Machine for machining gear teeth and gear teeth machining method

A machine for machining a workpiece having a central longitudinal axis is provided. The machine includes a chuck or fixture on which the workpiece is disposable, a grinding spindle to remove material from the workpiece, the grinding spindle having a central longitudinal axis about which the grinding spindle rotates and being disposed with the central longitudinal axes intersecting one another so as to create a continuous gear tooth on the workpiece and an electrochemical grinding (ECG) element configured to execute ECG processing on the grinding spindle and the workpiece to soften the workpiece as the gear tooth is being created by the grinding spindle.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to a machine and a machining method and, more particularly, to a machine for machining or grinding gear teeth and to a gear teeth grinding method.

Gears are used in various industrial and technological applications to permit power transmission from one rotating or translating element to another. Each gear generally includes an array of gear teeth that mesh with the gear teeth of another gear so that the rotation or translation of the first gear can be transmitted to the second. The shapes of the gear teeth can be varied with some gear teeth being linearly shaped, some being helically shaped and others being provided as double-helical or herringbone shaped, and still others being provided as arcuate shaped (or C-Gear) gear teeth.

Gears having gear teeth that are double helically (or herringbone) shaped include a side-to-side (not face to face) combination of two helical gears of opposite hands and, from a top-wise viewpoint, the helical grooves form a V formation with an apex in the middle. Whereas helical gears tend to produce axial loading, a side-thrust of one half of each gear is balanced by that of the other half. This means that gears having double helical or herringbone shaped gear teeth can be used in torque gearboxes without requiring a substantial thrust bearing. Gears having arcuate shaped teeth may also have self-aligning characteristics, which eliminate axial loads with the added benefit of reducing gear tooth end loading due to their inherent ability to adapt to axis misalignment.

However, while these shape gears are desired, due to manufacturing limitations, such gears can only be partially formed. Specifically, current manufacturing techniques use a large grinding wheel which forces a gap to be designed at the apex of the V formation since, when forming one tooth of the V formation, the grinding wheel would otherwise collide with the other tooth of the V formation. Thus, when using a grinding wheel, a true V formation is not formed since a space is required between adjacent teeth to allow for the size of grinding wheel. Further, as these wheels only provide straight line grooves, the resulting teeth are limited to linear shapes. Conversely, while non-wheel precision grinding shapes might allow more complex shapes such as curved lines, these non-wheel shapes do not allow for teeth production at a speed to be economical to create gears in a manufacturing setting. As such, there is a need for a grinding methodology which allows for the creation of gapless double helical/herringbone gear shapes and is sufficiently robust to be used in a manufacturing setting.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a machine for machining a workpiece having a central longitudinal axis is provided. The machine includes a chuck or fixture on which the workpiece is disposable, a grinding spindle to remove material from the workpiece, the grinding spindle having a central longitudinal axis about which the grinding spindle rotates and being disposed with the central longitudinal axes intersecting one another so as to create a continuous gear tooth on the workpiece and an electrochemical grinding (ECG) element configured to execute ECG processing on the grinding spindle and the workpiece to soften the workpiece as the gear tooth is being created by the grinding spindle.

According to another aspect of the invention, a gear including at least one of apex gap-less double-helical shaped teeth, apex gap-less herringbone shaped teeth and c-shaped teeth is provided and is machined by a process. The process includes disposing a workpiece having a central longitudinal axis on a chuck, disposing a grinding spindle having a central longitudinal axis such that the the central longitudinal axes of the workpiece and the grinding spindle intersect, executing ECG processing on the grinding spindle and the workpiece and using the grinding spindle to remove material from the workpiece.

According to yet another aspect of the invention, a method of machining a gear is provided and includes disposing a workpiece having a central longitudinal axis on a chuck, disposing a grinding spindle having a central longitudinal axis about which the grinding spindle rotates such that the central longitudinal axes of the workpiece and the grinding spindle intersect, executing ECG processing on the grinding spindle and the workpiece to soften an area of the workpiece and using the grinding spindle to remove material from the area of the workpiece to create a continuous gear tooth.

DETAILED DESCRIPTION OF THE INVENTION

In helicopter transmission design, transmission weight reduction is of considerable importance. Thus, since the gears inside a transmission are normally the heaviest components in a drive system, reducing gear size and numbers of gears can be useful in reducing transmission weight and volume. As will be described below, gear size reductions can be achieved by eliminating extraneous gear features, such as apex regions in a double helical (or herringbone) gear. Normally, such extraneous gear features are forced into use by manufacturing limitations.

