manulacture large. unique shapes which are utilized lor fabncation 01 industrial molds and tooling lor fiberglass and composile parts. Single Patts as large as 25 ...
Marine Technology, Vol. 39, No. 1, January 21332,pp. 48-53
CNC Robotic Manufacturing of Large Shapes for Fiberglass and Composite Production Tooling Donald L. Blount,’ Douglas L. Blount,’ and William L. Blount’ A Computer numeric controlled (CNC) robotic machining capabilily has demonslraled the capacity to manulacture large. unique shapes which are utilized lor fabncation 01 industrial molds and tooling lor fiberglass and composile parts. Single Patts as large as 25 m x 4.6 m x 2.9 m (82H x 15 R x 9.5 ft) can be machined trom low-density matenals by two 7-axis CNC robots Operating from parallel tracks. Large parts can be assembled by joining Several pieces. This capabiliiy demonstraled that rapid prototyping is possible via both limiled produnion and tull produclion lcmlinglmolds being machined from suriace andor solid Computer models. The paper also includes an ovewiew discussion 01 protoiype production methods that have evolved in lhe boalino and vachl buildino indust7ies in recent vears. Factors intluencino decisions ior the equipmeni ana matenais selened are disiussed The process’Is aiscussed trom design to man2 IactLre 01 a piug sLitable tor labncaling a productton mold
Introduction
Measnrements taken from the model lifts could be scaled hy ratios to provide full-scale dimensions for the construction of THE TIME of execution for the traditional boat design the vessel. Today, a s in early times, i t is possible to huild a through to the completion of the production process is being large, fair, 3-D shape without hard copy line drawings by materially reduced. Technology has advanced the building utilizing Computers in placc of thc lilt models carved hy process from scaling offsets from a hand-carved litt model to e r a h n e n . achieve the desired shape of a full-scale vessel to that of The evolution to a Computer-aided manufacturing (CAM) designen generating three-dimensional (3-D) shapes with pracess being described herein h a s developed through a path Computer graphics. From these 3-D Computer files, tcml path of other concepts worthy of comment. Tlie appropriate aptranslation can drive Computer numeric controlled (CNC) proach of the process to ohtain tooling for the manufacture of equipment to machine full-scale shapes a s Seen in Fig. 1. fiberglasdcomposite parts is determined by many factors. A With stereolithography, small study models may be produced Summary of alternatives for selecting an approach to develdirectly from the 3-D Computer files, if desired, to validate oping tooling may provide useful insight for making technical the defined shape prior to committing to manufacture of a and/or business decisions to meet requirements. This sumfull-scale shape. mary of approaches employs use of Computer-assisted techAR requiremcnts develop, various mcthods to produce pro- niques wiLh Lbc assumption iii “rapid prlyping.”that is, totype shapes have evolved within the constraints of eco- reducing time to develop tooling i s a worthy goal as well as nomic and scheduling realities from the ingenuity of cratts- possibly reducing man-hourdlabor costs. men utilizing materials and resources available. A sampling of methods presently being used to develop Prototype tooling for the manufacture of large fiberglass and/or composite 3-D Computer files parts are discussed. Developing 3-D Computer files thnt define the shape to be produced is a n essential and critical step in the process heBackground ginning with design and continuing through CNC manufacIn the early days af naval architecture, B.C. (before cam- turing. Without an efficient and skilied CAD resource, many Puters), artisans and designerdcraftsmen applied graphic advantages of CNC manufacturing may never be realized. practices to define 3-D shapes with two-dimensional draw- Thus, any analysis to study the feasibility of a CNC Operation ings to build boatdships t h a t had visually fair surface for manufacturing uniquelone-Off shapes must include the shapes. Shipwrights worked fmm offsets and templates de- resources necessary to generate 3-D Computer files at a rate veloped by full-scale lotting. The lottsman and mold loR were slightly greater than the machining capacity of the CNC vital components in m a n n e construction; today they are al- equipment. By so doing, an unexpected design Change to a most nonexistent. In even earlier times, hull models were part being machined can be replaccd by nnother part to keep hand carved from a stack of wood planks (litts) pinned to- a high utilization rate for the robots. Selecting design Software for developing 3-D Computer files gether with dowels. Craftsmenhuilders carved a shaped that “looked right” for the Service of the shiphoat to be built. for rapid pmtotyping and CAM is most important. It is necWhen the carving was finished and the dowels removed, the essary to develop a complete Computer file of the 3-D surface separated liRs defined water planes as depicted in Fig. 2. to be manufactured in Order to readiiy define accurate tool paths. Presently, this Operation employs the following, or a combination of the following, registered trademark Software to develop CAD and CAM files: SolidWorks, Multi’ Donald L. Blount and Assoeiates, Inc., Chesapeake, Virginia pmducts Surf, Anvil Express, Rhino, AutoCAD and SurEAM. 23320. Email: dlbawlba-inc.cam Three-dimensional surface wire frame geometry is generPresented at the March 8. 2001 meeting of the Hampton Roads &tim ofTHE SCCIETY OF NAVAL A R c ~ m m AND MARINEENGINEERS.ally snitable for driving a 2-D router for cutting jig frames. A 48
