Laser Rounding and Flattening of Cylindrical Parts

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims priority from United States Patent Application No. 11/188,158 filed July 22, 2005.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH [0002] Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0003] The present invention relates to a method for rounding and flattening hollow cylindrical parts which are out of round or are not flat due to non-uniform internal stresses.

2. Description of the Related Art

[0004] It is well known that steel parts can distort after heat treatment due to internal stresses created in the part during the heat-treat process. For instance, when a carbon steel part is quenched from above the austenitizing temperature, martensite is formed. The transformation of austenite to martensite is accompanied by an expansion in volume. As a result of the volume expansion, internal stresses are induced into the part. Any irregularities in the internal stresses can cause part distortion. In hollow cylindrical steel parts, the distortion can cause the parts to go out-of-round or cause the parts to lose flatness similar to a potato chip. The distortion is either tolerated in the application for the part or commonly the distortion is honed or ground out of the part at great expense. Thus, there is a need for a more cost effective method for rounding and flattening hollow cylindrical parts which are out of round and/or are not flat. [0005] Methods have been proposed for straightening truck structural members. Methods have also been proposed for straightening out of true shafts. Prior methods have been used where bent heat-treated shafts are straightened by back-bending. Methods for truing out bent shafts have been used wherein forces are applied to a bent shaft in a locally limited area, whereby these forces are sufficient to locally strengthen the shaft to cause compressive residual stress in a surface layer zone of the shaft for reducing the out of true bending of the shaft. [0006] The compressive residual stress may be generated in the surface layer zone of the bent shaft in various manners. Examples of means that have been

known for achieving compressive residual stress in a surface layer zone include case hardening, induction hardening, laser beam hardening, nitridation, and deep rolling. The compressive residual stresses are induced only in a surface layer zone of the shaft such that the induced compressive residual stresses cause a corresponding deformation of the shaft. The direction of this deformation depends on which specific surface areas of the shaft have induced compressive residual stresses. It is reported that in order to achieve a desired truing effect, the compressive residual stresses should be induced in the shaft in a defined locally bounded area. This may be achieved by means of a locally limited hardening process, or by means of a locally limited deep rolling operation. [0007] Case hardening in known methods required selective masking of the shaft to prevent surface portions of the shaft which must remain non-hardened from becoming hardened in the case hardening process. The case hardening method of hardening is both energy and labor intensive, and is therefore quite expensive. Nitridation suffers from similar drawbacks. In induction hardening, the shaft to be hardened is placed inside a coil through which a rapidly alternating current is flowing. In this method, it may also be difficult to prevent surface portions of the shaft which must remain non-hardened from becoming hardened in the induction hardening process. As such, induction hardening is also expensive and time-consuming. Deep rolling operations require complicated equipment and therefore, are also quite expensive.

[0008] Therefore, while methods have been proposed for straightening out of true shafts, there exists a need for more cost effective methods for rounding and flattening hollow cylindrical parts which are out of round and/or are not flat.

SUMMARY OF THE INVENTION

[0009] The present invention meets the foregoing needs by providing a method for rounding and/or flattening an annular part that is out of round and/or not flat due to non-uniform internal stresses typically caused by heat treatment. In the method, the annular part is rounded or flattened by introducing compressive stresses into selected areas on the lower surface, the inside diameter, or the outside diameter of the annular part such that the introduced compressive stresses cause deformation of the annular part thereby rounding and/or flattening the annular part.

[0010] In one aspect, the invention provides a method for rounding an annular part having an axis and having an inside surface and an outside surface wherein the part is out of round due to non-uniform internal stresses typically caused by heat treatment. In this method, it is first determined where the annular part is out of round. This can be done by measuring distances along reference lines from the axis of the annular part to corresponding sections of the inside surface of the annular part associated with each reference line. The annular part will be most out of round at a section of the inside surface furthest from the axis. Therefore, the method includes the step of identifying a first section of the inside surface of the annular part that is a greater distance from the axis than a second section of the inside surface. Compressive stresses are then introduced into the first surface section of the inside surface, whereby the introduced compressive stresses cause deformation of the annular part thereby rounding the annular part. The compressive stresses may be introduced into the selected sections of the annular part using laser heating, laser heating and quenching, induction heating, induction heating and quenching, laser shock peening, and shot peening methods. Preferably, each cross section in a part processed according to this aspect of the invention has balanced stress symmetric about the centroid after treatment such that the part will be round. In one version of the invention, hoop stresses are calculated in surface sections of the outside surface using finite element analysis to create a laser power pattern to apply to the inner surface sections. In one example, the laser power is increased when applied in the direction of outer surface sections having greater calculated stress than adjacent sections of the outer surface.

