GB2238266A

GB2238266A - Press die

Info

Publication number: GB2238266A
Application number: GB9020764A
Authority: GB
Inventors: Yoshimitsu Usuba; Yasuo Imaizumi; Shinji Hoshii
Original assignee: Fuji Jukogyo KK; Fuji Heavy Industries Ltd
Current assignee: Subaru Corp
Priority date: 1989-09-27
Filing date: 1990-09-24
Publication date: 1991-05-29
Also published as: JP2815417B2; DE4030314A1; JPH03114620A; GB9020764D0

Abstract

An upper die 11 is provided with a forming surface 11a and a holding surface 11b surrounding the forming surface 11a. A lower die 12 of the press die includes a lower die body 13 having a forming surface 13a, for pressing a work W in cooperation with the forming surface 11a, and a holding member 14, separate from the lower die body 13 and arranged around the lower die body 13. The holding member 14 has a holding surface 14a for holding a peripheral portion of the work W in cooperation with the holding surface 11b of the upper die 11 in a sandwiched manner. At least one surface out of the forming surfaces 11a, 13a and the holding surfaces 11b, 14a has a scabrous or ribbed area (21 Fig 2A) for reducing slip resistance thereof, so as to allow slippage of the work W in the press die when the work W is pressed. The area (21) may be formed by milling and coated with chromium (25, Fig 2B). <IMAGE>

Description

4 ? 3 a =_ E, C5 PRESS DIE The present invention relates to a press die

for pressworking sheet metal used for an outside or an inside panel of an automobile.

Forming and work holding surfaces of the press die and die blocks are conventionally machined by a ball end mill or a taper cutter of a profile or an NC milling machine. Then, cutter marks or cusps which have been formed in the machined surfaces of the die blocks due to the cutting operation are removed by electro-discharge, electro-chemical machining, ultrasonic polishing, a hand power tool. The process is performed to prevent the forming and the work holding surfaces of the press die from producing scars in a work as well as adjusting the clearance between the upper and lower dies. The forming and the work holding surfaces are plated with a hard chromium layer to enhance frictional resistance (see Japanese (unexamined) Patent Publication No. 63(1988)138924).

The scars are prevented from being produced in the work by grinding and smoothening the forming and the work holding surfaces of the press die in the above-mentioned manner. However, such operations necessitate so many steps and take considerably long production time, so that production cost rises. Moreover, if the forming surface of a lower die has a step portion, part of the work and an upper flat surface of the step portion make a. surface contact before bottom dead center in pressworking, so that an excessively large slip resistance to the work occurs. Therefore, another part of the work is locally stretched while the part of the work is not stretched. This causes the work to break. To avoid this problem, the press die must be reground, the clearance between the upper and lower dies is slightly adjusted, or the configuration of 1 the pressed work is modified.

Accordingly, it is an object of the present invention to provide the press die which enables to eliminate or at least reduce the manhours of the grinding operation thereof, thereby reducing production time and cost of the press die.

It is another object of the present invention to provide a press die which stretches the work in a whole area thereof by reducing frictional resistance between the work and the press die.

In view of these and other objects, the present invention provides a press die comprising: an upper die including a first forming surface and a first work holding surface surrounding the first forming surface; a lower die 0 0 body having a second forming surface for pressing a work in cooperation with the first forming surface of the upper die; and a work holding member, separate from the lower die body and adapted to be arranged around the lower die body, the work holding member having a second holding surface for holding a peripheral portion of the work in cooperation with the holding surface of the upper die, and wherein at least one of the first and second forming surfaces and the holding surfaces has a scabrous area for reducing slip resistance thereof, so as to allow a slip of the work in the press die when the work is pressed.

