Patent Document

BACKGROUND 
     This disclosure relates generally to drilling systems and methods for preparing composite material and substrate materials for assembly and, in particular, relates to tools and methods for deburring after drilling operations. 
     Composite components are being utilized in a wide variety of articles of manufacture due to their high strength and light weight. This is particularly true in the field of aircraft manufacturing. Typical materials used in the manufacture of composite components include glass or graphite fibers that are embedded in resins, such as phenolic, epoxy, and bismaleimide resins. A composite lamination can be built up by laying successive plies of fiber tows (e.g., carbon fiber tows preimpregnated with a thermoset epoxy resin) around a mandrel and then curing. As more advanced materials and a wider variety of material forms have become available, aerospace usage of composites has increased. 
     For example, composites are used in conjunction with metal substrates to form an assembly that may be used to construct a larger structure, for example, of an aircraft or other vehicle. The assembly may include a composite material and a structural metal substrate arranged in a stacked or layered orientation (referred to herein as a “stackup”). The composite material may, for example, be carbon fiber-reinforced plastic (CFRP) or other fiber-reinforced material. The structural metal substrate may, for example, be titanium, aluminum or steel. The metal substrate may be used to build a skeleton or frame, with the composite material attached to and covering the frame. For this reason the composite material is sometimes referred to as a skin. The metal and composite materials may be shaped, contoured, or curved into virtually any shape desired. 
     Of course the composite material and metal substrate have different physical attributes and properties, and exhibit different behavior in use. Due to those facts, attaching the composite material to the metal substrate can be challenging. For example, the materials may be joined to each another with a fastener that requires holes to be drilled in each respective material. Separate handling of the composite material from the metal substrate is undesirable. Especially for relatively large structures having many fasteners distributed over the structure, such as in the fabrication of an aircraft, avoiding separate drilling of the holes in each of the composite material and the metal substrate may result in appreciable reductions in production times and reduction in costs of fabricating the aircraft. 
     To avoid separate drilling, many machining applications involve drilling and/or reaming a hybrid stack-up of composite and metal materials. For example, certain aircraft require that a wing made from a composite material, such as CFRP, be joined to a titanium section of an aircraft body with fasteners that pass through holes made through the mating sections. When using fasteners to attach composite skins to metal substrates, coaxial holes must be drilled in both the skin and an underlying metal substrate. High-quality holes must be produced in such materials with dimensions within narrow tolerances. 
     The wing-to-body join task typically requires a three-step conventional drilling process comprising a pilot drill, followed by a step drill, followed by a finish diameter reamer. Frequently the reaming process is followed by a deburring operation. Various special tools are known for removing burrs from the circumferential edges surrounding openings of drilled holes and for adding chamfers thereto. In particular, mechanical hole-deburring tools are known which remove burrs on the front, back, or both sides of drilled holes in one pass, working from one side only. 
     The design of airframe structure dictates the elements in the stack. Metal components are often times “sandwiched” between CFRP components. The stack orientations are driven by structural loading requirements. The high-load areas at the wing-to-body interface typically have external metal components whereas the body section joins are mostly CFRP with interior metal components. Location and access are the primary drivers for determining from which direction one can approach the interface for deburring operations. 
     A known deburring blade has been used to perform metal material removal as part of a deburring operation within a drilled and/or reamed hole in a mixed material interface, e.g. CFRP/Ti or Ti/CFRP. The existing blade design provides for cutting force reaction in only a single direction. With the existing blade design, when attempting to deburr using the forward portion of a double-acting cutter tip, the cutting reaction forces “push” the blade back in the hole and a reduced amount of material is removed. 
     There is a need for a blade design that will enable loading of the cutter blade from either side of the cutter tip without any relative movement away from the metal interface as a result of cutting forces. 
