DESCRIPTION PRECISE COLUMNAR MEMBER AND METHOD OF PRODUCING THE SAME FIELD OF THE INVENTION The present invention relates to a metallic, precise columnar member having an outer diameter with high accuracy and a method of producing the same. In particular, the invention relates to a precise columnar member comprising a plurality of thread ridges or ring-shaped thread ridges provided on a circumferential surface of a metallic columnar blank in an axial direction and worked to be flat so that the tops of the thread ridges or ring-shaped thread ridges have a predetermined outer diameter, and to a method of producing the same.

BACKGROUND OF THE INVENTION In recent years, a need of metallic, precise columnar members having an outer diameter with high accuracy has increased rapidly. These precise columnar members are used for shafts, positioning pins and centering bolts in various precision machines, and control valves in a dripping structure for liquids. A high accuracy of an outer diameter dimension, for example, 0. 01 mm in columnar members having a diameter of about 10 mm and a length of about 35 mm, has been required for these applications.

The precise columnar members with high accuracy have conventionally been manufactured by cutting an outer periphery of a metallic blank with a turning process, and the final finishing of the blank by means of grindstone,

abrasive paper or free grinding particles such as buffing have been performed thereafter. When the machining of metals is used, it is possible to manufacture precise columnar members smith high dimensional accuracy of 0. 01 mm for columnar members of about 10 mm in outer diameter although the obtainable accuracy varies depending upon the outer diameter of columnar members.

With the machining of metals, however, much time is consumed in the cutting process and polishing process, which problematically leads to high processing cost. Since a metallic blank must be lager in outer diameter than a final product for the machining of metals, there is a problem associated with a poor yield of blank. Further, the machining of metals produces chips and ground powders, resulting in the worsening of working environment.

Plastic working has also been investigated to replace the machining of metals. One of the plastic working processes is diameter-reducing working, for example, drawing, using a rod-shaped or wire-shaped metallic blank.

The diameter-reducing working is defined as a method in which a rod-shaped blank is drawn through a die provided with a die hole which has a cross sectional area smaller than the blank to provide a product having the same cross sectional shape as that of the die hole. Since the plastic working can shorten working time as compared with the above-mentioned cutting and polishing processes, it is possible to reduce manufacturing cost.

However, since cold working is necessary to apply the diameter-reducing working to obtain products with high accuracy, there is caused a problem that a die tool is

short in service life due to a large working load on a die.

Heat treatment of annealing may be necessary to remove a large plastic deformation generated in a metallic blank after cold working. There is also caused a problem that since the metallic blank is expanded radially by spring back involved in cold working, it is difficult to obtain an outer diameter with high accuracy. Accordingly, the diameter-reducing working can not satisfy the accuracy requirement of 0. 01 mm as compared with the machining of metals. It is only possible to manufacture columnar members having an accuracy of 0. 1 mm.

Further, inventors partly common to those of the present invention have filed an international application with respect to the invention of a manufacturing method of columnar members, which application was published in the international publication (W099/39851). The invention relates to a method of forming uniformly a large number of longitudinal grooves around the outer periphery of a metallic columnar blank, and crushing uniformly the distal end portions of thread ridges into a flat shape to obtain a predetermined outer dimension. It is possible to manufacture precise columnar members having an accuracy of around 0. 01 mm.

With the method described above, however, only a portion being worked unstably contacts a rolling die when the longitudinal grooved blank is worked, for example, by means of rolling, whereby the grooves are liable to generate burrs, which limits a producible length of columnar members. When the second rolling is applied to crush the distal end portions of thread ridges into a flat

shape, the working leads to unstability because of the two manners in contact. One is that the rolling die contacts the distal end portions of thread ridges on one thread ridge and the other is that it contacts two thread ridges.

To solve the first problem, a splined wire blank produced by the spline drawing was investigated as a raw material for working of longitudinal grooves. However, the splined wire blank is expensive as compared with ordinary wire blank because of the short service life of a die in production. Also, when drawing is employed in place of the second rolling, a die for drawing suffers from a local abrasion since it only contacts longitudinal thread ridges during working.

SUMMARY OF THE INVENTION The present invention has been made to solve the problems in a metallic, precise columnar member in the prior art. The present invention provides a technique capable of inexpensively manufacturing a metallic, precise columnar member having the same dimensional accuracy as that obtained in the case of the machining of metals, without worsening of working environment and reduction in material yield. Hereupon, the metallic, precise columnar member means a precise columnar member manufactured from various metallic materials such as carbon steel, stainless steel, copper, and aluminum.

