Abstract:
The invention is a layout tool and a means for using the tool to fabricate multi-segment elbows and offset joints from rectangular cross-section fibrous air ductboard material. The tool has a triangular main body having at least one structural feature that defines a plane. At least one flange is connected to the main body, is oriented substantially perpendicular to the main body plane, and forms the first side of the triangle. The main body includes a first outer straight edge extending at an angle of 67.50 degrees from the flange forming the second side of the triangle. A second outer straight edge extending at an angle of 78.75 degrees from the flange forms the third side of the triangle. An inner structure of the main body forms a straight edge perpendicular to the plane of the flange. The outer angled and interior straight edges enable 22.5- and 45-degree miter joints to be easily marked and cut in pre-formed fibrous air duct. Markings on the main body indicate required distances between cuts to accomplish desired offset rises.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to the field of measuring and marking tools having a generally triangular configuration and a flange, perpendicular to one side, projecting above and below the upper and lower surfaces of the triangle, and upon which linear measurement markings are provided on the surfaces of the triangle along the respective edge portions. In particular, this invention relates to tools having a scalene triangular configuration in which the angular orientation of the sides relative to the flange are common angles used in mitered construction and markings on a surface of the triangle providing commonly used mitered air duct joint fabrication measurements.  
         [0003]     The use of fibrous ductboard material for heating and air conditioning ducts is well known. Such ductboard typically includes a layer of fiberglass attached to a composite outer covering. The outer covering is typically made up of a layer Kraft paper, a layer of scrim-like material, and a foil-like layer, the composition providing stiffness and forming an air-impervious outer layer for the duct. Fibrous ductboard material is commonly available in flat-sheet form or pre-formed into rectangular cross-section ducts in a variety of sizes. When flat-sheet material is used, it is conventional to cut, either by machine or the installer, a set of laterally-spaced-apart, longitudinal grooves in the fiberglass side of flat-sheet ductboard to form rectangular cross-section duct sections.  
         [0004]     It is also well known in the duct construction field that fittings, such as elbows and offsets, may be fabricated from the ductboard material using mitered cuts on the duct. For example, a simple 90-degree elbow can be formed by cutting the duct along a plane oriented 45 degrees to the longitudinal duct axis, rotating one segment one-half turn about its longitudinal axis, and connecting the two sections at the cut plane to form a 90-degree mitered elbow. Fittings having more gradual transitions, thereby imposing less resistance on air flow, are commonly formed by reducing the miter angle and increasing the number of duct segments comprising the fitting.  
         [0005]     In preparation for making a duct fitting, the user assembles tools including a marking pencil, an incrementally marked straight edge, a protractor, and a cutting knife and determines the dimensions of the desired fitting. The user then makes a series of cutting marks, or layout lines, on the ductboard material to define a cutting plane. Conventional flaming squares or carpenter&#39;s triangles are typically employed to make layout lines oriented perpendicular to the longitudinal axis of the duct. Non-perpendicular layout lines require the user to establish two points along the desired line with a protractor or other similar tool and then make a line using a straight edge. The duct can then be cut by drawing a cutting knife along the layout line, optionally using a straight edge as a guide, and cutting all the way thorough the ductboard material. Finally, the user aligns the resulting duct segments to form the desired fitting, reconnects the segments with adhesive, and seals the connection with duct tape. Drawbacks in using the above-described tools and method for making mitered elbows and offsets are the considerable length of time involved and the limited quality and accuracy of the resulting joint due to variations in measuring and cutting.  
         [0006]     It is therefore the principle objective of the present invention to provide a tool and a method for selecting and making a series of quick layout lines for the most common miter angles used in air duct fitting fabrication and for aligning the edge of a cutting tool in making the cuts. It is another objective of the present invention to provide a tool and a method for controlling the accuracy and quality of miter joint cuts.  
