Automated drill process for two-diameter holes in multi-layer variable thickness composite materials
A method of manufacturing a mobile platform. The platform includes a structure and a member assembled to the structure. Further, the member has a first and a second surface with a thickness defined there between that is varied. The method includes leaving the member on the structure. Also, the method includes advancing a tool through the member while on the structure. The tool is stopped before the tool advances into the structure by more than about a first pre-selected tolerance. Also, a mobile platform assembly is provided that includes a structure, a member, and a fastener fastening the structure and member via a hole through the structure and assembly. The hole has a first diameter through the structure and a second diameter through the member, wherein the diameter changes within an acceptable depth range. In another preferred embodiment, a boroscope, adapted for inspecting holes having two diameters, is also provided.
This invention was developed in the course of work under U.S. government contract No. F/A-18 E/F N00019-99-C-1226; FY00-04. The U.S. government may possess certain rights in the invention.
FIELD OF THE INVENTIONThis invention relates generally to the manufacture of mobile platforms and, more particularly, the assembly of skin panels to the airframes of aircraft.
BACKGROUND OF THE INVENTIONAircraft bodies are assembled by fabricating a frame and fastening panels to the frame. Typically, the frame is an aluminum or titanium structure with ribs, stringers, and the like to distribute the loads imposed by the aircraft's weight and aerodynamic forces that act on the aircraft. The assembly process continues with panels being fastened to the structure to form the skin of the aircraft. Because these skin panel fasteners carry much of the load on the aircraft as shear stress, the fastening of the skin panels to the aircraft frame is a factor in how efficiently the aircraft carries the loads.
Increasingly, composite skin panels are being used to lighten the aircraft and improve its load carrying capability. Assembling the composite panels to the aircraft requires that a liquid shim be applied to the cured panel to fill any gaps that might otherwise exist between the panel and the frame. These gaps arise because the composite panels generally will not match the shape of the frame exactly. Rather, some variation will exist between the frame and the panel that may be several thousands of an inch in magnitude. Thus, the liquid shim compensates for the variation. Once applied to the composite panel, the liquid shim begins curing and eventually forms a portion of the panel. Next, the panel is mounted to the airframe and fastener holes are drilled through the panel and frame for subsequent installation of a fastener.
If the material of the structure and the panel are the same, the hole may be sized with one constant diameter through the two sub-assemblies. However, metals and composites behave differently when subjected to loads. For instance, interference fits are frequently selected between metallic structures and fasteners to improve fatigue life in the tensilely loaded panels and frames. On the other hand composites generally require a clearance between the composite and the fastener to prevent the composite from delaminating while installing interference fit fasteners through the composite panel.
To provide the interference fit and the clearance, the panel must therefore be removed from the airframe after the fastener hole is drilled to the interference diameter. The hole in the composite panel is then reamed to a slightly larger size to create the clearance. Thereafter, the panel is placed on the structure once again. Then, the fastener is placed in the hole having the two diameters and tightened into place.
Unfortunately, the process of providing the clearance causes several disadvantageous results. First, removing and reinstalling the panel consumes time and resources that could be employed for other useful activities. Second, because the panel has been moved after the initial hole was formed, the reamer used to enlarge the hole in the composite panel may be positioned off of the longitudinal axis of the hole. Accordingly, the reamed enlargement may be off-center, or eccentric, with respect to the axis of the hole (through the panel). Moreover, perfect re-alignment between the panel and the structure may not be re-acquired either. Further with these previous assembly methods, any interlaminar metallic burrs generated during the enlargement process must be removed manually from the airframe structure before fastening the panel to the structure.
In the alternative, the reamer could be brought to the panel while it is still mounted on the airframe and the hole enlarged. In practice, this alternative has produced poor assemblies because the thickness of the composite panel varies from the theoretical thickness across the panel (both by design and due to variations inherent to fabrication of composite parts). Moreover, the liquid shim applied to the panel may vary in thickness because of the gaps between the airframe and the panel and because of variations in how the liquid shim is applied. Thus, the operator (or numerically controlled machine programmer) does not know at what depth to stop the reamer before it engages the airframe. If the reamer is advanced too far, it creates a clearance within the metal and weakens the joint. If the reamer does not advance far enough, it leaves an interference fit in the composite and weakens the joint.