In the drawings:

FIG. 1 is a vertical section of a press die according to the present invention; FIG. 2A is a fragmentary enlarged perspective view of a surface of the press die of FIG. 1; FIG. 2B is a fragmentary enlarged perspective view of the surface of FIG. 2A, in which the surface is coated for enhancing wear resistance; FIG. 3 is a fragmentary enlarged perspective view of 2 -9 1 -X a modified form of the surface of FIG. 2A; FIG. 4 is a fragmentary enlarged vertical section of the working surface of FIG. 3; FIG. 5 is a fragmentary enlarged vertical section of the surface of FIG. 3, in which the surface is also coated with a wear resistance layer; FIG. 6A is a schematic illustration of a press die of the prior art during press working of a work;

FIG. 6B is a schematic illustration of a press die of the present invention during pressing working of a work; and FIG. 7 is a vertical section of a press die according to the prior art.

Referring now to the drawings, corresponding parts are indicated by same reference characters.

Prior to the description of the preferred embodiments of the invention, known press dies discussed before will be described briefly below for a better understanding of the background art and disadvantages thereof.

FIG. 7 illustrates a typical example of known press dies, in which an upper die 11 is provided with an upwardly concave forming surface lla and an annular work holding surface llb integrally formed at a periphery of the forming surface Ila. The press die further includes a lower die 12 which is provided with a lower die body 13 and an annular work holding member 14 fitted around the lower die body 13. The lower die body 13 has an upwardly convex forming surface 13a to cooperate with the forming surface lla of the upper die 11. The work holding member 14 is provided with an annular work holding surface 14a to cooperate with the work holding surface llb of the upper die 11. The work holding member 14 is mounted to the lower die body 13 so that the work holding surface 14a surrounds the forming surface 13a as shown. The work holding member 14 is connected to a die cushion (not shown) which is 3 1 - 1.

actuated by hydraulic or pneumatic pressure through cushion pins 15, so that the work holding member 14 is normally held at an elevated position shown by a dots-anddash line at a predetermined height above a depressed position shown by the solid line.

In pressing operation, the upper die 11 is elevated to a top dead center position and then a work W is set to rest at a periphery Wa of the work W on the work holding surface 14a which is at the elevated position.

Thereafter, the upper die 11 is lowered so that the work holding surface 11b thereof comes into contact with the periphery Wa to hold the work W in cooperation with the work holding surface 14a when the upper die 11 comes to the position shown in FIG. 7. The upper die 11 further descends to lower the work holding member 14 against the upwardly biasing force by the die cushion to a pressed position. Thus, the concave forming surface 11a is forced to fit over the convex forming surface 13a of the lower die body 13 through the work W, so that the work W is formed into a predetermined shape.

To machine the forming surfaces and the work holding surfaces of the press die, die blocks are conventionally cut and then, cutter marks or cusps which have been formed in the machined surfaces of the die blocks due to the cutting operation are removed by grinding and smoothening the machined surfaces. In this manner, scars are prevented from being produced in the work W during pressing with the press dies. However, such grinding and smoothening operations necessitate many steps, which raise production time and cost. Moreover, in the pressing operation, part of the work W and the convex forming surface 13a of the lower die body 13 make a surface contact which provides a rather large slip resistance to the work W. Thus, the work W is locally excessively stretched when the lower die body 13 is at a position about 5 mm above the lower dead center. This causes 4 t breaking of the work W.

To overcome these problems, according to the present invention, the cusps formed in at least one of the forming surface lla, the forming surface 13a, the work holding surface llb and the work holding surface 14a are remained without being ground. In FIG. 1, a press die of the present invention is illustrated. Die blocks are machined by a profile milling machine or an NC milling machine to form the forming surface lla, the forming surface 13a, the work holding surface llb and the work holding surface 14a. The machining operation may be performed by using a ball end cutter with a 25 mm radius end mill at a pitch (pick-feed) 3 mm, for example. As shown in FIG. 2A, cusps 24 are formed in each of the die blocks by the milling operation and provide a scabrous or roughened surface 21 to each die block.

These scabrous surfaces 21 may be coated with a hard chromium layer 25 having a thickness from about 20 to about 30 micrometers for enhancing wear resistance as shown in FIG. 2B.

