panel 2 is removed from the aircraft substructure 4 (e.g. during maintenance etc.), the bolt 22 is retained in the first bore 10 against the aircraft panel 2. This advantageously tends to reduce the likelihood of bolts 22 being lost and causing FOD damage to aircraft, and also expedites the reattachment of the aircraft panel 2 to the aircraft substructure 4. Also, in examples in which the bolt 22 is specifically configured to fit through a particular bore, the retaining ring 24 retains the bolt 22 in the first bore 10 through which that bolt 22 is configured to fit.

Conventionally, the first packer layer 6 has to be at least as thick as the retaining ring 24 such that the second countersunk portion 14 in which the retaining ring 24 is housed can be formed in the first packer layer 6.

The measurement of the depth of a counterbore tends to be relatively difficult. The measurement of the depth of a counterbore that is formed in a curved surface (e.g. of an aircraft panel, a packer layer, or an aircraft substructure) tends to be particularly true.

Conventionally, a Vernier depth gauge is used to measure the depth of a counterbore at different points within the counterbore. These multiple measurements may then be used to provide a minimum depth measurement for the counterbore, for example, to ensure that a retaining ring of a fastener may be wholly contained within that counterbore.

SUMMARY OF THE INVENTION

S In a first aspect, the present invention provides a joint assembly comprising a first object comprising a first bore, a second object comprising a second bore, a fastener (e.g. a bolt or a screw) comprising an elongate shank extending through the first bore and the second bore, and a retaining element (e.g. a retaining ring) for retaining the fastener against the first object. At least part of the retaining element is located within a first counterbore, the first counterbore being formed in the formed in the first object coaxially with the first bore. Also, at least part of the retaining element is located within a second counterbore, the second counterbore being formed in the formed in the second object coaxially with the second bore. The fastener fastens (i.e. fixes or attaches) the first object and the second object together.

The joint assembly may be an aircraft skin construction.

The first object may be an aircraft panel assembly comprising an aircraft panel.

The second object may be an underlying load bearing aircraft structure assembly comprising an aircraft substructure.

The aircraft panel assembly may further include a first layer of packer material attached to the aircraft panel.

The first counterbore may be formed wholly in the first layer of packer material.

The underlying load bearing aircraft structure assembly may include a second layer of packer material attached to the aircraft substructure.

The second counterbore may be formed wholly in the second layer of packer material.

The first bore may be a through bore.

The fastener may comprise a head attached to one end of the elongate shank. The head may be located at a surface of the first object that is opposite to the surface of the first object in which the first counterbore is formed.

The first and second objects may be arranged such that the first and second bores are aligned.

The retaining element may be wholly contained within the first and second counterbores.

Half of the retaining element may be located within the first counterbore.

Half of the retaining element may located within the second counterbore.

In a further aspect, the present invention provides an aircraft comprising a joint assembly according to the preceding aspect.

In a further aspect, the present invention provides a method of forming a joint assembly comprising providing a first object, forming a first bore in the first object, forming a first counterbore in the first object coaxially with the first bore, providing a second object, forming a second bore in the second object, forming a second counterbore in the second object coaxially with the second bore, arranging a fastener such that an elongate shank of the fastener extends through the first bore and the second bore, and arranging a retaining element to retain the fastener against the first object in such a way that at least part of the retaining element is located within the first counterbore and at least part of the retaining element is located within the second counterbore.

The steps of forming the counterbores and arranging the retaining element may be performed such that retaining element is wholly contained within the first and second counterbores.

Forming a counterbore may include: providing a counterbore depth gauge, the counterbore depth gauge comprising: a housing having a bore, the bore having an opening in a surface of the housing, the surface of the housing being substantially the same shape as the surface of the object in which the counterbore is formed; an elongate probe slidably mounted within at least part of the bore, the probe having a first end retractably disposed inside the housing, and a second end opposite to the first end; a distance measurement device coupled to the housing and the probe; and an indicator coupled to the distance measurement device; positioning the counterbore depth gauge such that the surface of the housing is flush against the surface of the object and such that the probe is directed towards the counterbore; moving the probe from a first position to a second position, the first position being such that the probe is wholly within the axial bore, the second position being such that the second end extends from the opening beyond the surface of the housing and such that the second end contacts with a floor surface of the counterbore; measuring, by the distance measurement device, a distance moved by the probe between the first position and the second position; and, using the measured distance, verifying the counterbore has sufficient depth to accommodate a predetermined portion of the retaining ring.