Thus a need exists to improve the assembly of composite members to metallic structures.
SUMMARY OF THE INVENTIONIt is in view of the above problems that the present invention was developed. The invention provides improved mobile platform skin panel assemblies and methods of assembling the same.
More particularly, the present invention provides an assembly that includes a skin panel made from a composite material and a structure made from a metal. The assembly defines a hole through the panel and the structure with a first diameter through the metal and a second diameter through the panel, which has a thickness profile known in advance. The transition between the two diameters occurs within a pre-selected tolerance from the surface of the metal that is adjacent to the panel and another pre-selected tolerance from the surface of the panel that abuts the structure. In other words, the clearance reliably extends into the metal only to the extent of the first tolerance, whereas the interference fit reliably extends into the composite only to the extent of the second tolerance. Thus, the present invention provides superior joints between composite skin panels and metallic structures. In addition, the present invention provides assemblies with a structure having a first material and a member having a second material and a fastener hole therethrough, the materials requiring the hole to have two different diameters.
In another preferred embodiment, the present invention provides a method of assembling composite panels to metallic structures. The method includes leaving the panel on the structure while a hole, which extends through the panel and structure, is enlarged down to between a pre-selected tolerance from the surface of the panel and another pre-selected tolerance in the structure. Thus, the present invention provides a superior method of assembling composite panels to metallic structures.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGSThe accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and together with the description, serve to explain the principles of the invention. In the drawings:
Referring to the accompanying drawings in which like reference numbers indicate like elements,
The hole 22 includes an overall depth 32 that includes a first depth 34 defined by the first diameter 24 portion, a second depth 36 defined by the second diameter 26 portion, and a third depth 38 defined by the transition 28. Since the transition 28 may be a step, the depth 38 will henceforth be treated as being generally negligible. Similarly, the assembly 10 (e.g. the wing of
Also,
As mentioned previously, the thickness 42 of the panel 14 also varies. The variations in the thickness 42 of the panel(s) arise from the complexity of the composite panels 14 and the design and manufacturing requirements, or preferences, for a given panel. For instance, the number of plies (e.g. carbon epoxy plies) in the panel typically varies with location on the panel 14. Also, manual lay-up methods and autoclave cure cycles are likely to cause further deviations from the theoretical thickness of the panel 14. To account for these variations, the following data may be loaded into a database for a numerically controlled machine prior to performing the drilling and reaming operations described for the present embodiment: a hole identification number, a hole location on the panel, and the actual panel 14 thickness 42. The methods discussed herein may also be performed manually, although automated machining is preferred. Whether the machining is performed manually or automatically, the machining parameters (e.g. drill feeds and speeds) can be changed as the tool(s) progress through the various layers of the assembly based on the stack-up information and the theoretical and actual thicknesses associated with a given hole identification number. As a result, it is possible to machine at the optimum rate for each material in the assembly and, thereby, reduce the cycle time for each hole. In particular, the machining speed may increase as progress is made through the softer materials (e.g. composites and aluminum) as compared to the speed preferred for the harder materials (e.g. titanium).
In accordance with the principals of the present invention, the variable thickness 54 region is defined by materials (the liquid shim 18 and the sealant 20) for which neither an interference fit, nor a clearance, with the fastener 16 (see
More particularly,
In the example shown by
The results showed that the average transition 128 occurred within the liquid shim 118 in accordance with the principals of the present invention. That is to say, the transition 128 lies within the variable thickness of the liquid shim 118 where the joint requires neither a clearance nor an interference fit. The process was also shown to possess a Cpk (process capability index) of 1.45 that is satisfactory for most applications, even here in the realistic worst-case example.
