August 14, 2003
It is not unusual for me to perform a few magic tricks when holding a conference on stamping die drawing, troubleshooting, or processing. Later on, I may disclose the magicbehind the trick. I do this for a couple of reasons: first, to entertain the conference attendees, and second, to show them that there is no such thing as magic, only physics.
This article focuses on a few tricks of the tooling trade that have been used for years. It is very important for toolmakers, troubleshooters, and technicians to make rational data based decisions regarding permanent corrective action. These tricks are intended solely for data gathering purposes so that permanent corrective action can be implemented. However, in very rare cases, these tricks or industrial band aidscan be implemented in a short run production setting. A strong word of caution—overusing some of these tricks may result in die damage. Pay close attention to the cautionary statements for each trick.
Wrinkles are the result of insufficient stretch or uncontrolled compression. If the blank is wrinkling on the blank holder surface, the cause is most likely one of four factors: insufficient blank holder force; a poor fit between the die face and blank holder surface; an equalizer block that is holding the die face apart from the blank holder surface; or severe blank holder deflection.
With the exception of severe blank holder deflection, these factors can be corrected by making minor permanent adjustments, such as increasing the blank holder force, re-adjusting the equalizer height, and refitting or spotting the blank holder or the die face.
However, in the case of deep-drawn contoured parts, wrinkling in the product area can occur from insufficient sheet material stretching. To simulate the effects of a draw bead, a process that restricts metal from flowing into the cavity, grit cloth or sandpaper can be placed on one or both sides of the blank in a specific area. The abrasiveness of the grit cloth and the increased pressure exerted on the blank simulate the draw bead effect and help reduce wrinkling in the product area (See Figure 1). Once you determine that limiting the metal flow will reduce or eliminate the wrinkling, a permanent draw bead can be added to the problem area on the blank holder. Caution: excessive use of the simulated draw bead process will result in die erosion. Do not use if the blank holder bottoms out. Doing so could cause the blank holder to break. Do not use on coated dies.
When grit cloth or sandpaper cannot be used, increasing the blank size in a particular area on the blank also can help. Increasing the blank size affects the draw ratio. The draw ratio is the direct relationship between the draw punch and the blank edge—the greater the distance between the draw punch and the blank edge, the greater the resistance to flow, primarily because of the additional surface area of sheet metal clamped between the die face and the blank holder.
Try experimenting with different blank sizes and profile shapes. Caution: although adding metal to a blank may decrease wrinkling, it also may add cost to each part. Permanent corrective action can include adding a draw bead, reducing a die entry radius, or increasing the blank holder force in affected area (See Figure 2).
In many cases, draw dies are designed without equalizers. Equalizers are spacer blocks mounted on either the blank holder surface or the die face that maintain a specific gap between the die face and blank holder surface. Without these critical blocks, metal flow must be controlled through the use of blank holder pressure alone, which can present a serious problem with parts that are both wrinkled and torn.
Increasing the blank holder pressure may help eliminate the wrinkles but most likely, will increase the severity of the tear. Reducing the blank holder force may reduce the tearing but increase the severity of the wrinkling. In this case, it may be necessary to add an equalizer block. The blocks will deflect the blank holder slightly and can be adjusted to facilitate different holding pressures in various area of the blank holder. They will allow the die troubleshooter to squeeze the blank tighter in one area (reducing part wrinkling) and let it flow more in another area (reducing tearing).
To simulate an equalizer, place a small portion of the sheet material that you are using plus a shim equal to approximately 10 percent of the metal thickness outside of the blankon the blank holder surface. By adjusting the thickness of the additional shim, you can control metal flow (See Figure 3). The permanent fix is to mill pockets and add fixed equalizer blocks in strategic areas around the draw punch.
Using equalizers also can assist in solving tearing problems. Increasing the shim thickness increases the gap between the die face and blank holder and allows more metal to flow inward.
Lubricants greatly affect metal flow. To simulate a super lubricant in a drawing operation, use a thin film barrier lubricant. Garbage bags and Saran Wrap® work well, but high strength grocery sacks work best.
Place the film on both sides of the blank, either on the entire blank or on areas in which you want to increase flow. Using this special lubricant reduces the friction and may reduce or eliminate tearing problems. Keep in mind that this procedure should be used as an experimental process to collect data for permanent tool changes. Although, a few isolated companies use this in production, it is not the most cost-effective means of reducing friction. Permanent fixes include increasing a radius, reducing the blank size, coating the tool, or adjusting the equalizer block.
Have you ever produced a part in which one surface needed to be a little bit higher or lower? Say hello to aluminized tape, crude and unusual but effective. Aluminized tape can be placed in certain areas of a re-strike die. This process simulates the effects of welding the tool and changing the die's geometry. It can be used to bump or tweak part surfaces temporarily. Caution: adding aluminized tape to a punch or cavity will most likely requiring grinding the mating half respectively. In addition, the tape can fall off during production and cause die damage. The permanent fix is to change the die or cavity shape by re-machining or welding, if necessary (See Figure 4).
To repeat, these tooling tricks of the trade are not intended to be permanent solutions to stamping problems. They should be used to collect data that will allow you to make intelligent, permanent fixes that can improve your bottom line.