Abstract:
An apparatus and method for routing radii and for producing full-scale layouts is disclosed. The apparatus consists of a guide of smaller radius than the work, a carriage adapted to trace the guide, and a trammel adjustably extending from the carriage. The trammel is adapted to mount a tool for performing the work at the desired radius. The trammel extension corresponds to the difference between the radius of the work and the radius of the guide. The apparatus thus provides a simple method for working with large radius work pieces within confined spaces. Further, the device is usable by one worker and produces accurate results. Finally, the device is inexpensive to produce and may be adjusted to work with virtually any size radius.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to woodworking tools generally. In particular the invention provides an inexpensive device and method for working within a confined space to accurately rout curved work pieces of large radius. The invention may also be used to draw full-scale layouts. 
     2. Description of the Related Art 
     Methods of routing curved work pieces and of producing full-scale layouts are well known. However, when working with large radii these methods are either inconvenient because they require a large work space and waste a great deal of material, or very expensive. 
     One method for routing at large radii is to use a CNC (Computer Numerical Control) machine. This type of machine is capable of working with pieces of very large radius while producing very accurate results. The principal drawback to this machine however is that it is very expensive, costing upwards of $100,000. Furthermore, farming the work out to a bigger shop that owns a CNC machine is also prohibitively expensive, costing $200 or more depending on the size and type of the order. Due to the nature of the construction industry, where work projects are awarded to the lowest bidder, it is critical to keep costs down as much as possible. A subcontractor who uses a CNC machine often will not be the low bidder. 
     An inexpensive method of both routing at large radii and producing full-scale layouts is to use a trammel. This device is simply a long arm that is rotatably fixed at one end and adapted to mount a router or marking instrument on the other end. U.S. Pat. Nos. 4,685,496 and 5,486,076 disclose trammels for mounting a router. The length of the trammel arm corresponds to the radius of the work piece. The trammel is pivotally secured at a point on the trammel spaced from the router, and the trammel acts as a guide to allow the router to trace a circumference on the work piece. 
     This method has a number of drawbacks. First, because the trammel arm must be the same length as the radius of the work piece, very large work spaces are needed to work with the trammel when routing large radii. Floor space in a workshop is of course very valuable. Therefore it is not practical for most shops to devote a large section of the shop floor to routing large radius pieces. Nor is it practical for the shop to spend the time necessary to clear an area for this operation. The space is better suited for ongoing projects. 
     Second, constructing a long trammel requires a great deal of material and requires a significant number of man-hours. Furthermore, trammels of various lengths must be built to meet the needs of each job. If a new trammel is to be built for each job, a significant cost in materials and man-hours must be absorbed. Conversely, if a number of trammels of different lengths are to be built and stored for future use, costs in wasted storage space arise. 
     Third, trammels over twenty feet long require two workers to operate, and the results are generally of poor quality. Long trammels tend to flex during operation, which results in flat spots being formed along the circumference of the work piece. 
     Trammels may also be used to produce full-scale layouts. However, the same difficulties described above in connection with routing must be overcome. Another known method of producing full-scale layouts that solves these problems is to use a computer and CAD (computer-aided design) software. The layout is drawn on the computer screen and then put on paper using a plotter. This method produces very high quality results, but requires a large and expensive investment to purchase the necessary hardware and software. 
     U.S. Pat. No. 3,393,453 discloses a device for scribing large radii on layouts that overcomes some of the problems outlined above in connection with trammels. The device uses a trammel in combination with a template to produce large radii without the need to use a correspondingly large trammel. 
     The device of the &#39;453 patent suffers from three principal drawbacks. First, the template is not adapted to be secured to a work surface during use. The operator must hold the template in place, where it is obviously prone to shift and produce poor quality arcs. 
     Second, the device may only be used to produce a limited range of arc radii, depending on what size templates are available. The carriage is adapted to ride on only a specific type of template, and this template is not readily producible by the average person. 
     Third, the invention may only be used to draw layouts. The trammel is not adapted to mount a router or other power tool. 
     Therefore, a device and method of routing at large radii and producing full-scale layouts that is inexpensive, easy for one person to use, and produces high quality results would be of great benefit in the woodworking industry. 
     SUMMARY OF THE INVENTION 
     The invention provides an apparatus and method for routing radii and for producing full scale layouts. The device is particularly well-adapted to work with large radii, because it is usable within confined spaces and requires only one person to operate, but produces exceptional results. 
     The device comprises a plurality of guides, a carriage assembly and an adjustable trammel arm. The guide is a curved section of material that is secured to a flat work surface. The radius of the guide is determined by the radius of the work piece, as discussed below. In operation, the guide steers the carriage as the carriage travels along a length of the guide. 