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Verification of meetinglexceeding requirements Validation of productivn Prototype Various techniques for hull prototypiiig arc: 1. Oue-ofi male Cold mvlded Composite Hand lay-up Pre-preg + Heat + Vacuuiii Plaster or sprayed foam 2. Teiiipor:ir.y kCiii:iic ninid 2-11 frtiiiicd CNC 3-11 iiincliining 3. CNC 3-D iiiacliining iiialc plug Female mold fabricated from plug
One-Off male hull plug Several methods are employed for huilding male hull plugs from which fiberglass production molds may he fahricated. Having developed 3-D hull lines using CAD techniques, transverse jig frames can he nested to be Cut by 2-D routers from sheets of plywvod. The jig frames are “keyed” to properly interlock at assemhly. These jig frames, a s depicted iii Fig. 3, are nested prior to cutting and then assembled. The assernbled jig frames are aligned and typically spaced a t about 2 ft (0.6 m) as Seen in Fig. 4; longitudinal hattens are attached to the frames to define thc shape vf the hull surface. Various approaches are uscd 10 covcr thc huil surfacc 10 obtain the quaiity, texturc (smoothnessi and desired accuracy. 1. Batteiis clvsely spaced. 2. Diagonal cold-moldcd pianking is applied over widely spaced battciis. 3. Compvsite Lore material i s attached vver widely spaced hattcns. Several layers of fiberglass are then honded. 4. A screen incsh is attached 10 the battens and coated with plaster ur sprayed foam and faired tu templates. solid modeler produccs ii ciimputer file requiring the least preparation of tool I>ntlx Liir :I-D CNC machining. A 3-D surface modeler also rcsiilts i n :I suilahle Computer file when no unconnected surfiicc carallcl willi attachment taking place late in the process. The significant advantages related to rapid prototyping are: 1. Timeischedule conipression 2. Minimize expenditure vf production rcsourccs 3. Provf vf conccpt Demonstrate einerging technology Demonstrate prvducihility
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Fig. 4 Onmii mld molded Lmal conslwciion
Temporary female hull mold
CNC 3-D machining male hull plug
Again, assuming3-D CAD hull lines are available, external (female)jig frames are nested and Cut from sheets ofplywood (similar to Fig. 31, assemhled, aligned and spaced a t about 2 ft (0.6 m). Longitudinal battens are attached to the frames and surfaced with wood planks or plywwd dependingon local shape as depicted in Fig. 5 . Typically the wood surface is coated with fiberglass for durability and sanded tn the desired surface texture. After the surface of the temporary female hnll mold is properly prepared for faimess and smoothness, and a release agent is applied, a fiherglasdcomposite hull can he layed-up. The finished hull, See Fig. 6, may he fitted with a simple engine installation and trimmed with appropriate ballast for sea trials. Thus, hull suitability for the desired Service may be evaluated, hull refinements made and retested before committing to the cost of permanent tooling. The equipment is then removed from this "test bed plug" and the surface can then be properly prepared for lifting a production mold. Temporary female hull molds can he machined directly with a manufacturing process analogous to the hull plug method described later with the difference that separate port and starboard parts must he milled. The port and starboard female parts are joined a t the hull centerline. Then a hull may be layed-up. Hull shapes such a s tumblehome, oiten Seen on sportfishing boats which would "capture" (not release) in a single part mold may be readiiy praduced with this split-mold method. The materials used in this process should permit multiple p a r k to be manufactured from these temporary molds; the numher ofreleases is projected ia be a t least five but this is yet to be validated in practice for this newly developed tooiing Polyester compound.
The preceding paragraphs described 2-D CNC router machining applications heginning with 3-D Computer files. Some manual labor and time are saved in the plug fabrication process with some imprnvement in accuracy for added value relative tn a total manual build-up o f t h e shape necessary for a production mold. T h e application of CNC machines has taken the plug manufacturing processes to a new level. There are two predominate technologies now being used in the CNC machining of large plugs: 5-axis gantry routers and I-axis rohotic arms. This paper will focus on the 7-axis method.
