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Often, a good bar lifter system allows higher press speeds because feed problems are eliminated. Although the initial cost is more than round lifters, performance is better and setup time is reduced.
As the strip is started into the lead-in gauges, the strip should be able to feed automatically through all the following die stations without requiring manual alignment in each set of gauges and lifters. The strip also must be balanced on the lifters so that it does not fall to one side during feed. A retainer cap can be mounted on the top of the outside bar lifters. This produces a groove that captures the strip during feed and prevents strip buckling.
Gauging and lifter conditions can be simulated during die design by cutting a piece of transparent paper to the width of the strip. The lead edge of the paper is placed over the plan view of the die design at the location the strip will be for the first press stroke. Then the paper is marked with all of the operations that will be performed at the first die station--for example, notching and punching. The paper strip then is moved to the second station on the drawing and the operations for both the first and second stations are marked. This process is repeated through all the die stations to illustrate what the real part strip will look like when it is started into the die and helps determine the adequacy of gauges and lifters.
To transport the strip from one station to the next in a progressive die, some material must be left between the parts on the strip. This carrier material may be solid across the width of the strip, or may be one or more narrow ribbons of material, see part carriers sidebar.
Many parts require the edge of the blank to flow inward during flanging, forming or drawing operations. This may require the carrier to move sideways or flex vertically, or both, during the die operation. A flexible loop must be provided in the carrier to allow flexing and movement of the blank without pulling the adjacent parts out of position, Fig. 2.
Another concern is the vertical "breathing" of parts in die stations during the closing and opening of the die in the press stroke. For example, vertical breathing takes place between the draw stations of parts requiring more than one draw to complete the part, Fig. 3. Vertical breathing also occurs when a flange is formed "up" in a progressive die station that is adjacent to stations that use upper pressure pads to hold the adjacent parts down.
It is important to consider the flexing of the carrier during the upstroke of the press as well as during the downstroke because the action may be different. This can be simulated in the design stage by making an outline of the cross-section of the part, the pressure pads and the stationary-mounted steels on separate sheets of paper and then placing these sheets on top of each other in layers over the die section views. This will show the relative position of the part as the die closes and during the reverse action as the die ram opens
Part Carriers
A common feature in all progressive stamping dies is the material that transports the parts from station-to-station as it passes through the die. This material is known by various terms, such as carrier, web, strip, tie, attachment, etc. In this instance, we will use the term carrier, of which there are five basic styles:
Solid carrier--All required work can be accomplished on the part without preliminary trimming. The part is cut off or blanked in the final operation.
Center carrier--The periphery of the part is trimmed; leaving only a narrow tie near the middle of the part. This permits work to be performed all around the part. A wide center carrier permits trimming only at the sides of the part.
Lance and carry at the center--The strip is lanced between parts, leaving a narrow area near the center to carry the parts. This eliminates scrap material between parts.
Outside carriers--The carriers are attached to the sides of the part so that work can be done to the center of the part.