Abstract:
The board cutter includes a T-square and a knife. The T-square has a head with a board edge contact surface and a board face contact surface. A long leg of the T-square is attached to the head. A board face contact surface on the bottom of the long leg slides along a board face during use. A slider is slideable along a slot in the long leg and is clamped in any position along the slot. The knife is pivotally connected to the slider for pivotal movement about an axis that is parallel to the slot in the long leg. Penetration of the cutter into a board is controlled manually as the T-square is slid along surfaces of the board.

Description:
TECHNICAL FIELD 
     The invention relates to a board cutter for cutting gypsum board and foam insulation board and more particularly to a combination T-square guide and mat knife for accurately cutting large boards. 
     BACKGROUND OF THE INVENTION 
     Builders employ gypsum board for interior walls and ceilings and foam insulation board for insulation coverings of various surfaces in home and commercial building construction. These boards are generally about four feet wide and eight, twelve or sixteen feet long. The dimensions vary somewhat if the sheets are metric. The boards are cut for attachment to wall studs and other structural beams. 
     The cuts in gypsum board and foam insulation board must be fairly accurate to fit on the studs and to provide good joints between boards. The cuts must also be made quickly to reduce building costs. Both gypsum board and foam insulation boards are severed by making a shallow cut and then breaking the boards along a line where the boards are weakened by the shallow cut. 
     Cuts across the width of a board are made quickly and accurately using a T-square with a long leg that is up to four feet long and a mat knife. A four-foot T-square is easily transported without damage. 
     Making a straight cut that extends the length of gypsum and foam insulation boards is relatively difficult. Transportation of long beams with a straight edge is also difficult. Frequently long straight edge members are bent and destroyed. Long beams with a straight edge may require an additional person to hold the straight edge beam during cutting with a mat knife. 
     Long cuts have been made by construction workers using a T-square and a knifeblade. Numerous T-square and knife combinations have been tried over the years. These combination T-squares and knifeblades have drawbacks that result from having a knifeblade that is rigidly secured to the long leg of the T-square. The knifeblade tends to raise the long leg up off of the surface of the board that is being cut when hard material spots in the board are encountered. Up and down movement of the long leg of the T-square makes it somewhat more difficult to accurately guide the knifeblade and cut the board along a straight line occasionally construction workers like to make a second deeper cut when hard material is encountered in a board. To increase the depth of a knifeblade that is rigidly clamped to a T-square, it is necessary to loosen the blade, reset the position of the blade and then clamp the blade back to the blade holder. This procedure takes time. It can also be difficult to determine a depth setting of the blade. 
     Knifeblade penetration forces and cutting forces are transmitted between the knifeblade and the long leg of a T-square when the knifeblade is rigidly secured to the T-square. The T-square must be somewhat stronger and weigh more to withstand these extra forces. The transmission of force through the T-square to the knifeblade to obtain penetration and to cut material makes guidance of the blade by the T-square somewhat more difficult and less accurate. 
     SUMMARY OF THE INVENTION 
     The combination T-square and mat knife cutter for making straight cuts in long sheets of gypsum board and foam insulation boards includes a head with a board edge contact surface and a board face contact surface. A long leg is connected to the head. A bottom surface of the long leg is a board face contact surface that slides along the face surface of a board that is being cut. A slider is slideably mounted on an upper surface of the long leg. A clamp fixes the position of the slider in selected positions along the length of the long leg. A knife having a handle and a knifeblade is pivotally connected to a knife support on the slider for pivotal movement about a knife axis. The knife axis is parallel to a long axis of the long leg of the T-square and spaced from the board face contact surface. 
     The connection of the knife to the T-square for pivotal movement about a knife axis allows a construction worker and others to manually control the depth of cut and the pressure exerted on the knifeblade. The T-square guides the knifeblade only. A board face contact surface on the long leg can remain in sliding contact with a board that is being cut. Forces exerted on the T-square by the mat knife during cutting are minimized. Reducing the force exerted on the T-square permits a reduction in T-square weight and improves the accuracy of a cut in a board at the same time. If the construction worker wishes to make a second deeper cut in the same location, the T-square and mat knife are moved back to the starting position and another cut is made. No additional adjustment is required. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The presently preferred embodiments of the invention are disclosed in the following description and in the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of the T-square and knife cutter; 
     FIG. 2 is an enlarged sectional view taken along line  2 — 2  in FIG. 1; 
     FIG. 3 is an expanded perspective view of the slider, the mat knife and the long leg; and 
     FIG. 4 is an enlarged section view taken along line  4 — 4  in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The board cutter generally designated by reference number  10  includes a T-square  12 , a slider  14  and a mat knife  16 . The T-square  12  has a head  18  and a long leg  20 . 
     The head  18  is a T-shaped extrusion as shown in FIG.  4 . The trunk  22  is a short member with a board edge contact surface  24  that extends the length of the head  18 . The board edge contact surface  24 , as shown in FIGS. 1 and 4 is in contact with an edge  26  of a gypsum board or foam insulation board  28 . A cross bar  30  is integral with the trunk  22  and is transverse to one end of the trunk. A long wing  32  of the cross bar  30  forms a hand grip for sliding the T-square  12  along the edge  26 . A short wing  34  of the cross bar  30  has a board face contact surface  36  that is in contact with the face  38  of board  28 . The board face contact surface  36  extends to both ends of the head  18 . However, the center portion of the short wing  34  is removed to form a cutout  40  shown in FIG.  1 . The knife blade cutout  40  makes it possible to move the mat knife  16  up to the board edge contact surface  24 . 
