METHOD FOR THE MANUFACTURE OF FRONT FORK OF SINGLE UNIT TYPE FOR BICYCLES FROM CENTER BUTTED FERROUS TUBULAR MATERIAL

Abstract: A front fork blank of the single unit type is manufactured from a center butted tube, the wall thickness of its middle-por­ tion exceeding that of both end portions of the tube, as a start­ ing material. The center butted tube is produced by a known machine including a mandrel and a die. The center butted tube is bent at two positions with the right angle to form a tubular fork. The tubular fork is set in a sectional die which has a con­ cave recess corresponding to a protuberant socket, and sub­ jected to the known oil hydraulic bulge-forming process in or­ der to bulge-form the socket at the middle of the top of the tu­ bular fork. The socket is opened to receive a stem.

Description: Method for the Manufacture of Front Fork of Single Unit Type for Bicycles from Center Butted Ferrous Tubular Material The present invention relates to a method for the manufacture of a front fork blank for use in bicycles, and more particularly to a method for manufacturing a front fork of the single unit type from a center butted ferrous tubular material as a starting material in which a crown member to which a pair of blades are joined by brazing and the like is no longer required. The method for manufacturing the front fork of the single unit type conforms to present requirements of the saving of material, energy and labor. As is known in the bicycle industry, the front fork com prises a stem,-a crown, and a pair of blades. The stem, crown, and blades are manufactured separately. Except for the stem, the front fork is assembled by joining two blades to the crown. The manufacture of the crown itself requires a somewhat complicated step. In bicycle tubing, the term "butted tube" refers to a tube having a wall which is thicker at a certain part along its length than the remainder. There exist three different types of butted tubing: single, double, and differential (or triple) butted tubes. Single butted tubes are thicker at one end only, double butted tubes are thicker at both ends, and. differential butted tubes have ends of varying thickness. The butted tubes are butted because the stress in a tubing frame is highest at the ends where the tubes are joined. By butting, the tube can be made'thinner and lighter as steel is removed from the middle of the tube where the'stress is relatively low. The term "center butted tube" used in the present invention refers to a tube wall thickness of which exceeds only in the middle that of its two end portions. In the bicycle tubing, however, the center butted tube has never been mentioned, because the center butted tube may be of no use for bicycles. As described above, it is generally considered that the butted portion of the tube is thicker than the other portion thereof, and is thus also stronger than the other portion. Accordingly, the present invention is directed to the appiiaation of the'thick and strong portion of the center -butted tube to a crown portion of'the single unit front fork. According to US-A-4 305 269 and JP-A-47375/81 both end portions of a tubular ferrous blank are tapered, and itsmiddle portion is shaped ellipsoidal; the tubular blank so shaped is bent at two positions of the middle portion at right angles to form a two-blade fork;- the tubular fork formed is set in a sectional die of top and bottom molds, and is subjected to the oil hydraulic bulge-forming step to bulge-form a prbtuberant socket in the middle. The socket can receive a stem to form a front fork blank of the single unit type. It sould be noted that the center butted tube is not applied as a starting material in the above U.S. -and JP-prior art documents. Accordingly, it is a prime object of the present invention to provide a method for the manufacture of a front fork blank of the single unit type for bicycles by applying a center butted tubular ferrous material as a starting material. It is another object of the invention to provide a method for the manufacture of a front fork blank of the single unit type for bicycles by applying the center butted tubular ferrous material as a starting material and applying also the oil hydraulic bulge-forming process thereto with a view to saving material, energy, and labor, and thus markedly reducing the manufacturing cost. Further objects of the invention will be better understood from the following detailed description with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a ferrous tubular starting material of this invention showing a longitudinal welded seam represented as a dotted line. Figure 2 is a sectional view wherein one of the machines for making a center butted tube having an outer thick wall in the middle is shown in a very simplified manner. Figure 3 is a sectional view showing the same center butted tube having the same outer thick wall in the middle as the tube of Fig. 2. Figure 4 is a sectional view