[0011] In another aspect, the invention provides a method for rounding an annular part having an axis and having an inside surface and an outside surface wherein the part is out of round due to non-uniform internal stresses typically caused by heat treatment. In this method, it is first determined where the annular part is out of round. This can be done by measuring distances along reference lines from the axis of the annular part to corresponding sections of the outside surface of the annular part associated with each reference line. The annular part will be most out of round at a section of the outside surface furthest from the axis. Therefore, the method includes the step of identifying a first section of the outside

surface of the annular part that is a greater distance from the axis than a second section of the outside surface. Compressive stresses are then introduced into a surface section of the inside surface that is opposite the first section of the outside surface, whereby the introduced compressive stresses cause deformation of the annular part thereby rounding the annular part. The compressive stresses may be introduced into the selected sections of the annular part using laser heating, laser heating and quenching, laser shock peening, induction heating, induction heating and quenching, and shot peening methods. Preferably, each cross section in a part processed according to this aspect of the invention has balanced stress symmetric about the centroid after treatment such that the part will be round. [0012] In still another aspect, the invention provides a method for rounding an annular part having an axis and having an inside surface and an outside surface wherein the part is out of round due to non-uniform internal stresses typically caused by heat treatment. In this method, it is first determined where the annular part is out of round. This can be done by measuring distances along reference lines from the axis of the annular part to corresponding sections of the outside surface of the annular part associated with each reference line. The annular part will be most out of round at a section of the outside surface furthest from the axis. Therefore, the method includes the step of identifying a first section of the outside surface of the annular part that is a greater distance from the axis than a second section of the outside surface. Compressive stresses are then introduced into at least one surface section of the outside surface other than the first section of the outside surface, whereby the introduced compressive stresses cause deformation of the annular part thereby rounding the annular part. The compressive stresses may be introduced into the selected sections of the annular part using laser heating, laser heating and quenching, laser shock peening, induction heating, induction heating and quenching, and shot peening methods. Preferably, each cross section in an annular part processed according to this aspect of the invention has the same internal stress distribution around the part after treatment such that the part will be round.

[0013] In yet another aspect, the invention provides a method for flattening an annular part having an axis and a reference plane normal to the axis and having a first end surface and a second end surface wherein all points on the first end

surface are not equidistant from the reference plane due to non-uniform internal stresses typically caused by heat treatment. In this method, it is first determined where the annular part is not flat. This can be done by measuring distances along normal reference lines from the reference plane to surface sections of the first end surface associated with each normal reference line. The part will be most out of flat at a section of the first end surface furthest from the lower reference plane. Therefore, the method includes the step of identifying a first surface section of the first end surface that is a greater distance from the reference plane than a second section of the first end surface. Compressive stresses are then introduced into a surface section of the second end surface opposite the first surface section of the first end surface whereby the introduced compressive stresses cause deformation of the annular part thereby flattening the annular part. The compressive stresses may be introduced into the selected sections of the annular part using laser heating, laser heating and quenching, laser shock peening, induction heating, induction heating and quenching, and shot peening methods. Preferably, each cross section in an annular part processed according to this aspect of the invention has the same internal stress distribution around the part after treatment such that the part will be flat.

[0014] In still another aspect, the invention provides a method for rounding an annular part having an axis and having an inside surface and an outside surface wherein the part is out of round due to non-uniform internal stresses typically caused by heat treatment. In this method, it is first determined where the annular part is out of round. This can be done by measuring distances along reference lines from the axis of the annular part to corresponding sections of the outside surface of the annular part associated with each reference line. The annular part will be most out of round at a section of the outside surface furthest from the axis. Therefore, the method includes the step of identifying a first section of the outside surface of the annular part that is a greater distance from the axis than a second section of the outside surface. Compressive stresses are then relieved in the first surface section of the outside surface, whereby relieving the compressive stresses causes deformation of the annular part thereby rounding the annular part. The compressive stresses may be relieved by laser tempering the first surface section of the outside surface.

