Patent Application
20070141962
The present disclosure relates generally to methods and related apparatuses for grinding a workpiece, and relates more particularly to a method and apparatus whereby primary and secondary relative motions of a grinding wheel are superimposed while grinding the workpiece.
Grinding, polishing and related forms of workpiece modification are integral parts of many manufacturing processes. Developing a particular surface finish, for example, is often necessary for optimal functioning and/or fitting of manufactured parts. Similarly, the abrasive removal of material from the exterior of a workpiece is often a preferred way of ensuring that components are dimensioned within required tolerances. Various sanders, grinding belts and grinding wheels are widely used for post-production modification of parts, to render the parts into a finished state suitable for an end use or further modification.
As an example, metallic gear components, such as gear wheels and gear racks, are sometimes modified by grinding following an initial forming step such as casting, milling, cutting, forging, etc. A final forming step such as grinding with a rotating grinding wheel achieves the necessary surface finish and tolerances that may be required for a particular gear application. This final grinding step can be crucial for ensuring that the gear components are capable of meshing together and operating in a desired manner. Besides using grinding as a finishing operation in gear production, grinding is also sometimes used to rough form complex gear tooth profiles may not be cost effectively rough formed with other rough forming processes such as hobbing or milling. While grinding has been shown to be quite effective and reliable in the production of gears and other types of components, there are some inherent difficulties and challenges relating to the use of grinding.
For example, a rotating abrasive grinding wheel modifies the surface of a metallic part, such as a gear, by removing tiny chips of material via abrasives adhered to the surface of the grinding wheel, or incorporated into the material matrix of which the wheel is composed. These chips tend to “clog” the grinding wheel in that they often weld or otherwise adhere to the wheel after removal from the workpiece. Upon subsequent contacting of the grinding wheel surface with the workpiece some of the abrasive material can be obscured by the adhered chips, and the grinding effectiveness can be compromised. Engineers have addressed this problem by removing the adhered material with a high-pressure fluid spray. The fluid pressure must typically be quite high to successfully remove the chips from the grinding wheel surface. It is also common to use a dressing tool to scrape off chips of material, sharpen dull grains and also to reform the exterior of the grinding wheel, as its shape may wear when material is removed from the grinding wheel itself. The scraping of the wheel is known in the art as “dressing” or “redressing.” Grinding, especially with large depth of cut requires great amounts of energy and fluid flow to support a grinding operation.
Another problem in many grinding processes relates to the heat generated by frictional forces between the rotating grinding wheel and the surface of the workpiece. The heat, which is often relatively intense, is typically dissipated in part through coolant fluid. However, the volume of fluid that must be sprayed onto the rotating grinding wheel and workpiece tends to be relatively large, often on the order of hundreds of gallons per minute for certain grinding processes. In addition, the relatively high amounts of heat that must be dissipated from the fluid require a relatively large and sophisticated heat exchanger/chiller system and often require substantial floor space and energy demand to support a grinding operation. In some instances, the fluid flow and cooling apparatus occupies more space on a factory floor than the grinding machine it supports. High heat loads further require a relatively lengthy process time for grinding a workpiece, to prevent thermal damage to the workpiece (grind burn).
Moreover, the grinding wheel itself must typically be relatively porous to allow space for chips and fluid to be transported through the contact length between the wheel and the workpiece. Higher grinding wheel porosity generally results in decreased stiffness, higher cost and a relatively shorter working life of the grinding wheel. Such grinding wheels are also typically larger making the contact length longer than what it ideally could be, and thus make the grinding forces higher requiring relatively robust bearing supports and powerful drive motors.
One known gear grinding method is discussed in U.S. Pat. No. 4,780,990 to Cody, Jr. et al. Cody, Jr. et al. describe a method and machine for forming longitudinally curved tooth surfaces in bevel and hypoid gears. The machine of Cody, Jr. et al. includes a dish-shaped grinding wheel rotated about its axis, and also about a parallel cradle axis in a timed relationship with a reciprocating work gear. Cody, Jr. et al. recognize that rotating a grinding wheel while oscillating it through an arc corresponding to desired gear tooth shape can promote coolant access between the gear and grinding wheel, however, the disclosed design and method is applicable only to certain gear types, and is relatively complex.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.