Fig. 5 Temprary lemale hull mold
flg. 6 Hull being removed IrOm iemporary mold
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Robot System description This System consists of a pair of ABB IRB-6400 7-axis rohots mounted on parallel tracks ( 6 degrces of freedom on the arm and one axis of linear track travel), see Fig. I. This System provides an optimized working envelope of 25 m x 4.6 m x 2.9 m (82 ft x 15 ft x 9.5 ft) as depicted in Figs. 8 and 9. To ohtain the desired working envelope, the width and the height are dependent on the rohotic arm and the length is independent of the robot since additional track can be added. The working envelope height is a function ofthe width due to the robot's ability to reach (axes 4 and 5).As the height of the envelope is increased, the width is decreased. This envelope, Fig. 8, was optimized to support the cutting of a 40-ft hull in one Piece. The robots trace along tool paths, or Software rnutines directing the robot motion, that are developed using the S u d CAM 2000' sottware package to convert the 3-D Computer model to machinable surfaces. The tool paths determine the
MARINE ECHNOLOGY
dous amount of material in a short penod of time. The polyester tooling resin is tougher, and slower linear machining rates are required (feed rates -0.5 &sec). The tooling resin is miiled nsing a DieBold milling head with Carbide cutting tools with diameters of 1.5 in. (4 cm) or iess (highiy dependent on geometry h i n g machined). Final cuts are made with tools of the appropriate shape and diameter to ohtain the desired detail, surface finish, and part accuracy. The accuracy of this System of approximately 0.060 in. (1.5 mm), with a 95% confidence level taken from over 2000 data Points, as measured hy a third party using SMX measurement technology.
Axir
Axir
Fig. 7 Definition 01 7-axes 01 mbol conlralled by lwl palh Mltware
Plug build-up The piug structure serves as the male shape for the final mold tooling to he constructed, and thus must serve several critical functions in order to perform properly. The plug must be structurally stable to support movement and transportation while maintaining the designed shape and dimension and be tough and resilient enough to resist surface damage. Finally, the plug materiais must he easily machinable to allow timely and cost-efficient construction. Plugs are built in a three-Stage process: rough plug construction, final surface machining, and final surface finishing.
Rough plug construction
Fig. ü Envelope 01 Single Part machining capaciiy
The plug hegins with a steel and/or wocden skeletal substructure constructed on a steel base frame which will provide the primary structural support for the object. Plywood is then fastened to the suhstructure to provide additional support and adequate surface area to adhere the poiyisocyanurate foam hlocks with a Polyurethane adhesive. The foam blocks [two to three Pounds per cubic foot (32 to 48 kg/m3)1 form the rough inner skin of the plug. An example of the build-up of the rough plug is shown in Fig. 10. The type of foam used must hc rigid (to resist the shnnkage of thc surface resins), chemically resistant to styrene, and thermally Stahle. The rough huilt-up part is thcn uiidercut 0.3 to 0.6 in. (8 io 15 m m )bclow ihc final siirracc using Llic rolmiic iiiillinc cquipmciii shown in Vig. 11. Find surface machining After the foain iü undcrcut, cliopped strand liberglass mat is applied to the foam surface. The part is then sprayed with a filled Polyester tooling compound (sec Fig. 12) to 0.25 in. (6 mm) over the final surface dimension. The tooling resin (Poly-Fair T-27 developed by ATC Chemical, Inc.) f o r m the outer skin. This material was selected a s i t has greater ductiiity than traditional tooling resins, thus it resists cracking.
Fig. 9 RoboliC machining arrangerneni. (Photo courtesy 01 Jones 8 Rollins)
critical Operations of the machining that include rohot motion over the surface, tool type (length, diameter. and geometry), cutting rate, and step ovcr distance or the width of the path. If necessary, adjustments may be made hy the Operator as the path is run. Due to the differences in material densities being machined, two types of milling heads are used with the robots. The rough-Cut material is a closed cell foam (described in the next section), which can be cut a t very fast linear Speed (feed rates over 2 &sec) using a Bornomann Spindle. This spindle uses case-hardened tool steel cutting tools as long 8s 18 in. (46 cm) and 2 in. in diameter ( 5 cm) which remove a tremenJANUARY 2w2
Fig. 10 Rough plug build-up lor slatboard 5ide 01 a deck and cwkpil
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surface finishing. Many builders linisli tlie plugs to the final levels in their faciiity, and only request an 80-grit finish (Duratec base coat) to be delivered as Seen in Fig. 14. To obtain the necessary surface an tlie plug to produce quality mold tooling, additional primer coats (Duratec EZ Sanding Pnmerl and top coat (Duratec High-Gloss Top Coat) are applied and finished to the desircd level of polish. After the plug is finished, mold coiistructioii begins with the release agent application followed by gel coats before build-up of the mold Skin.
Financial information Flg. 11 Rough cuning 01 law-densily faam blmk for Sea-Pro 23 hull
Flg. 12 Spraying pdyester lwling mmpaund
The resin has a relatively low giass filler content which extends cutting tml life (Carbide) and a low exotherm, which allows application layers 0.10 to 0.25 in. (3 to 6 mml thick on each Spray pass. After a minimum of 24 h r curing, the Part is Cut ta the final surface dimensions as shown in Fig. 13.
The price of a hull or deck plug depends on the machinable surface area, complexity of shape, and surface finish of the plug. The machinahle surface area includes the surface area of the shape, and a "pul1 flange" which is usually added for ease of removing the final part from thc mold. Complexity of shape refers to the number of individual CAM tool path programs that must be developed to nxicliine the shape. Each surface plane and radius require a separate CAM program. Therefore, a hull shape has less surface planes and radii than a deck shape, and requires less time to develop the CAM programs and machine the shape. With some small and local details, such as hatch wnterways,