     The carriage assembly comprises a substantially rectangular support frame with four rotating members on its underside. The rotating members provide a low-friction interface between the guide and carriage as the carriage rides from one end of the guide to the other. 
     The trammel arm is adjustably secured to the carriage so that it defines an extension of the guide radius. In this orientation, the trammel is adjustable so that the sum of the trammel extension and the guide radius is equal to the radius of the work piece or layout. The end of the trammel opposite the carriage is adapted to mount a router for cutting or a marking instrument for drawing. Other types of power tools may be mounted to the trammel arm, such as jigsaws or circular saws, but routers are preferred to produce the highest quality finished product. 
     To practice the method of using the apparatus, first an appropriately sized guide is secured to the work surface. Second, the carriage is positioned atop the guide and secured in place. Third, a router or marking instrument is secured to the trammel. Fourth, the work piece or layout is secured to the work surface at the appropriate distance from the guide. Fifth, measurements are made and the trammel arm is adjusted to the appropriate length and secured into place. Finally, the arc is drawn or the work piece is routed by moving the carriage along the guide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a plan view of a preferred embodiment the device for routing radii and producing full-scale layouts according to the present invention. 
     FIG. 1B is a side elevation view of the device. 
     FIG. 1C is a rear elevation view of the device. 
     FIG. 2 is a detail view of the carriage arm assembly from a top perspective. 
     FIG. 3 is a detail view of the carriage arm assembly from a side perspective. 
     FIG. 4 is a detail view of the front left portion of the carriage arm and adjustable runner from a front perspective. 
     FIG. 5 is a detail view of the trammel arm and fixed runner from a front perspective. 
     FIG. 6 is a detail view of the right rear portion of the carriage arm and fixed runner from a front perspective. 
     FIG. 7 is a detail plan view of the router end of the trammel arm. 
     FIG. 8 is a detail side elevation view of the router end of the trammel arm. 
     FIG. 9A is a plan view of the slider. 
     FIG. 9B is a side elevation view of the slider. 
     FIG. 10A is a detail plan view of the router end of the trammel arm, illustrating an alternative configuration for mounting a marking instrument. 
     FIG. 10B is a detail cross-sectional view of the router end of the trammel arm, illustrating an alternative configuration for mounting a marking instrument. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1A-C the invention comprises three major components: a guide  20 , a carriage assembly  22  that traces the guide  20 , and a trammel  24 , which is connected to the carriage  22  and extends from the guide  20  to the work piece  26 . 
     The guide  20  is an arc-shaped strip of material, preferably wood, that is well adapted to be firmly secured to a flat work surface. The guide  20  has a concave edge  28  and a convex edge  30  that define concentric arcs. The edges  28 ,  30  guide the carriage  22 , which rides over the guide  20  as the work is performed. 
     The carriage  22  consists of two parallel arms  32  connected to one another by two parallel runners  34 ,  36 . The arms  32  are shown in detail in FIGS. 2-4, and  6 . The upper surface of each arm  32  is shaped as a dovetail tab  38  (FIGS.  4  and  6 ). Each arm  32  contains a through-bore  40  near one end. An elongate through-slot  42  extends along a length of the longitudinal axis of the arm  32  from a point spaced from the bore  40 . 
     The runners  34 ,  36  are shown in detail in FIGS. 2-6. Each runner  34 ,  36  contains three through-bores  44 , one near each end and one near the center. The bores  44 , are illustrated in FIGS. 4-6. Also near each end, each runner  34 ,  36  has a dovetail slot  46  (FIGS. 4 and 6) adapted to mate with the dovetail tab  38  of each arm  32 . The dovetail slot  46  at each end of the rear runner  36  slidably engages the dovetail tab  38  of one arm  32 , and is secured to the arm  32  with a threaded member  48 , preferably a bolt, that extends through an end bore  44  in the runner  36  and through the bore  40  on one end of the arm  32 . Thus, the relative positions of the arms  32  and the rear runner  36  remain fixed. 
     The dovetail slot  46  at each end of the forward runner  34  also slidably engages the dovetail tab  38  of one arm  32  and is secured to the arm  32  with a threaded member  48  that extends through the end bore  44  in the runner  34 . These members  48  extend through the elongate slot  42  running along the longitudinal axis of each arm  32  and engage internally threaded knobs  50 . Thus, the forward runner  34  may be moved forward and backward along a length of the arms  32 , and secured in a desired position by tightening the knobs  50  down on the threaded members  48 . 
