March 2012 Archives

While investigating the process of a drawing operation (consider cylindrical drawing), although the number of drawings is determined from the development of the blank and the drawing ratio, it is also possible to continue investigations even if the plate thickness is omitted. However, when we consider the actual drawing, if we assume that the blank diameter obtained during blank calculations is 100 mm, nobody will think that the ease of drawing is the same when the plate thickness is 1 mm as when it is 0.1 mm. "Relative plate thickness" is an attempt to judge the state of drawing from the relationship between the blank diameter and the plate thickness. The relative plate thickness (see Fig. 1) is expressed as follows.

The result of calculation will be in the range of 0.1 to 2.0 in the case of usual products. A product with a smaller value is more difficult to draw, and a product with a larger value is easier to draw.
When the blank diameter is 100 mm and the plate thickness is 0.1 mm, the relative plate thickness becomes 0.1 and is difficult to draw. The meaning of this is that, wrinkles or cracks appear when a small change is made in the conditions such as the wrinkle suppression force, etc.
When the blank diameter is 100 mm and the plate thickness is 1.0 mm, the relative plate thickness becomes 1.0, and with this value, it is possible to draw without much difficulty using a die with a wrinkle suppressor.
When the relative thickness is 3.0 or more, it is possible to draw without a wrinkle suppressor.
When the relative thickness becomes still larger, the plate cracks and it becomes difficult to draw the product.
Using relative thickness, it becomes possible to make judgments in this manner. It is possible to know still more detailed conditions when combined with the drawing ratio.

A die of the quick die change (QDC) type is a die which can be changed quickly. The schematic of quick die changing system is shown in Fig. 1. It consists of a QDC holder and a QDC die.

The QDC holder has a structure in which the die set has clamps and location pins. The location pins are raised or lowered by knobs. The location pins can be thought of as movable type dowel pins (notch pins) which are inserted in the die set and die. The clamp is of the mechanical type or of the hydraulic type, and techniques have been used so that it can be locked with a single touch.

A QDC die is assembled in the base plate. The base plate is matched with the location pin of the QDC unit. A QDC die can be changed in a short time, and is also intended to reduce the cost of setting up the dies. It was proposed for small volume production of a large variety of products. Since changing the dies becomes difficult if the die becomes heavy, dies that can be held in the hand easily are used very frequently.
In a QDC system with this structure, a die matching jig will be needed at the time of preparing the QDC dies.

The concept of changing the dies in a short time is spreading to ordinary dies as well, and the improvement called "simple die setup" came to be used widely. This is the technique of making it possible to complete the setting up of dies within 10 minutes. The major points of improvement are the die height, feed line height, clamp height, clamping method, and the method of leveling the die, etc.

A safe die (see Fig. 1) is a die which has been prepared so that the gaps (A or B) in which a hand or finger can get in is 8 mm or less in order to ensure that any part of the body (mainly finger tips) of the person carrying out the work does not enter dangerous regions (between the punch and the die). Recently, since the hands of women are small, there is also the opinion that "8 mm is too large a gap and the appropriate gap would be 6 mm". However, if such dies are prepared, the actual work becomes very difficult. An automated type while incorporating the concepts of safe dies has the structure of Fig. 1. This is called the tap-fit type. The top die and the bottom die are connected by stripper bolts, and the top die is lifted up by a return spring. The return spring has to have a spring force equal to or more than the stripping force + weight of top die.

When this structure is used, even the work of preparing the dies becomes fast, and also the relationship between the punch and the die becomes stable. It is suitable for products that only need to be blanked. The precaution required is that, while the tip die is pressed (tapped) by the slide of the press machine, since the slide moves by its stroke length, it is necessary to protect using bellows, etc., so that the operator's hand does not get caught in this region.

Fig. 1 shows a die for a product having an upward bend and a downward bend. In this structure, movable punch and die are used which are supported by springs. Such dies and punches are called floating dies and floating punches.

Let us explain why such cumbersome structure is necessary. Consider the downward bend in the structure of Fig. 1. The floating punch becomes a pad. The floating punch (= pad) presses the material, and after that, the spring behind the pad flexes and the downward bend punch starts the downward bending operation. The bending is not possible unless the floating punch (pad) and the downward bending punch are not in this relationship.

When we consider the upward bending, the floating punch is the upward bending punch. The floating die becomes the pad for upward bending. The floating punch presses the material, and starts the upward bending by pushing down the upward bending pad (floating die).

It can be understood that the floating punch and the floating die are compound components having the function of pads. Since a pad is a component that moves while pressing, it is supported by a spring and is in the floating state. Since this component is simultaneously used as a punch or a die, the term floating punch or floating die have come to be used. This is a die structure with such floating components which is used frequently for complex forming or progressive forming.

The relationship between the forces of the spring A and the spring B in Fig. 1 is discussed below.

1. When A=B and both are stronger than the bending force, the upward bending and the downward bending progress simultaneously. After the floating punches press the material, since the springs A and B flex equally. However, since it is very difficult to prepare this state, normally this kind of design is not done.
2. In the state in which A>B, if the spring force of the spring A is more than the bending force + the spring force of B, when the floating punch presses the material, the spring B below the floating die starts to flex and the upward bending starts. The upward bending is completed when the floating die has hit the bottom. The spring A starts flexing from this state and the downward bending starts, and the upward bending and downward bending both are completed in the state shown in Fig. 2.
3. In the state in which A<B, if the spring force of the spring B is more than the bending force + the spring force of A, the downward bending starts first and when the floating punch hits the bottom, the spring B starts flexing and the upward bending starts. The bending progresses with relationships opposite to that of (2) above and the upward and downward bending both are completed in the state shown in Fig. 2.