In pressing the work W, the press die of this embodiment makes a line contact or a point contact with the work W since the forming surface lla, the forming surface 13a, the work holding surface llb and the work holding surface 14a are scabrous surfaces 21 wih many cusps 24. Thus, the press die provides a low slip resistance to the work W as compared to the press die of the prior art which makes a surface contact with the work W. For this reason, in this embodiment the work W is stretched substantially in the whole area of the forming surface 13a except a small area a of a central portion of the work W as illustrated in FIG. 6B, and hence the press die exhibits an excellent drawing property. On the other hand, in the prior art the work W is not'stretched substantially at an area b thereof, which is larger than the area a of the present invention.

( X In this embodiment, the work W is also allowed to stretch between the work holding surfaces llb and 14a to further enhance drawability of the press die and hence an excellent pressworking is achieved.

As shown in FIG. 3, cusps 24 may be truncated to form flat top surfaces 24a to effectively prevent scars from being produced in the work W during the pressing operation. When the work W is machined by a ball end cutter with a 25 mm radius end mill at a pitch (pick-feed) 3 mm, the cusps 24 are truncated preferably at a height H about 5 to about 10 micrometers from the bottom 24b thereof as illustrated in FIG. 4. In this case, arcuate surfaces 22 formed by the milling operation have a radius R about 25 mm. The scabrous surfaces 21 thus formed may be also coated with a hard chromium layer 25 about 20 to about 30 micrometers thick for improving wear resistance (FIG. 5).

In the preceding embodiments, all the surfaces of the press die, i.e., the forming surface lla, the forming surface 13a, the work holding surface llb and the work holding surface 14a are formed to have scabrous surfaces 21, but a suitable portion or portions of the press die may be provided with scabrous surfaces 21 according to configurations and materials of the work W. The cusps 24 may be formed in appropriate directions.

While the presently preferred embodiments of the present invention have been shown and described, it is to be understood that these disclosures are for the purpose of illustration and that various changes and modifications may be made without departing from scope of the invention as set forth in the appended claims.

6 1 1

Claims

Claims:

1. A press die of the type including: an upper die having a first forming surface and a first holding surface surrounding the first forming surface; a lower die body having a second forming surface for pressing a work in cooperation with the first forming surface of the upper die; and a holding member, separate from the lower die body and adapted to be arranged around the lower die body, the holding member having a second holding surface for holding a peripheral portion of the work in cooperation with the work holding surface of the upper die, characterized in that:

at least one of the first and second forming surfaces and the work holding surfaces has a scabrous area for reducing slip resistance thereof, so as to allow a slip of the work (W) in the press die when the work (W) is pressed.

2. A press die as recited in Claim 1, wherein:

the scabrous area is provided by cutting the at least one surface; and the scabrous area comprises arcuate surfaces, adjacent arcuate surfaces intersecting to form a cusp,

3. A press die as recited in Claim 2, wherein the cusps are truncated to form flat top surfaces, respectively.

4. A press die as recited in Claim 3, wherein:

each of the arcuate surfaces - is formed with a radius (R) about 50 mm and a pitch about 3 mm; each of the truncated cusps has a height (H) ranging from about 5 to about 10 micrometers from a bottom thereof.

5. A press die as recited in Claim 4, wherein the scabrous area comprises a wear resistance layer coated over the cusps.

6. A press die as recited in Claim 5, wherein the 7 wear resistance layer is a plated hard chromium layer having a thickness from about 20 to about 30 micrometers.

1 1

7. A press die substantially as hereinbefore described with reference to and as shown in figures 1 to 5 and figure 6B.

1 K 8 Published 1991 at The Patent Office. State House. 66/71 High Tiolborn. London WCIR47P. Further copies may be obtained from Sales Branch, Unit 6. Nine Mile Foint.. Cwmfelinfacb. Cross Keys, Newport, NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray, Kent