         [0007]     2. Description of Related Art  
         [0008]     Numerous measuring and marking tools of the right-triangular type are known in the prior art. U.S. Pat. No. 4,513,510, by Swanson, discloses a right-triangular-shaped layout tool with a T-flange base on one side, and which is adaptable with a layout bar to provide a means for repeated marking of predetermined angles as are common in marking of stair stringer boards. U.S. Pat. No. 5,727,325, by Mussell, discloses a right-triangular-shaped tool with a T-flange on one side and markings to facilitate aligning the tool on workpieces at selected angles commonly used in rafter and stair stringer framing. U.S. Pat. No. 6,622,394, by Werner, discloses a right-triangular-shaped measuring tool with a T-flange for aligning the tool base to the workpiece. Indicia along the hypotenuse in conjunction with a defined origin allow marking of acute angles commonly used in deck construction. U.S. Pat. No. 6,688,014, by Allemand, discloses a right-triangular-shaped measuring and marking tool that includes internal structures for marking frequently used wood frame construction dimensions and a method of using the tool to mark layout lines common in wood frame construction.  
         [0009]     These measuring and marking tools have limited efficacy compared with the present invention. All are based on a right triangle having a flange perpendicular to one leg of the triangle useful for rapidly aligning the tool with an edge of the workpiece. The limitation with this right-triangular design is that the second leg of the right triangle is always perpendicular to the flange, leaving only the hypotenuse available for non-perpendicular layout lines. To overcome this limitation, each tool includes structures for marking other common framing angles. However, using these features is a multi-step operation. One method requires making a pair of marks to define a line, moving the tool, and using a straight edge to draw the desired line. Another method requires visually aligning two or more points on the tool with a reference edge of the workpiece to establish the desired angle and then drawing the desired line. In the former method, making a layout line is a three-step process; the second method requires two steps and fails to take advantage of the T-flange for quick and accurate tool alignment with the workpiece.  
         [0010]     In addition to framing tools, drawing instruments are known in the prior art. U.S. Pat. No. 2,610,407, by McQuaid, discloses a drafting instrument incorporating straight edges inclined at angles commonly used in making axonometric projection drawings. U.S. Pat. No. 4,455,760, by Arceneaux, discloses a drafting instrument incorporating straight edges inclined at angles commonly used in making isometric projection drawings. These instruments depart from a standard right-triangular-shaped design and incorporate interior structures thereby increasing the number of straight edges offered in a single instrument. As drafting instruments, these instruments do not incorporate a T-flange to align the instrument against a workpiece corner. Alignment to a reference line is commonly performed using a T-square or similar drafting apparatus.  
         [0011]     Layout and fabrication tools specifically useful for working with fibrous ductboard material are also well known in the prior art. U.S. Pat. No. 4,179,808, by Smith, discloses a movable tool guide for cutting and removing wedge-shaped pieces from sheet-form fibrous ductboard material enabling air duct transition pieces of a range of sizes to be formed. U.S. Pat. No. 4,608,902, by Ivey, discloses a portable measuring and cutting tool guide for cutting parallel V-grooves in fibrous ductboard material enabling rectangular air duct to be formed from flat-sheet material. These tools are designed to make layout lines and cuts on ductboard material in sheet form needed to form rectangular cross-section ducts, but they are not suited to working with pre-formed rectangular cross-section ducts.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention is a measuring and layout tool providing straight edges oriented such that layout marks and cuts commonly used in fabricating elbows and offsets in rectangular cross-section duct can be made more easily. The invention essentially comprises a main body structure having a triangular outer perimeter with a flange affixed perpendicular to a base edge of the main body structure. The base-edge flange allows quick and consistent alignment of the tool with a corner edge of a rectangular cross-section duct workpiece so that the main body structure lies across a face of the workpiece.  