The second end of the probe may be substantially the same shape as the floor surface of the counterbore.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration (not to scale) used to illustrate conventional attachment of an aircraft panel to an aircraft substructure; Figure 2 is a schematic illustration (not to scale) showing an example conventional fastener; Figure 3 is a schematic illustration (not to scale) showing a conventional bolted joint in which the aircraft panel is fastened to the aircraft substructure; Figure 4 is a schematic illustration (not to scale) used to illustrate the attachment of the aircraft panel to the aircraft substructure according to an embodiment of the invention; Figure 5 is a schematic illustration (not to scale) of an embodiment of a bolted joint in which the aircraft panel is fastened to the aircraft substructure; Figure 6 is a schematic illustration (not to scale) of a side-view cross-section of an embodiment of a counterbore depth gauge; Figure 7 is a process flow chart showing certain steps of an embodiment of a process of using the counterbore depth gauge to measure the depth of a counterbore; Figure 8 is a schematic illustration (not to scale) of the counterbore depth gauge and the second packer layer at a certain step of the process of Figure 7; and Figure 9 is a schematic illustration (not to scale) of the counterbore depth gauge and the second packer layer at a certain step of the process of Figure 7.

DETAILED DESCRIPTION

In the following description, like reference numerals refer to like elements.

The following description is based on embodiments of the invention and should not be taken as limiting the invention with regard to alternative embodiments that are not explicitly described herein.

It will be appreciated that relative terms such as top and bottom, upper and lower, and so on, are used merely for ease of reference to the Figures, and these terms are not limiting as such, and any two differing directions or positions and so on may be implemented.

Figure 4 is a schematic illustration (not to scale) used to illustrate an embodiment in which the aircraft panel 2 is attached to the aircraft substructure 4.

In this embodiment, the aircraft panel 2 and the first packer layer 6 include a through bore (hereinafter referred to as the third bore" 40). The third bore 40 passes transversely through the aircraft panel 2 and the first packer layer 6 from the upper surface of the aircraft panel 2 to the lower surface of the first packer layer 6.

The third bore 40 comprises a third countersunk portion 42, a fourth countersunk portion 44, and a further intermediate portion 46.

The third countersunk portion 42 is contiguous with the upper surface of the aircraft panel 2 and is configured to receive the head 30 of the bolt 22.

Thus, in this embodiment, the third countersunk portion 42 is substantially the same as the first countersunk portion 12 of the conventional assembly described above with reference to Figures 1 to 3.

In this embodiment, the third countersunk portion 42 is a countersink.

The fourth countersunk portion 44 is contiguous with the lower surface of the first packer layer 6. In this embodiment, as described in more detail later below with reference to Figure 5, the fourth countersunk portion 44 is configured to, in use, house only half of the retaining ring 24 of the fastener assembly 20.

In particular, in use, the upper half of the retaining ring 24 is located in the fourth countersunk portion 44. Thus, in this embodiment, the depth of the fourth countersunk portion 44 (i.e. the distance that the fourth countersunk portion 44 extends into the first packer layer 6 from the lower surface of the first packer layer 6) is substantially equal to half the depth of the second countersunk portion 14 of the conventional assembly described above with reference to Figures ito 3.

In this embodiment, the fourth countersunk portion 44 is a counterbore.

The further intermediate portion 46 connects together the third countersunk portion 42 and the fourth countersunk portion 44.

In this embodiment, the aircraft substructure 4 and the second packer layer 8 include a through bore (hereinafter referred to as the "fourth bore" 50).

The fourth bore 50 passes transversely through the aircraft substructure 4 and the second packer layer 8 from the upper surface of the second packer layer 8 to the lower surface of the aircraft substructure 4.

The fourth bore 50 comprises a fifth countersunk portion 52 and an elongate portion 54.

In this embodiment, the fifth countersunk portion 52 is a counterbore.

The fifth countersunk portion 52 is contiguous with the upper surface of the second packer layer 8. In this embodiment, as described in more detail later below with reference to Figure 5, the fifth countersunk portion 52 is configured to, in use, house half of the retaining ring 24 of the fastener assembly 20. In particular, in use, the lower half of the retaining ring 24 is located in the fifth countersunk portion 52. In this embodiment, the depth of the fifth countersunk portion 52 (i.e. the distance that the fifth countersunk portion 52 extends into the second packer layer 8 from the upper surface of the second packer layer 8) is substantially equal to the depth of the fourth countersunk portion 44.