[0015] In yet another aspect, the invention provides a method for flattening an annular part having an axis and a reference plane normal to the axis and having a first end surface and a second end surface wherein all points on the first end surface are not equidistant from the reference plane due to non-uniform internal stresses typically caused by heat treatment. In this method, it is first determined where the annular part is not flat. This can be done by measuring distances along normal reference lines from the reference plane to surface sections of the first end surface associated with each normal reference line. The part will be most out of flat at a section of the first end surface furthest from the lower reference plane. Therefore, the method includes the step of identifying a first surface section of the first end surface that is a greater distance from the reference plane than a second section of the first end surface. Compressive stresses are then relieved in the first surface section of the first end surface, whereby relieving the compressive stresses causes deformation of the annular part thereby flattening the annular part. The compressive stresses may be relieved by laser tempering the first surface section of the first end surface.

[0016] It is therefore one advantage of the invention to provide a method for rounding an annular part that is out of round due to non-uniform internal stresses wherein compressive stresses are introduced into a surface section of the inside surface of the annular part opposite a section of the outside surface of the annular part that has internal stresses typically caused by the heat treatment. [0017] It is another advantage of the invention to provide a method for rounding an annular part that is out of round due to non-uniform internal stresses wherein compressive stresses are introduced into at least one surface section of the outside surface of the annular part other than a first section of the outside surface of the annular part that has internal stresses typically caused by the heat treatment.

[0018] It is yet another advantage of the invention to provide a method for flattening an annular part that is not flat due to non-uniform internal stresses wherein compressive stresses are introduced into a surface section of the second end surface of the annular part opposite a section of the first end surface of the annular part that has internal stresses typically caused by the heat treatment.

[0019] It is still another advantage of the invention to provide a method for rounding an annular part that is out of round due to non-uniform internal stresses wherein compressive stresses are relieved by tempering a first section of the outside surface of the annular part that has internal stresses.

[0020] It is yet another advantage of the invention to provide a method for flattening an annular part that is not flat due to non-uniform internal stresses wherein compressive stresses are relieved by tempering a surface section of an end surface of the annular part that has internal stresses.

[0021] These and other features, aspects, and advantages of the present invention will become better understood upon consideration of the following detailed description, drawings, and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 shows a top plan view of an out of round annular part before processing in accordance with a method of the invention. [0023] Figure 2 shows a side elevational view of an annular part that is not flat before processing in accordance with a method of the invention. [0024] Figure 3 shows a top perspective view of a half model of a ring gear that is out of round.

[0025] Figure 4 shows a top perspective view of a half model of a ring gear similar to the ring gear of Figure 3, wherein the ring gear of Figure 4 is not flat. [0026] Figure 5 shows a ring gear used in tests that illustrate the method of the invention.

[0027] Figure 6 shows a Watts vs. Strain Factor graph used to calculate laser power levels in a test that illustrates the method of the invention. [0028] Figure 7 shows a laser power pattern used in a test that illustrates the method of the invention.

[0029] Figure 8 is a graph comparing the radius of an untreated part and the same part treated with a method of the invention.

[0030] Figure 9 shows a Watts vs. Strain Factor graph used to calculate laser power levels in another test that illustrates the method of the invention. [0031] Figure 10 is another graph comparing the radius of an untreated part and the same part treated with a method of the invention.

[0032] Figure 11 shows a laser power pattern used in another test that illustrates the method of the invention.

[0033] Figure 12 is yet another graph comparing the radius of an untreated part and the same part treated with a method of the invention.

[0034] Figure 13 shows a laser power pattern used in another test that illustrates the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0035] There are many different ways of heat-treating metallic parts, and heat- treating processes can cause internal stresses in metallic annular parts. For example, in one steel heat-treating process, a carbon steel part is heated above the austenitizing temperature at which the ferrite transforms into austenite, and the part is rapidly quenched such that harder martensite is formed. In quenching, the face centered cubic austenite spontaneously changes to body centered martensite which results in an increase in the volume of the part. As a result of the volume change, internal stresses are induced into the part. Any irregularities in the martensitic volume change results in irregularities in internal stress that can cause part distortion. For instance, in hollow cylindrical steel parts, the distortion can cause the parts to go out-of-round or cause the parts to lose flatness similar to a potato chip.

[0036] Carbon steel annular parts may be thru hardened, or may be surface hardened, as in case hardening where carbon is allowed to diffuse into surface sections of a steel part and the steel part is heated and quenched to form a surface layer of hard martensite. Also, carbon steel annular parts may be locally hardened in certain areas like the inside diameter or outside diameter using, for example, case hardening masking techniques or a laser. Any irregularities in the martensitic volume change in thru hardening, surface hardening or local hardening results in irregularities in internal stress that can cause part distortion. [0037] In the present invention, after an annular part is heat-treated (and possibly tempered at a low temperature), the part is checked for out of round and/or out of flat conditions due to non-uniform internal stresses typically caused by the heat treatment. Out of round parts are then rounded by introducing compressive stresses into selected surface sections of the part whereby the introduced compressive stresses cause deformation of the annular part thereby

