Patent Application
20060048615
1. Field of the Invention
This invention relates to tool shafts that are used in releasable holders, especially shanks for rotary tools such as machining cutters and drills and in stationary tools such as boring bars in lathes.
2. Description of Prior Art
Shafts of rotary tools are commonly cylindrical. A cylindrical shape can be held in a variety of holder types. However, in one of the most commonly used holders, called an end mill holder, there is a drawback to using a cylindrical shaft. This type of holder has a bore into which a tool shaft is slidably inserted. It is fixed in the holder by a clamping element. This is typically a setscrew that threads through the side of the holder and bears laterally against the shaft to pin it in place. The clamping element presses against the cylindrical surface of the shaft or against a flat portion on its surface. The shaft must be smaller in diameter than the holder bore so that the shaft can be readily inserted or removed by hand.
The prior art views of
A common method used to reduce this effect is to minimize the clearance. However, when the clearance is reduced below a certain point the shaft cannot easily be assembled into the bore by hand. Another problem with this strategy is the fact that manufacturers need practical tolerances for production of both holders and of tool shafts. Industry standards require that for a given nominal diameter, the largest diameter shaft within the tolerances will assemble with the smallest diameter holder bore. However, this means that the smallest allowable shaft will be comparatively loose when assembled with the largest allowable holder bore.
After extended use, the area 25 of the holder bore 21 opposite the clamping element 22 can become worn or deformed. This allows a cylindrical shaft to shift and vibrate even more readily under dynamic loads. This worn condition also causes the center line 27 of a shaft held in the bore to be even further displaced 34 from the bore center line 29.
Another solution to the above problems is to use a “shrink-fit holder” which has a bore very slightly smaller than the tool shaft it holds. The holder is heated, causing the bore to expand and allowing the shaft to be inserted, and then it cools and shrinks to securely grip and accurately center the shaft. Drawbacks of this system include high cost, both for the holders and for the heating system needed; heating and cooling processes slow the tool exchange process, and they can't be performed with the holder installed in the machine; Risk of burns to the operator.
U.S. Pat. No. 2,362,053 of Danielson provided a shaft with three points of contact in a bore, but it was only suitable for use in fixed bore type holder. It would not be dynamically stable or true in other types of holders such as segmented collets. Also, the Danielson shaft design had up to nineteen surfaces, making it difficult and expensive to produce.
An object of the invention is provision of a tool shaft that overcomes the inherent weaknesses of conventional cylindrical shafts when held in an end mill type holder, and provides the following advantages:
Another object is improved performance and longevity of any tool carried by this shaft. Another object is practicality of production at minimal cost.
These objects are realized in a shaft for clamping in the bore of a holder. In cross section, the circumference of the shaft is a series of arcs with different centers. These arcs intersect in sequence at their end points. Each intersection forms a slight ridge along the length of the shaft. Two of the ridges flank the point opposite the clamping element. These two ridges contact the bore, and they lie at the distance from the axis of the tool equal to the radius of the bore of the holder. Thus, they precisely locate the center of the shaft at the center of the bore. Two additional ridges on the shaft flank the clamping element. These additional ridges clear the holder bore just enough so that when the clamping element is released, the shaft can readily slide out of the bore and also can be easily re-inserted.
The invention is a tool shaft 1 for use with a working end 4 of a rotary tool such as a drill, milling cutter, or bore bar, or for use with a stationary tool such as a work-holding arbor or the like. The working end 4 of a rotary tool is driven in rotation about an axis of rotation 2 to perform cutting machining, torque transmission and the like. The shaft is designed to be inserted into, and precisely fixed in, a releasable supporting holder of a drive machine.
For this purpose the shaft is not cylindrical. It has a cross sectional circumference comprised of a series of arcs A1-A4. The curve A1 can optionally be flat. This segmented circumference extends along the portion of the shaft to be fixed in a holder. The arcs meet sequentially at their end points to form corners or edges E1-E4, two of which, E2 and E3, contact the bore 21 of the holder. A clamping element, such as a set screw 22 in the holder, presses against the midpoint of the first arc A1. This locks the shaft in and against the bore in a triangulated manner.
The simplicity of the multi-radial shaped shaft makes it very easy and inexpensive to produce. Each arc can be produced by cylindrical form grinding, in a process similar to that used to produce cams. The procedure to make a rotary cutting tool such as an end mill using the described multi-radial shape shaft is as follows:
This is similar to the process to make a tool with a conventional cylindrical shaft. However, for a conventional cylindrical shaft, the grinding wheel does not vary in distance from the center of rotation of the blank as it turns. Instead, the entire rough blank is uniformly ground to a size slightly under nominal that allows it to slip into a holder bore. The multi-radial shaft form grinding process will cost slightly more than the cylindrical grinding process, but the benefits far outweigh any additional cost.
The following sequence of steps can be used to determine the location of the ridges and arcs forming the circumference of the present multi-radial shaft that permits its releasable insertion and clamping in fixed bore holders, and in non-fixed-bore holders also.
For this discussion, IDH is the exact diameter of the holder bore. Angles are centered on the centerline C0 of the shaft in a section plane normal to the centerline. Zero degrees is on the line from C0 through the midpoint of arc 1, or at 12 o'clock in the drawings. For example, the nine o'clock position in the drawings is 270 degrees.
One of the major benefits of the present shaft is that when it is gripped in a segmented collet chuck holder or a shrink fit holder, it runs nearly true (centered). Correctly centered and solidly gripped tools are not only more productive, but they last longer as well. An additional benefit of this shape is that when it is held in a segmented collet type holder or other non-end mill type holders it runs almost perfectly true. A shaft as described in Danielson would not run acceptably true in these other holders.
It is beneficial for a cutter to run as true as possible, but virtually all collet holders, including milling chucks, have some degree of eccentricity. Some collet chuck makers claim that tools run true to within two ten thousandths of an inch in their holders, but most of them are actually less true than that, and there is little that can be done to correct the eccentricity when using a tool with a cylindrical shaft. Tapping or hammering the tool into alignment is sometimes attempted, but under load the tool can creep back to the original eccentric position.
When a multi-radial shaft according to the present invention is held in a rare, perfectly true-running collet holder, the tool will run very slightly eccentric, but to a generally acceptable degree. However, in the usual case with a slightly eccentric running collet holder, the eccentricity in the holder can be at least partially corrected by rotating the multi-radial shaft in the holder to a position where the eccentricities of the holder and shaft offset each other, thus improving the centricity of the tool. This adjustment cannot be done with cylindrical shafts or the shaft of Danielson.
Although the present invention has been described herein with respect to preferred embodiments, it will be understood that the foregoing description is intended to be illustrative, not restrictive. Modifications of the present invention will occur to those skilled in the art. All such modifications that fall within the scope of the appended claims are intended to be within the scope and spirit of the present invention.