With reference toFIG. 1, a conventional double helical gear1is provided. The conventional double helical gear1includes a first side2having a helical gear pattern of a first hand, a second side3having a helical gear pattern of a second hand opposite the first hand and an apex gap4defined axially between the first and second sides. The double helical gear1has a relatively high gear contact ratio owing to the presence of the helical gear patterns of the first and second sides2and3. As a result, a gear mesh of the double helical gear exhibits increased strength and reduced noise signature as compared to that of a straight spur gear. The apex gap4is formed as a result of processes used to shape and precision grind the gear flanks and roots of the helical gear teeth. The apex gap4may add a considerable weight and size penalty to an overall transmission system in which the double helical gear1resides.

As will be described below, a gear grinding machine is provided and incorporates the use of a high speed grinding spindle with its center axis intersecting a center axis of the gear. Electrochemical grinding (ECG), and super abrasives, such as cubic boron nitride (CBN), may be utilized in a creep feed, deep cut, grinding process allowing for almost any conceivable gear flank design. The gear grinding machine produces hyper smooth ground surfaces of less than 1 micro inch Ra, burr free edges, with low heat generation and has the ability to grind exotic high hardness conductive materials. ECG allows for a very small grinding wheel with extremely low tool wear.

With reference toFIGS. 2 and 3, a machine10is provided for machining a workpiece11. The workpiece11may have a substantially cylindrical initial shape with a first central longitudinal axis110. The machine10includes a chuck or fixture20on which the workpiece11is disposable, and a grinding spindle30. The grinding spindle30is configured to remove material from the workpiece11and has an elongate shape with a second central longitudinal axis300. The grinding spindle30is disposable relative to the chuck20and the workpiece11such that the first and second central longitudinal axes110and300may or may not intersect one another. The machine10further includes an electrochemical grinding (ECG) element40, which is configured to execute ECG processing on the grinding spindle30and the workpiece11.

As shown inFIG. 2, the grinding spindle30may include a wheel31, a spindle body32and an insulator33. The wheel31is disposed to be rotatable about the second central longitudinal axis300of the grinding spindle30and includes a main wheel portion310, which extends axially outwardly from an end of the spindle body32, and a tip311defined at a distal end312of the main wheel portion310. Abrasive34may be attached to the tip311. The spindle body32is disposed to drive rotation of the wheel31about the central longitudinal axis300of the grinding spindle30and the insulator33is disposed to electrically insulate the wheel31from the spindle body32.

In accordance with embodiments, the abrasive34may include a super abrasive, such as cubic boron nitride (CBN), diamond, etc. In addition, the tip311may be pencil-shaped or substantially conical and may have an involute profile313. That is, an outer surface of the tip311may curve inwardly from an edge of the main wheel portion310with a radius of curvature that decreases with increasing axial distance from the edge of the main wheel portion310. At the axial end of the tip311, the radius of curvature may flip direction such that the end-most portion of the tip311has a blunt, rounded surface.

The ECG element40includes a first electrical lead41, a second electrical lead42and a dispenser43. The first electrical lead41is configured to positively charge the workpiece (anode)11, the second electrical lead42is configured to negatively charge the grinding spindle (cathode)30and the dispenser43is configured to dispense electrolytic fluid430toward the workpiece11. The opposite electrical charging of the workpiece11and the grinding spindle30in combination with the dispensation of the electrolytic fluid430toward the workpiece11causes a material of the workpiece11to soften by a substantial degree. This softening permits the grinding spindle30to remove material from the workpiece11in various forms or configurations. In some cases, the softening facilitates removal of material from the workpiece11by the grinding spindle to a desired depth in only a single pass and more rapidly than could be done without the softening.

The machine10further includes a machine body50and a controller51. The machine body50may be provided, for example, as one or more support structures500and robotic arms501that are coupled to the chuck10, the grinding spindle30and the ECG element40to position the various elements with respect to one another for grinding internal or external gears. The controller51may be provided as a computer numerical control (CNC) element. Where the controller51is provided as the CNC element, the machine body50is formed to define four axes (e.g., rotational axis B and spatial axes X, Y, Z, as shown inFIG. 3) and is capable of performing multi-axis synchronous motion. The axes may include the rotary axis B for indexing the workpiece11, the vertical axis Y running parallel to the first central longitudinal axis110of the workpiece11(i.e., a cutter path), the horizontal axis X for centrality adjustments between the wheel31of the grinding spindle30and the workpiece11and the fore and aft axis Z to control a cutting depth of the grinding spindle30. In accordance with the embodiments, the ECG element40may be integral the machine body50and the controller51.