rounding the annular part. Alternatively, out of round parts are rounded by relieving compressive stresses in selected surface sections of the part whereby the relieving of compressive stresses causes deformation of the annular part thereby rounding the annular part. Out of flat parts are flattened by introducing compressive stresses into selected surface sections of the part whereby the introduced compressive stresses cause deformation of the annular part thereby flattening the annular part. Alternatively, out of flat parts are rounded by relieving compressive stresses in selected end surface sections of the part whereby the relieving of compressive stresses causes deformation of the annular part thereby flattening the annular part. While out of round and/or out of flat conditions due to non-uniform internal stresses are typically caused by the heat treatment, the invention is not limited to correcting out of round and/or out of flat conditions caused by the heat treatment.

[0038] An example method according to the invention for rounding an out of round annular part, such as a ring, can be explained with reference to Figure 1. In Figure 1, there is shown a top plan view of a heat treated out of round annular part before processing in accordance with a method of the invention. The annular part 10 has an inside surface 13, an outside surface 21 , an upper surface 27 and an axis A . Also shown in Figure 1 are a circular dashed line 15 that represents the inside surface of a perfectly round part and a circular dashed line 23 that represents the outside surface of the same perfectly round part. The perfectly round part has the same axis A as annular part 10. The deviation of the inside surface 13 of the annular part 10 from dashed line 15 and the deviation of the outside surface 21 of the annular part 10 from dashed line 23 shows that annular part is out of round due to non-uniform internal stresses typically caused by the heat treatment.

[0039] One method according to the invention for rounding the annular part 10 proceeds as follows. First, distances are measured along reference lines from the axis A of the annular part 10 to sections of the outside surface 21 of the annular part 10 wherein each section of the outside surface 21 is associated with a reference line. These measurements can be made using conventional measuring equipment such as an optical comparator or a vision system. Looking at Figure 1, imaginary reference lines R ₁ , R ₂ and R3 are shown that extend from the axis A to

corresponding sections OS ₁ , OS ₂ , OS ₃ , of the outside surface 21 respectively. Of course, an infinite number of reference lines are possible that correspond to an infinite number of corresponding sections of the outside surface 21 of the annular part 10. If annular part 10 were perfectly round, imaginary reference lines R ₁ to R _n corresponding to all sections OSi to OS _n Of the outside surface 21 would be equal. In the method, at least one of the reference lines R ₁ , R ₂ and R ₃ should extend beyond the circular dashed line 23 that represents the outside surface of the perfectly round part.

[0040] Still referring to Figure 1 , the length of reference lines R ₁ , R ₂ and R ₃ is then compared, and the section OS ₁ , OS ₂ , OS ₃ of the outside surface 21 that most outwardly deviates from perfectly round condition will be the section that corresponds to the longest reference line. For example, in Figure 1 , reference line R ₁ has the greatest length and therefore, section OS ₁ of the outside surface 21 deviates most outwardly from perfectly round condition in comparison to sections OS ₂ , OS ₃ of the outside surface 21. It can be appreciated that if an infinite number of reference lines corresponding to sections of the outside surface 21 of the annular part 10 were used, the imaginary reference line having the longest length would correspond to the section of the outside surface 21 that most outwardly deviates from perfectly round condition.

[0041] Because section OS ₁ of the outside surface 21 deviates more outwardly from perfectly round condition in comparison to sections OS ₂ , OS ₃ of the outside surface 21 , annular part 10 can be rounded by introducing compressive stresses into surface section IS ₁ of the inside surface 13 shown in Figure 1. The surface section IS ₁ of the inside surface 13 has a perimeter IS _P surrounding the intersection of the reference line R ₁ and the inside surface 13 of the annular part 10 and preferably, compressive stresses are introduced within perimeter IS _P of the inside surface 13. Most preferably, the compressive stresses are introduced into a surface section of the inside surface that is 180 degrees opposite from the section of the outside surface corresponding to the reference line having the longest length.

[0042] Alternatively, distances can be measured along reference lines from the axis A of the annular part 10 to sections of the inside surface 13 of the annular part 10 wherein each section of the inside surface 13 is associated with a

reference line. When a section of the inside surface 13 deviates more outwardly from perfectly round condition in comparison to other sections of the inside surface 13, annular part 10 can be rounded by introducing compressive stresses into the surface section of the inside surface 13 that deviates more outwardly from perfectly round condition.