showing another center butted tube having-an inner thick wall in the middle. Figure 5 is a sectional view showing an additional center butted tube having an #inner-and outer thick wall in the middle. Figs.6-8 are-front views of front fork blanks bent and formed from straight center butted tubes of Figs. 3-5, - respectively. Figure 9 is a front view of a front fork blank of the single unit type manufactured in accordance with the. method of the invention. Figure 10 is a perspective view of a conventional crown member available on the market; Figure 11 is a schematic view showing three components of a conventional front fork before assembly as compared with that of the single unit type de picted in Fig. 9. Figure 12 is a simplified schematic view showing how each component of the oil hydraulic bulge forming press machine according to this invention is formed; Figure 13 is a schematic plan view showing how a tubular fork blank set in a bottom mold of a sectional die is subjected to the oil hydraulic bulge forming step, and how a working oil under press ure is delivered to the die. Figure 14 is a detailed plan view of the bottom mold of the sectional die in which the tubular fork blank is set, and a mandrel is inserted into the open ing of the die. Figure 15 is a schematic view of the mandrel with a concave recess which corresponds to the bulge-formed socket of the front fork blank. and Figure 16 is a schematic view of the sectional die consist ing of top and bottom molds having an opening into which the mandrel is to be inserted. Referring to the drawings in more detail, Fig. 1- shows the tubular ferrous stock 1 with the longitudinal welded seam 1a, which'is specified in JIS (Japanese Industrial Standard) G 3445" Carbon Steel Tubes for Machine Structural Purposes" STKM 11A-13A. The steel described therein contains less than 0.25 % C, less than 0.35 % Si, 0.30-0.90 % Mn, less than 0.040 % P, less than 0.040 % S, the remainder being Fe; and has the following physical properties: its tensile strength is more than 38 kg/mm2, and its elongation more than 35 %. As for the welded seam, a spirally welded seam is not suitable, because in charging the welded tubular blank into the sectional die described hereinafter, care should be taken that the'welded seam la is located outside the area where a single component is to be bulge-formed. A seamless tube may be used as a tubular blank, offering the advantage that no attention may be paid to the welded seam so that the seamless tube may be automatically charged into the die,- though, on the other hand, the seamless tube is more expensive than the welded tube. Fig. 2 shows one of the known machines for making a butted tube, the invention and development of which are quite old. The "Bicycle Technical Information Bulletin" No. 13 of Japan reports that the "butted tube" (developed in 1890) was imported from British Reynolds Co. into Japan as a steel tube having a thick end wall. Rauch et al.: "Fahrradtechnik" published in West Germany 1980 by Bielefelder Verlagsanstalt KG describes that the "butting process was invented in 1935 by Reynolds Co., and further developed (-p. 48). - As mentioned hereinbefore, the center butted tube whose wall is particularly thick in the middle of the tube length has never been applied for use in the bicycle -tubing. If the center butted- tube is conceived as a tube as whose wall is only thick in the middle, three different types are proposed as follows: (1) The middle portion of the center butted tube protrudes, which is called "center butted tube with an out er thick wall" (see Fig. 3). (2) The-middle portion of the center butted tube is plain 'and straight, but its inside middle portion protrudes: "center butted tube with an inner thick wall" (Fig.4). (3) The middle portion of the center butted tube, and its inside middle portion protrudes: "center butted tube with both inner and outer thick walls" (Fig. 5). From Fig. 2, it can be seen that the center butted tube 2 having the outer thick wall 2a can in principle manufactured by a known mechanism consisting of a die 14 which surrounds the tube 2 and a mandrel 15 closely inserted into the inside of the tube 2. The outside diameter of the mandrel 15 is almost equal to the inside diameter of the tube 2 so that the mandrel 15 can be inserted into the inside of the tube to be processed so as to support the external force of the die 14 imposed on the wall of the tube. The tube 2 into which the mandrel 15 has been inserted is then subjected to the cold drawing or extruding process by the die 14 in order to reduce the wall thickness of the tube 2 -except for the middle portion, namely the butted portion 2a, to about 60 % of the original wall thickness, whereby the center butted tube 2 with the outer thick wall 2a is formed. It is understood that Fig. 2 shows the fundamental principle of the known metal processing mechanism for manufacturing a butted tube. In addition, there are many other