     SUMMARY 
     A tool is disclosed that facilitates deburring interfaces in any stack orientation when access is limited to one side of the structure. This tool has a cutter blade designed to perform metal material removal as part of a deburring operation within a drilled hole in a mixed material stackup. The blade design provides the capacity to enter hole features and remove metallic burrs from mixed material interfaces regardless of stack orientation, i.e., removes metallic entrance or exit burrs within the stack. In accordance with one embodiment, the blade design incorporates a slotted feature that hinges on a steel pin. The slot orientation is generally parallel to the metal/composite interface in the stackup. The resultant forces of metal material removal are reacted by the walls of the slot, allowing material removal on either the front side or back side of the double-acting cutter tip. 
     This blade design enables loading of the cutter blade from either side of the blade tip without any relative movement away from the metal/composite interface as a result of cutting forces. In a single design, the new blade can deburr equal amounts of metal material from a mixed material interface regardless of material orientation in the stack, e.g. CFRP/Ti or Ti/CFRP. The “trapping” of the blade position within the hole provides a more reliable and precise deburring operation without the potential negative consequences of linear movement. 
     Other aspects of the invention are disclosed and claimed below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a sectional view of a drilled hole in a metal/composite stackup, the hole having a cutter blade of a deburring tool inserted therein and in position to commence deburring the metal/composite interface. 
         FIG. 2  is a diagram showing a sectional view of the same hole shown in  FIG. 1 , except that the cutter tip is shown in a typical position as it is deburring the interface. 
         FIG. 3  is a diagram showing a sectional view of the same hole shown in  FIGS. 1 and 2 , after completion of the deburring operation and removal of the deburring tool from the hole. 
         FIG. 4  is a diagram showing a sectional view of a deburring tool having a pilot/blade assembly wherein the cutter blade incorporates a slot having sides oriented generally perpendicular to the blade axis. The pilot in  FIG. 4  is shown in a partially extended position. 
         FIG. 5  is a diagram showing an isometric view of a deburring blade with cutting insert in accordance with one embodiment. 
         FIG. 6  is a flowchart showing a sequence of steps of a deburring operation employing the tool shown in  FIG. 4 . 
     
    
    
     Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals. 
     DETAILED DESCRIPTION 
       FIGS. 1 through 3  are sectional views showing three stages in a process for deburring in a circumferential region where an interface  8  intersects the wall of a drilled hole  10  in a stackup  2  made of dissimilar materials. For the purpose of illustration, it will be assumed that the stackup  2  comprises a composite material  4  and a metal substrate  6 . The composite material  4  may be CFRP or other fiber-reinforced material. The metal substrate  6  may, for example, be titanium, aluminum, steel or another structural metal. Any metal burr formed during drilling the hole is removed during the deburring operation. Adjacent composite material is also removed. 
     The deburring operation is performed by an automated system comprising a tool having a pilot/blade assembly.  FIG. 1  shows a cutter blade  12  that is disposed in a slot of a pilot  14 . Parts of the deburring tool other than portions of the blade and pilot are not shown in  FIGS. 1-3 . A double-acting cutter tip  22  (e.g., coated with carbide) is attached to a pocket  25  in the distal end of the cutter blade  12  by brazing. The pilot  14  is axially movable relative to the cutter blade  12 . The cutter blade  12  can pivot relative to a portion of the tool (not shown in  FIGS. 1-3 ) which does not move with the pilot. The structures which facilitate axial movement of the pilot and pivoting of the cutter blade will be explained later with reference to  FIG. 4 . 
       FIG. 1  shows the state wherein the pilot/blade assembly is properly positioned to begin the deburring operation. In the proper position, the apex of the cutter tip  22  is aligned with the metal/composite interface  8  within a specified tolerance. This alignment is achieved using known automated procedures for locating an interface between dissimilar materials. The system and method for detecting the material interface incorporates electrical equipment (not shown) and an electric circuit, plus data processing to accurately identify the position, relative to a drilling machine, of the interface  8  between two materials of dissimilar electrical characteristics (e.g., the composite material and the metal substrate) when in the electric circuit. The differing electrical properties and characteristics of each material result in changes to electrical conditions in the electric circuit. By monitoring the changes in electrical conditions in the circuit, the interface  8  between the two materials  4  and  6  may be detected and located. For example, the interface  8  can be located using the following automated procedure disclosed in United States Patent Application Publ. No. 2009/0129877, the disclosure of which is incorporated herein in its entirety. 