     It will be understood by one of skill in the art that the arms  32  and runners  34 ,  36  need not be secured to one another using dovetail joints. Simply using threaded fasteners, as shown, without the dovetail joints would also be effective. The dovetail joints, however, provide the overall assembly with greater stability, and are the preferred construction. 
     Rotatable members  52  are disposed on the threaded members  48  beneath the carriage arms  32 . These rotatable members  52  aid the movement of the carriage  22  along the guide  20  as the work is performed. Any type of substantially cylindrical part with a central bore is suitable for this member  52 , but precision bearings are preferred. 
     The trammel  24 , preferably constructed of wood, aluminum or plexiglass, comprises an elongate arm having a square end near the guide  20 , and a rounded end near the work piece  26 . The rounded end consists of a circular platform  54  containing a central through-bore  56 , as shown in detail in FIGS. 7-8,  10 A-B. The platform  54  is configured to support a router (not shown) such that the cutter extends through the bore  56 . A V-shaped notch  58  on the edge of the platform provides a convenient point to retain a marking instrument when the device is used to draw full-scale layouts. 
     A second preferred embodiment, shown in FIGS. 10A-B, provides an alternative configuration for mounting the marking instrument on the edge of the circular platform. This configuration includes an extension member  60 , which is preferably constructed of aluminum bar stock. In order to maintain the flat surface of the circular platform  54 , the extension member  60  sits in a channel carved in the upper surface of the platform  54 . A similarly shaped channel in the lower surface houses a plate  62  having two threaded holes  64 . The extension member  60  is mounted to the platform  54  with two flat head screws  66  that extend through bores in the extension member and into the plate  62 , with a section of the platform  54  sandwiched in between. Other means of fastening the extension member  60  to the trammel, such as nails, rivets, adhesive, cooperating joints, etc., would also be effective. 
     A vertical cylinder  68  is mounted to the far end of the extension member  60 . The cylinder  68  includes an externally threaded extension that engages an internally threaded bore in the extension member  60 . The cylinder  68  includes a central through bore that is sized to accept a standard writing utensil  70 , such as a #2 pencil. A threaded pin  72  that penetrates the side wall of the cylinder  68  engages the writing utensil  70  and maintains its position within the cylinder  68 . The pin  72  includes a knurled knob  73  to facilitate hand-tightening of the pin  72 . 
     A dovetail tab  74  extends along the upper surface of the trammel  24  from the edge of the circular platform  54  to the square end. This tab  74  is adapted to mate with dovetail slots  76  disposed on the undersides of the runners  34 ,  36 , near their centers. A slot  78  extends along the trammel&#39;s longitudinal axis of symmetry from near the guide  20  end to near the work end. A dado  80  extends along the length of the trammel  24  underside. 
     The trammel  24  is secured to the carriage  22  with threaded fasteners  82 . The fasteners  82  extend through the trammel slot  78  and the central bores  84  in the runners  34 ,  36 . In order to protect the wood on the underside of the trammel  24 , a slider  86 , adapted to fit within the dado  80  provides a buffer between the threaded fasteners  82  and the trammel  24 . 
     The slider  86 , shown in detail in FIGS. 9A-B, is preferably constructed of aluminum or steel, or other durable material that slides easily against the wood surface of the trammel  24 . A bore  88  engages the threaded fastener  82  that is secured to the fixed runner  36 . An elongate through-slot  90  engages the fastener  82  that is secured to the adjustable runner  34 . The slider  86  also includes two countersunk holes  91 , one near each end. The holes  91  are used when relatively short radii are to be cut or drawn, as explained below. 
     Although FIG. 5 depicts the threaded fastener  82  as a bolt extending upward into an internally threaded 3-star knob  50 , alternatively a 3-star knob having an externally threaded extension may be provided. The threads would be adapted to engage internal threads provided on the slider, or an internally threaded nut disposed on the underside of the slider. Other alternatives are also possible, as one of skill in the art will understand. 
     To rout a work piece  26  using the invention, first a guide  20  is secured to the work surface. Since the same work surface will ideally be used to rout many work pieces  26 , and guides  20  will ideally be reusable, the guide  20  should be secured in such a way that it may easily be removed without damaging the guide  20  or the work surface. Screws are preferred. The radius of the guide  20  is determined by the radius of the work piece  26  to be routed, as explained below. 
     Next, the knobs  50  on the threaded fasteners  48 ,  82  are loosened to enable free sliding movement of the adjustable runner  34  and the trammel  24 . The carriage assembly  22  is placed over the guide  20  such that the rotatable members  52  of the fixed runner  36  are on the concave side  28  of the guide  20  and the rotatable members  52  of the adjustable runner  34  are on the convex side  30  of the guide  20 . The fixed rotatable members  52  are then pressed against the concave side  28  of the guide  20  while the adjustable runner  34  is slid towards the opposite side of the guide  20  until all four rotatable members  52  are in firm contact with the guide  20 . To secure the rotatable members  52  in this position, the knobs  50  on the threaded members  48  joining the adjustable runner  34  and the arms  32  are tightened. 