machines which may be used in the manufacture- of any of the center butted tubes listed under (1)-(3) above: Initially, the length of a tubular starting material is about 60-65 cm. It is elongated to about 90 cm when being subjected to a series of cold rolling, drawing, swaging, and tapering steps, etc. The length of the middle portion of the tubular blank is selected to be about 10-15 cm, and the length of each blade portion is equal to about¯ half the balance, namely, about 34 - 40 cm, respectively. The outside diameter of the tubular material is in the range of 23.8-25.4 mm and the wall thickness is 2-2.3 mm. This size and dimension is merely illustrative, and it is understood that the present invention should not be limited thereby. The wall thickness of the butted portion of the center butted tube listed in (1) and (2) is in the range of 2-2.3 mm which is the original thickness of the tubular starting material, and while the thickness of the two end portions which are reduced by 40 % is in the range of 1.2 - 1.38 mm, which is 60 % of the original thickness. The wall thickness of the butted portion of the center butted tube of (3) is almost the same as that of (1) and (2). The details are omitted. Next, both reduced or thin portions of the center butted tube are subjected to the tapering process by a known swaging machine and the like to form tapered blades. In addition, the section of the butted portion may be shaped elliptically or ovally, as desired. The center butted tubes depicted in Figs. 3-4 are then subjected to the bending process producing about right angles in the same direction and in the same plane at two positions so as to form tubular fork blanks as shown in Figs. 6-8. It should be noted that the butted portion of the tube includes both bent shoulders at two positions as illustrated in Figs. 6-8 in order to give the shoulders sufficient strength to support both blades. It is also to be noted that the longitudinal welded seam -of- the tubular material should face downwardly, in other words, the seam of the butted portion faces downwardly so that the seams of both blade portions between the two blades face each other. The bending operation wherein the straight center butted tube is bent at two places, producing nearly the right angle to form a tubular fork shape with two prongs or blades, is performed by a known method and machine. The bending operation may be carried out either in two steps, bending one blade first and then the other or in a single step bending two blades simultaneously. Fig. 9 is a simplified view showing a front fork blank 8 with two blades 8a and 8b of the single unit type. A socket 8c is to be formed by the oil hydraulic bulgeforming process which-is described in detail hereinbelow. It is to be noted that the front fork blank 8 of the single unit type as shown in Fig. 9 should be compared with a disassembled front fork of the prior art illustrated in Fig. 11. The conventional front fork is manufactured by joining two blades to a crown member shown in Fig. 10 by brazing and the like. As can be seen the front fork of the single unit of the present invention requires neither a crown nor a brazing step. Thus it is clear that the manufacturing cost of this invention can be considerably reduced as compared with that of the prior art. As regards the manufacture of a crown member depicted in 10, such a crown member is at present generally manufactured in two methods: (a) a molten metal is cast into a crownshaped mold, and (b) a metal plate is subjected to the metal drawing process in a press machine, and the thus formed product is welded together'to'obtain a finished crown. Accordingly, the- manufacturing cost of the crown should not be negligible. Next, the tubular fork blank of the single unit type shown in Figs'. 6-8 -is set into a sectional die 13 (Fig. 16) consisting of a top-half 13b and a bottom half 13c, and is subjected to the oil hydraulic bulge-forming process to form a socket 8c (Fig. 9). The oil hydraulic bulge-forming process to expand a component of a desired shape from the tubular stock by inserting it into the sectional die 13 (Fig. 16), and introducing a working oil under high pressure into the tubular blank in the die to expand the above component is disclosed in Japan Patent Nos. 417,728; 417,730; 418,055 (1963), and also in "Industrie Anzeiger", 1966, pp. 770 and 1001. Fig. 12 shows a schematic view of a bulge-forming press machine 21 comprising an arch frame 22, a vertically reciprocating ram 23 suspended from the top, and a pair of horizontally reciprocating opposed rams 24,25 at either side. The top half 13b of the die 13 is secured to the vertical ram 23 with its recess facing downwardly, and the bottom half 13c is bolted onto the bed 26 with its recess facing upwardly so that the tubular fork blank is set within the die 13 when the top half 13b is exactly put on the bottom half 13c. It is to be noted that Fig. 13 of the drawings of this