     In one specific implementation, a resistance sensing routine is used to locate the interface between a metal substrate and a layer of CFRP material. During the resistance sensing routine, the pilot/blade assembly is not rotating. The resistance sensing routine comprises the following steps: (1) the pilot/blade assembly (blade retracted) enters the hole to a preset depth in the CFRP; (2) the pilot is extended, causing the blade to extend until the cutter tip contact the hole wall. (3) The electrical resistance of the contacted material is measured. 
     If the measured electrical resistance indicates the presence of CFRP, then the following steps are performed: (4) the pilot is retracted, thereby retracting the blade; and (5) the pilot/blade assembly is moved one increment (e.g., 10 mils) toward the metal. Then steps (2) and (3) are repeated. If the measured electrical resistance still indicates the presence of CFRP, then steps (4), (5), (2) and (3) are repeated in that order. This procedure is repeated until step (3) indicates the presence of metal. 
     If the measured electrical resistance indicates the presence of metal, then step (4) is repeated; thereafter the following steps are performed: (6) the pilot/blade assembly is moved a smaller increment (e.g., 2 mils) toward the CFRP; and then steps (2) and (3) are repeated. If the measured electrical resistance again indicates the presence of metal, then steps (4), (6), (2) and (3) are repeated in that order. This procedure is repeated until step (3) indicates the presence of CFRP. When CFRP is present, the interface can be effectively located. For example, if the advancement increment was 2 mils and before the advance, the detected material was metal, whereas after the advance, the detected material was CFRP, then the control computer knows that the cutter tip is within ±2 mils of the interface. After CFRP has been detected, step (4) is repeated and the system is ready to perform the deburring operation. 
     To enable deburring, first the pilot/blade assembly is rotated. Then the pilot is extended an amount sufficient to extend the cutter blade to its maximum protrusion while the assembly is still rotating. As the rotating cutter tip presses against the hole wall, material in the area where the interface intersects the hole wall is removed. 
       FIG. 2  shows the changed position of the cutting tip  22  relative to the pilot  14  during deburring. Although not apparent from a comparison of the respective orientations of the cutter blade  12  seen in  FIGS. 1 and 2 , the change in position of the cutting tip  22  is produced by pivoting the cutter blade  12  about a pin (see item  52  in  FIG. 4 ) that is attached to a portion of the tool that rotates with the blade  12 , but does not move axially with the pilot  14 . The cutter blade  12  pivots in response to forward axial movement of pilot  14  relative to cutter blade  12 . The pilot has a ramped camming surface  16  at its distal end. As the pilot  14  is moved forward from the retracted position seen in  FIG. 1  to its extended position seen in  FIG. 2 , the ramped camming surface  16  slides under the heel of the cutter blade  12  and cams the blade heel upward. This deflection occurs while the cutter blade  12  and shaft  14  are rotating. Therefore, as the cutter tip  22  moves further out of the pilot slot, it removes material from the target region. The maximum protrusion of the cutter tip  22  produces chamfers having the desired depth. When the desired depth has been reached, i.e., sufficient material has been removed, the pilot is retracted, causing the blade to return to its retracted (i.e., home) position. Then the assembly exits the hole. 
     Upon completion of the deburring operation, ideally both materials will have been chamfered in the area of the interface/hole intersection as seen in  FIG. 3 . One side of the cutting edge forms a chamfer  18  in the composite material  4 , while the other side of the cutting edge forms a chamfer  20  in the metal substrate  6 . During the deburring process, any metal burr at the intersection of interface  8  and the wall of hole  10  is removed. The removal of composite material is incidental to the metal deburring process. The metal removal produces higher reaction forces on the cutter blade than are produced by the removal of composite material. 
