A Review of Methods and Precautions against Slug Pulling

Headnote

Abstract: This article deals with slug pulling and methods and precautions against the effect in detail. Cutting, one of forming processes, can meet a few problems. One of them is slug pulling. Slug pulling is the concomitant effect of punching or blanking. Punching and blanking are similar operations differing only in the scrap and part understanding. Slug pulling occurs if a cut slug is caught to the punch face and afterward the punch pulls the slug up from the die. Slug pulling is a cause of problems at sheet metal cutting for instance press failure, time losses, financial losses and so on. Nowadays slug pulling trouble us more than in the past because of high cutting speeds, automation and robotization innovations, economization, price cutting, material costs savings, cost reduction etc. Causes of slug pulling are dependent on parameters of a punch, of a die, of relationship between the punch and the die, of material and of other cutting factors. Fortunately, there are several successful methods to remove the slug from the punch. The aim of the article is to give a review of the methods and precautions against this unpleasant effect. Foremost it is needed to avoid improper parameters of cutting process such as excessive cutting clearance, punch and die edge wear etc. Among the best methods precluding slug pulling belongs to the aeration punch, air blow punch, ejector punch, shear ground punch, vacuum die, die with protrusions, die with grooves, eclipse die, die with reverse taper and others. This type of a cutting problem is possible successfully solve. Presented information can be used for selection a suitable method for customer's needs and for decreasing of downtime and financial losses. We would never underestimate the influence of the effect on the cutting process in the stamping shop.

Keywords: slug pulling, punching, blanking, a punch, a die

(ProQuest: ... denotes formulae omitted.)

1. Introduction:

Many parts made in industry contain holes. The holes are manufactured by various ways of hole making process. Drilling, laser cutting, ultrasonic cutting, water jet cutting, plasma cutting, magnetic field cutting and punching belong to hole making ways [16]. Punching is an operation of shearing. Shearing is the process of cutting (parting) material, which is subjected to shearing stresses between the blades of shears of the edges of a punch and die of a shear tool. Various operations based on the shearing process are performed. Thus they may be explained as punching and blanking. Punching and blanking are the operations of cutting a flat shape from a strip of metal in a die. There is also perforating. Perforating is actually punching a number of small holes in a sheet [11]. Punching is more productive and cheaper process than other processes. That is why sheet metal is perforated by punching. In [15] it is proven that it is possible to punch a hole in the ordinary way, if the ratio of the hole size and sheet metal thickness is equal or greater than one:

... (1)

where dh is the hole diameter and s is the sheet metal thickness.

The difference between punching and blanking is shown in Fig. 1. The punches and dies manufactured by a company are mostly for the two basic sheet metal cutting operations: blanking and punching. Blanking is a cutting operation by which a part is cut from a sheet metal stock such that the cut touches no edge of the sheet metal stock. The cut-out part from the sheet metal stock is called a blank. Although the punching operation is quite similar to blanking, there is a small difference between the two operations. In the blanking operation, the part cut from the original sheet metal stock is the usable portion; in punching, the part that is cut out (called a slug) is scrapped as shown in Fig. 1 [2].

In the world of high-volume sheet metal cutting and production, challenges are often encountered. Cutting, although a simple and trouble-free operation at first glance, can blind-side us. Slug pulling, a concomitant effect of sheet cutting, is one such unpleasant surprise. The drive to produce more and more products on shorter timescales by punching, perforating or blanking sometimes leads to slug pulling [3].

The slug falls down through the die opening or the slug remains caught in the die opening in ordinary circumstances. Slug pulling occurs if a slug sticks on the punch face, holds and remains there. Then the punch pulls the slug from the die. For most companies involved in the punching or blanking of sheet metal, it can be a recurring, undesirable side-effect of punching or of blanking.

2. Causes of Slug Pulling:

Causes of slug pulling are depicted in Ishikawa diagram in Fig. 2. The causes of slug pulling are possible to divide into five main groups. The causes are on the side of a punch, of a die, of the relationships between the punch and the die, of a kind of material and of other parameters of cutting process.

Main factors responsible for slug pulling are magnetization of punches after grinding the punch face, vacuum between the punch face and the slug and a suction which is created between the punch and the die during the punch withdrawal in high speed stamping. Another factor responsible for slug pulling is adhesion between the punch face and the slug.

Magnetized punches attract ferromagnetic alloy such as ferromagnetic steel which can be slugs of. Punch edge wear and die edge wear leads to the slug pulling problem. Extremely fast piercing operations lead to slug pulling too [7].

High die land also leads to slug pulling. An influence of penetration is not clear. More entry will create excessive wear and can also cause slug pulling. The further a punch is entered, the more vacuum it creates at withdrawal. This vacuum will likely pull slugs. [6]. On the other hand, the source [13] claims that the solution suggested is to maximize die penetration to avoid slug pulling. In addition, a possible solution to avoid slug pulling is to increase punch penetration into the die [12]. This ambiguity is the reason why the influence of penetration is not involved in Fig. 2.