For purposes of demonstration, an abnormal situation, wherein little (or no) liquid shim (gap) exists in the joint at the locale of the hole 122 was also tested, as shown by
Moreover, because the panel 114 and the structure 112 remain generally adjacent each after the application of the liquid shim 118 and the sealant 120, no manufacturing debris, or other contaminants, will be found in the joint. Thus, the overall joint is stronger than provided by previous methods of assembly. Moreover, the drilling and reaming may be performed by using the same conventional 6-degree of freedom robot that can remain stationary relative to the axis 130 of the hole 122 (except as it traverse the axis 130). Thus, eccentricity of the clearance diameter 126 portion of the hole 122 (with respect to the interference diameter 124 portion) is significantly reduced over that of previous processes. For the joints illustrated by
In the alternative to the one-up method, the present invention may also be employed where no sealant, or bonding agent, is applied between the panel 14 and the structure. For instance, the panel 114 may be clamped to the structure 112, an initial one-diameter hole drilled, and the hole may then be enlarged through the panel 114. Thereafter, the panel 114 may be unclamped from the structure 112 and, if desired, removed for de-burring and other operations prior to subsequent assembly operations.
With respect to
While the exemplary fastener holes previously discussed were generally orientated perpendicularly to the mating surfaces of the structure 12 and panel 14 (see
Lighting may also be provided internal to the instruments 200 and 300 to enable the user to view the visible differences between the structure 212, the panel 214, the liquid shim 218, and the sealant 220. The instruments 200 and 300 also enable the user to inspect the sides of the hole 222 to determine whether any chips or cuttings were caught between the outer pitch of the drill bit (and reamer) and the sides of the hole 322 by turning the instrument while traversing the axis of the hole 322. Thus, the composite panels 220 and 320 may be inspected for internal machining damage that would otherwise be hidden.
In another preferred embodiment, the present invention provides a dial indicator for inspecting the depth of the transitions. The dial indicator includes a plunger operatively connected to a depth dial gage. Further, the end of the plunger is adapted to engage the transition of a hole, thereby enabling the inspection. To inspect a hole, the indicator is zeroed by fully depressing the plunger against a hard surface. Then the plunger is inserted into the hole and allowed (by a biasing member such as a spring) to extend to the depth at which it stops. Generally, the depth at which the plunger stops indicates the location of the transition. However, debris in the hole may cause the dial indicator to indicate a transition depth shallower than the true transition depth. Also, erosion (particularly of the liquid shim and sealant) caused by chips being caught between the drill bit, or reamer, during the machining of the hole, may allow the plunger to extend beyond the true transition, thereby indicating a transition depth larger than the true transition depth. Accordingly the boroscope based instruments 200, 300, and 400 are preferred over the dial indicator of the current embodiment.
With reference now to
In view of the foregoing, it will be seen that the several advantages of the invention are achieved and attained. In particular, satisfactory joints are provided with the panels remaining on the airframe once placed thereon. Thus, the present invention reduces the cost of assembling aircraft. For the same reason, the present invention provides joints having superior mechanical properties (e.g. strength, fit, noise or rattling because of poor “fit up”). Additionally, the present invention provides improved inspection tools over those previously available.
The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.
As various modifications could be made in the constructions and methods herein described and illustrated without departing from the scope of the invention, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. For example, whereas the foregoing discussion involved composite members being assembled to metallic structures, the present invention is not limited thereby. Rather, any assembly with materials requiring having fastener holes with two different diameters is within the scope of the present invention (e.g. titanium and aluminum). Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.
Claims
1. A method of manufacturing a mobile platform, the mobile platform including a structure and a plurality of members assembled to the structure, the members each having a first and a second surface with a thickness defined there between that varies with a location on each member and which varies between members, the method comprising:
- leaving a member with a varied thickness on the structure in such a manner that the second surface is generally adjacent to the structure;
- advancing a tool through the member in a direction generally from the first surface toward the second surface; and
- stopping the advance before the tool advances into the structure by more than about a first pre-selected tolerance.
2. The method according to claim 1, the stopping further comprising being after the tool advances to within about a second pre-selected tolerance of the second surface.
3. The method according to claim 2, wherein the first and the second pre-selected tolerances are the same.
4. The method according to claim 1, further comprising inspecting the path of the tool to determine where the tool stopped advancing.
5. The method according to claim 4, the inspecting further comprising using a dial indicator adapted to engage a feature in the member formed by the tool at about the time the tool stopped advancing.
6. The method according to claim 4, the inspecting further comprising using a boroscope to view engage a feature in the member formed by the tool at about the time the tool stopped advancing.