         [0013]     One of the remaining two outer edges of the main body structure is offset 22.5 degrees from a line perpendicular to the base-edge flange. Miter cuts of 22.5 degrees are used to form 45-degree miter joints in ducts. Two 45-degree miter joints may be combined to form a three-segment, 90-degree elbow fitting. Forty-five-degree miter joints may also be use to fabricate sharp transition offset fittings useful for making parallel shifts, or offsets, of the longitudinal duct axis.  
         [0014]     The remaining outer edge of the main body structure is offset 11.25 degrees from a line perpendicular to the base-edge flange. Miter cuts of 11.25 degrees are used to form 22.5-degree miterjoints in ducts. Four 22.5-degree miter joints may be combined to form a 90-degree elbow fitting having a smoother transition than the elbow fitting made using 45-degree miter joints. Smooth transition offset fittings using 22.5-degree miter joints may also be fabricated using the 11.25-degree outer edge of the layout tool.  
         [0015]     A portion of the main body structure is removed to form an interior structure having a straight edge oriented perpendicular to the base-edge flange. The perpendicular straight edge aids in layout and cutting of duct faces that must be cut perpendicular to the longitudinal duct axis to form a mitered duct joint. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The invention is shown in  FIGS. 1 through 4 .  
         [0017]      FIG. 1  is a front view of the invention showing the main body structure.  
         [0018]      FIG. 2  is an edge view of the invention showing the base flange in relation to the main body structure.  
         [0019]      FIG. 3  shows a table located on a planar surface of the main body structure with common duct elbow fabrication measurements.  
         [0020]      FIG. 4  shows a table located on a planar surface of the main body structure with common offset riser fabrication measurements.  
         [0021]      FIGS. 5 through 16  show the method of using the invention and typical air duct fittings that can result. The invention and workpiece are shown in solid lines in  FIGS. 5 through 8  and the layout lines are shown in dashed lines.  
         [0022]      FIG. 5  shows a workpiece with the layout tool in position to make a first layout line.  
         [0023]      FIG. 6  shows the workpiece with the layout tool in position to make a second layout line.  
         [0024]      FIG. 7  shows the workpiece with the layout tool in position to make a third layout line.  
         [0025]      FIG. 8  shows the workpiece with the layout tool in position to make a fourth layout line.  
         [0026]      FIG. 9  shows two segments of the workpiece after cutting along the plane defined by the four layout lines.  
         [0027]      FIG. 10  shows a two-segment mitered duct joint fitting.  
         [0028]      FIG. 11  shows a workpiece with layout lines for a three-segment, 90-degree elbow having a specified throat dimension.  
         [0029]      FIG. 12  shows a completed three-segment, 90-degree elbow fitting.  
         [0030]      FIG. 13  shows workpiece with layout line for a five-segment, 90-degree elbow having a specified throat dimension.  
         [0031]      FIG. 14  shows a completed five-segment, 90-degree elbow fitting.  
         [0032]      FIG. 15  shows a workpiece with layout lines for an offset fitting.  FIG. 16  shows a completed offset fitting of a selected rise. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     The preferred embodiment shown in  FIG. 1  is a layout tool  10  consisting of a main body structure  12  having a generally triangular periphery and a substantially planar surface. The periphery of main body structure  12  is formed by base edge  14 , 22.5-degree straight edge  16 , and 11.25-degree straight edge  18 . In the preferred embodiment, base edge  14  measures approximately 14-⅝ inches long, 22.5-degree straight edge  16  is approximately 25-⅝ inches long, and 11.25-degree straight edge is approximately 24-⅛ inches long. A portion of main body structure  12  is removed to form an interior opening that includes 90-degree straight edge  20 . The layout tool  10  has a first planer surface  24  facing in one direction and a second planar surface  26  facing in the opposite direction, said surfaces being parallel and separated by a short distance ranging from about ⅛ inch to ¼ inch. Though the invention may be made of any suitably rigid material, such as steel, aluminum, or plastic, the preferred embodiment is made of plastic.  