In this embodiment, the aircraft panel 2 and the aircraft substructure 4 are attached together using the conventional fastener assembly 20 described in more detail above with reference to Figure 2.

Figure 5 is a schematic illustration (not to scale) showing the elements described above with reference to Figure 4 fastened together by the fastener assembly 20. The shank of the bolt 22 extends transversely through the aircraft panel 2 and the aircraft substructure 4. The fastening together of the elements shown in Figure 4 is performed as follows.

Firstly, the bolt 22 is positioned through the third bore 40 such that the head 30 of the bolt 22 is located within the third countersunk portion 42 and such that the shank 32 of the bolt 22 passes through the further intermediate portion 46 and fourth countersunk portion 44 and extends from the lower surface of the first packer layer 6.

Secondly, the bolt 22 is fixedly attached to the aircraft panel 2 by fixing the retaining ring 24 to the bolt 22 such that the retaining ring 24 is partially contained within the fourth countersunk bore 44. In particular, the bolt 22 is fixedly attached to the aircraft panel 2 by fixing the retaining ring 24 to the bolt 22 such that the upper half of the retaining ring 24 is located within the fourth countersunk bore 44. This may be performed by, for example, by screwing the retaining ring 24 along the threaded portion of the shank 32 of the bolt 22.

Thirdly, the aircraft panel 2 and the first packer layer 6 (with the bolt 22 and retaining ring 24 fixed thereto) is moved in the direction indicated by solid arrows in Figure 4 such that: -the lower surface of the first packer layer 6 contacts with the upper surface of the second packer layer 8; -the threaded portion of the shank 32 of the bolt 22 passes through the fourth bore 50; and -the lower half of the retaining ring 24 is located within the fifth countersunk bore 52.

Thus, an embodiment in which the aircraft panel 2 is connected to the aircraft substructure 4 is provided.

In this embodiment, the retaining ring 24 provides that, when the aircraft panel 2 is removed from the aircraft substructure 4 (e.g. during maintenance etc.), the bolt 22 is retained in the third bore 40 against the aircraft panel 2. This advantageously tends to reduce the likelihood of bolts 22 being lost and causing FOD damage to aircraft, and also expedites the reattachment of the aircraft panel 2 to the aircraft substructure 4. Also, in embodiments in which the bolt 22 is specifically configured to fit through a particular bore, the retaining ring 24 retains the bolt 22 in the third bore 40 through which that bolt 22 is configured to fit.

In this embodiment, the retaining ring 24 is partially located within a counterbore in the packer layer attached to the aircraft panel 2 (i.e. the fourth countersunk portion 44), and partially located within a counterbore in the packer layer attached to the aircraft substructure 4 (i.e. the fifth countersunk portion 52).

As only part (i.e. half in this embodiment) of the retaining ring 24 is housed within the counterbore in first packer layer 6, the depth of the counterbore in first packer layer 6 may be less than is used conventionally. In other words, the depth of the fourth countersunk portion 44 may be less than the depth of the second countersunk portion 14. This tends to provide that the thickness of the first packer layer 6 in this embodiment may be substantially less than that in conventional assemblies. In other words, the thickness of the first packer layer 6 in the assembly described above with reference to Figures 4 and 5 is substantially less than the thickness of the first packer layer 6 in the conventional assembly which is described above with reference to Figures 1 to 3. Over the entire aircraft, this tends to provide a significant weight reduction.

Thus, aircraft fuel costs tend to be reduced.

Furthermore, the thinner first packer layer 6 tends to provide that the first packer landings 6' in this embodiment may be smaller than those in conventional assemblies. Thus, the size of the first packer layer 6 (i.e. the footprint of the first packer layer 6 on the aircraft panel 2) in the assembly described above with reference to Figures 4 and 5 is substantially less than that of the first packer layer 6 in the conventional assembly. Over the entire aircraft, this tends to provide a further significant weight reduction. Thus, aircraft fuel costs tend to be further reduced.