7. The method according to claim 6, the using the boroscope further comprising determining whether the tool stopped before advancing into the structure by more than about the first pre-selected tolerance.
8. The method according to claim 1, wherein the structure is an airframe and the member is a wing skin panel.
9. The method according to claim 1, further comprising receiving the varied thickness.
10. The method according to claim 1, the advancing further comprising using the varied thickness to guide a numerically controlled machine.
11. The method according to claim 1, further comprising drilling a hole through the structure and member.
12. The method according to claim 11, the advancing further comprising a reaming of the hole.
13. The method according to claim 12, further comprising fastening the member to the structure with a fastener placed in the hole, the fastener to have an interference fit with the structure and a clearance with the member when the fastener is in the hole.
14. The method according to claim 1, wherein the structure includes a metallic material and the member includes a composite material.
15. The method according to claim 1, wherein the direction of advance forms an acute angle with respect to an orientation of the second surface.
16. The method according to claim 1, further comprising applying liquid shim to the member, the liquid shim forming at least a portion of the second surface and defining at least a portion of the varied thickness.
17. The method according to claim 1, further comprising applying a sealant to the structure to seal the structure and the member.
18. The method according to claim 1, further comprising changing a parameter associated with the tool while advancing the tool.
19. An assembly for a mobile platform comprising:
- a structure;
- at least one fastener, and
- at least one member, each member having a first and a second surface with a thickness defined there between that varies with a location on each member and which varies between members, the at least one member fastened to the structure by the at least one fastener in such a manner that the second surface is generally adjacent to the structure, the assembly defining at least one hole with a first diameter generally within the structure and a second diameter generally within the member, the second diameter extending to between a first pre-selected tolerance into the structure and to within a second pre-selected tolerance from the second surface into the member.
20. The assembly according to claim 19, wherein the first and the second pre-selected tolerances are the same.
21. The assembly according to claim 19, wherein the structure is an airframe and the member is a wing skin panel. thickness.
22. The assembly according to claim 19, wherein the first diameter causes an interference fit between the structure and the fastener and the second diameter causes a clearance between the member and the fastener.
23. The assembly according to claim 19, wherein the structure includes a metallic material and the member includes a composite material.
24. The assembly according to claim 19, wherein an axis of the hole forms an acute angle with respect to an orientation of the second surface.
25. The assembly according to claim 19, further comprising a liquid shim defining at least a portion of the second surface and defining at least a portion of the varied thickness.
26. The assembly according to claim 19, further comprising a sealant between the structure and the member.
27. The assembly according to claim 19, wherein the extension of the second diameters of the at least one holes defines a standard deviation of less than about 0.0028″ (twenty eight ten thousandths of an inch) in the presence of the varied thicknesses.
28. The assembly according to claim 19 wherein the hole is orientated at an acute angle with respect to at least one of the structure and the at least one member.
29. A boroscope for inspecting assemblies including a structure, at least one fastener, and at least one member, each member having a first and a second surface with a thickness defined there between that varies with a location on each member and which varies between members, the at least one member fastened to the structure by the at least one fastener in such a manner that the second surface is generally adjacent to the structure, the assembly defining at least one hole with a first diameter generally within the structure and a second diameter generally within the member, the second diameter extending to between a first pre-selected tolerance into the structure and to within a second pre-selected tolerance from the second surface into the member, the boroscope comprising:
- a body including a proximal and a distal end, the distal end adapted to engage the first diameter and the second diameter;
- at least one index mark on the body and spaced apart by at least one of the first and second tolerances; and
- a mirror positioned in such a manner so as to allow a user to view the at least one index mark and the first and the second diameters if the second diameter extends to between the first pre-selected tolerance into the structure and to within the second pre-selected tolerance from the second surface into the member.
Type: Application
Filed: Jul 14, 2004
Publication Date: Jan 19, 2006
Inventors: Ali Salour (Fenton, MO), Don Grzina (O'Fallon, MO), Robert Barclay (Foristell, MO), David McCoy (Creve Coeur, MO), Mark Feldman (St. Charles, MO), Susan Miller (St. Louis, MO)
Application Number: 10/890,619
International Classification: B21D 47/00 (20060101);