In one aspect, the present disclosure provides a method of modifying a workpiece, including grinding the workpiece with a rotating grinding wheel. The method further includes, while grinding the workpiece, moving the axis of rotation of the grinding wheel relative to the workpiece in a primary feed motion along a path oriented perpendicular to the axis, wherein moving the axis of rotation includes superimposing onto the primary motion a second, oscillatory motion of the axis relative to the workpiece.
In another aspect, the present disclosure provides a method of grinding a profiled form member, including grinding at least one groove in the member with a rotating grinding wheel. The method further includes, moving the axis of rotation of the grinding wheel relative to the member in a feed direction oriented perpendicular to the axis, including moving the axis via an oscillatory motion thereof relative to the member.
In still another aspect, the present disclosure provides a motorized apparatus for grinding a workpiece, including a workpiece holding device which holds thereon a workpiece to be modified. The apparatus further includes a grinding wheel which rotates about a first axis, the grinding wheel having a radial face and two opposed axial faces, each of the axial faces having a circumferential edge being joined to the other by the radial face. The first axis rotates about a second axis that is generally parallel to, and offset from, the first axis, wherein the workpiece holding device and the grinding wheel are moved toward one another in a direction of feed that is normal to the first axis permitting the radial face to grind against the workpiece.
Referring to
Returning to
Shaft 16 will typically be rotatable about a central axis C and journaled via a first set of bearings 22. Shaft 16 is also disposed within an eccentric 20 which is rotatable about another axis E and journaled by a second set of bearings 24. An eccentric drive motor 13 may be coupled with eccentric 20 via an output shaft 15 operable to drive eccentric 20 via a belt 17, for example. Those skilled in the art will appreciate that a variety of different drive motor configurations might be used to rotate shaft 16 and eccentric 20. Moreover, in the embodiment of
Simultaneous rotation of shaft 16 about axis C and rotation of eccentric 20 about axis E will allow grinding wheel 40 to rotate against and intermittently contact workpiece W1 while axis C travels about a path defined by the shape/eccentricity of eccentric 20. Distance h denotes a relative offset of axes C and E, in turn defining the radius of the path traversed by axis C. Offset h may be a distance less than about 0.02 inches, and may in certain embodiments be less than about 0.01 inches. In still further embodiments, offset h may be in the range of about 0.001 to about 0.005 inches. In practice, an observer may not be able to perceive the compound motion of grinding wheel 40 merely by watching the grinding process.
It will generally be desirable to set the relative offset of axes C and E to a distance that shortens the contact length and is sufficient to allow coolant to be pumped between the workpiece and grinding wheel 40, as described herein. However, if the offset between axes C and E is too large, undesirable shaking of apparatus 10 may be induced. In one contemplated embodiment, grinding wheel 40 will rotate about axis C in a first direction, and axis C will rotate about axis E in an opposite direction. In other embodiments, the directions of rotation of axis C and grinding wheel 40 may be the same.
It is contemplated that the absolute value of the ratio of the rotational speed of grinding wheel 40 about axis C to the rotational speed of axis C about axis E may be in the range of about 0.8 to 1.6, and may further be in the range of about 1.05 to 1.4. In one embodiment, the ratio of the absolute value of the rotational speed of grinding wheel 40 about axis C to the rotational speed of axis C about axis E may be about 1.2. It will generally be desirable for the rotational speed of axis C about axis E to be as fast as is practicable, to minimize the instantaneous contact length between grinding wheel 40 and workpiece W1, described herein. The maximum rotational speed of axis C about axis E will generally be limited by the capability of the associated bearings and drive motor, and in certain embodiments the surface speed of grinding wheel 40 may be in the range of about 3600 to about 4500 feet per minute. In any event, however, different rotational speeds of grinding wheel 40 itself versus the speed of axis C about axis E will be desirable. If the respective speeds are the same, substantially the same portion of grinding wheel 40 will repeatedly contact the workpiece at each revolution, resulting in uneven wear on grinding wheel 40 and reduced wheel life. In one contemplated embodiment, an appropriate orbital speed (RPM) of axis C about axis E may be determined by dividing the grinding wheel rotational RPM by 1.2.