[0043] The compressive stresses are introduced into surface section ISi of the inside surface 13 so that the internal stress is uniform in the cross section of the annular part 10 between section OS ₁ of the outside surface 21 and surface section ISi of the inside surface 13. The introduced compressive stresses in surface section IS ₁ of the inside surface 13 cause deformation of the annular part thereby rounding the annular part. The rounding magnitude of the annular part 10 is dependent on how strong the introduced compressive stresses are and how deeply the introduced compressive stresses reach into surface section ISi of the inside surface 13 of the annular part 10. However, in order to achieve a desired rounding effect, the compressive stresses should be introduced into the inside surface 13 of the annular part 10 in a defined locally bounded area. For example, it may be necessary to introduce compressive stresses within 90 degrees of the surface section IS ₁ of the inside surface 13 of the annular part 10. [0044] The compressive stresses may be introduced into surface section IS ₁ of the inside surface 13 using different methods. For instance, an industrial laser or similar beam of radiation can be used for rapid heating of the surface section IS-i of the inside surface 13. The laser induces strain into the surface section IS ₁ of the inside surface 13 by very locally heating the section. The thermal expansion from the heat plastically strains the surface section IS ₁ resulting in a change in the internal stress distribution.

[0045] When the annular part 10 comprises a carbon steel, an industrial laser or similar beam of radiation can be used for rapid heating of the surface section ISi such that energy transfers from the laser beam to heat energy within the surface section ISi. By using a laser to quickly heat the surface section IS-i, the surface section IS-i will self-quench forming martensite, due to the extremely high heat differential between the shallow surface section IS ₁ heated by the laser and the bulk of the annular part 10. The transformation of austenite to martensite in the surface section IS ₁ is accompanied by an expansion in volume. As a result of

the volume expansion, stresses are induced into the surface section [S ₁ of the annular part 10 such that the internal stress is uniform in the cross section of the annular part 10 between section OSi of the outside surface 21 and surface section ISi of the inside surface 13. Alternatively, the surface section IS ₁ may be heated using a high frequency induction heating system.

[0046] The compressive stresses may be introduced into surface section ISi of the inside surface 13 using laser shock peening. Multiple radiation pulses from a high power pulsed laser are used to produce shock waves on surface section ISi of the annular part 10 similar to methods disclosed in U.S. Patent Nos. 5,131,957, 4,401,477 and 3,850,698. Alternatively, the compressive stresses may be introduced into surface section ISi of the inside surface 13 using shot peening. [0047] Another method according to the invention for rounding the annular part 10 proceeds as follows. First, distances are measured along reference lines from the axis A of the annular part 10 to sections of the outside surface 21 of the annular part 10 wherein each section of the outside surface 21 is associated with a reference line. These measurements can be made using conventional measuring equipment such as an optical comparator or a vision system. Looking at Figure 1 , imaginary reference lines R ₁ , R ₂ and R ₃ are shown that extend from the axis A to corresponding sections OSi, OS ₂ , OS ₃ , of the outside surface 21 respectively. In the method, at least one of the reference lines Ri, R ₂ and R ₃ should extend beyond the circular dashed line 23 that represents the outside surface of the perfectly round part.

[0048] Still referring to Figure 1, the length of reference lines R ₁ , R ₂ and R ₃ is then compared, and the section OS ₁ , OS ₂ , OS ₃ of the outside surface 21 that most outwardly deviates from perfectly round condition will be the section that corresponds to the longest reference line. For example, in Figure 1, reference line R ₁ has the greatest length and therefore, section OS ₁ of the outside surface 21 deviates most outwardly from perfectly round condition in comparison to sections OS ₂ , OS ₃ of the outside surface 21.

[0049] Because section OS ₁ of the outside surface 21 deviates more outwardly from perfectly round condition in comparison to sections OS ₂ , OS ₃ of the outside surface 21, annular part 10 can be rounded by introducing compressive stresses into surface sections of the outside surface 21 other than section OS-i of the

outside surface 21. The compressive stresses are introduced into surface sections of the outside surface 21 so that the internal stress is uniform around the outside surface 21 of the annular part 10. The introduced compressive stresses in the surface sections of the outside surface 21 cause deformation of the annular part thereby rounding the annular part. The rounding magnitude of the annular part 10 is dependent on how strong the introduced compressive stresses are and how deeply the introduced compressive stresses reach into surface sections of the outside surface 21 of the annular part 10.