A larger than conventional clearance value generally results in the slug pulling problem [10]. Too large cutting clearance increases the bending of the slug. Generally, tight clearances in the 3-5% per side range result in fewer tendencies for the slug to be pulled from the die opening [21]. Thin and easily bendable material such as copper alloy or some steels allow creating large volume of a vacuum pocket. Physical and mechanical properties of metals (as bendability) are dependent on chemical composition, on a grain size, on a grain shape and on grain organization [24]. Round and oval shapes of holes can easily create tighter sealing around the vacuum pocket than other rugged shapes of holes. That is why these shapes of holes are prone to slug pulling. Thin and light material for example aluminum alloy has low weight resulting in weaker effect of gravity and the slug can harder fall down from the punch face.

It also can be caused by poor setup techniques, lack of operator training, faulty tooling components, lack of seal breakers of the punches, insufficient spring pressure on the seal breakers, improper drafton the die sections, excessive or lack of lubricant, improper die set up and dulling to name a few [22].

3. Forces in Slug Pulling:

During cutting process, various kinds of forces are created between the punch face and the slug. These forces have two effects on relationship between the slug and the punch. Either they have the effect causing sticking the slug to the punch face, or they have the effect causing removing the slug from the punch face. The slug is pulled by the punch from the die due to punch withdrawal.

Forces responsible for slug sticking:

* Magnetic force Fm

* Vacuum force Fv

* Adhesion force Fa

Forces responsible for slug removing:

* Gravitational force Fg on the slug

* Scraping force Fs caused by sheet metal or by a stripper

* Frictional force Ff between interior die walls and side slug walls

The other important forces between the slug and the punch face are inertial force Fi caused by punch moving and force caused by press vibrations. All these forces have various sizes and the forces do not affect equally during the cutting process.

The forces responsible for slug sticking to the punch face are denoted Fsp. The forces responsible for slug removing from the punch face are denoted Fre. If

... (2)

Then the slug sticks to the punch face and slug pulling arises. If

... (3)

Then the slug falls down from the punch face and slug pulling can not arise.

4. Consequences of Slug Pulling:

For most companies involved in the stamping of sheet metal, slug pulling can be a permanent, undesirable side-effect responsible for downtime, financial losses, press failure, punch breakage, poor geometry precision and low roughness quality etc. These adverse consequences are shown in diagram in Fig. 3. A fallen slug from a punch can cause indentation on formed parts or on the stamping die. The slug can fall into the interior of a press and can cause an expensive press failure. The caught slug on the punch face is the cause of double shearing and the often consequence is breakage of the punch or an inaccurate shape of the hole. Nowadays slug pulling problem trouble us more than in the past. It is caused by automation and robotization, economization, faster production, greater productivity, price cutting, strong competition, material costs savings, cost reduction etc.

5. Methods and Precautions against Slug Pulling:

Slug pulling is a type of a problem which can be almost totally successfully solved. Nowadays we have knowledge and information about this problem which can help to solve it. Slug-pulling, therefore, is a problem that can be prevented during design by using one of several applicable techniques [17].

The foremost operation to avoid slug pulling at ferromagnetic materials punching or blanking is demagnetization of the punch. Demagnetization also relates to other parts of cutting tool if possible. It is made with a demagnetization device [14].

According to [12] possible solutions for slug pulling prevention are decreasing die clearance on small holes or increasing die clearance on holes greater than 2" (51 mm), using of thinner lubricant and so on. There are even special lubricants solving slug pulling in the market. In the first place, it is important to eschew the parameters described in section 2 and in Fig. 2.

So far invented methods are possible to divide into two main parts. The first ones are methods solving slug pulling with a special type of a punch and the other ones are methods solving slug pulling with a special type of a die. J. Szumera in his book [22] divides methods into conventional and nonconventional methods. Another partition of the methods is into professional solutions, which are more effective than the latter group amateur methods used in a pinch, which are less effective than those of the former group.

One possible solution could be the flexible punching method using an elastic tool instead of a metal punch as it was tested in [23].

Aeration punches (see Fig. 4a) rank among underestimated methods against slug pulling. Particularly if the main cause of slug pulling is the vacuum force then this type of a punch can work effectively. The aeration punch allows air to aerate vacuum through a vent on the side of the punch and then air can leads through the hole in the body of the punch.

Nicking of the punch face (see Fig. 4b) works on a similar principle to that of the aeration punch. It allows venting the vacuum pocket between the punch face and the slug. However, more effective method is an air blow punch (see Fig. 4c), which with an air blast blows away the slug from the punch face. Instead of a blast of a common air it is better to blow aerosol of air and lubricant. It is effective but costly and harmful to working environment.