invention is a modified plan view illustrated in Fig. 3 of US-A-4 051 704 entitled, "Method for the Manufacture of Ornamental Head Lug of Single Unit Type for Bicycles" invented by S. Kumura. In this US-A-4 051 704, a straight tubular blank is set in the sectional die and subjected to the oil hydraulic bulge-forming process to expand two pro tuberant components with the hydraulic oil delivered from both# end openings of the tubular blank. However, in this invention,-when a tubular fork blank is subjected to the oil hydraulic bulge-forming process, the left side of the plan view of Fig 13 is no longer required, because the hydraulic oil should be delivered from the right side only as both end openings of the tubular fork are directed to the same right side. Therefore the oil and pressure lines depicted at the left side of the view of Fig. 13 are closed to be inoperative. The tubular fork blank 8 has two tubular blades 8a,8b, with the result that there is a hollow void from one end opening of the one blade 8a to the other end of the other blade 8b as seen- from the top. First, the hydraulic oil which is not pressurized is introduced into one end opening of the blade in the direction of-the arrow as shown in Fig. 14 with the other end opening being closed to fill the whole tubular fork 8 in the die 13 with the oil. When both end openings are tightly closed, the oil is put under pressure by applying the high pressure pump 18 instead of the oil supply pump 19 to attain a desired pressure value of 950-1200 kg/cm2. Recent research has proved that the highest value of the oil pressure reaches up to 1500 kg/cm2. In Fig. 13, piston 16, 16a are housed in cylinders 17,17a, respectively, but the piston 16 is inoperative and a passage 20 is closed as described hereinbefore. The oil and pressure are delivered from a central bore shown as a dotted line and provided within the piston 16a. It is to be noted that the central bore of the piston 16a is divided into two parts, namely, tow end openings of two blades. In Fig. 16, the sectional die 13 is provided with an opening 13a at its end, and a mandrel 12 shown in Fig. 15 is so designed that it can be inserted into the opening 13a. The mandrel 12 has a flange 12b at its one end while a concave recess 19a is formed at other end. In carrying out the bulge-forming process of this invention, both processes as shown in Figs. 13-14 are performed in a similar way. The process of Fig. 13 is somewhat different from that of Fig. 14 in that the mandrel of Fig. 15 is not used in the former process while the mandrel must be applied in the latter. The sectional die 13 is provided with a recess corresponding to a desired shape, and in the present invention the recess is formed as a socket 8c at the middle of the top or shoulder of the tubular fork blank 8. As soon as the working oil 27 with a pressure of 950-1500 kg/crn2 is forced into the tubular fork blank 8 in the die 13, the middle top 8c is bulge-formed into the socket-like recess 8c of the die 13 (shown as dotted line in Fig. 14). At the very beginning, the production of a bicycle part or similar product from a tubular material by the bulge-forming process was considered almost infeasible, but mass production of various bicycle parts by the bulge-forming process has been in full swing for the past twenty years. The height of the bulge-formed socket 8c may be 5-10 mm, as desired. Next, the bulge-forming process in accordance with the teaching of Fig. 14 will be described hereinbelow. As soon as the pressure of the hydraulic oil filled in the tubular fork blank set in the die 13 has reached the value of 950-1500 kg/cm2, the mandrel 12 with the recessed female die 12a is caused to impinge on the middle top of the tubular blank 8 in order to bulge-form the socket 8c. As can be seen, the concave recess 12a of the mandrel 12 corresponds to the contour of the socket 8c to be bulgeformed, in other words, the socket 8c acts as a male die while the recess 12a acts as a female die. It is understood that the die 13 of Fig. 13 is provided with the recess 8c which corresponds to the contour-of the socket 8c. It is to be noted that the socket 8c should not be opened simultaneously at the beginning of the high pressure oil bulge-forming step, but should be opened by severing its closed top after the bulge-forming step. The tubular front fork blank thus manufactured by the bulge-forming process from the center butted tube is an unfinished front fork product, from which a finished front fork article consisting of stem (not shown), socket on the crown integral with two blades, and fork ends (not shown) is obtained by the known various working step. Finally, the capacity of the rams now in operation is described: vertical ram 23 is 120-360 t.; horizontal ram 24 or 25 is 60-180 t.; and the oil hydraulic pressure is 950-1500 kg/cm2. The ferrous tubular blank can be bulgeformed with the die being subjected to the combined load mentioned above.