[0050] The annular part may be rounded by introducing compressive stresses into at least one surface section of the outside surface other than the section OSi of the outside surface 21, whereby the introduced compressive stresses cause deformation of the annular part 10 thereby rounding the annular part 10. Preferably, compressive stresses are introduced into each surface section of the outside surface other than the section OSi of the outside surface 21 so that compressive stresses in surface sections of the outside surface other than the section OSi of the outside surface 21 are substantially equal to compressive stresses in the section OSi of the outside surface 21. Most preferably, the compressive stresses are introduced into surface sections of the outside surface other than the section of the outside surface corresponding to the reference line having the longest length.

[0051] The compressive stresses may be introduced into surface sections of the outside surface other than the section OS ₁ of the outside surface 21 using the laser heating, laser heating and quenching, laser shock peening, and shot peening methods described above. When laser hardening occurs, the mass of the part self-quenches the heated area.

[0052] Referring still to Figure 1 , when section OSi of the outside surface 21 deviates more outwardly from perfectly round condition in comparison to sections OS ₂ , OS3 of the outside surface 21, annular part 10 can also be rounded by relieving compressive stresses in surface section OS ₁ of the outside surface 21. The compressive stresses are locally relieved in surface section OS ₁ of the outside surface 21 so that the internal stress is uniform around the outside surface 21 of the annular part 10. This causes deformation of the annular part thereby rounding the annular part. The compressive stresses may be relieved in surface

section OS ₁ of the outside surface 21 by laser tempering the surface section OS ₁ of the outside surface 21. Tempering is particularly advantageous in carbon steel parts.

[0053] Methods according to the invention for flattening an annular part that is not flat can be explained with reference to Figure 2. In Figure 2, there is shown a side elevational view of an annular part that is not flat before processing in accordance the invention. The annular part 10 has an outside surface 21, an upper surface 27 providing a first end surface, a lower surface 35 providing a second end surface, and an axis A. Also shown in Figure 2 are a perfectly flat dashed line 29 that represents the upper surface of a perfectly flat part and a perfectly flat dashed line 37 that represents the lower surface of the same perfectly flat part. The lower surface of the perfectly flat part defines a lower reference plane for the annular part 10. The deviation of the upper surface 27 of the annular part 10 from dashed line 29 and the deviation of the lower surface 35 of the annular part 10 from dashed line 37 shows that annular part is not flat due to non-uniform internal stresses typically caused by the heat treatment. [0054] This method according to the invention for flattening the annular part 10 proceeds as follows. First, distances are measured along reference lines from the dashed line 37 that defines the lower reference plane of the annular part 10 to surface sections of the upper surface 27 of the annular part 10 wherein each section of the upper surface 27 is associated with a reference line. These measurements can be made using conventional measuring equipment such as an optical comparator or a vision system. Looking at Figure 2, imaginary reference lines L ₁ , L ₂ and L ₃ are shown that extend normally from the dashed line 37 that defines the lower reference plane to corresponding sections US ₁ , US ₂ , US ₃ of the upper surface 27 respectively. Of course, an infinite number of reference lines are possible that correspond to an infinite number of corresponding sections of the upper surface 27 of the annular part 10. If annular part 10 were perfectly flat, imaginary reference lines L ₁ to L _n corresponding to all sections US ₁ to US _n Of the upper surface 27 would be equal.

[0055] Still referring to Figure 2, the length of reference lines L ₁ , L ₂ and L ₃ is then compared, and the section US ₁ , US ₂ , US ₃ of the upper surface 27 that most upwardly deviates from perfectly flat condition will be the section that corresponds

to the longest reference line. For example, in Figure 2, reference line Li has the greatest length and therefore, section USi of the upper surface 27 deviates most upwardly from perfectly flat condition in comparison to sections US2, US ₃ of the upper surface 27. It can be appreciated that if an infinite number of reference lines corresponding to sections of the upper surface 27 of the annular part 10 were used, the imaginary reference line having the longest length would correspond to the section of the upper surface 27 that most upwardly deviates from perfectly flat condition.

[0056] Because section USi of the upper surface 27 deviates more upwardly from perfectly flat condition in comparison to sections US2, US ₃ of the upper surface 27, annular part 10 can be flattened by introducing compressive stresses into surface section LSi of the lower surface 35 shown in Figure 2. The surface section LSi of the lower surface 35 has a perimeter LS _P surrounding the intersection of the normal reference line Li and the lower surface 35 of the annular part 10 and preferably, compressive stresses are introduced within perimeter LS _P of the lower surface 35. Most preferably, the compressive stresses are introduced into a surface section of the lower surface that is 180 degrees opposite from the section of the upper surface corresponding to the normal reference line having the longest length.

