BACKGROUND OF INVENTION
1. Field of the Invention
The invention relates generally to a spinning metal processing method and apparatus therefor, and articles manufactured according to the method. More specifically, the invention comprises a two-step tube-necking spinning method in which a series of primary forming rollers used in a first step is followed by a series of secondary or finishing forming rollers used in a second step, the rollers having specially profiled section shapes, and product produced thereby.
2. Description of Related Art
Spinning is a metalworking process by which a disc or tube of metal is rotated at high speed and formed into an axially symmetric part. It typically involves the forming of axisymmetric components over a rotating mandrel using rigid tools or rollers. Tube-necking spinning is one type of spinning method in which a metal tube is processed to form a section of reduced diameter.
SRF (Superconducting Radio Frequency) cavities are the heart of advanced particle accelerator systems. The International Linear Collider (ILC) is one of the highest priorities in High Energy Physics (HEP). Extensive research and development has been completed on the SRF cavities for this purpose. Other uses of SRF cavities in HEP facilities and industrial fields include: material analysis, other nuclear physics applications, military applications such as Free Electron Laser (FEL), medical and industrial isotope generation, and sub-critical power generation devices. FIG. 1 shows an example of a nine-cell SRF cavity—the most widely used TESLA type SRF cavity. Its inner diameter is 206.9 mm at equator 1e (or 1c) and 78.1 mm at iris 1d and tube end 1f; length L is 1355 mm; wall thickness is 3.0 mm; material is RRR Niobium (Nb) or other superconducting material. 1a is one full cavity cell while 1b is the end cell. Most SRF cavities are traditionally fabricated by deep drawing of half-cells from sheet material and electron beam (EB) welding of the half cells. Obviously, an advanced fabrication method of seamless multi-cell cavities that avoids the welding has many advantages. First, the seamless cavity does not have the risk of equator weld contamination. This could improve the reliability for reaching high gradients. Secondly lower cost and high efficiency can be expected, especially for mass production. There is a great need to provide a high efficiency, lower cost and reliable seamless fabrication method for fabrication of SRF cavities.
Known fabrication methods for SRF cavities have several limitations, especially for mass production of seamless multi-cell SRF cavities. By reviewing and investigating the manufacturing methods by which seamless multi-cell SRF cavities can be fabricated, it is found that the most feasible process for mass production is tube-necking spinning followed by tube hydroforming starting with a seamless blank tube of an intermediate diameter between cavity iris 1d and equator 1c. In the tube-necking spinning, the tube diameter in the iris area and at the tube end is reduced; then in the following tube hydroforming, the tube diameter in the equator area is expanded to cavity shape. After this, a calibration-hydroforming may be added to further calibrate the cavity shape and dimensions. The development of the tube diameter reduction (tube-necking) at the iris area needs more efforts than expansion (hydroforming) at equator area. Tube-necking spinning remains the main challenge for mass production of seamless multi-cell SRF cavities.
FIG. 2 shows an example of spun work piece in which the initial blank tube diameter has intermediate diameter 2e between cavity iris 1d and equator 1c. 2a is one full spinning cell while 2b is the end spinning cell. In tube-necking spinning, iris area 2d and tube end 2f are reduced; in the followed tube hydroforming, equator section 2c is expanded to cavity shape 1c. When initial tube diameter 2e equals 150 mm, the diameter reduction ratio in tube-necking spinning is 1.92 (or 48%). If not using hydroforming to expand the equator, only using spinning, starting initial tube diameter 2e from around 1e (around 207 mm), then the diameter reduction ratio is larger than 2.65 (or 65%). It is very high.
The basic principle of the reduction at the iris and tube end is the same as in conventional tube-necking spinning. The conventional spinning apparatus, as illustrated in FIGS. 3-5, consists of metal tube blank 2, roller 3, clamps 5 and a mandrel (not shown) for spinning one full cell, while FIG. 6 illustrates apparatus for spinning multi-cells (conventional spinning mandrel not shown). FIG. 3 is a front view; FIG. 4 is an isometric view; FIG. 5 is a cross-sectional view showing the shape of roller 3 having simple full-radius 3a, roller path 4, and the section shapes of the target spun work piece 2. 2o, 2m and 2i are the target sections of the outer layer, mid layer and inner layer, respectively, for spun work piece 2. Driving by the spinning equipment, the metal tube 2 rotating around a longitudinal axis (X), the necking shape is gradually obtained by driving roller 3 along roller path 4 by moving in radial and axial directions. As the roller moves along roller path 4 and passes on the work piece, the blank tube gets smaller-and-smaller in diameter, until the target diameter is reached.