     While the deburring tool is shown inserted into hole  10  from the left as seen from the vantage point in  FIGS. 1-3 , it should be appreciated at the outset that the deburring tool disclosed herein can alternatively be inserted into hole  10  from the right with equivalent deburring performance. More specifically,  FIG. 2  depicts a situation wherein the deburring tool is inserted in the hole  10  from the left and the forward portion of the double-acting cutter tip  22  deburrs the metal material. In this case, the cutting reaction forces tend to “push” the cutter blade  12  leftward in  FIG. 2 , i.e., rearward relative to the rest of the tool (not shown). Conversely, if the tool were inserted from the right, the metal material would be deburred by the rearward portion of the double-acting cutter tip  22 . In the latter case, the cutting reaction forces would again tend to “push” the cutter blade  12  leftward in  FIG. 2 , but this movement would be in a forward direction relative to the rest of the tool. The deburring tool shown in  FIG. 4  is designed to accommodate and counteract cutting force reactions in both directions. In particular, the cutting tip is “biased” to the CFRP side of the interface so that more CFRP material than metal material is removed. 
     Referring to  FIG. 4 , the deburring tool in accordance with one embodiment comprises a non-rotating assembly and a rotatable assembly rotatably supported by bearings  38  and  40  mounted to the non-rotating assembly. The non-rotating assembly comprises: an air cylinder body  30 ; a removable port connector  44  attached to the air cylinder body  30 ; a piston assembly  32 / 33  slidably nested inside the air cylinder body  30 ; and a flange  34  which holds bearing  38 . The port connector  44  comprises two air cylinder ports  46  and  48 . 
     The rotatable assembly comprises: a hollow body  36  having a cylindrical bore and pilot holder  34  having a hollow piston portion which is slidably nested in the cylindrical bore of hollow body  36 . A dowel pin  60  aligns and prevents rotation of parts  34  and  36 . These two parts can rotate together and as the air cylinder extends and retracts, allows linear motion as well. The pilot holder  34  has an opening which receives a proximal end of the pilot  14 . The pilot  14  is retained in the pilot holder  34  by a set screw  50 . The hollow body  36  is rotatably supported by bearing  38 ; the pilot holder  34  is rotatably supported by bearing  40 . The rotatable assembly further comprises the cutter blade  12  and the pilot  14 . 
     As previously described, the pilot  14  (and pilot holder  34 ) can be alternatingly extended and retracted. This is accomplished by means of a piston assembly comprising an air cylinder piston  32  and a piston flange  33  attached to the rearward outer peripheral portion of air cylinder piston  32 . The air cylinder piston  32  holds bearing  40 . The piston assembly  32 / 33  is nested inside the air cylinder body  30  and axially movable relative thereto. More specifically, the piston assembly  32 / 33  is free to slide back and forth along the cylinder axis between opposite limit positions inside the air cylinder body  30 . As the piston assembly moves, the pilot holder  34  and pilot  14 , which are supported by bearing  40 , also move. The air cylinder body  30  and piston assembly  32 / 33  are configured in conventional manner to provide two annular chambers which are in respective fluid communications with ports  46  and  48 . When pressurized air is supplied to port  46 , the air pressure drives the piston assembly  32 / 33  (and pilot holder  34 ) forward, thereby extending the pilot  14 . Conversely, when pressurized air is supplied to port  48 , the air pressure drives the piston assembly  32 / 33  (and pilot holder  34 ) rearward, thereby retracting the pilot  14 . Leakage of pressurized air between air cylinder body  30  and piston assembly  32 / 33  is prevented or reduced by a pair of lip seal  42  and by an O-ring  58 . 