Also shown in
Grinding wheel 40 will remove material from workpiece W1 as it rotates, and the removal of material will allow axis C of grinding wheel 40 to be moved generally along feed direction A toward workpiece W1. Arrow B represents a secondary, oscillatory motion of axis C relative to workpiece W1, resulting from the motion of axis C about its path defined by distance h. Thus, the secondary, oscillatory motion will typically be a non-linear motion, although represented in
During operation, primary loads on shaft 16 resulting from urging grinding wheel 40 against workpiece W1, i.e. moving axis C in the feed direction, will be reacted by bearings 22 and 24. This contrasts with known apparatuses (e.g. Cody, Jr. et al. discussed above) wherein primary loads from urging a grinding wheel against a workpiece are reacted by thrust bearings in a direction parallel the grinding wheel axis of rotation.
Referring now to
Turning to
The eccentric path of axis C of grinding wheel 40 will allow radial faces 43 and 45 to intermittently simultaneously contact side walls S1 and S2 of groove G. The approximate position of such contact is shown in phantom in
Turning to
In a related embodiment, existing worktable position adjustment controls may be used to move W3 in an appropriate manner. Many common grinding machines utilize Computer Number Control (CNC) to position a workpiece for grinding. For example, a grinding machine microprocessor may be programmed to adjust the left-right position of a worktable (and hence a workpiece) as well as the up-down position of the worktable in such a manner that the workpiece travels in a non-linear path. Motion along other axes of such a worktable may also be used to produce the desired relative motion between the workpiece and grinding wheel.
In either of the above embodiments involving moving the workpiece to provide the desired oscillatory motion, the direction, frequency and radius of the travel path defined by the workpiece may be similar to the embodiments described above with respect to
Referring to the drawing Figures generally, a process of grinding/modifying of a workpiece will typically take place by positioning a workpiece W1, W2 on a workpiece holding device, such as a worktable 50 as shown in
The secondary, oscillatory motion of grinding wheel 40 may be produced by rotation of eccentric 20 about axis E, causing axis C of grinding wheel 40 to orbit around axis E. This orbiting of axis C causes the outer radial face 42 of grinding wheel 40 to move generally back and forth relative to workpiece W1, W2.
The eccentric motion of grinding wheel 40, and hence the corresponding eccentric motion of its outer radial face, will facilitate the reduction of the contact length and pumping of coolant fluid between grinding wheel 40 and workpiece W1, W2, in contrast to certain earlier designs for creep feed and parallel axis gear grinding as described herein. In other words, rather than a constant contact path which is a segment of a circular line, outer radial face 42 of grinding wheel 40 will alternately plunge toward and away from workpiece W1, W2, contacting it along an arcuate contact path having a path length defined at least in part by the offset between axes C and E. A relatively larger offset may correspond generally with a relatively shorter arcuate contact path, although it should be appreciated that the contact path length may depend also on RPM and feed rate. Each time that grinding wheel 40 is brought into contact with workpiece W1, W2, it may grind a facet thereon. Continuous rotation of grinding wheel 40, and movement of axis C about its path will grind a series of adjacent facets along the contact path Y resulting in a finished surface. The length of each individual facet will be based at least in part on the offset between axes C and E, and could also be based on RPM of axis E and feed rate in the D direction. A smaller offset may generate relatively longer facets, and vice versa. A relatively faster feed rate may generate longer facets, whereas a relatively faster RPM may generate relatively shorter facets.
As grinding proceeds, and material is removed from workpiece W1, W2, workpiece W1, W2 and grinding wheel 40 will continue to be moved relative to one another in a selected feed direction. In the context of a gear rack, for example, grinding wheel 40 might be moved back and forth across the workpiece for multiple passes, each time removing another layer of material from the workpiece. The use of plural dihedral edge portions, as shown in
It is contemplated that the presently described methods and apparatuses will be particularly well suited to creep feed grinding, for example, of a gear rack or similar article, and also to parallel axis gear grinding. The present disclosure further provides advantages over known gear grinding and workpiece modifying methods using oscillatory motions which, while applicable to certain specific gear types, are unsuitable for creep feed and parallel axis grinding processes.
