Patent Application
20090085307
Machine tools, such as lathes, screw cutters, boring devices, and the like, as they are commonly known in the art, provide for a rigidly held work piece which is rotated or otherwise manipulated around or along some axis, while a fixed tool such as a holder with a tool bit therein is introduced to the work piece for removing unwanted material from said work piece. It is imperative that both the tool and the work piece be rigidly held with respect to each other, except for the motion introduced to allow the tool to operate on the work piece.
Alternatively, the work piece is fixed as the tool is rotated or manipulated for the above purpose.
A tool bit, which is the part of the tool effecting said removal of material, is both expensive and difficult to form properly. The tool bit is normally made of a special hardened material to allow removal of material without excessive wear or damage to the tool bit or damage to the work piece due to excessive vibration of the tool bit.
Consequently, the tool bit is made as small as is practical to minimize both material and preparation expense. However, the tool bit must not be unduly fragile, due to the possibility of breakage with high stresses, and the need to suppress vibration in the bit to avoid damage to the work piece. As disclosed above, the tool bit is normally held in a tool bit holder, which helps to minimize the material required for the bit, and allows for easier mounting and use. The size of the tool bit is determined in large part by the ability of the holder to contain the bit properly.
One type of tool bit holder useful to those in the art is the so-called chuck. The chuck is designed to hold a bit axially within the chuck, normally by means of “fingers”, or multiple extensions from within the chuck along the long axis of the bit, with the fingers being urged against the bit to secure the bit firmly. The chuck does not readily allow for any other angle than axial with the chuck for the deployment of the bit.
A simple holder, as opposed to a chuck, rigidly holds a work piece, such as other than axially with the holder, and when the work piece is rotated or otherwise manipulated around or along some axis, the tool bit in the holder is introduced to the work piece for removing unwanted material from said work piece.
Alternatively, the work piece is fixed and the holder with a tool bit is rotated or otherwise manipulated for the above purpose.
A primary purpose of the holder is to provide both a secure connection to the machine tool and a secure connection to the tool bit. Normally, the bit is inserted into the holder, such as in a groove or other type of receptacle for the bit, and is secured there by some stable means. The securing means is normally a set of set screws which thread through the holder and force the bit against an edge of the holder, the edge being formed for that purpose. Since the areas of the set screw ends are comparatively small, holding the bit securely requires substantial force on the screws, both for clamping the bit in place and for minimizing vibration of the bit. These requirements often determine a minimum size of bit that can be used without an unacceptable risk of damage to the bit or vibration of the bit. A larger bit is more expensive and may require a larger holder, which is disadvantageous, both for the expense involved and for the limitations in operating in confined spaces which are introduced thereby.
An alternative holder of the current art is the “shrink fit” holder. With the shrink fit holder, the recess in the holder is too small to allow the bit to be inserted, until the holder is heated with respect to the bit or the bit is cooled with respect to the holder, or both. The differences in the thermal expansion of the holder and the tool bit, when the holder is heated relative to the bit, allows the bit to be inserted. When the holder and bit are allowed to stabilize at the same temperature the bit is securely held in the holder.
With the shrink fit holder, a smaller bit may be used, with corresponding savings in both material cost and bit preparation expense. The smaller bit is made practical because the securing force is spread over a larger area and less force is applied to any one point on the bit. Also, the larger area of contact, relative to the size of the holder and the size of the bit, minimizes vibration.
A problem with the shrink fit holder is that it is very difficult to remove the bit from the holder when it is desired to use another bit, such as with bit wear or breakage. As a result, the shrink fit holder normally requires special equipment, or a holder must be dedicated to a specific bit. Another problem is that the process for inserting the tool bit into the holder is relatively time-consuming and expensive, since heating and cooling operations are required.
In this invention, a holder having jaws on deformable arms is provided. The jaws of the holder are of a substantial enough nature to hold the tool bit securely with a relatively large area of contact, but the holder is small compared to prior art holders for the area of contact achieved. The jaws are arranged on the arms of the holder in an opposing manner, with the arms being relatively wide in a direction parallel to both of the arms. In this way, when a bit is held, the two holder arms form a triangular structure with good stability, even with relatively flexible arms. The width of the arms provide lateral support, whereby each arm stabilizes the other and the bit is held securely thereby.
The jaws of the holder of this invention are at an extreme end of said holder, and in the initial state are so disposed as to hold the tool bit with a light touch, or “slip fit”. The two arms, or arm-like extensions, hold the jaws essentially on the end of cantilever levers, and the jaws can be slightly flexed by the user when it is desired to clamp the tool bit. The extensions so formed may be further thinned at the far end of the arms from the jaws for allowing preferential flexing at that point. The thinning at the far end of the arms is normally by removal of some of the holder material, allowing the thinning to be accomplished such as by drilling a hole through the holder parallel to said width of said arms at said far end of said arms from said jaws.
As disclosed above, the jaws are flexed together when it is desired to clamp a tool bit, such as by applying pressure on the sides of the jaws or on the sides of the arms. In a preferred embodiment, this pressure is provided by tightening a set screw mounted on said arms between said ends of said arms, the set screw forming a slip fit with a first arm, and threaded into a second arm. A head of the set screw applies the desired pressure on an outside surface of the first arm when the screw is tightened into the second arm.
While the tool bit is normally rectangular in cross-section, it will be clear that tool bits having other cross-sections can also be accommodated, such as by introducing a contour into the jaws where they meet the tool bit in order to hold, for example, a round bit.
The preferential flexing so allowed provides advantages similar to a shrink fit holder by providing a relatively large area and secure contact with a tool bit, as compared to the bit size or holder size, but further allows the tool bit to be easily inserted into or released from the holder by simply relieving the flexure, such as by loosening a set screw.
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