     The pilot  14  in  FIG. 4  is shown in a partially extended position. During extension or retraction of the pilot  14 , the cutter blade  12  and hollow body  36  do not move axially. The cutter blade  12  is coupled to a blade attachment pin  52 , which is attached to and projects from the wall of a radial slot formed in the wall of the hollow body  36  at a forward end thereof. The proximal end of the cutter blade  12  has a slot  24 , which receives the blade attachment pin  52  when the blade  12  is installed in the tool. The sides of the slot  24  are mutually parallel and generally directed perpendicular to the axis along which the pilot is moved. The abutment of the sides of slot  24  against the blade attachment pin  52  prevents axial displacement (i.e., forward/aft movement) of the cutter blade  12  that might otherwise occur due to an axial reaction force component generated during deburring. To keep the blade  12  in place so that slot  24  does not rise off of the pin  52 , a spring-loaded round nose plunger  54  presses against a fillet  26  formed on the proximal end of blade  12 . The fillet  26  is shaped to receive a portion of the round nose of plunger  54 . More specifically, the fillet  26  is in contact with a surface area of the round nose plunger  54  which is not centered along the axis of rotation of hollow body  36 . The plunger  54  is biased in a forward axial direction by the spring  56 . The orientation and position of fillet  26  are such that the axial spring force produces a transverse biasing force component acting on the fillet  26  to retain the end of slot  24  against pin  52 . The plunger  54  is slidably nested in a hollow shaft of the pilot holder  34  (which shaft is slidably nested in a cylindrical bore of the hollow body  36 ). When the pilot holder  34  moves forward during extension of the pilot  14 , the plunger  54  does not move, but rather the spring  56  compresses and continues to urge the plunger  54  against the fillet  26  on the proximal end of blade  12 . 
     The maximum extend distance of the pilot  14  can be set using a threaded rod  64 . The forward end of threaded rod  64  is threadably coupled to the rearward end of the hollow shaft portion of the pilot holder  34 . A nut  65  is threaded onto the rearward end of the threaded rod  64 . Abutment of the forward face of nut  65  against the rearward face of hollow body  36  determines the maximum extend distance of the pilot  14 . 
     The distal end of the pilot  14  has a ramped camming surface  16  formed thereon. A major portion of the shaft of cutter blade  12  is disposed in a longitudinal slot formed in pilot  14  (indicated by the unhatched portion of pilot  14  in  FIG. 4 ). When the piston  32  is displaced in a forward direction, the resulting extension of pilot  14  and its ramped camming surface  16  causes the cutter blade to rotate about pin  52 . Although the spring-loaded plunger  54  produces a force component on the fillet  26  which resists pivoting of the cutter blade  12  about the pin  52 , that force component is insufficient to prevent such pivoting. As the cutter blade pivots, the cutter tip  22  protrudes from the slot in the pilot. 
     The structure of the cutter blade  12  in accordance with one embodiment is shown in  FIG. 5 . The distal end of cutter blade  12  has a pocket  25  in which a carbide cutter tip  22  is brazed. In this example, the angle of the cutter tip  22  is 90 degrees. The proximal end of the cutter blade  12  has a slot or notch  24  and a fillet  26  as previously described. 
     The above-described assembly includes features that prevent axial displacement of the cutter blade during the deburring operation. The basic steps in that deburring operation are outlined in  FIG. 6 . First, the distal end of the pilot/blade assembly is inserted in the drilled hole (step  66 ). Then the metal composite interface is located using the technique previously described (step  68 ). It should be understood, however, that locating the While the pilot and blade are in their retracted positions, the pilot/blade assembly is rotated (step  70 ). More specifically, in response to a command to begin the deburring operation, the entire rotatable assembly (described above) is rotated by conventional means, e.g., an electric motor (not shown). While the pilot/blade assembly are rotating, the pilot is extended (step  72 ), which causes the blade to extend. During deburring, the interaction of a slot formed in the cutter blade with a blade attachment pin inside the deburring tool prevents the blade from moving axially. More particularly, if the metal burr at the metal/composite interface is being removed by the forward edge of the double-acting cutter tip, the blade will not move toward the tool body, whereas if the metal burr at the metal/composite interface is being removed by the rearward edge of the double-acting cutter tip, the blade will not move away from the tool body. When deburring at the interface has been completed, the pilot is retracted (step  74 ), which causes the blade to retract. The pilot/blade assembly then exits the hole (step  76 ). 
     While a deburring tool has been described with reference to a particular embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the teachings herein. In addition, many modifications may be made to adapt a particular situation to the teachings herein without departing from the essential scope thereof. Therefore it is intended that the claims set forth hereinafter not be limited to the disclosed embodiment. 
     The method claims set forth hereinafter should not be construed to require that the steps recited therein be performed in alphabetical order or in the order in which they are recited. Nor should they be construed to exclude any steps being performed concurrently.

Technology Category: 7