With reference toFIGS. 4 and 5, it is to be understood that the machine10can be employed to machine a gear with outwardly facing gear teeth (seeFIGS. 3 and 4) or inwardly facing gear teeth (seeFIG. 5). In the latter case, as shown inFIG. 5, the robotic arms501may include a hook structure502. The hook structure502extends forwardly along the fore and aft axis Z (seeFIG. 3) from the robotic arm501, downwardly along the vertical axis Y (seeFIG. 3) and then reversely along the fore and aft axis Z. The grinding spindle30is disposed at the distal end of the hook structure502.

With the machine10provided as described above, the workpiece11may be ground or cut by the grinding spindle30in various forms and configurations. For example, the grinding spindle30may provide the workpiece11with gear teeth in one or more of an apex gap-less double-helical shape (or an apex gap-less herringbone shape) and a c-shape.

An example of a gear60that can be formed by the machine10to have gear teeth in an apex gap-less double-helical shaped formation is shown inFIGS. 6, 7 and 8. The gear60includes a body61defining a central longitudinal axis that would be aligned with the second central longitudinal axis300, first and second opposite axial faces62,63and a circumferential face64. The circumferential face64is formed by the machine10and includes a first annular array65of helical gear teeth651and helical gear lands652of a first hand and a second annular array66of helical gear teeth661and helical gear lands662of a second hand, which is oppositely oriented with respect to the first hand. The first and second annular arrays65and66converge such that each helical gear tooth651abuts a corresponding helical gear tooth661and each helical gear land652abuts a corresponding helical gear land662.

As shown inFIGS. 6, 7 and 8, the abutment of each helical gear tooth651with the corresponding helical gear tooth661and of each helical gear land652with the corresponding helical gear land662may be achieved with little to no apex region defined between the first and second annular arrays65and66and without an interruption in the respective shapes of the helical gear teeth651,661or the helical gear lands652,662in the region of the abutment (seeFIGS. 6 and 8). Also, a shape of the helical gear teeth651,661may be reflective of the tip311of the grinding spindle30(seeFIG. 7). In addition, it is to be understood that although the gear60is illustrated inFIGS. 6 and 7with the region of the abutment being linear and axially centered, other embodiments exist. For example, the region of the abutment may be offset from an axial center of the gear60and the abutment itself may be staggered relative to the axial center of the gear60.

With reference toFIG. 9, another example of a gear70that can be formed by the machine10is provided. The gear70has gear teeth in an apex gap-less c-shaped formation. The gear70includes a body71defining a central longitudinal axis that would be aligned with the second central longitudinal axis300, first and second opposite axial faces72,73and a circumferential face74. The circumferential face74is formed by the machine10and includes an annular array75of c-shaped gear teeth751and c-shaped gear lands752. Each c-shaped gear tooth751has an arcuate face with nearly constant involute transverse profiles corresponding to the shape of the tip311of the grinding spindle. In practice, this configuration would be expected to provide a nearly constant pressure angle across the length of the c-shaped gear tooth751. Like the gear60, the gear70would possess a self-alignment characteristic but with the added benefit of reducing gear tooth end loading due to their inherent ability to adapt to multiple axis misalignment.

With reference toFIG. 10, a method of machining gear teeth such as the gear teeth described above is provided. As shown inFIG. 10, the method first includes a rough grinding of the gear teeth from a solid, such as a workpiece, using the ECG pencil grinding method described above (operation100). Once the rough grinding is fully or partially completed, the method further includes a carburization and hardening of the workpiece (operation101) and a finishing grind of the gear teeth using the ECG pencil grind method described above (operation102). Of course, it will be understood that the ECG grinding method of operations100and102need not be limited to the ECG pencil grinding method and can be replaced by an ECG grinding method designed to form any tooth shape (e.g., a tooth shape that is reflective of the tip311of the grinding spindle30having an involute profile313).

With the machine10, gearboxes for helicopters and other weight limited applications, may be designed with higher power densities. This is due to the fact that every pound of weight that is removed from a transmission design as a result of using the machine10to fashion gears with apex-less configurations translates into better performance characteristics.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. By way of example, while described in the context of gearboxes used in power dense environments, aspects of the invention can be used to create intermeshing gears in other contexts, such as clock machinery, elevator machinery without limitation. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.