Advantageously, housing half of the retaining ring 24 within the counterbore in second packer layer 8 tends not to require increased thickness of the second packer layer 8 compared to that used conventionally. This is because, in this embodiment and in conventional assemblies, the second packer layer 8 is machined such that its upper surface is located at a pre-specified IML for the aircraft. Thus, the second packer layer 8 in this embodiment will have substantially the same thickness as that in conventional assemblies.

What will now be described is device for measuring the depth of a counterbore (i.e. countersunk portion of a bore), such as the fourth and fifth countersunk portions 44, 52 described in more detail earlier above with reference to Figures 4 and 5.

Figure 6 is a schematic illustration (not to scale) of a side-view cross-section of an embodiment of a counterbore depth gauge 60.

In this embodiment, the counterbore depth gauge 60 comprises a distance measurement module 62, a tubular housing 64, and a counterbore probe 66.

The distance measurement module 62 comprises a digital display 68.

The distance measurement module 62 is fixedly attached to one end of the tubular housing 64.

Also, the distance measurement module 62 is connected to the counterbore probe 66. The distance measurement module 62 is configured to measure a distance between the distance measurement module 62 and the counterbore probe 66 along the length of the tubular housing 64.

In this embodiment, the tubular housing 64 is cylindrical in shape. The tubular housing 64 houses the probe 66 in such a way that the probe 66 may be moved along the length of the tubular housing 64 in the directions indicated in Figure 6 by the double headed arrow and the reference numeral 72. Thus, the probe 66 may be closer to, or further away from, the distance measurement module 62.

The tubular housing 64 has a proximal end 69 at which it is connected to the distance measurement module 62, and a distal, free end 70 opposite to the proximal end 69.

In this embodiment, the distal end 70 of the tubular housing 64 has a curved profile. In other words, when the cross section of the counterbore depth gauge is viewed from the side (as shown in Figure 6), the distal end 70 of the tubular housing 64 is curved.

In this embodiment, the curved profile of the distal end 70 is such that, as described in more detail later below with reference to Figures 7 to 9, the distal end 70 may be placed flush against, i.e. contiguous with, a curved surface in which the counterbore to be measured is formed.

In this embodiment, the upper surface of the second packer layer 8 is a curved surface. Also, the counterbore depth gauge 60 is configured to measure the depth of the fifth countersunk bore 52 which is formed in the upper surface of the second packer layer 8. Thus, the shape of the curved distal end 70 of the tubular housing 64 is substantially the same as the shape of the curved upper surface of the second packer layer 8 such that the distal end 70 may be placed flush against the upper surface of the second packer layer 8.

In this embodiment, the upper surface of the second packer layer 8 is a convex surface and the distal end 70 is a corresponding concave surface.

Production of the counterbore depth gauge 60 may comprise determining a curvature of a surface in which a counterbore to be measured is formed.

Using the determined curvature, the tubular housing 64 of the counterbore depth gauge 60 may then be produced to have, at its distal end 70, a correspondingly curved surface. The determination of a surface curvature may be performed in any appropriate way such as by measuring the curvature of the surface, for example, using a coordinate measuring machine (CMM) or laser tracker, or by using a digital model of the surface in which the counterbore is formed.

In this embodiment, the probe 66 is substantially cylindrical in shape. The probe 66 is housed within the tubular housing 64 in such a way that the longitudinal axes of the tubular housing 64 and the probe 66 are substantially aligned. The probe 66 is connected at its first end 74 to the distance measurement module 62, and has a free, second end 76 opposite to the first end 74. In this embodiment, the second end of the probe is a substantially flat (i.e. planar) surface.

In this embodiment, the probe 66 may be moved along the length of the tubular housing 64 between a first position and a second position. The counterbore depth gauge 60 may include any appropriate actuation means which a user may operate to move the probe 66 along the length of the tubular housing 64.

In the first position, the second end 76 of the probe 66 does not extend out of the tubular housing 64 past the distal end 70 of the tubular housing 64. In other words, in the first position, the probe 66 is wholly located within the tubular housing 64. For example, in the first position, the second end 76 of the probe 66 may be level with the distal end 70 of the tubular housing 64.

In the second position, the second end 76 of the probe 66 extends out of the tubular housing 64 beyond the distal end 70 of the tubular housing 64.