         [0034]     Linear measure markings  28  located on first planar surface  22  along 22.5-degree straight edge  16  and along 11.25-degree straight edge may be used to define distances in the manner as a ruler.  
         [0035]     A first data table  30  is marked on first planar surface  24  and provides information useful for elbow-fitting layout. First data table  30  is shown in  FIG. 3 . A second data table  32  is also marked on first planar surface  24  and provides information useful for offset fitting layout. Second data table  32  is shown in  FIG. 4 . Use of first data table  30  and second data table  32  is described later in this specification.  
         [0036]     Referring to  FIG. 2 , base flange  22  is shown affixed to main body structure  12 . Base flange  22  is oriented perpendicularly to main body structure  12  such that a first portion of base flange  22  extends outwardly from first planar surface  24  approximately ⅝ inch and a second portion of base flange  22  extends outwardly from second planar surface  26  approximately ⅝ inch. The length of base flange  22  is approximately equal to the length of base edge  14 .  
         [0037]     Fabricating rectangular cross-section duct fittings requires the user to make at least one miter cut of the duct. Using layout tool  10  improves the efficiency of the layout and cutting steps. Forming a miter cut of a rectangular cross-section duct requires the user to cut each of the four faces of the duct. The user must cut two faces opposite of each other at an acute angle measured relative to the longitudinal axis of the duct. The remaining two faces of the duct must be cut along lines that are perpendicular to the longitudinal axis of the duct; however, the cuts through the ductboard wall thickness must be angled to match the acute angle used for the first angled cuts.  
         [0038]     The user first determines the orientation of the desired bend in relation to the duct dimensions to identify a starting point  110 . Staring point  110  is located on a first longitudinal corner  112  of workpiece  50 . If the duct cross-section is square, the location of starting point  110  is immaterial. For rectangular cross-sections, the user&#39;s determination is based on whether the miter bend occurs on the major side or on the minor side of the duct. For major-side miters, the user makes cuts displaced by an angle from a line perpendicular to the longitudinal duct axis on the major sides of the duct to form the miter. Cuts on the minor sides are made perpendicular to the longitudinal duct axis. For minor-side miters, the user makes the angled cuts on the minor sides and perpendicular cuts on the major sides.  
         [0039]     Next, the user determines the miter angle necessary for the desired fitting. Layout tool  10  is useful for making 22.5-degree and 45-degree miter bends in rectangular cross-section ducts. A 45-degree miter bend is used in this description. Forming a 22.5-degree miter bend from this description requires only substitution of 22.5-degree straight edge  16  with 11.25-degree straight edge  18  in the following description.  
         [0040]     Once starting point  110  is determined, the user positions layout tool  10  on workpiece  50  so that the intersection of base flange  22  and main body structure  12  rests along first longitudinal corner  112 , main body structure  12  rests on first face  114 , and 22.5-degree straight edge  16  is aligned with starting point  110  oriented in the desired direction of the miter cut as shown in  FIG. 5 . First planar surface  24  faces away from workpiece  50 , but if the desired miter is in the opposite direction, layout tool  50  could be turned over so that first planar surface  24  is adjacent to workpiece  50 . With layout tool  10  positioned, the user makes first layout line  116  on first face  114  along 22.5-degree straight edge, extending from first longitudinal corner  112  to second longitudinal corner  122 . Second point  120  is located at the intersection of first layout line  116  and second longitudinal corner  122 . The user cuts the ductboard material along first layout line  116  with the cutting blade oriented perpendicular to the plane of first face  114 .  
         [0041]     In  FIG. 6 , the user positions layout tool  10  on workpiece  50  so that the intersection of base flange  22  and main body structure  12  rests along second longitudinal corner  122 , main body structure  12  rests on second face  124 , and 90-degree straight edge  20  is aligned with second point  120 . Second face  124  is adjacent to first face  114 . With layout tool  10  positioned, the user makes second layout line  126  on second face  124  along 90-degree straight edge  20 , extending from second longitudinal corner  122  to third longitudinal corner  132 . Second point  120  is located at the intersection of first layout line  116  and second longitudinal corner  122 . The user cuts the ductboard material along second layout line  126  with the cutting blade oriented parallel to first layout line  116 .  
