Transfer die design considerations

After receiving an assignment to design a die for a transfer operation, a die designer considers many questions and factors concerning the specific project. These questions help the designer to create the best possible design for all the factors comprising the project. If annual volume, press availability, or economic factors change, the die design may require major alteration.

What Information Is Needed to Start?

To begin designing transfer die properly, a designer needs several pieces of information. They are:

1. Press specifications—tonnage, bed size, strokes per minute (fixed or variable), stroke length, shut height, type of drive, scrap opening locations, size of windows.

2. Transfer specifications—make, type of drive (servo or mechanical), pitch length (minimum and maximum), clamp length (minimum and maximum), lift height (minimum and maximum), speed or control limitations, weight of bar and fingers (if available), length of part life in years.

3. Part specifications—material, thickness, complete data on part shape, tolerances, volume required per hour/day/month.

4. Miscellaneous information—quick die change system with description, frequency of changeover, feed method (coil or blank) and the feed method's accuracy, drop-off or finished part, lubrication specifications and amount, critical finish areas.

5. Sample parts or sight model—either styrofoam or wood.

Why Use a Transfer?

Often, customers requesting a die specify a transfer-type die for the following reasons:

1. The customer has a transfer press or a press equipped with a transfer system and wants to keep the system busy with a number of parts.

2. The part size dictates the use of a transfer system as opposed to a progressive system.

3. The volume is high enough to preclude the use of line, or stage, processing.

4. Cost reductions are required on the part to stay competitive.

Other reasons for requesting a transfer die may include the following:

1. Material savings of very thick or expensive material such as stainless or high-strength, low-alloy (HSLA) steel can be increased. This savings can be significant, depending on the volume.

2. The part rotation required to produce a part cannot be done in a progressive die. (A transfer system can rotate as much as 180 degrees in a front-to-back axis as compared to a maximum of about 90 degrees with progressive-type dies.) This part rotation might be required because of burr locations or if the part shape makes access specific operations difficult.

How Will the Material Be Loaded?

Material can be loaded in several ways.

Figure 1

Coil Feed. This method, of course, requires a coil handling system and a window in the press large enough for the required coil width. (Coil can be loaded at right angles to part transfer without a window, but this is unusual and uses a lot of press space for the loading station.)

A coil feed works well with square or rectangular blank shapes, but other shapes can result in inefficient material use. The addition of a carrier on a progressive die can increase material use, so customers may choose to use a transfer when they are stamping expensive material (see Figure 1). A zig-zag feed system sometimes can be used to improve material loss and nest the blanks on the strip.

Coil/Transfer Hybrid. Another option is a combination coil-fed progressive die for the blanking operations and a transfer system for the remaining operations. This type of system can use material inefficiently with some blank shapes, but it eliminates the need for a blank destacker.

Often, these hybrids are in-die special mechanical, pneumatic, or hydraulic transfers rather than in-press transfer systems. They are designed and produced by die shops rather than by transfer manufacturers.

Blank Destacker. This system offers the most efficient material use because blanks can be nested in various configurations and produced from coil in the blanking line/press at a faster rate. In some cases, purchased blanks are a good, cost-effective alternative. A blank destacker also allows quick change of material for a part change or for material quality problems in a particular stack.

A blank destacker also eliminates one or more stations in the transfer and may allow the transfer to fit in a smaller press than otherwise possible. Other advantages of a destacker include uninterrupted production, output controlled by equipment rather than an operator, and improved safety.

What Is the Sequence of Operations?

The sequence of operations is developed and reviewed to evaluate the feasibility of producing the part in the press requested by the customer. If the number of stations required times the pitch length (usually dictated by the blank length) exceeds the capability of the press, a different press or other manufacturing techniques, such as off-line operations, will be required.

Two-, Three-Axis System. Is a two- or three-axis transfer required? Two axes usually allow faster operations on a piece-per-minute basis because of the transfer mechanism's decreased travel length. Sometimes, the part configuration will work well with a two-axis transfer, and, in other cases, the part shape does not allow two-axis operation, so a three-axis system is required.

A two-axis transfer needs supports between the operations to allow the parts to slide. If the part has a flat bottom surface similar to a World War I military helmet or automotive hubcap, it can be slid on the supports or bridges between the die stations. Other part shapes tend to tip when sliding and so are not suitable for this type of transfer.

Figure 2: The location of the part must be maintained in all axes after the fingers release it. In a two-axis system, this can be accomplished by using hold-down pins that hold the part in position.

Often, the three- or two-axis decision is irrelevant because the customer already has a press or transfer system with a three-axis system. Many of these systems have variable travel with a minimum stroke on each axis, and the only concern is if the part requires more than the maximum stroke available. In some cases, the die design can be simplified with a three-axis transfer because the lifter mechanisms required to lift the parts for a two-axis transfer are not required.

Part Location. Another important factor is accurate part location when the parts are transferred to a new station. When the fingers release the part, the location must be maintained in all axes, including the rotational axis.

