Optimum Carrier Profile
The optimum carrier profile is affected by some of the following conditions:
lSpace available between parts: Try to keep the carriers within the stock width and pitch required for the blank. If this is not possible then the designer must add to the width and/or the progression of the material to provide adequate carrier room.
lAttachment points to the part: If two carrier are used, try to keep the profile and length of the carriers somewhat the same so that any effect of carrier flexing is closed to being balanced.
lClearance for punch and die blocks: Punch blocks that extend below the stock or die blocks that extend above the stock when the die closes will require clearance in relation to the parts and the carriers. If a loop of the carrier interferes with blocks it may be possible to form the loop vertical to provide clearance.
lThickness of the material: Large parts with thin material may require stiffener beads to add strength to the carrier for stock feeding. Another stiffening and strip guiding method is to lance and flange the edge of the stock, which also can be used as a progressive notch.
The total of the strip: Heavy parts in long dies require more force to push the strip through the progressive die. However, the weight is usually thick material, and thick material is stiffer than thin material. As a rule thumb, flexible carriers for materials of 0.020 in. are about 3/16 in. to 5/16 in. wide. For stock thickness above and below this thickness range, carrier width is a ”best judgment call.”
Depending on all the die factors involved, under normal conditions the carriers should be a consistent width for their full length, but especially in the area of flexing. Since nearly every stock feeder pushes material through the die rather than pull the material, the carrier must be strong enough to push the parts all the way through the die.
A detection switch actuated by a complete feed of the strip at the exit of the die can detect buckling. If action of the progressive die during closure or opening of the press requires the carriers to flex, design the carrier with loops that are long enough to flex without breaking, but still strong enough to feed all the parts to their full progression. If two flex carriers are not strong enough to feed the strip, consider three carriers.
Try to make the radii in flex loops as large as practical. Sharp corners or small radii will concentrate stress of flexing, making it the first point to fracture during flexing of the carrier. Also avoid any steps or nicks in the edges of the carrier.
Upper Pressure Pads
Because of size or function, many progressive dies required two more pressure pads in the upper die. Each may require a different travel distance to perform the work in the individual die station, such as trimming or forming or drawing.
However, the upper pressure pads often are used to push the material lifters down by pressing against the strip, which pushes the lifters down. In this situation, all of the pressure pads that push material lifters down should have the same travel distance. If the upper pressure pads travel different distances, the strip will not be pushed down evenly. This can adjacent parts out of the progression, making it difficult to locate the parts in their proper station position after the feed cycle.
If the part requires a flange to be formed up, the part carrier must have a flex loop to allow for vertical breathing of the part or provide a pressurized punch/pad with the same travel as the other pressure pads. The force required by the pressurized punch/pad has to be adequate to from the flanges up during the down stroke while the punch/pad is in the extended position. This keeps the strip from breathing vertically as it is pushed down from the feed level to the normal work level.
When the strip reaches the work level, the pressurized punch/pad stops its downward motion while the upper die continues down for punching, trimming, down flanging and other operations. Springs or nitrogen cylinders can be used for pressure in these pressurized punch/pad stations, but they must have enough preload force to form the flanges up to collapse the lower gripper pad before the upper punch/pad recedes.
Drawn shells are produced when strip material is changed from a flat plane to a cylindrical shape. During the draw operation, the “diameter” of the blank is reduced to the “circumference” of the shell. As the circumference is being reduced during the flow of material inward, the outer portion of the material goes into side or edge compression.
When this compression becomes too great for the material to stay flat, it begins to fold or wrinkle. To prevent this, the material is allowed to flow in a controlled gap between a draw ring and a pressure pad. The two main caused of failure in drawing a shell are to exceed the percentage that the blank (or shell) is reduced in diameter and an improper draw ring radius.
There is limit to how far inward metal will flow when drawing from the blank diameter to the first draw diameter and from a drawn shell diameter to a smaller shell diameter. This is expressed as a percentage of draw reduction. The maximum percentage of reduction is limited by the flow of material inward that causes the metal to go into compression, which in turn causes a resistance to flow. Too much resistance will cause fracture near the cap of the shell, which is the weakest area in tension.
The percentage of reduction varies with the metal thickness. For example, for a deep drawing steel blank, the percentage of reduction to the first draw shell diameter varies from 32 percent for 0.015-in. thick material to 48 percent for 0.125-in. thick material.
There is a minimum and maximum draw radius on the draw ring that will control the flow of material. For deep drawing steel parts, the correct radius on the draw ring that will control the flow of material. For deep drawing steel parts, the correct radius varies from 5/32-in. minimum to 1/4 in. maximum for 0.015-in. stock, and 11/32-in. minimum to 15/32 in. maximum for 0.125-in. stock.
If the radius is too small, the metal will not flow well, which increases the resistance to flow, causing excessive thinning or fractures near the cap of the cup. If the radius is too large, the metal will wrinkle after it leaves the point of pinch between the draw ring and the pressure pad, and before it is formed into the vertical wall of the cup.
The normal tendency is to make the radius too small because “it’s easy to make the radius larger during die tryout and it’s difficult to make a smaller radius.” The result is that needles stress is put on the cup, which results in excessive thinning or fracturing. Many times the problem of an improper percentage of reduction or improper draw radius in the first draw station will not show up in the first draw station, but in a later redraw station, with the result that considerable time is spent trying to fix the wrong station.
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