     The long leg  20  of the T-square  12  is rigidly secured to the center of the head  18  and extends past a free edge  42  of the short wing  34 . A free end  44  of the long leg  20  is preferably between 24 inches and 48 inches from the board edge contact surface  24 . However, if the width of the boards  28  is not 48 inches, the length of the long leg  20  may require some adjustment. The long leg  20  should have a length that is at least half the width of boards  28  that are to be cut. Maximum length of the long leg  20 , from the edge contact surface  24  to the free end  44  should not exceed the width of a board  28 . If the long leg  20  is too long, a board  28  cannot be cut when the board is on a floor and leading against a wall. 
     A handle  48  may be attached to the head  18  as shown in FIGS. 1 and 4. The long wing  32  may serve as a handgrip as mentioned above. However, a person grasping the wing  32  may have inadequate finger space when cutting a board leaning against a wall or setting on a floor. The added handle  48  provides additional space for holding and sliding the T-square on the edge  26  of a board  28 . 
     The long leg  20 , as shown in the drawing, is a tubular member with a slot so extending through the upper wall  52 . The slot  50  is parallel to a central axis  54  of the long leg  20 . Top walls  56  and  58  extend outward from both sides of the slot  50 . Side walls  60  and  62  extend at right angles to the top walls  56  and  58 . A bottom wall  64  is integral with both side walls  60  and  62  and has a leg board face contact surface  66 . The slot  50  is defined by side walls  68  and  70 . Flanges  72  and  74  are integral with the side walls  68  and  70  on either side of the slot  50  and have clamp engaging surfaces  76  and  78 . 
     The slider  14  has a channel-shaped base  80  with a web  82  and two side flanges  84  and  86 . The web  82  contacts the top walls  56  and  58  of the long leg  20 . Side flanges  84  and  86  of the base  80  contact the side walls  60  and  62 . A bolt  88  has a shank  90  that is received in the slot  50  of the long leg  20  and a head  92  that engages the clamp engaging surfaces  76  and  78 . The shank  90  also passes through an aperture  98  through the channel-shaped base  90 . A wing nut or clamp  100  screws onto the bolt  88  and is tightened to fix the position of the slider  14  in any one of an infinite number of positions along the length of the long leg  20  and the slot  50 . A carriage bolt  88  to keep the shank  90  from rotating in the slot  50 . A carriage bolt can be used in place of the bolt  88  to keep the shank  90  from rotating in the slot  50 . An upright flange  102  on one end of the channel-shaped member has three mat knives mounting apertures  104 ,  106  and  108 . 
     The mat knife  16  has a handle  110  and a blade  112 . A mounting aperture  114  passes through the handle  110 . A pivot bolt  116  passes through the aperture  114 . A nut  118  screws onto the bolt  116  to a position in which the handle is axially fixed on the bolt but free to rotate relative to the bolt. The bolt  116  is inserted through one of the knife mounting apertures  104 ,  106  or  108  and a wing nut  120  is screwed onto the pivot bolt  116  and tightened. In the described position, the axis of the pivot bolt  116  is parallel to the axis  54  of the long leg  20 . The axis of the pivot bolt  116  is also spaced from the long leg  20  as shown in FIG.  2 . 
     The pivot bolt  116  is held in the center aperture  116  with a free end of the handle  110  of the mat knife  16  extending toward the aperture  104  as shown in FIGS. 1 and 2. This is the proper position for a right-handed person to cut gypsum board. The blade  112  does not penetrate very deep into a gypsum board due to the density of the gypsum. A relatively shallow cut through the paper cover on the board  28  and into the gypsum allows a board to be broken and divided into two pieces. Foam insulation board  28  is also cut partway through the board to score the board and then the board is broken. However the cut should be substantially deeper than the cut in gypsum boards. The blade  112  of the mat knife  16  is penetrating into the board  28  about as deep as it can as shown in FIG. 2. A slight increase in the depth of penetration, obtained by pivoting the mat knife  16  about the axis of the pivot bolt  116 , will move the handle  110  into contact with the channel-shaped base  80 . To obtain increased penetration of a foam insulation board  28 , the pivot bolt  116  is moved from the center aperture  106  to the aperture  108 . 
     A left handed person, using the board cutter  10  would reverse the mat knife  16  on the pivot bolt  116  and insert the pivot bolt back into the center aperture  106  to cut a relatively hard board such as gypsum board. For a left-handed person to cut foam insulation board  28 , the pivot bolt  116  is moved to the aperture  104 . 
     A relatively thin foam insulation board does not need to be cut any deeper than a gypsum board. When a requirement arises to sever a thin foam insulation board, the pivot bolt  116  can remain in the center aperture  106 . 
     Measurement scales  130  are engraved in side walls  60  and  62  of the long leg  20 . These scales  130  can be in inches or metric units. Scale indicators  132  are engraved in both side flanges  84  and  86  of the channel-shaped base  80  as shown in FIG.  3 . If desired the side walls  60  and  62  could extend inwardly and upwardly from the bottom wall  64  to make it easier to read the measurement scale  130 . The shape of the slider  14  is changed to conform to the shape of the long leg  20  if the long leg  4 s modified as describe above. 
     The short wing  34  of the head  18  has a board face contact surface  36  as explained above. This board face contact surface  36  is spaced from the board face contact surface  66 , on the long leg  20 , a distance that is substantially equal to the decrease in thickness of gypsum board sheets adjacent to their long edges. Gypsum board sheets  28  have a reduced thickness along their long edges to provide space for forming a joint seam having a surface in a common plane with the surface of two adjacent gypsum board sheets. The slight lifting of the head  18  of the T-square  12  when cutting a board with a uniform thickness does not change the accuracy of the board cutter. The free end of the long leg  20  still slides on the surface of the board that is being cut. 
     The disclosed embodiments are representative of presently preferred forms of the invention, but are intended to be illustrative rather than definitive thereof. The invention is defined in the claims.