Sometimes, on a two-axis system, accurate part location is achieved by using hold-down pins that hold the part in position when the fingers retract and continue to hold the part until it is trapped when the die closes (see Figure 2). In some cases, on a three-axis system, the part shape itself helps to maintain the location. An example of part shape that is easily gauged is a cone-shaped part that accurately and automatically nests in the proper locations.

Transfer Stroke Distances. The part is studied carefully to determine the amount of pitch, clamp, and lift motion required. Then these characteristics are compared to the transfer system requested to determine if the part requirements and the transfer system are compatible.

Speed. Because pitch length is usually the longest transfer length, it is often the limiting factor in transfer speed. For maximum speed and because of press space constraints, dies are located as closely together as possible, and, ideally, the parts are oriented with the shortest dimension in the pitch axis.

Steel Grain Orientation. Another factor is the orientation of the part in respect to the grain of the steel. If a coil feed is used, the orientation may result in excessive material loss and excessive scrap costs. In some cases, the grain must go in one direction because of the part length and the coil width that can be used.

Scrap. During trimming, many pieces of material must be moved away from the dies. These small pieces of scrap must be disposed of quickly and automatically. One possibility is to add idle stations near the scrap chutes to keep the pitch length short. Of course, this can be done only if the press length can accommodate extra stations.

What Are the Details of Manufacturing Processing?

After the basic evaluation is completed, the manufacturing processing of the part from station to station is analyzed. This encompasses a step-by-step, detailed analysis of each of the following.

Part Orientation. Decisions must be made at every station about the orientation of the part for ease of forming, life span of the die components, and accurate location of pickup during transfer. Often, a small amount of tilt allows a punch to go squarely through the material rather than hit on an angle, causing side loading and potential punch breakage. Other times, burr locations or extruded holes require major reorientation of the part, sometimes as much as 180 degrees or more.

Figure 3: The location of the scrap holes in the press bolster is one of several factors that influence scrap removal.

Feed Line Height. At the same time that orientation is decided, feed line height must be determined to minimize transfer distance and maximize the speed of the system. Care must be exercised to make sure that the part orientation allows a satisfactory pickup point for the part in all stations, before and after the stamping operation. Lifters must be provided to allow access for the fingers without losing location or control of the part.

Weight. The weight and size of the part determine the acceleration and deceleration that can be used without losing control. Of course, the finger design also plays a part in this control. Excessive weight limits the peak speeds, which in turn affect the final average transfer time or speed.

In addition to part weight, the weights of the transfer arm, transfer fingers, and mechanism (or inertia) are factors in the ultimate speed of a unit. To minimize these weights, many transfer designers use high-strength, lightweight materials such as high-strength aluminum or ultrahigh-molecular-weight (UHMW) urethane for part contact fingers. This has the added value of eliminating or minimizing die damage in case the fingers are caught inside the die during tryout or because of a system failure.

Scrap. Scrap must be removed quickly and automatically from the trim dies or wherever it is generated. Scrap removal is affected by the location of scrap holes in the press bolster (if any exist), locations of scrap chutes outside the press, the size of the scrap, and many other factors (see Figure 3). The main point is to eliminate the jam-up of scrap, as well as any manual scrap removal, if at all possible. This will keep the system running at maximum efficiency with minimum downtime.

Mechanism Supports. On some transfers, supports exit at various points along the transfer bar to eliminate shake or excessive vibration. These locations and the locations of any die posts must be known to eliminate interference points during the design phase of the project.Grippers. In some cases, the part must be held with a hand-type gripper so that rotation can occur. This type of gripper is usually avoided, if possible, because of its cost, weight, and unreliability.

Finger Return Path. One of the most critical items of the transfer die design is the finger return path. The clearance between the fingers and the die components during the return stroke of the transfer must be analyzed to ensure there is no interference. If the transfer is mechanical, this is even more critical. Servo-type systems can vary the return profile of the fingers, allowing more clearance possibilities.

Figure 4: To eliminate obstacles to the transfer of parts, the die set pins are almost always located in the upper shoe.

Post Locations. To eliminate obstacles to the transfer of parts, the die set pins are almost always located in the upper shoe. This allows clearance for the transfer fingers to work as soon as possible during the upstroke to allow the maximum time for finger retraction during the downstroke (see Figure 4).

Are Any Additional Efforts Required in Project Management?

Usually, no extra project management effort is required, particularly if the transfer press or system is already operating at the customer's plant. If a new transfer is to be purchased, the situation can be somewhat different.

Because more than one company is involved—the transfer manufacturer, the die designer/builder, and the customer—communication and decision making among the involved companies can be somewhat difficult and time-consuming. If answers to critical questions are not received on a timely basis, delays can accumulate and affect delivery deadlines.

One answer to this dilemma is to assign the transfer responsibilities to the die designer/builder. The cost of the transfer might increase slightly if handling or markup is required, but communication and decision making will be much smoother, and delivery dates will most likely be met.

Summary

Transfer dies are not much different than line dies, with the exception of pin or post location, scrap handling, accurate automatic part location, and pitch alignment from one station to another.

When compared with other types of stamping, transfer dies can decrease labor cost, increase material savings, and eliminate secondary operations.