     Next the work piece  26  is secured to the work surface at an appropriate distance from the guide  20 . For example, if the work piece  26  is an arc having a  30 ′ radius, and a  25 ′ radius guide  20  is used, the work piece  26  must be placed so that the router blade will engage the work piece  26  and the  30 ′ radius cut will be made at the desired position on the work piece  26 .  5 ′ from the convex edge  30  of the guide  20  would determine this position. In order for the router to trace the edge of the work piece  26 , the work piece  26  must be placed on the work surface in an orientation such that the guide  20  and work piece  26  define concentric arcs. 
     The work piece  26  is secured by any appropriate means to prevent undesirable damage to the finished piece. If the final product is to be cut from a larger work piece  26 , screws may be used to secure the work piece  26  onto the work surface by positioning the screws in a part of the work piece  26  that is to be discarded. Care must be taken to ensure that the screws, or other suitable fasteners, are not in the path of the router blade. 
     Next, the trammel arm  24  is adjusted to the appropriate length to allow the router blade to trace the desired radius. For example, assume that the work piece  26  is a rectangular sheet of plywood from which a  5 ″ wide,  20 ′ radius arc is to be cut. A  15 ′ radius guide  20  may be selected, so that  5 ′ of trammel  24  extension is needed. Measuring from the convex side  30  of the guide  20 , a pencil mark is placed at  55 ″ from the guide  20 . A second mark is placed at  60 ″ from the guide  20 . Sliding the trammel arm  24  outward, the router is positioned to that the cutting blade touches the inside of the  55 ″ mark. The two knobs  50  on the trammel  24  are tightened to secure the router in this position. The  55 ″ arc is then routed by making two passes. Finally, the router is repositioned so that the cutting blade touches the outside of the  60 ″ mark and routing proceeds as described. Care must be taken so that the rotatable members  52  do not disengage the guide  20  when beginning and ending cuts. 
     Following the steps just described, pieces of radius from zero to near infinity may be produced by simply using an appropriately sized guide  20 . Beginning with only guides  20  of small radius, guides  20  of progressively larger and larger radius are produced using the invention until the required radius is reached. Guides in increments of  7 ′, such as  7 ′,  14 ′,  21 ′, etc., are preferred. As each work piece  26  is cut, it then is used as the guide  20  to make the next larger guide  20 . Additionally, small radii may be produced using the invention by simply reorienting the carriage  22  so that the trammel  24  projects from the concave edge  28  of the guide  20 . 
     If relatively short radii (shorter than the overall length of the trammel  24 ) are to be cut or drawn, the guide  20  and carriage  22  may be removed so that the trammel  24  may be used on its own to trace the desired radius. After removing the guide  20  and carriage  22 , a screw is driven through one of the countersunk holes  91  in the slider  86  to rotatably secure the trammel  24  directly to the work surface. Measurements are then made from the center of the countersunk hole  91  to the work piece  26 , and the knobs  50  are tightened to lock the trammel  24  at the desired extension. Operation of the invention then proceeds as described above. 
     To use the invention to draw full-scale layouts, the router is replaced with a marking instrument. The steps outlined above are then followed with appropriate adjustments. 
     The invention provides a device that is versatile, easy to use, and produces high quality results. The trammel  24  is adapted to mount various tools, from a simple marking instrument to a power router. Because the invention uses a trammel arm  24  that is significantly shorter than the radius to be cut or drawn, one worker may operate the device with ease. The shorter length of the trammel  24  also helps reduce flex, which boosts the quality of the finished product by removing flat spots from the curve. The securely fastened guide  20  also increases quality by preventing shifting of the carriage  22  during operation. Because an operator using the invention may easily produce guides  20  of progressively larger and larger radius, the device may be used to work with pieces having a virtually infinite range of radii. 
     The above presents a description of the best mode contemplated for carrying out the present invention, and of the manner and process of making and using it, in such full, clear, concise, and exact terms as to enable any person skilled in the art to which it pertains to make and use this invention. This invention is, however, susceptible to modifications and alternate constructions from that discussed above which are fully equivalent. Consequently, it is not the intention to limit this invention to the particular embodiments disclosed. On the contrary, the intention is to cover all modifications and alternate constructions coming within the spirit and scope of the invention as generally expressed by the following claims, which particularly point out and distinctly claim the subject matter of the invention.