The diameter of second end 76 of the probe 66 is less than or equal to the diameter of the bottom surface of the counterbore to be measured (i.e. the fifth countersunk portion 52). In some embodiments, the diameter of second end 76 of the probe 66 is substantially the same size as the bottom surface of the fifth countersunk portion 52. Preferably, the diameter of second end 76 of the probe 66 is slightly smaller than the bottom surface of the fifth countersunk portion 52.

Figure 7 is a process flow chart showing certain steps of an embodiment of a process of using the counterbore depth gauge 60 to measure the depth of a counterbore. In this embodiment, the counterbore being measured is the fifth countersunk portion 52 in the second packer layerS of the assembly described above with reference to Figures 4 and 5. In this embodiment, the upper surface of the second packer layer 8, which is contiguous with the fifth countersunk portion 52, is a curved surface.

At step s2, the user positions the counterbore depth gauge 60 such that the distal end 70 of the tubular housing 64 is contiguous with (i.e. is flush against) a portion of the upper surface of the second packer layer 8. The distal end 70 conforms to the curved upper surface of the second packer layer 8 against which it is positioned.

In this embodiment, the portion of the upper surface of the second packer layer 8 against which the distal end 70 is positioned is proximate to, but spaced apart from, the fifth countersunk portion 52.

In this embodiment, at step s2, the probe 66 is positioned at its first position, i.e. such that the probe 66 is wholly located within the tubular housing 64.

At step s4, the user operates the counterbore depth gauge 60 such that the second end 76 of the probe 66 is in contact with the upper surface of the second packer layer 8.

Figure 8 is a schematic illustration (not to scale) of the counterbore depth gauge 60 and the second packer layer 8 at step s4.

At step s6, the user operates the counterbore depth gauge 60 such that the distance measurement module 62 measures the position of the probe 66 relative to the distance measurement module 62. In this embodiment, the distance measurement module 62 measures the distance between the first end 74 of the probe 66 and the distance measurement module 62.

In this embodiment, the measurement of the position of the probe 66 relative to the distance measurement module 62 taken at step s6 is stored, by the distance measurement module 62, as a "zero" position. The measurement of the position of the probe 66 relative to the distance measurement module 62 taken at step s6 provides a datum against which the depth of the fifth countersunk portion 52 is to be measured.

Steps s2 to s6 described a process of "zeroing" the counterbore depth gauge 60 against the upper surface of the second packer layer 8.

At step s8, the user positions the counterbore depth gauge 60 such that the distal end 70 of the tubular housing 64 is contiguous with a portion of the upper surface of the second packer layer 8 that surrounds the fifth countersunk portion 52. Thus, at step s8, the counterbore depth gauge 60 is positioned such that the annular distal end 70 surrounds the fifth countersunk portion 52.

At step sI 0, the user operates the counterbore depth gauge 60 such that the probe 66 is moved away from the distance measurement module 62 until the second end 76 of the probe 66 is in contact with the bottom surface 52' (or floor) of the fifth countersunk portion 52. In this embodiment, the bottom surface 52' of the fifth countersunk portion 52 is located opposite to an opening of the fifth countersunk portion 52, the opening being in the upper surface of the second packer layer 8. The bottom surface 52' of the fifth countersunk portion 52 is separated from the opening of the fifth countersunk portion 52 by walls of the counterbore.

Figure 9 is a schematic illustration (not to scale) of the counterbore depth gauge 60 and the second packer layer 8 at step si 0.

In this embodiment, the probe 66 is moved away from the distance measurement module 62 until it contacts with the uppermost point of the bottom surface 52' of the fifth countersunk portion 52, at which point the probe 66 is stopped by the bottom surface 52' of the fifth countersunk portion 52. Thus, the shallowestor minimum depth of the fifth countersunk portion 52 is measured.

At step s12, the user operates the counterbore depth gauge 60 such that the distance measurement module 62 measures the position of the probe 66 relative to the datum generated at step s6. The distance between the probe 66 and the datum is measured. The measured distance is the minimum depth of the fifth countersunk portion 52.

At step s14, the display 68 displays the distance measured at step s12, i.e. the depth of the fifth countersunk portion 52, to the user.

Using the displayed counterbore depth, the user may perform any appropriate action such as verifying or otherwise that the fifth countersunk portion 52 has a required minimum depth to house a predetermined portion (e.g. half) of the retaining ring 24.

Thus a process in which the counterbore depth gauge 60 is used to measure the depth of a counterbore is provided.

