February 2012 Archives

Insert type die shown in Fig. 1 is a generic name given to die using an insert type plate. Insert type plates are used frequently as die plates or as stripper plates. Very often dies using this type of plates are called precision dies, and from the point of view of accumulation of error, the positioning accuracy is lower compared to integrated dies. The reasons why they are considered precision in spite of this are the following.

1. It is possible to machine the inserted components with high accuracy.
2. It is possible to adjust the positioning accuracy by moving the insert plate.

In recent precision dies, it has become very common to assemble the dies with the stripper as the reference. Stripper reference means that the relationships such as the tolerances, etc., are adjusted by adjusting the die to correspond to the punch which is guided by the stripper. The adjustment becomes easy because of using the insert type. The changes in the method of preparing inserts are explained below.

1. Although the shape of the hole for an integrated block type insert is prepared by the wire cut electric discharge method (WEDM), etc., which is not different from the method of preparing an integrated type insert. This is used when wanting to increase the positioning accuracy.
2. A split type insert is intended to be prepared by machining the shape by grinding. This is used for increasing the accuracy of the shape of the hole or for increasing the accuracy of the second draught, etc. The positioning accuracy is the same as that of integrated type inserts.
3. Inserts using spacers are designed for the purpose of making it easy to adjust the positioning accuracy. The positioning adjustment of inserts is made using shims (thin plates). Expertise is required for the handling of shims. This can be said to be an insert type which has been improved by making it easy to carry out adjustments using thick spacers. Position adjustment is not possible in the case of round inserts.

In the case of insert type dies, when the tip of a die gets worn out, only the insert is ground. The level is adjusted by putting a shim corresponding to the extent of grinding. In the case of integrated type inserts, since the grinding is done entirely, the die height changes.
In the case of insert type dies, by using materials with high wear resistance (cemented carbide, etc.) for the inserts, it is possible to extend the life of the dies.
Although round blocks are suitable from this aspect and are suitable for regrinding, there are problems in using them from the point of view of increasing the positioning accuracy.

The backing plate is a very important die component which is also called a back up plate (perhaps this is a more accurate name), a back plate, or a backing, etc. It is used as shown in Fig. 1.

However, a backing plate is not necessary for all dies. When a large force acts on a small area such as in the case of a hole opening punch, the punch gets pressed and can dent the punch holder. A backing plate is necessary in such situations. It is not necessary as long as such a condition does not arise.

Because of this purpose of use, a certain amount of hardness is required of the backing plate, such as a hardness of about 56 HRC. If it is too hard, it can get cracked. Generally tool steel (SK steel) or special tool steel (SKS steel) is used.

Among the plates constituting dies, a backing plate is the one that goes mostly unnoticed. Therefore, it does not get much attention. Changes in its condition are very rarely checked during die maintenance, and not much attention is paid even if there are any dents or deformations in it. However, paying it as much attention as that given to punches or dies and maintaining its surface in good condition leads to maintaining the die quality.

In a press machine, there is a maximum height of the die that can be installed. If this maximum height is exceeded, it is not possible to install the die in the press machine. There are two methods of expressing this height, namely, die height and shut height.

These are explained using Fig. 1. When the slide is at the bottom dead center position and the slide adjustment screw has been raised to the topmost position, the distance from the bottom surface of the slide to the surface of the bolster plate is called the "die height". When the bolster plate is removed from the press machine, the surface on which the bolster plate was is called the "head surface", and the "shut height" is the distance from this head surface to the bottom surface of the slide.
Some people might have thought that these two heights are the same, but they are not, and it is important to understand the difference between them. However, recently, since the bolster plate is almost never removed from the press machine, there may not be any problem if these two words are being used interchangeably.

However, why this shut height is used is because in the olden days it was difficult to possess a press machine because it was very costly. In those days, the bolster plate was being changed to suit the application. For example, when carrying out a blanking operation, since the work becomes easy if the blanked part is made to fall through a hole in the bolster plate, a bolster plate with a hole at the center was being used. In compression forming, since it is necessary to firmly support the bottom die, a bolster plate without any hole in it was being used.

For the sake of reference, when a die lower than the maximum adjustable height is installed, it is easy to install the die if the height is within the adjustment range of the slide adjustment screw. In the case of a die with a height smaller than the adjustment range of the slide adjustment screw, the height is adjusted using spacers (which is normally a block called "Geta" in Japanese). The work of installing a die in the press machine is called "die setting up", and this height adjustment is a big factor that affects the time taken for this die setting up. As far as possible, the height of installation of the dies should be made uniform.

Blanking operation is a typical one among press forming operations. In such a forming operation, a frame is created around the blank as is shown in Fig. 1. The part of this frame in the width direction of the material is called the "edge bridge" and the part in the direction of feeding is called the "feed bridge". These are necessary for forming the contour shape of the blank in blanking. Since these bridges become scrap, making them large wastes the material. Therefore, they are made as small as possible. The limit would be the smallest dimensions that cause no abnormalities in blanking.

Fig. 2 shows the progressive blanking operation. In progressive blanking, the blanking is done successively while moving the material which is kept connected by the bridge part. The "bridge" in this case is not important for the blanking operation but is mainly intended to hold the blank in a stable manner, and to feed the blank to the next process. Therefore, in order to distinguish this from the blanking bridges, most often such "bridges" are called "carriers" in progressive blanking. The three types of carriers shown in Fig. 2 are the double side carrier, the single side carrier, and the center carrier. The double side carrier holds the blank in a stable manner. Depending on the method of using, the instability remains in the cases of the single side carrier and the center carrier.

Although in general the part of the material that connects the blanks is called a "bridge", depending on the application, if the method of using is mistaken, there will be abnormalities in the product quality. Do not make simple judgments merely based on the way it is called.