A precise columnar member according to the present invention characteristically comprises a plurality of flat thread ridges or ring-shaped thread ridges, provided on a circumferential surface of a metallic columnar blank in an axial direction and worked to be flat to meet a

predetermined outer diameter. Also, a method of manufacturing a precise columnar member, according to the invention, comprises the steps of using rolling to form a plurality of thread ridges or ring-shaped thread ridges on a circumferential surface of a metallic columnar blank in an axial direction to obtain a preformed blank, and using rolling or diameter-reducing working to make the tops of the thread ridges or ring-shaped thread ridges of the preformed blank flat to obtain a precise columnar member having a predetermined outer diameter. The rolling or diameter-reducing working for the preformed blank can be also applied to a part of the preformed blank in the axial direction.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side view showing a method of manufacturing a precise columnar member, according to a first embodiment of the invention.

Fig. 2 is a side view showing a method of manufacturing a precise columnar member, according to a second embodiment of the invention.

Fig. 3 is a side view showing an application of the precise columnar member obtained in the first embodiment with a part thereof in cross section.

Fig. 4 is a side view showing the case, in which the precise columnar member obtained in the first embodiment is used as a centering bolt, with a part thereof in cross section.

Fig. 5 is a cross sectional photograph showing material flow, in which distal end portions of thread ridges or ring-shaped thread ridges of the precise columnar member manufactured by the method of the invention flow into thread grooves.

DETAILED DESCRIPTION OF THE INVENTION The present invention will be explained below in detail with reference to the drawings.

Fig. 1 is a view showing a method of manufacturing a precise columnar member, according to the first embodiment of the present invention. First, a metallic columnar blank 1 is prepared, and a plurality of thread ridges 2 are formed on a circumferential surface of the columnar blank in an axial direction by the first rolling using a rolling die. The first rolling presses metal in areas corresponding to the thread grooves 3 of the columnar blank 1 to make the same protuberant to form the thread ridges 2, whereby an outer diameter of the distal ends of thread ridges of a preformed blank 4 increases relative to that of the initial columnar blank 1. The process using rolling forms no chips.

The raw material of a metallic columnar blank can meet allowance of wire diameter and radial deviation prescribed in normal carbon steel wire for cold heading and cold forging (JIS G3539). In the invention, an outer diameter of the distal ends of thread ridges of a preformed blank increases relative to that of a columnar blank. When the outer diameter of product is, for example, 10 mm, a steel wire having the outer diameter of 9.5 to 9.7 mm can

be used as the blank.

The thread ridges 2 formed by the first rolling are not specifically limited in cross sectional shape, for example, flank angle of thread (half angle of thread).

However, taking into account of the cost-performance ratio in manufacture of dies, a normal pattern of a rolling die, that is, threads having a flank angle of 30° is preferable.

Then, the second rolling or diameter-reducing working is used to flatten the distal ends of thread ridges of the preformed blank 4 so that the tops of the respective thread ridges 2 have a predetermined diameter, whereby a precise columnar member 5 is obtained. The second rolling can use a flat type rolling die or a roller-rolling die. A ratio of a width of a flat portion to a width of a groove is not specifically limited but can be varied in accordance with use. To obtain a precise columnar member having a dimensional accuracy of 0. 01 mm, the die spacing is adjusted in the case of a rolling die and an inner diameter tolerance is adjusted to 0.005 to 0.01 mm in the case of the diameter-reducing working.

Fig. 5 is cross sectional photographs showing material flow, in which the distal end portions of thread ridges or ring-shaped thread ridges of the precise columnar member manufactured by the method of the invention flow into thread grooves. As shown in the figure, in the case where the second rolling is used to flatten, the distal ends of thread ridges flow into both sides of thread grooves. In the case where the drawing is used to flatten, the distal ends flow to an opposite side to a direction of

drawing, so that the distal ends of thread ridges fill the thread grooves. Therefore, the width of openings of the thread grooves is narrowed, and so the thread grooves decrease in cross sectional area. Accordingly, there is produced no spring back, which is observed in the case of a conventional plastic working, whereby it is possible to easily manufacture a column member having a dimensional accuracy of 0. 01 mm in outer diameter, which is the same order as that in a conventional machining of metals. The present work will not cause chips and ground powders and is high in yield of product. Further, since the work involves small local plastic deformation, working load on a metallic die mold is small and the metallic mold can be extended in service life.