[0057] The compressive stresses are introduced into surface section LS ₁ of the lower surface 35 so that the internal stress is uniform in the cross section of the annular part 10 between section USi of the upper surface 27 and surface section LSi of the lower surface 35. The introduced compressive stresses in surface section LSi of the lower surface 35 cause deformation of the annular part thereby flattening the annular part 10. The flattening magnitude of the annular part 10 is dependent on how strong the introduced compressive stresses are and how deeply the introduced compressive stresses reach into surface section LSi of the lower surface 35 of the annular part 10. However, in order to achieve a desired flattening effect, the compressive stresses should be introduced into the lower surface 35 of the annular part 10 in a defined locally bounded area. [0058] The compressive stresses may be introduced into surface sections of the lower surface 35 using the laser heating, laser heating and quenching, laser shock peening, and shot peening methods described above.

[0059] Referring still to Figure 2, when section USi of the upper surface 27 deviates more upwardly from perfectly flat condition in comparison to sections US ₂ , US ₃ of the upper surface 27, annular part 10 can be flattened by locally relieving compressive stresses in section USi of the upper surface 27 shown in Figure 2. This causes deformation of the annular part thereby flattening the annular part. The compressive stresses may be relieved in section USi of the upper surface 27 by laser tempering section US ₁ of the upper surface 27. Tempering is particularly advantageous in carbon steel parts. [0060] It should be appreciated that any combination of the above methods is also within the scope of the invention. Stresses may introduced and/or relieved in any combination of the outside diameter, the inside diameter, the top surface and the bottom surface of the part to change the internal stresses in the part. For example, one could stress relief a surface section of the inside diameter and harden (thereby introducing stresses into) a surface section of the outside diameter to complete the rounding and/or flattening process. Also, the invention is not limited to specific methods for determining flatness or roundness.

EXAMPLES

[0061] The following Examples have been presented in order to further illustrate the invention and are not intended to limit the invention in any way.

Example 1

[0062] Computer modeling was used to show the effects of a surface section volume expansion on the roundness of an annular part. The "Double mark laser shaping" chart shown in Figure 3 is a half model of a ring gear (teeth not modeled) which is approximately 10 inches in diameter. Small areas 71, 72 on the outside diameter 180 degrees apart were increased in volume by 0.23% to simulate hardening in these local areas. The part went out-of-round by 0.026 inches. This model shows the effect of non-uniform heat-treatment and also that the part could be rounded by further heat-treatment at points HT-i, HT ₂ , HT ₃ , HT ₄ , HT ₅ , HT ₆ around the outside diameter so that the internal stresses would be uniform around the circumference of the part. Alternatively, if the inside diameter were heat- treated in areas IDi and ID ₂ corresponding to the area on the outside diameter, the internal stress would be uniform in each cross section around the annular part.

[0063] Thus, there are at least two ways to round the annular part as a result of heat-treat distortion. First, if the internal stress distribution in each cross section around the part is the same, then the part will be round. Second, if each cross section in the part has balanced stress symmetric about the centroid, the part will be round. In the invention, one measures the out-of-round to determine where to introduce compressive stresses into the part.

Example 2

[0064] Computer modeling was used to show the effects of a surface section volume expansion on the flatness of an annular part. The "Double mark laser flattening" chart shown in Figure 4 is a half model of the same ring gear (teeth not modeled) as Figure 3, which is approximately 10 inches in diameter. Small areas 81, 82 on the top of the gear 180 degrees apart were increased in volume by 0.23% to simulate hardening in these local areas. The part warped 0.0025 inches from flat. Again, this shows the effect of non-uniform heat-treatment and also how by heat-treating the corresponding areas B ₁ , B ₂ on the bottom of the part, all cross sections of the part would have a uniform stress distribution.

Example 3

[0065] In the first step, metal strips were lasered with the concept of an almen strip to determine induced strain. The bend and the strain in the strips was plotted to provide direction for the second step.

[0066] In the second step, a group of SF1009 ring gears (see an example gear in Figure 5) were lasered in four places equally spaced on the inside diameter of the part at a given laser power setting. A series of these parts were made at different power settings. The resulting change in roundness was compared to an finite elements analysis (FEA) simulation of the individual test parts. [0067] This data was used to create a series of strain factor to laser power (watts) equations. At the lower laser power levels and/or higher speeds (IPM), the equations are linear, as the power is increased the curve of the equation bends back and less total strain occurs in the part. The linear part of the curve is due to tempering, while in the curved transition area there is rehardening. The Watts vs. Strain Factor graph is shown in Figure 6.

[0068] In the third step, parts were laser rounded. In the laser rounding process, the roundness of the part is first measured, an FEA model was run to

round the part and then a strain factor equation was applied to create the laser pattern needed to round the part, see the Laser Power Pattern of Figure 7. The laser power pattern correlates to the outside diameter hoop stress. The laser power pattern is inversely related to the inside diameter hoop stress. [0069] Success was achieved in rounding fully dense steel parts with a case hardened surface. Figure 8 shows a graph comparing the radius of an untreated part and the same part treated with the method of the invention. The chart of part BT37 shown in Figure 8 reduced the out-of-round by 75%. Without intending to be bound by theory, the main phenomena responsible for the rounding was believed to be tempering. In this process, if care is used in directing the laser beam, it is likely that there will not be a heat-affected zone. There will be a variation in the hardness in the lasered area.

[0070] On powder metal test parts, only step 2 (lasered 4 equal locations) parts were run, there was a small amount of shape change. These step 2 parts, which were evaluated, were at 6.8 g/cc density. These tests assumed and used the power range of the SF1009 ring gear (Fig. 5) strain factor to power factor curves. As a result, these parts were lasered at power settings in the non-linear area of the curve.

[0071] Thus, in a laser rounding process, the process steps developed work well as follows: (1) Measure 100 points around the inside diameter of the part. (2) Input the data into a computer spreadsheet. (3) Round the part in an FEA model. (4) Use the FEA inside diameter hoop stress and outside diameter hoop stress to develop the laser pattern around the part. (5) Laser 360 degrees of the part inside diameter with the laser pattern. (6) Measure 100 points around the inside diameter of the part and compare to the original shape.

Example 4

[0072] This example used powder metal parts. The basic laser rounding procedure used for the powder metal parts was the same as used for the fully dense parts as tested in Example 3. Tests were run with the laser surfaces painted with a dark paint designed to create a uniform absorptivity and minimize variation.

[0073] The first step in rounding a new part is to develop a laser wattage to strain factor relationship. The laser wattage to strain factor relationship is shown

in Figure 9 which is developed from testing parts. The "Z" area of the graph is a mixture of fresh martensite and tempered martensite in the heat affected zone which produces an unpredictable result; the area to the left of the "Z" is the result of tempering.

[0074] This wattage to strain factor relationship is used in step 5 below to create the laser pattern required to round a part. The laser rounding procedure is as follows: (1) Measure 100 points around the inside diameter of the part. (2) Input the data into a computer spreadsheet for data calculations. (3) Round the part in an FEA model. (4) Use the FEA inside diameter hoop stress and outside diameter hoop stress to develop the basic laser pattern. (5) Use the information in step 4 and the wattage to strain factor relationship to create the magnitude of the laser pattern. (6) Laser 360 degrees of the part inside diameter with the laser pattern from step 5. (7) Measure 100 points around the inside diameter of the part and compare to the original shape.

[0075] Success was achieved rounding powder metal steel parts with a case hardened surface. Figure 10 is a graph comparing the radius of an untreated part and the same part treated with a method of the invention. The chart of part PRE16 shown in Figure 10 reduced the out-of-round by over 90%. Without intending to be bound by theory, the phenomena responsible for the part rounding was believed to be tempering. The laser pattern used to round PRE 16 is shown in Figure 11. The laser power pattern tracks the outside diameter hoop stress. The laser power pattern is inversely related to the inside diameter hoop stress. The maximum wattage used was 830 watts at 20 IPM, which can be located on the chart in Figure 9.

[0076] The laser-rounding algorithm to create a laser pattern works well. Figure 12 shows the progressive use of the laser pattern. Shape "A" was the result of a first pass pattern while "B" shape was a second pass with the same laser pattern on the same part in a different area of the inside diameter. The progressive rounding shows that the out-of-round can be uniformly and progressively reduced with a laser pattern developed from steps 4 and 5 above. Figure 13 shows the laser pattern used to progressively round PRE18. [0077] Although the present invention has been described in considerable detail with reference to certain embodiments, one skilled in the art will appreciate

that the present invention can be practiced by other than the described embodiments, which have been presented for purposes of illustration and not of limitation. Therefore, the scope of the appended claims should not be limited to the description of the embodiments contained herein.

INDUSTRIAL APPLICABILITY

[0078] The invention provides methods for rounding and/or flattening annular parts, such as rings, which are out of round and/or are not flat due to non-uniform internal stresses.