         [0042]     The method of making a layout line and cutting third face  134  is shown in  FIG. 7 . Third face  134  is adjacent to second face  124  and opposite of first face  114 . The user positions layout tool  10  on workpiece  50  so that the intersection of base flange  22  and main body structure  12  rests along third longitudinal corner  132 , main body structure  12  rests on third face  134 , and 22.5-degree straight edge  16  is aligned with third point  130  and oriented such that it is parallel with first layout line  116 . Aligning 22.5-degree straight edge so that it is parallel with first layout line  116  generally requires the user to turn layout tool  10  over so that the planar face that was adjacent to workpiece  50  to make the first and second layout lines now faces away from workpiece  50 . As shown, second planar surface  26  faces away from workpiece  50 . With layout tool  10  positioned, the user makes third layout line  136  on third face  134  along 22.5-degree straight edge, extending from third longitudinal corner  132  to fourth longitudinal corner  142  and parallel to first layout line  116 . Fourth point  140  is located at the intersection of third layout line  136  and fourth longitudinal corner  142 . The user cuts the ductboard material along third layout line  136  with the cutting blade oriented perpendicular to the plane of third face  134 .  
         [0043]     As shown in  FIG. 8 , the user positions layout tool  10  on workpiece  50  so that the intersection of base flange  22  and main body structure  12  rests along fourth longitudinal corner  142 , main body structure  12  rests on fourth face  144 , and 90-degree straight edge  20  is aligned with fourth point  140 . In this position, straight edge  20  also aligns with starting point  110  on first longitudinal corner  112 . Fourth face  144  is adjacent to first face  114  and opposite of second face  124 . With layout tool  10  positioned, the user makes fourth layout line  146  on fourth face  144  along 90-degree straight edge  20 , extending from fourth longitudinal corner  142  to first longitudinal corner  112 . The line should intersect first layout line  116  at starting point  110 . The user cuts ductboard material along fourth layout line  146  with the cutting blade oriented parallel to first layout line  116 .  
         [0044]     In  FIG. 9 , cutting plane  150  is defined by first layout line  116 , second layout line  126 , third layout line  136 , and fourth layout line  146 . Cutting plane  150  is oriented at miter-cut angle  152  which is measured from a plane perpendicular to the longitudinal axis of the duct, 22.5 degrees in the described example. Cutting workpiece  50  along cutting plane  150  results in a first segment  160  and a second segment  170 .  
         [0045]      FIG. 10  shows mitered fitting  180 . Mitered fitting  180  is formed by joining first segment  160  and second segment  170  at cutting plane  150  after rotating either first segment  160  or second segment  170  by 180 degrees about its longitudinal axis. The two segments are joined at cutting plane  150  using conventional adhesives and tape. Miter angle  154  is the angular displacement of the longitudinal axis of mitered fitting  180 . The value of miter angle  154  is twice the value of miter-cut angle  152 .  
         [0046]     Users can form a variety of duct fittings by making additional cuts specifically oriented relative to first cutting plane  150  and selecting an appropriate miter angle for the cutting plane using layout tool  10  and the method described. Referring to  FIG. 11 , a three-segment, 90-degree elbow can be formed by marking a second cutting plane  250  on workpiece  50  using layout tool  10  such that the angle between first cutting plane  150  and second cutting plane  250  is bisected by a plane perpendicular to the longitudinal axis of the duct. First cutting plane  150  and second cutting plane  250  are separated by inside measure  164 . The user determines inside measure  164  by selecting the desired throat  166  to suit construction needs. Throat  166  may also be referenced as an inside radius of the elbow fitting. The user locates the desired throat on first data table  30  and selects the corresponding inside measure  164 . When the user cuts workpiece  50  along the first and second cutting planes, three segments result, shown in  FIG. 11  as first segment  160 , second segment  170 , and third segment  162 . Third segment  162  is in the shape of an isosceles trapezoid when a face on which miter cuts are made is viewed perpendicularly. By rotating third segment  162  by 180 degrees about its longitudinal axis and joining the three segments at the cuttings planes, the user can form a 90-degree, three-segment elbow  280  having two 45-degree miter joints and a desired throat  166  as shown in  FIG. 12 .  