The precise columnar member 5 thus obtained comprises the tops 2 of a plurality of thread ridges, which are flattened such that flat portions have a predetermined outer diameter, so that it can be lightened in weight by, for example, 2 to several percents as compared with a solid columnar member. Since it is possible to make a dimensional accuracy of outer diameter the same as in the conventional machining of metals, the column member is suitable for shafts and positioning pins in various precision machines.

Further, since the obtained precise columnar member 5 is formed on its outer peripheral surface with the thread ridges 3, it can be used as a control valve for dripping liquids little by little from a container or a piping as shown in Fig. 3. In this case, since the thread grooves can be made smaller in cross sectional area than those produced by a conventional threading work, the dripping

amount of liquid can be controlled in a much small region.

In addition, while Fig. 3 illustrates an example of a bolt- shaped precise columnar member, in which a head 11 is provided integrally on one axial end of the columnar member 5, the head 11 can be formed by the normal cold pressing in the manufacture of bolts.

Fig. 2 is a view showing a method of manufacturing a precise columnar member, according to the second embodiment of the present invention. This embodiment is the same as the first embodiment except that a preformed blank 4 having a plurality of ring-shaped thread ridges 2 is formed by means of rolling using a rolling die. Accordingly, the ring-shaped thread ridges 2 can be made to have the same cross sectional shape as that of a normal thread ridge except that they are not spiral.

The preformed blank 4 is further subjected to the second rolling or diameter-reducing working in the same manner as in the first embodiment such that the distal ends of the ring-shaped thread ridges 2 are collapsed to form flat tops to provide a precise columnar member having a predetermined outer diameter at the tops.

It is not necessary that the precise columnar member according to the present invention be a columnar member having flat tops on thread ridges over the whole length of an axial direction, as shown in Fig. 4. The normal threads 12 are formed on a columnar blank 1 by means of rolling. A flat type rolling die is then used to flatten a part of the thread ridges or the ring-shaped thread ridges 2 to rolling, thus enabling tops 2 on the threads according to the invention.

In this case, the normal threads 12 are rolled coaxially with the tops of the threads of the invention.

Accordingly, when the precise columnar member 5 is mounted to a member 13 through the threads 12 to be used as a centering bolt, it is possible to obtain a core (axis) of a columnar member having excellent centering accuracy. The centering bolt is suitable for use, in which it is necessary to reduce a clearance of a bolt hole 15 when a member 13 being clamped is fixed to a casing in a correct position. The severe accuracy of hole diameter in correct positioning of a bolt is needed in the case where a plurality of clamping locations are included in a single part and the case of a pump casing where precision equipment casings are clamped to internal contents.

Example 1 As a columnar member, a chromium steel blank for steel wire for cold heading and cold forging (JIS G4104, SCr415) having a wire diameter of 8.7 mm was cut to a length of 22 mm. The blank was then subjected to roller rolling to provide ring-shaped threads having a pitch of 1.0 mm and a lead angle of 0°. The outer diameter of the threads formed was 9.3 mm¢. Subsequently, the preformed blank was subjected to drawing and finally manufactured to columnar members having an outer diameter of 9.035 mmf and a length of 22 mm. Five columnar members had an outer diameter in the range of 9.034 to 9.035 mm and a deviation in outer diameter among them was only 0.001 mm.

Example 2 A columnar blank having a wire diameter of 7.7 mm

and a length of 20 mm was used to manufacture a preformed blank in the same manner as Embodiment 1 except that the tops of ring-shaped thread ridges had an outer diameter of 8.3 mm The preformed blank was then subjected to drawing and finally manufactured to columnar members having an outer diameter of 7.997 mm and a length of 20 mm. Five columnar members thus manufactured had an outer diameter in the range of 7.997 to 7.998 mm and a deviation in outer diameter among them was only 0.001 mm.

INDUSTRIAL APPLICABILITY As described above, the characteristic method in the invention can manufacture a precise columnar member with high accuracy because of the avoiding spring back, involves small work load on a metallic mold and necessitates neither cutting nor polishing, so that it is possible to maintain a favorable working environment. Further, since it is possible to use a metallic material having an outer diameter smaller than that of a final product, yield is very high. Further, since the precise columnar member has narrow thread grooves, the weight thereof can be reduced.

The invention has various advantages in combination in addition to the precise columnar member having a high dimensional accuracy.

The precise columnar member according to the invention is applicable to shafts, positioning pins and centering bolts in various precision machines, and control valves used in a dripping structure for liquids.