The above described counterbore depth gauge is advantageously simple to use and provides a measurement of a minimum depth of a counterbore relatively quickly.

Conventionally, a minimum depth measurement for a counterbore tends to require multiple measurements to be taken using. For example, a Vernier depth gauge. The above described counterbore depth gauge 60 advantageously tends to solve this problem by allowing for the minimum depth of a counterbore to be determined using only a single measurement. This tends to result from the size and shape of the second end 76 of the probe being the same size and shape (or only slightly smaller) than the bottom surface of the counterbore being measured.

The distal end 70 of the tubular housing 64 tends to provide that the depths of counterbores formed in curved surfaces may be accurately measured.

Having the curved distal end 70 conform to the surface in which the counterbore being measured is formed advantageously tends to provide that, during zeroing and measurement processes, the counterbore depth gauge 60 is securely and stably anchored against that surface. Thus, a reliable and repeatable datum or "zero" is generated against which a depth measurement may be taken. Also, reliable and repeatable measurements of the depth of the counterbore are provided.

Advantageously, the tubular housing 64 may be detachable from the rest of the counterbore depth gauge 60. The tubular housing 64 may be replaced by a different tubular housing which may have a distal end that has a different curvature to the distal end 70 of the removed tubular housing 64. This allows for the accurate measurement of the depths of counterbores that are formed in surfaces having different curvatures to the curvature of the upper surface of the second packer layer 8.

In the above embodiments, the fourth countersunk portion is configured to receive half of the retaining ring. Similarly, the fifth countersunk portion is configured to receive half of the retaining ring. However, in other embodiments, the fourth countersunk portion is configured to receive a different proportion of the retaining ring. Similarly, the fifth countersunk portion may be configured to receive a different proportion of the retaining ring. For example, in some embodiments, the fourth countersunk portion is configured to receive a quarter of the retaining ring, while the fifth countersunk portion is configured to receive three quarters of the retaining ring.

In the above embodiments, an aircraft panel is attached to an aircraft substructure. In other embodiments the aircraft panel is replaced by a different type of entity. Also, in other embodiments, the aircraft substructure is replaced by a different type of entity. The entities that are fastened together may comprise sacrificial portions, such as layers of packer material, in which the counterbores for housing the retaining ring are formed.

In the above embodiments, the joint in which the aircraft panel and the aircraft substructure are fastened is a bolted joint, i.e. the aircraft panel and the substructure are fastened together by a bolt and a nut. Also, the retaining ring retains the bolt against the aircraft panel. However, in other embodiments, the aircraft panel may be fixed to the aircraft substructure by a different type of joint such as a screw joint, i.e. a screw may replace the bolt and be screwed into to aircraft substructure. In some such embodiments, a nut is not used. In such embodiments, the retaining ring may retain the screw against the aircraft panel.

In the above embodiments, the fastener is retained against the aircraft panel by a retaining ring. However, in other embodiments, a different type of retaining element is used.

In the above embodiments, the aircraft panel is made of CFC material.

However, in other embodiments, the aircraft panel is made of a different material instead of or in addition to CFC material.

In the above embodiments, the aircraft substructure is made of aluminium. However, in other embodiments, the aircraft substructure is made of a different material instead of or in addition to aluminium.

In the above embodiments, the packer layers are made of a composite material. However, in other embodiments, the packer layers are made of a different material instead of or in addition to a composite material.

In the above embodiments, the fastener is made of metal. However, in other embodiments, the fastener is made of a different material instead of or in addition to metal.

In the above embodiments, the counterbore depth gauge comprises a digital display for indicating a depth measurement to a user. However, in other embodiments, the counterbore depth gauge comprises a different type of indicator for indicating a depth measurement to a user. -20 -

The components of the counterbore depth gauge may be made of any appropriate material such as metal or plastic. -21 -