         [0047]      FIGS. 13 and 14  show a five-segment, 90-degree elbow fitting that can be formed by selecting a miter cut angle  152  of 11.25 degrees which may be accomplished by using 11.25-degree straight edge of layout tool  10  to lay out and cut the angled faces of workpiece  50 . Five-segment, 90-degree elbows are desirable since their use results in less resistance to air flow in a completed duct. A five-segment, 90-degree elbow  380  can be formed by marking a second cutting plane  250 , a third cutting plane  152 , and a fourth cutting plane  252  on workpiece  50  using layout tool  10 . The angle between first cutting plane  150  and second cutting plane  250  is bisected by a plane perpendicular to the longitudinal axis of the duct. Third cutting plane  152  is oriented parallel to first cutting plane  150 . Fourth cutting plane  252  is oriented parallel to second cutting plane  250 . Each cutting plane is separated from the adjacent cutting plane by inside measure  164 . Inside measure  164  is determined by selecting the desired throat  166  to suit construction needs. Throat  166  may also be referenced as an inside radius of the elbow fitting. The user determines inside measurement  164  by locating the desired throat on first data table  30  and selecting the corresponding inside measurement  164 . When the user cuts workpiece  50  along the cutting planes, five segments result, shown in  FIG. 13  as first segment  160 , second segment  170 , third segment  162 , fourth segment  262 , and fifth segment  352 . Third segment  162 , fourth segment  262 , and fifth segment  362  have the same geometric shape and dimensions. The segments have the shape of an isosceles trapezoid when a face on which miter cuts are made is viewed perpendicularly. By rotating third segment  162  and fifth segment  362  by 180 degrees about their longitudinal axis and joining the five segments at the cuttings planes, the user forms a 90-degree, five-segment elbow  380  having four 22.5-degree miter joints and a desired throat  166  as shown in  FIG. 14 .  
         [0048]      FIGS. 15 and 16  show an offset transition fitting fabricated using layout tool  10  and the described method for making a cutting plane. Offset transition fitting  480  may be fabricated by making a second cutting plane  250  on workpiece  50  such that second cutting plane  250  is oriented parallel to first cutting plane  150 . First cutting plane  150  and second cutting plane  250  are separated by offset measure  264 . The user determines offset measure  264  by selecting the desired offset  266  and miter angle  152  to suit construction needs. A miter angle of 22.5 degrees results in 45-degree miter joints and is referred to as a sharp offset. A miter angle of 11.25 degrees results in 22.5-degree miter joints and is referred to as a gradual offset. The user locates the desired offset dimension on second data table  32 , selects either sharp offset or gradual offset, and selects the corresponding offset measure  264 . When the user cuts workpiece  50  along the first and second cutting planes, three segments result, shown in  FIG. 15  as first segment  160 , second segment  170 , and third segment  168 . Third segment  168  is in the shape of a parallelogram when a face on which miter cuts are made is viewed perpendicularly. By rotating third segment  168  by 180 degrees about its longitudinal axis and joining the three segments at the cuttings planes, the user forms a three-segment offset  480  having a desired offset  266  as shown in  FIG. 15 .  
         [0049]     It is to be understood that the form of this invention as shown is merely a preferred embodiment and the methods described are ones most commonly used. This invention may be embodied in several forms without departing from its function. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims.