Abstract:
A hand tool for use by one hand provides double compound leverage of the force exerted on the handles to the jaws. This is achieved by adding additional pivots and lever arms between the handles and the jaws. The jaws may take the form of a sheet metal snip or a pruning shear.

Description:
TECHNICAL FIELD 
   The present invention pertains generally to hand tools, and more particularly to hand tools that are operated using one hand. 
   BACKGROUND OF THE INVENTION 
   Hand tools having two pivotally connected handles for moving jaws that a user operates with one hand are well known in the art. The basic tool has three parts: a unitary first member having a first handle on one end and a first jaw on the other, a unitary second member having a second handle on one end and a second jaw on the other, and an axle pivotally connecting the two together. Scissors and sheet metal snips are examples of such hand tools operated by one hand. 
   In certain applications the mechanical advantage offered by the leverage of such a three part hand tool is insufficient making it difficult or impossible to achieve the desired result with one hand. For example, cutting a thick gauge of sheet metal with a simple sheet metal snip can require more hand pressure than the user can deliver, particularly when the cutting must be performed over a protracted period of time. 
     FIGS. 1 and 2  illustrate side elevation views of a prior art hand tool  500  shown in open and closed positions. The hand tool shown is generally referred to as an aviation snip. A second set of levers between the handles and the jaws compounds the force created by a person squeezing the handles when the force is transmitted to the jaws. This allows the user to cut thicker materials that would otherwise be difficult or impossible to cut with the strength of one hand. Hand tool  500  has a first jaw member  502  having a first end  504 , an opposite second end  506 , and an intermediate portion  508 . Hand tool  500  also has a second jaw member  510  having a first end  512 , an opposite second end  514 , and an intermediate portion  516 . Intermediate portion  516  of second jaw member  510  is pivotally connected to intermediate portion  508  of first jaw member  502  at a first pivot P 1 . Hand tool  500  also has a first handle member  518  having a first end  520 , an opposite second end  522 , and an intermediate portion  524 . Intermediate portion  524  of first handle member  518  is pivotally connected to second end  514  of second jaw member  510  at a second pivot P 2 . Hand tool  500  also has a second handle member  526  having a first end  528 , an opposite second end  530 , and an intermediate portion  532 . Intermediate portion  532  of second handle member  526  is pivotally connected to second end  506  of first jaw member  502  at a third pivot P 3 . First end  520  of first handle member  518  is pivotally connected to first end  528  of second handle member  526  at a seventh pivot P 7 . It is noted that pivot P 7  is disposed between pivot P 1  and pivots P 2  and P 3 . Or put another way, when viewed as shown with pivot P 1  the uppermost pivot, pivot P 7  is below pivot P 1  and above pivots P 2  and P 3 . Hand tool  500  also includes a torsion spring  534  which biases first handle member  518  and second handle member  526  apart so that hand tool  500  resides in the open position of  FIG. 1 . A pivoting lock  536  cooperates with a shaft located at P 2  to lock hand tool  500  in the closed position. The first jaw member  502  and second jaw member  510  are shaped and dimensioned so that they combine to form an aviation snip. They cross over each other as the hand tool is closed providing a cutting action such as in scissors. It is further noted that hand tool  500  is designed to be operated using only one hand. 
   While a simple metal shear may be satisfactory for certain applications where the material to be cut is relatively thin, and while a compound metal shear of the type shown as hand tool  500  above may be satisfactory for other applications where the material to be cut is somewhat thicker, it would be advantageous to have available yet another metal shear having even greater mechanical advantage for cutting material with one hand that could not be cut with either a simple or compound shear such as those shown in the prior art. Furthermore, such a metal shear having a greater mechanical advantage could also be used over a longer period of time by one hand to cut materials that could be cut by either of the other prior art devices for the short term. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed to a hand tool that has an increased mechanical advantage. The hand tool of the present invention includes additional pivots and lever arms that increase the mechanical advantage of the hand tool. This is accomplished by adding center members between the jaw members and handle members of prior art hand tools. 
   In accordance with a preferred embodiment of the invention, a hand tool includes first and second jaw members which are pivotally connected at their intermediate portions. First and second center members are pivotally connected at their intermediate portions to the ends of the first and second jaw members, and the ends of the first and second center members are pivotally connected. First and second handle members are pivotally connected at their intermediate portions to the opposite ends of the first and second center members, and the ends of the first and second handle portions are pivotally connected. In all, the hand tool of the present invention includes six members that are connected at seven pivot points. 
   The jaws of the present invention can be adapted to perform a variety of tasks such as cutting sheet metal, cutting vegetation, and other uses. 
   Other aspects of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevation view of a prior art hand tool shown in the open position; 
       FIG. 2  is a side elevation view of the prior art hand tool shown in the closed position; 
       FIG. 3  is a side elevation view of a hand tool in accordance with the present invention shown in the open position; 
       FIG. 4  is a side elevation view of the hand tool of the present invention shown in the closed position; and 
       FIG. 5  is a side elevation view of a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 3 and 4  illustrate side elevation views of a hand tool  20  in accordance with the present invention shown in the open and closed positions, respectively. Hand tool  20  includes a first jaw member  22  having a first end  24 , an opposite second end  26 , and an intermediate portion  28 . Hand tool  20  also has a second jaw member  30  having a first end  32 , an opposite second end  34 , and an intermediate portion  36 . Intermediate portion  36  of second jaw member  30  is pivotally connected to intermediate portion  28  of first jaw member  22  at a first pivot P 1 . 
   Hand tool  20  also has a first center member  38  having a first end  40 , an opposite second end  42 , and an intermediate portion  44 . Intermediate portion  44  of first center member  38  is pivotally connected to second end  34  of second jaw  30  member at a second pivot P 2 . Hand tool  20  also has a second center member  46  having a first end  48 , an opposite second end  50 , and an intermediate portion  52 . Intermediate portion  52  of second center member  46  is pivotally connected to second end  26  of first jaw member  22  at a third pivot P 3 . First end  40  of first center member  38  is pivotally connected to first end  48  of second center member  46  at a fourth pivot P 4 . Fourth pivot P 4  is disposed between first pivot P 1  and second and third pivots P 2  and P 3 . That is, in the shown view, pivot P 4  is below pivot P 1  and above pivots P 2  and P 3 . 
   Hand tool  20  also has a first handle member  54  having a first end  56 , an opposite second end  58 , and an intermediate portion  60 . Intermediate portion  60  of first handle member  54  is pivotally connected to second end  42  of first center member  38  at a fifth pivot P 5 . Hand tool  20  also has a second handle member  62  having a first end  64 , an opposite second end  66 , and an intermediate portion  68 . Intermediate portion  68  of second handle member  62  is pivotally connected to second end  50  of second center member  46  at a sixth pivot P 6 . First end  56  of first handle member  54  is pivotally connected to first end  64  of second handle member  62  at a seventh pivot P 7 . Seventh pivot P 7  is disposed between (1) second and third pivots P 2  and P 3 , and (2) fifth and sixth pivots P 5  and P 6 . Stop pins  70  limit the open distance first and second handle member  54  and  62  can move from each other. Hand tool  20  also includes a torsion spring  55  which biases first handle member  54  and second handle member  62  apart so that hand tool  20  resides in the open position of  FIG. 3 . 
   In the shown embodiment, first jaw member  22  and second jaw member  30  are shaped and dimensioned so that they combine to form the particular form of a sheet metal snip known as an aviation snip. However, it may be appreciated that hand tools  20  having the same structural arrangement of pivots and levers may be used for other purposes. One such hand tool  20  is depicted in  FIG. 5  and the discussion pertaining thereto. As with prior art hand tool  500 , hand tool  20  is designed to be operated using only one hand. 
   It is noted that the addition of first and second center members  38  and  46  only adds slightly to the overall length of hand tool  20  when compared to the prior art hand tool  500  of  FIGS. 1-2 . But the mechanical advantage MA of hand tool  20  is significantly increased over that of prior art hand tool  500 . The mechanic advantage MA of hand tool  20  is more than twice as great as the mechanical advantage MA of prior art hand tool  500 . 
     FIG. 5  is a side elevation view of a second embodiment of the present invention. In this embodiment, first jaw member  22  and second jaw member  30  are shaped and dimensioned so that they combine to form a pruning shear. It may be appreciated that other embodiments of first jaw member  22  and second jaw member  30  are also possible to perform other types of work. 
   The preferred embodiments of the invention described herein are exemplary and numerous modifications, variations, dimensional variations, and rearrangements can be readily envisioned to achieve an equivalent result, all of which are intended to be embraced within the scope of the appended claims.