From the view of cost-efficient manufacturing, processing time must be shortened, especially for spinning products like multi-cell SRF cavities in which processing time is very long. In spinning, the processing time is determined by a feeding rate, rotating speed and the total length of the roller path. In order to decrease the processing time, it is necessary to increase feeding rate and rotating speed, and shorten the length of roller path. For a certain material, the rotating speed is limited, and feeding rate is also limited for the given roller shape. Because of the limitation of high rotating dynamics in the spinning, it is impossible to greatly increase the rotating speed. So, the possible and feasible way to shorten processing time is to find an effective way to increase feeding rate and shorten the length of roller path.
The present inventor has found that it is very difficult to reduce the processing time in the conventional spinning processing methods. The conventional tube-necking spinning commonly uses very simple roller shape (such as full radius section shape 3a in FIG. 5) and a simple one-step or several-step method. Even using several steps, the rollers in each step use very simple shapes. That is, simple rollers are used to spin a whole necking shape in one or several pressing operations. The main drawbacks of conventional spinning include: very long processing time so the efficiency is very low. First, the feeding in each spinning pass is small when using very simple roller shapes in which the maximum feeding is limited. On the other side, the total length of roll path is too long, especially in cases like SRF cavities in which the diameter reduction ratio is very high. Secondly, a mandrel is needed, and only the very complicated collapsible mandrel like disclosed in U.S. Pat. No. 5,500,995 can be used for multi-cell tube-necking shapes, such as SRF cavities. Thirdly, it is very difficult to obtain ideal spun profiles as the simple roller shape, especially if a mandrel is not used in the spinning. For SRF cavities, as the interference zone exists and the mandrel difficulty issue exists when starting from a tube blank, it is too difficult and too expensive to use a mandrel in spinning full multi-cell shapes.
The following is about the prior art related to tube-necking spinning and SRF cavities fabrication methods in which the present inventor is aware, and its differences and distinctions from the present invention. U.S. Pat. No. 5,500,995 to Vincenzo Palmieri et al, titled “Method of Producing Radiofrequency Resonating Cavities of the Weldless Type,” describes a spinning method and apparatus of producing seamless SRF cavities, in which, starting from a disc sheet blank, in the first step spinning the sheet onto a die having the shape of a frustum of a cone; in the second step, the die (also called mandrel) having exactly the internal shape of SRF cavity is used to spin the residual half-cell and the second cut-off tube, with a fast annealing at a temperature of the copper lower than 600° C. The above steps are repeated to obtain multi-cells. Although it is a spinning method of producing SRF cavities in the present invention, it differs in that: (1) the former method uses a disc sheet blank, while the present invention uses seamless tube as a blank; (2) the former method forms a half-cell by half-cell to get the full cell shape, while the present invention directly spins the whole full cell shape; (3) the former method uses complicated mandrels (or dies), especially a collapsible mandrel, and uses fast annealing in a second step, while the present invention does not use a mandrel for spinning the full cell shapes, only uses a very simple mandrel for spinning two ends, and does not need any annealing during spinning; (4) the former method uses conventional simple rollers and does not mention any high-efficiency or high-feeding-rate measures, while the present invention focuses on high efficiency processing and using specially profiled rollers in each step. For a single cell cavity the former method worked, but necking of the multi-cells was less successful. For a long tube it was difficult to achieve the uniform wall thickness at the necking area. With the present invention using a tube as a blank, spinning each cell having almost exactly the same condition to obtain uniform shapes, does not have this problem; Prior work has been directed to the low-cost production of a low volume of SRF cavities, while the present invention focuses on cost-effectiveness for mass production of SRF cavities.
U.S. Pat. No. 8,042,258 B2 to Katsuya Sennyu et al, titled “Method for Producing Superconducting Accelerator Cavity,” describes producing a SRF cavity by a method in which production cost is reduced by reducing the number of welding points. It is basically still a traditional forming and welding method but with lower cost.
WO2007062829A1 (and EP1955404B1) issued on Jun. 7, 2007 to Xenia Singer et al, titled “Method for Production of Hollow Bodies for Resonators”, describes a method to make SRF cavities, but it is still makes half-cells first, not using the seamless method.
WO2009021698A2 issued on Feb. 19, 2009 to Waldemar Singer et al, titled “Process and Device for Producing Radio-Frequency Resonators Which Are Free from Weld Seams,” describes a seamless method and device to make seamless SRF cavities. The process is a swaging preform followed by tube hydroforming In the swaging it uses special swaging devices, in which the main tool contacting the tube is torus-shaped. The torus tool is rotating eccentrically around the blank tube to make the tube deform to necking shapes. It needs a special device that may cause vibration problems.
The main difference between Singer (WO2009021698A2) and the present invention is that Singer uses a swaging process and a special swaging device other than spinning rollers, while the present invention uses a two-step spinning method and conventional spinning equipment, not requiring special equipment. The main differences from conventional spinning is that the present invention uses a two-step method and two sets of specially profiled rollers, rather than the simple step and/or simple roller-shape. The advantage of the present invention is that it may greatly increase feeding rate, shorten roller paths and accurately control spun shapes. The specially profiled rollers in the present invention are still spinning rollers and do not have vibration issues.
U.S. Pat. No. 7,316,142 B2 issued on Jan. 8, 2008 to Paul B. Lancaster, titled “Metal Spin Forming Head”, describes a metal spin forming head including two sets of rollers, with each set having a series of individual metal working rollers therein. It uses a mandrel, while the present invention does not. Its rollers' shapes are still very simple, similar to conventional spinning roller shapes; while the present invention uses specially profiled rollers. It uses two sets of rollers but still uses a one-step method, while the present invention uses a two-step method and each step uses specially profiled rollers. Its two sets of rollers are unable to be used to produce SRF cavities, while the present invention is especially effective for producing SRF cavities.
In his patent, Lancaster listed and compared many other patents which are mostly related to spinning of the catalytic converters. There are also many patents cited that relate to spinning of the catalytic converters, such as U.S. Pat. No. 6,386,010 B1, U.S. Pat. No. 5,937,516, and U.S. Pat. No. 4,953,376. All those are completely different from the present invention. All those are effective or useful to products like catalytic converters that are only similar to the end shape of SRF cavities and the diameter reduction ratio of the former is much less. They do not have the ability to produce the full cell shapes of multi-cell SRF cavities. In addition, all those patents and art do not use the two-step method and specially-profiled rollers like in present invention.
Thus, an efficient tube-necking spinning method and apparatus therefor solving the aforementioned problems are needed.
SUMMARY OF THE INVENTION
The tube-necking spinning process remains the main challenge to solve and to innovate for mass production of SRF cavities. It is an object of the present invention to provide apparatus and method for spinning processing which solves the above problems and difficulties, increases the feeding rate and shortens the roller path to greatly reduce processing time and accurately control the spun profile.
The tube-necking spinning method and apparatus therefor of the present invention are configured to provide high efficient and accurate spin forming of metal tubes and similar work pieces. While the present invention may be adapted for use in spin forming a generally tubular necking shape of any practicable size and for any practicable purpose, it is particular well suited for spinning the necking or iris shapes of seamless multi-cell SRF cavities for particle accelerators.
The spinning processing method of the present invention comprises applying two specially-profiled rollers and a two-step tube-necking spinning processing method for spin forming tubular metal components. By means of the driving mechanism of the spinning equipment, a work piece or blank metal tube is rotated around its longitudinal axis. To spin one full cell shape, in first step, a plurality of primary forming rollers with a selected profile shape move along primary roller paths to form the first part of the necking shape; then in a second step, a plurality of secondary forming rollers with a selected profile shape move along secondary roller paths, to draw the metal tube and form a section of reduced diameter. By applying a certain axial load P at one end along longitudinal axis of the metal tube in both steps, it is easier to achieve the accurate spun profile. No mandrel is required for spinning the full cell shape. To spin multiple full cells, the two-step method for each full cell is repeated. To spin an end shape like SRF cavity ends, this two-step method is still applied, but the roller path is required to change to fit the end shape and a simple mandrel with the same diameter of inside of target end is required to be applied.
The spinning processing apparatus according to the invention comprises means for supporting and rotating a metal tube round its longitudinal axis thereof; means for supporting and driving two series of profiled rollers: a plurality of primary profiled rollers in first step and a plurality of secondary profiled rollers in second step, respectively; and means for applying axial loads along the longitudinal axis of the metal tube. With this apparatus, the processing method of present invention described above can be carried out. Among components in the processing apparatus, except the primary profiled rollers and secondary profiled rollers, all other components are same as that in conventional spinning and can still use the conventional spinning equipment and related tools, which is commercially available from multiple manufacturers.
The present invention has several advantages and features that are not provided in earlier developed devices and methods of the prior art. The present invention provides much bigger feeding rate in each spinning passes by the profiled forming rollers. These features greatly decrease processing time. The secondary forming rollers that have a similar cross-section shape to the target spun shape accurately control the profile of a spun work piece and eliminate the spinning mandrel, which is a big difficulty in the spinning of multi-cell SRF cavities.
BRIEF DESCRIPTION OF THE DRAWING(S)
FIG. 1 is a view showing a part of a nine-cell SRF cavity. FIG. 1 also illustrates the final part shape of the work piece of FIG. 2.
FIG. 2 is a view illustrating an example of a work piece for a nine-cell SRF cavity. FIG. 2 is the preform shape of the part of FIG. 1.
FIG. 3 is the front view showing the essential part of a conventional tube-necking spinning processing apparatus for spinning one full cell (not showing conventional mandrel).
FIG. 4 is the isometric view showing the essential part of a conventional tube-necking spinning processing apparatus for spinning one full cell (not showing conventional mandrel).
FIG. 5 is the cross-section view showing the shapes of the target spun work piece with the roller and roller path.
FIG. 6 is illustrates the essential parts of a conventional tube-necking spinning processing apparatus for multi-cell SRF cavities (not showing conventional spinning mandrel).
FIG. 7 is the front view showing the target spun work piece 2 and sections of primary forming roller 6 and secondary forming roller 8;
FIG. 8 is the isometric view of the first step showing the target spun work piece and the primary forming roller.
FIG. 9 is the isometric view of the second step showing the target spun work piece 2 and the secondary forming roller.
FIG. 10 is the section view illustrating the present invention, showing the section shapes of the target spun work piece, primary forming roller and primary roller path, as well as a secondary forming roller and secondary roller path.
FIG. 11 is the section profile of the primary forming roller according to the present invention.
FIG. 12 is the section profile of the secondary forming roller according to the present invention.
FIG. 13 is a side view illustrating an example similar to FIG. 7 according to the present invention, showing relative circumferential positioning of two sets of rollers for each spinning steps.
FIG. 14 is a side view illustrating another example similar to FIG. 7 according to the present invention, showing relative circumferential positioning of three sets of rollers for each of the spinning steps.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 7-10 show work piece 2, primary forming rollers 6, secondary forming rollers 8 and clamps 5. Only one primary forming roller 6 and one secondary forming roller 8 are shown in FIGS. 7-10, but either may be a plurality of rollers. FIG. 7 is the front view, showing target spun work piece 2 and the section shapes of profiled rollers 6 and 8. FIG. 8 and FIG. 9 show isometric views of the first step using primary forming roller 6 and the second step using secondary forming roller 8, respectively. Work piece 2 is supported and clamped by clamps 5, rotating round its longitudinal axis (X). Its left end 2g is mounted on spinning fixtures (not shown in FIG.). The left end 2g can be rotated freely, but cannot be moved along axis (X). Its right end 2h can be rotated round axis (X) freely, be moved along axis (X) to provide required material, and a certain axial load P along axis (X) (not shown in FIG.) can be applied if preferred. Commercial spinning apparatus may be employed, such as standard CNC spinning machines of any brand and/or standard CNC lathes of any brand.
The initial tube blank diameter is the same as left end 2g (or right end 2h) and equals 2e in FIG. 2. It is an intermediate diameter between cavity equator 1c and iris 1d of the SRF cavity 1 in FIG. 1. After spinning, the diameter at central iris of FIG. 7 is the same as 2d (or 2f) in FIG. 2, and is close to cavity iris 1d (or 1f) in FIG. 1.
The target necking shape is obtained gradually by driving the roller along the roller path, moving in radial and axial directions. To simplify and clarify, herein the same definitions of the roller pass and roller path as that in conventional spinning are used. Each motion curve of a roller from left to right or from right to left is called a roller pass. A combined curve of all motion curves for the same roller in a processing operation is called a “roller path.” The roller path limit is the envelope curve of roller shape at moving limit points of all roller passes of the same roller. A roller pass is a feeding curve. In spinning, there are different types and shapes of passes or feeding curves. Driving the roller along a roller path to provide feeding and forming force forms the tube to the target shape. So, the design of the roller path is critical. There are different types of roller paths. The basic principle of roller path design is the same as that in conventional spinning, and may be obtained from manuals or handbooks, such as ASM Handbook Volume 14B. For tube-necking spinning of SRF cavities, the most effective roller path is a set of multi-pass, multi-direction feeding curves. FIG. 7 and FIG. 10 illustrate the example of roller path 7 for roller 6 and roller path 9 for roller 8, respectively. Roller paths are different from handbook values, but the design principles are the same. To design a roller path, first, according to the target spun work piece profile and roller shape, determine the roller path limit. Then, using the determined feeding (per feeding rate in radial and axial directions and rotating speed), design the passes and intersect with the roller path limit. Join all roller passes to form a roller path as a combined curve. As shown in FIG. 10, the roller path 7 (or 9) is composed of multi-passes. It is a combined curve of multi-pass, multi-direction feeding curves.
The operation may be carried out by means of a driving mechanism for the spinning equipment, rotating a metal tube around its longitudinal axis (X) as work piece 2 or blank tube. To spin one full cell shape, in the first step, the primary forming roller 6 with profiled section shape 6a in FIG. 11 moves along roller path 7 to draw the metal tube to form a section of reduced diameter, and finally form the first part of a necking shape; then, in the second step, the secondary or finishing forming roller 8 with profiled section shape 8a in FIG. 12 moves along roller path 9 to draw the metal tube and continue forming a section of reduced diameter, forming the residual partial shape to obtain a final necking shape. To control and easier achieve the accurate spun profile, it may be helpful to apply a selected axial load P at right end 2h (FIG. 7) along longitudinal axis (X) of the metal tube in both steps (axial load P not shown in FIG. 7). No mandrel is required for spinning the full cell shape. To spin multiple full cells, this two-step method may be repeated for each full cell. To spin the end shape like SRF cavity ends, this two-step method is still applied, but the roller path is required to change to fit the end shape and a simple mandrel with the same diameter that equals the inside diameter of the target end is applied.
Alternatively, the operation of tube-necking may be carried out by means of a driving mechanism for the spinning equipment, maintaining a metal tube stationary around its longitudinal axis (X) as work piece 2 or blank tube and rotating the forming rollers around the metal tube. FEA results show that this method also results in very good spun profiles at low thinning
The roller section profile 6a of primary forming rollers 6 in the present invention may be as shown in FIG. 11. The primary forming roller 6 includes three nose shapes: 6c, 6l and 6r. 6l and 6r are preferably symmetric to the center point of 6c. As 6l and 6r are preferably at same level, both have a disc radius (that is, the distance between the nose and 6e, 6e being the center line) differences of 6d. When roller 6 moves toward the left, both 6l and 6c provide feeding on the tube; when roller 6 moves toward the right, both 6r and 6c provide feeding on the tube. Roller 6 always has two feedings on the tube for each roller moving. As a result, for each pass, roller 6 provides much bigger feeding than conventional spinning. The increased feeding amount equals 6d for each pass, and at least is half of conventional radial feeding. As roller 6 always has two noses working and in total has three nose shapes, the roller width is much bigger than conventional roller 3. To form the same width of the necking shape, the distance of roll 6 moving on each pass is much shorter than that of conventional roller 3, less than half of each passes of roller 3, so the total path length of roller 6 is much shorter than that of roller 3, as shown in FIG. 10 versus FIG. 5. The rapid feeding and much shorter roller path of the present invention greatly increase the processing efficiency.
The best spun profile is obtained by using specially profiled secondary roller 8 moving in radial and axial directions, that is, moving along secondary roller path 9. The roller section profile 8a of secondary forming rollers 8 (its central line is 8e) in the present invention is illustrated in FIG. 12. The section profile 8a is roughly the same as the outside section profile of the cavity iris or that of the target spun work piece. This special profiled shape can accurately control the spun profile of the work piece and at the same time decrease the processing time in each pass, as its roller passes 9 (FIG. 10) are much shorter than that of conventional roller 3 (4 in FIG. 5).
When the necking width is very big, more step roller noses similar to 6r or 6l can be added to roller 6 to further increase feeding and shorten the roller path, and/or a three or more step method can be used, in which different profiled rollers are used in each step to further speed up the processing.
FIG. 2 is a schematic view showing an example of a nine-cell SRF cavity according to an embodiment of the present invention. In this example, the starting tube diameter 2e is an intermediate diameter between equator 1e and iris 1f, and the tube-necking spinning is as a preform process, followed by hydroforming to form to final part shape of FIG. 1.
FIG. 1 is the final part shape of FIG. 2, and FIG. 1 is also another example of a nine-cell SRF cavity according to an embodiment of the present invention. In this example, the starting tube diameter roughly equals equator diameter 1e, and the tube-necking spinning is the main forming process, directly spinning to the part shape of FIG. 1. Following this, only calibration operation by hydroforming or other forming is required to achieve accurate final part shape.
For mass production, the present invention can also be used on separate spinning equipment to further increase production: applying the primary forming rollers 6 and the first step to one machine, and applying the secondary forming rollers 8 and the second step to another machine. Similarly, the two-step method of present invention may be applied to each of multiple machines, or some for first step and others for second step.
Similar to the preferred embodiment, the present invention can be used in spinning where primary profiled rollers and/or secondary profiled rollers are used for one step, two-step, and multi-step processing.
FIG. 13 and FIG. 14 show the roller configurations of two examples of the present invention. FIG. 13 applies two sets of rollers for primary and for secondary forming rollers, while FIG. 14 applies three sets of rollers. Using two sets or three sets of rollers can save more processing time than using one set of rollers. Depending on the spinning equipment\'s capability, the present invention can use one set or multi-sets of rollers to do the processing.
The present invention does not require special equipment or machine to carry out. The present invention may use conventional spinning equipment. By using two series of rollers comprising primary forming rollers 6 and secondary forming rollers 8 and the two-step method, the present invention greatly increases feeding rate in each spinning pass, shortens each pass\'s length, greatly increases the processing efficiency, and accurately controls the spun shapes. Moreover, it eliminates the complicated mandrel of conventional spinning. It is especially cost-effective to produce multi-cell SRF cavities and other types of tube-necking products.
Comparing conventional tube-necking spinning (FIG. 3) to present invention (FIGS. 7-10) for the same SRF cavities, we may find: the one-step forming processing is replaced by two-step processing of the present invention; one form roller 3 with simple section shape as in FIG. 5 is replaced by primary profiled roller 6 and secondary profiled roller 8 in FIG. 7-FIG. 10; and roller path 4 in FIG. 5 that has much longer length of each passes is replaced by much shorter roller path 7 and path 9 as in FIG. 10. To transfer multi-cell spinning shown in FIG. 6 to the present invention, the same two-step method may be used to take the place of the one-step method, replacing rollers and roller path with those of the present invention. The conventional processing requires a very complex mandrel, while the present invention does not need the mandrel for the full cell spinning. It only needs a very simple mandrel for the two ends.
Finite element analysis (FEA) has demonstrated the feasibility and applicability of the present invention. The FEA results have shown that the present invention can obtain a very good profile of a spun work piece, well meet the thinning requirement and greatly decrease the processing time. Moreover, mandrel-free operation is feasible for the full cell tube-necking spinning of SRF cavities.
The present invention solves the difficulties in tube-necking spinning for SRF cavities, providing a high efficiency, low cost and reliable seamless fabrication method, and has high a potential market value. As the key components of modern particle accelerators, the SRF cavities are widely utilized in HEP facilities and industrial fields. The demand of the ILC project alone would be higher than $1 billion at current market value. The applications include nuclear physics, military, material analysis, medical and industrial isotope generation, and power generation devices.
Although the present invention has been described with respect to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except to the extent that they are included in the accompanying claims.