Claims

CLAIM S1. A joint assembly comprising: a first object (2, 6) comprising a first bore (40); a second object (4, 8) comprising a second bore (50); S a fastener (22) comprising an elongate shank (32) extending through the first bore (40) and the second bore (50); and a retaining element (24) for retaining the fastener (22) against the first object (2, 6); wherein at least part of the retaining element (24) is located within a first counterbore (44), the first counterbore (44) being formed in the formed in the first object (2, 6) coaxially with the first bore (40); and at least part of the retaining element (24) is located within a second counterbore (52), the second counterbore (52) being formed in the formed in the second object (4, 8) coaxially with the second bore (50).
2. A joint assembly according to claim 1, wherein: the joint assembly is an aircraft skin construction; the first object (2, 6) is an aircraft panel assembly comprising an aircraft panel (2); and the second object (4, 8) is an underlying load bearing aircraft structure assembly comprising an aircraft substructure (4).
3. An joint assembly according to claim 2, wherein the aircraft panel assembly further includes a first layer of packer material (6) attached to the aircraft panel (2); and the first counterbore (44) is formed in the first layer of packer material (6). -22 -
4. An joint assembly according to claim 2 or 3, wherein the underlying load bearing aircraft structure assembly includes a second layer of packer material (8) attached to the aircraft substructure (4); and the second counterbore (52) is formed in the second layer of packer material (8).
5. A joint assembly according to any of claims 1 to 4, wherein the fastener (22) is a threaded fastener selected from the group of threaded fasteners consisting of a bolt and a screw.
6. A joint assembly according to any of claims 1 to 5, wherein the retaining element (24) is a retaining ring.
7. A joint assembly according to any of claims 1 to 6, wherein the first bore (40) is a through bore; the fastener (22) comprises a head (30) attached to one end of the elongate shank (32); and the head (30) is located at a surface of the first object (2, 6) that is opposite to the surface of the first object (2, 6) in which the first counterbore (44) is formed.
8. A joint assembly according to any of claims 1 to 7, wherein the first and second objects are arranged such that an axis of the first bore (40) and an axis of the second bore (50) are aligned.
9. A joint assembly according to any of claims 1 to 8, wherein the retaining element (24) is wholly contained within the first and second counterbores (44, 52). -23 -
10. A joint assembly according to any of claims 1 to 9, wherein half of the retaining element (24) is located within the first counterbore (44); and S half of the retaining element (24) is located within the second counterbore (52).
11. An aircraft comprising a joint assembly according to any of claims 1 to 10.
12. A method of forming a joint assembly, the method comprising: providing a first object (2, 6); forming a first bore (40) in the first object (2, 6); forming a first counterbore (44) in the first object (2, 6) coaxially with the first bore (40); providing a second object (4, 8); forming a second bore (50) in the second object (4, 8); and forming a second counterbore (52) in the second object (4, 8) coaxially with the second bore (50); arranging a fastener (22) such that an elongate shank (32) of the fastener (22) extends through the first bore (40) and the second bore (50); and arranging a retaining element (24) to retain the fastener (22) against the first object (2, 6) in such a way that at least part of the retaining element (24) is located within the first counterbore (44) and at least part of the retaining element (24) is located within the second counterbore (52).
13. A method according to claim 12 wherein the steps of forming the counterbores (44, 52) and arranging the retaining element (24) are performed -24 -such that retaining element (24) is wholly contained within the first and second counterbores (44, 52).
14. A method according to claim 12 or 13, wherein a step of forming a counterbore (44, 52) includes: providing a counterbore depth gauge (60), the counterbore depth gauge (60) comprising: a housing (64) having a bore, the bore having an opening in a surface (70) of the housing (64), the surface (70) of the housing (64) being substantially the same shape as the surface of the object in which the counterbore (44, 52) is formed; an elongate probe (66) slidably mounted within at least part of the bore, the probe (66) having a first end (74) retractably disposed inside the housing (64), and a second end (76) opposite to the first end (74); a distance measurement device (62) coupled to the housing (64) and the probe (66); and an indicator (68) coupled to the distance measurement device (62); positioning the counterbore depth gauge (60) such that the surface (70) of the housing (64) is flush against the surface of the object in which the counterbore (44, 52) is formed and such that the probe (66) is directed towards the counterbore (44, 52); moving the probe (66) from a first position to a second position, the first position being such that the probe (66) is wholly within the axial bore, the second position being such that the second end extends from the opening beyond the surface (70) of the housing (64) and such that the second end (76) contacts with a floor surface of the counterbore (44, 52); measuring, by the distance measurement device (62), a distance moved by the probe (66) between the first position and the second position; and -25 -using the measured distance, verifying the counterbore (44, 52) has sufficient depth to accommodate a predetermined portion of the retaining ring (24).
15. A method according to claim 14, wherein the second end (76) of the probe is substantially the same shape as the floor surface of the counterbore (44, 52).