December 2011 Archives

The operations of assembling and disassembling a die vary depending on the method of affixing the parts constituting the die.
Fig. 1 shows an example of affixing the parts of a die without any considerations. While there are three punches, the method of fixing is different in each of them.
Removing the punch (a) is not possible unless the fixing screw and the dowel pin are detached, and the die is disassembled.
The punch (b) is removed by detaching the stripper bolt, detaching the stripper plate, inverting the top die, and detaching the punch fixing screw.
The punch (c) can be removed by detaching the stripper bolt and the stripper plate, and then detaching the punch without having to turn the top die upside down.
In this manner, if there are many methods of fixing present in a single die, the operations become confusing leading to problems.

It is good to design a die considering the nature of the die and with aim towards unifying the assembling and disassembling methods.

Fig. 2 is a structure in which the punch is fixed from the punch holder side. This is the method of fixing a punch seen very often in the case of dies having punches that are prepared by wire cut discharge machining. A round punch with a flange is fixed considering the ease of detaching it.

Fig. 3 is a structure that makes attaching and detaching the punch easy from the stripper side. When the punch material is an ultra hard alloy, very frequently fixing it using a key is common, and hence this structure is used very often.

The stripper bolt is a part that is related to the method of fixing a punch. It is good to match even the stripper bolt with the method of fixing the punch. There are some people who are afraid of using the stripper bolt (internal thread type) of Fig. 3 because it drops out, but there is no problem unless somebody has forgotten to tighten the bolt properly. Even measures for preventing loosening of the bolt have progressed, by using such improved bolts it is possible to obtain a still higher feeling of assurance.

Although there is no problem with fixing screws for the plates constituting a die, there will be complaints that it is difficult to carry out the work during assembling or maintaining the dies.
Let us consider the reason for this, which is explained using Fig. 1. The standard form of screw fixing will be as shown in (a) for both the top die and the bottom die. There will be no problems with the form shown by (a) in the case of a small die because it is possible to easily separate or turn upside down the top and bottom dies.

As the die becomes larger, separating or turning upside down the top die and the bottom die becomes difficult. Sometimes this work will require two persons. In such cases, if the fixing screw of the bottom die is of the form (a), the screws have to be tightened after turning the dies upside down. If the work has to be done in this condition, doing this work will become difficult. In view of this, if the fixing is done as shown in (b) or (c) the work can be done without turning the dies upside down.

Since very often, after the top die and the bottom die are separated, the top die is inverted and put down, the work becomes easy if the method of fixing shown in (b) or (c) is used. At this time, if the stripper plate is of the movable type, since in the case of (b) the screw tightening work cannot be done unless the stripper plate is removed, the form of fixing shown in (c) may be good.

When fixing the punch backing plate using the form of screw fixing shown in (c), the two plates, the punch plate and the punch backing plate are integrated using the fixing method of (d) or (e). Integrating the backing plate and the body plate or integrating the stripper plate and the die plate are also done in a similar manner. When the surface of a die plate, etc., is machined without separating the backing plate, etc., it will be difficult to carry out the machining work if the method of using the fixing screws is like (e).

Even fixing by screws of parts that go on top of the die plate, such as the guide plate, will be like (f) or (g), but the escape on the side of the stripper plate becomes cumbersome, or providing this escape may be forgotten sometimes. If there is no problem with the thickness of the plate, it is good to sink the head part of the screw as in (f). Some techniques can be thought of since there are some screws available with thin heads.

Although it looks obvious when explanations such as the above are given while showing the figures, in actual dies fixing screws are very often used without thinking. It is necessary to grasp the features of the die structure and to use the fixing screws that make the work easy. Sometimes, because of this, it may also be good to change the structure of the dies.

The stud part of the punch (shaft = D) is held and supported when inserted into the hole in the punch plate. The methods of supporting are divided into two types of the fixed stripper structure shown in Fig. 1 (including even a movable stripper structure in which the tip of the punch is not guided by the stripper) and the structure shown in Fig. 2 in which the tip of the punch is guided by a movable stripper. These are explained below.

In a mold having the fixed stripper structure of Fig. 1, since it is necessary to maintain the position and perpendicularity of the punch by the punch plate, the hole for inserting a round punch has to be a press-fit hole. The "tightening margin" is very often set at 0.005 mm to 0.01 mm and it is very rare to use a mating tolerance. When a small number of punches are to be inserted in a punch plate, there is no problem even if a mating tolerance is used. However, when a large number of punches are to be inserted, the press-fit becomes too strong if a mating tolerance is used and the punch plate can become warped. Because of this problem, the conditions of the holes are determined by keeping the tightening margin fixed (the machining of the hole becomes more difficult compared to when a mating tolerance is used), thereby making the effect due to press-fitting small, and also, making the supporting of the punch very firm.
Although the seating surface dimension (H) has been taken as 5.3 mm, this has been determined because the tolerance of the flange dimension is +0.3/0 for a standard round punch. If possible it is better to finish the punch to have a flange dimension of 5.00 mm 0/-0.02 and an H dimension of about 5.00 mm. It is not good if the play between the H dimension and the punch flange dimension is too large and the punch moves in the axial direction.

Fig. 2 shows the different hole diameters for the structure when the stripper guides the tip of the punch (stripper reference). With respect to the dimension P of the punch, the stripper hole is made about equal to the dimension P1. The size of the number and the removing clearance (the plate thickness of the work to be formed) are proportional to each other. This is made loose with respect to the punch plate hole (D1). This method of fixing the punch is also called the punch free fixing method. Punch supporting with the stripper as the reference is considered better than when the relationship between the punch and the die is a fixing method based on the punch plate as the reference.
The relationship with the dimension H does not differ from that of Fig. 1. There is also a structure that is used in which the tip of the punch is guided by the stripper, and also the shaft of the punch is press-fitted into the punch plate, but this makes the preparation of dies considerably difficult.