Moreover, the present disclosure provides substantial improvements over conventional, non-oscillatory creep feed and parallel axis gear grinding processes, which tend to be slow, produce excessive amounts of heat, and require complex and expensive apparatuses for dressing and thermal management. By superimposing a primary and a secondary, oscillatory motion of the grinding wheel axis of rotation, the contact length will be smaller than in conventional creep feed and parallel axis gear grinding. Chips removed from the workpiece will typically be thicker and shorter, resulting in lower forces between the grinding wheel and the workpiece. Chips removed from the workpiece are also easier to remove from the grinding wheel in the wheel cleaning process, as they are less likely to weld thereto. This is due at least in part to the lesser heat amount produced due to the shorter contact path of the grinding wheel with the workpiece. The relatively shorter contact path of the grinding wheel with the workpiece and its intermittent contact therewith also better enables the chips to be cleared from the contact zone as compared from conventional creep feed and parallel axis gear grinding. The shorter contact length also provides for better cooling given the larger amount of space for coolant to pass between the grinding wheel and workpiece, and the coolant pumping action of the grinding wheel as it oscillates.
Increased coolant delivery to the contact zone between the workpiece and the grinding wheel also prevents overheating of the surface of the part, drastically reducing the risk of grinder burn. The grinding wheels employed in apparatuses and processes according to the present disclosure may also be made less porous than in certain earlier designs, as the amount of fluid that needs to be passed through the grinding wheel is reduced. This allows a more durable grinding wheel to be used, capable of spinning at higher speeds and having a longer working life. The more durable grinding wheel, coupled with easier chip removal can reduce the need for dressing of the grinding wheel. Thus, only intermittent contact while grinding, or periodic treatment with a dressing roller while the eccentric is locked, for example, may be sufficient to maintain grinding wheel 40. Still further advantages relate to smaller and less costly supporting hardware, including a reduction in the necessary grinding shaft power, including a smaller shaft motor, and better management of loads on the shaft, as well as a greater overall apparatus working life given the lower stress and heat levels.
Faster processing time of parts is also made possible by the present disclosure, as less heat goes into the workpiece, reducing the risk of grind burn. In the context of parallel axis gears, there is less time required between grinding of the first and last tooth, increasing tooth spacing quality due to a reduction in thermal differences among the teeth during grinding. Cold working of the part is also possible via the application of the grinding wheel at lower temperatures. Where the subject gear or other part is ground with a relatively cooler grinding wheel, compressive stresses can be introduced into the surface zone of the part, analogous to pre-stressing cold forming processes for metallic components known from related technical fields.
R1 illustrates a theoretical radius of grinding wheel 40. In other words, R1 may be thought of as the radius of a circle swept by the outer radial face (or edge) of grinding wheel 40 as it makes a complete rotation via rotation of the grinding wheel itself and the secondary, oscillatory motion of axis C. R2 represents an actual radius of grinding wheel 40. Line V represents an approximate depth of grind of grinding wheel 40 in workpiece W, whereas Y denotes the arcuate contact path of grinding wheel 40 with workpiece W without the oscillation motion. Arrow Z refers to a direction of rotation of axis C, whereas arrow T identifies the direction of rotation of grinding wheel 40. Although grinding wheel rotation is shown opposite to the rotation direction of axis C, it should be appreciated that the present disclosure is by no means thereby limited. Arrow A represents the feed direction of workpiece W relative to grinding wheel 40. In the embodiment shown in
As represented in
The present description is for illustrative purposes only, and should not be construed to narrow the breadth of the present disclosure in any manner. Thus, those skilled in the art will appreciate that various modifications might be made to the presently disclosed embodiments without departing from the intended spirit and scope of the present disclosure. For instance, while much of the foregoing description relates to grinding processes for metallic gears, the present disclosure is by no means thereby limited. A multiplicity of other workpieces and workpiece modification processes may benefit through the application of the present disclosure. Rather than a gear grinding process, the present disclosure contemplates an apparatus and method for grinding/modifying a surface on other types of workpieces. Further, the workpiece need not be metallic but instead might be wood, a composite or some other type of material, and need not be a geared member but could be some other type of profiled member or a member that is not profiled at all. Other aspects, features and advantages will be apparent upon an examination of the attached drawings and appended claims.