An often solution is an ejector punch shown in Fig. 4d. The ejector punch consists of an ejector pin, of a spring, of a screw and of a punch with an aeration hole. The spring can be made of elastic material or a spiral spring or the best one is a pneumatic spring. Designed to combat troubles related to slug pulling, also exist elastic pins with a screw at the end made of special elastic material - perfect for stamping softmaterials such as aluminum, copper and Tbrass alloys. With the system called shed it, stampers can increase tolerances to as much as 10 percent per side. The result is less tool wear and more parts per sharpening. They do not require another screw and spring. The ejector also can replace the spring in a traditional spring and pin ejector [18].

Insertion of a small urethane pin into the punch tip (see Fig. 4e) has quite similar effect. These elastic urethane pins push the slug from the punch face. But according to latest research of effectivity of methods stopping slug pulling, they do not work well.

A punch with a small hemisphere or just with a metal piece on the punch face (see Fig. 4f) does not allow creating tight seal in the vicinity of possible vacuum. But this solution is not such effective as one could think.

The effective solution can be a shear ground punch shown in Fig. 4g. There are a lot of types of shear ground punches. A stepped face, shear angle face, balled face, cone face etc. decreases the cutting force and leads to a smoother cutting process without shocks causing noise. However, more complicated production of the shear ground punch face and its grinding are the reasons for a less often use in production of stamped parts.

Besides special punches, there are a few of special dies designed to reduce slug pulling. Deposited hard metal on the die land (see Fig. 4h) not only reduces slug pulling but it also increases die land life. The metal deposit is executed with a special deposition device.

Another die against the slug pulling problem is a vacuum die shown in Fig. 4i. Commercially available vacuum units can be incorporated in the piercing operation. These units create a vacuum and pull the slug downward into the matrix. In a pinch, try a simple wet and dry vacuum. It works fairly well. However, keep in mind that these vacuum units typically are not meant to run for hours and hours. Even the higher-quality models burn up quickly [9].

A die with barbs (see Fig. 4j) can be the right solution for every plant meeting slug pulling. This unique design of the die contains at least one barb, but it usually has four barbs. One disadvantage of the die is higher price. With a slug-hugger button, barbs in the matrix impale themselves into the slug. An industrial quick-fix alternative to the slug hugger is to place tiny weld spatter on the inside of the pierce hole [8].

Slug retention die with grooves is an effective die. In a round die opening, the grooves are helical or straight (see Fig. 4k). If helical, then, the grooves have oppposite helixes. If straight, they are terminated at the depth of punch penetration. Non round shapes have angular grooves or straight grooves similar to the round shapes. The inability of the small discontinuity on the slug to follow the groove locks the slug into the die section. This method may leave a slight burr on both the slug and opening that may be objectionable in some cases [19].

A die with a unique bore, named the eclipse die which is shown in Fig. 4l, uses another principle. It is based on the mechanical influence of the die on the slug, which slightly tumbles in the die bore. It helps to fall the slug from the punch face and thus this die prevents to occur slug pulling. The utilization of the die is limited for round holes and it is not possible to use shear ground punches for this application [4].

Another method to help stop slug pulling is a die with reverse taper also called the bell mouthing die bell-mouthing the die, which essentially involves cutting a funnel shape around the hole (see Fig. 4m). If done properly, it needs to be done just once, when a die is new or any time after a steel or carbide section is ground. It does not need to be done again after the next sharpening. The bellmouthing method can eliminate slug pulling, increase punch/die clearance, retain the slug, and increase punch life and press uptime [5]. A die design that has an opening with a constriction point below the surface prevents the slug from returning after it passes through this point. A constriction point prevents slug pullback when the punch retracts. After the slug separates from the punch, it is free to fall away from the punching area. This eliminates slug pulling [20].

Besides the solutions previously described, there are other systems that control the whole cutting process based on an acoustic emission or similar principle. These systems involve sensors which output a signal to an operator's monitor [3]. Acoustic sensors mount on each corner of the stripper plate and as the die runs in the press, acoustic signals route to the monitor. The monitor immediately recognizes out-of-ordinary acoustic pulses generated by cracks, slugs or unejected scrap and breaks in tooling, and immediately shuts down the press [1]. The sensors automatically give an order to stop the press as soon as the slug is stuck on the punch face or if the slug has fallen onto the sheet metal to preclude possible damage [3].

There are many methods against slug pulling. Some of them are shown in Fig. 4. There are several methods using by special dies and punches designed for reduction of this effect and its consequences.

6. Conclusion:

We have to be aware that besides slug pulling, there are many more failures at cutting process. We can also mention other annoying issues of sheet metal cutting such as slug jamming, burrs, and die breakage, for example. There are ways to eliminate their influence as it is to see in this paper concerning slug pulling solutions.

If you make holes in perforating material you should always pay attention to slug pulling. Especially if you cut too fast and you cut very costly parts, slug pulling happens very troublesome and permanent problem, namely at high volume production. Therefore you would use a solution to avoid this type of a problem. The consequence of slug pulling will be more expensive than the costs of any other used special punch and die. Special punches and dies will bring benefit by means of time saving, costs saving, etc. It could be a way to defeat competition across the market and to have an advantage over competition in the field of stamped parts.

References

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