Claims: Claims: 1. A method for the manufacture of a front fork blank of the single unit type for use in bicycles from a center butted ferrous tubular material having a middle butted portion and both thin end portions produced by sub jecting a straight ferrous tubular material to a known metal drawing process in a machine provided with a mandrel inserted into said straight tubular material and a drawing die, comprises a series of steps of bending said two thin end portions of said center butted tubular material in the same direction and in the same plane re lative to said middle butted portion constituting the top to form a tubular fork having two parallel blades; providing an opening at one end of a sectional die consisting of a top half and a bottom half and a mandrel with a concave recess designed to mate with a protuberant socket to be bulge-formed on said middle butted portion of said center butted tubular fork, said mandrel being capable of being inserted into said opening of said die; inserting said center butted tubular fork into said die designed to bulge-form said protoberant socket on said middle portion of said tubular fork; subjecting said tubular fork in said die to the combined load of a vertical compressive load, axial compressive loads, and a high pressure oil supplied into the interior of said tubular fork from both end openings of said tubular blades; and apply ing the known oil hydraulic bulge-forming process to bulge-form said protuberant socket on the middle of said tubular fork material. 2. Method as claimed in Claim 1 wherein said sectional die has no opening at one end, the interior of said sectional die is provided with a concave recess de signed to mate with said protuberant socket to be bulge-formed on said middle butted portion of said tubular fork material, wherein no mandrel is provided. 3. Method as claimed in Claims 1 or 2 wherein said butted middle portion of said center butted tube is formed with a wall outwardly protruding relative to said straight thin wall of said center butted tube. 4. Method as claimed in Claims 1 or 2 wherein said butted middle portion of said center butted tube is formed with a wall inwardly protruding relative to said straight thin wall of said center butted tube. 5. Method as claimed in Claims 1 or 2 wherein said butted middle portion of said center butted tube is formed with walls both outwardly and inwardly protruding relative to the middle longitudinal line of said center butted tube. 6. Method as claimed in any of Claims 1 to 5 wherein said ferrous tubular fork blank for use in the bicycle has about 90 cm in length, the wall thickness of said center butted portion is in the range of 2-2.3 mm, the length of said center butted portion is in the range of 10-15 cm, the outer diameter of said center butted portion is in the range of 23.8-25.4 mm, the wall thickness of both thin end portions is in the range of 1.2-1.38 mm, and the original length of said straigth ferrous tubular material is in the range of 60-65 cm; and its chemical composition is less than 0.12 e C, 0.35 % Si, 0.25-0.60 % Mn, 0.040 & P, and 0.040 % S maximum, respectively, the remainder being Fe; and its physical properties being as follows: tensile strength 32-42 kg/cm2, elongation 30-40 %.

Inventors: KIMURA, ISAO
Applicants: KIMURA, ISAO
Application year: 1983
Main classification: B21D53
IPC codes: B21D53