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Strip Growth
A progressive die is completed and mounted into an appropriate press. The strip of material is introduced and a skeleton is produced one careful stroke at a time. The very first blanks are created. There is a great deal of satisfaction that the strip fed all the way through the progressive die the first time, although meticulously fed by hand. On the first try a complete blank was produced.
After a visual inspection, the next step is the preliminary measurement of the part. What is discovered is that the part—a sample drawing is in Fig. 1—is not central. The formed legs are uneven and the ends of the part are not central.
Okay, let’s find the problem! First, the dimensions on the design are checked for errors, because that’s the easiest thing to do. A recalculation verifies the correctness of the design. It is assumed the problem is with the die.
So the next logical step is to put the die onto a jig borer, coordinate measuring machine or another appropriate measuring device and physically measure the progression in the die. All is well.The die progression measures what it should be.
How can the design and die be correct and the parts come out wrong?
Let’s inspect the skeleton.
Affix the skeleton onto the measurement device, align it carefully and measure it. First of all, measure the progression in the early stages to make sure you don’t have “drift.” Drift is when the pilots aren’t doing an adequate job and one progression might be . long, another 0.0005 in. short and so on. If all the progressions are 0.001 in. long 10 stations later you are 0.010 in off center.
For this instance we will assume the pilots are doing their job and the progression is correct. That is, correct until the station where a rather large gutting takes place and the strip grows, see Fig. 2.
Evidence of strip growth is usually displayed by the difference of the cut edge on the large gutting. One side will have a textbook edge of 1/3 shear and 2/3 break (in most materials) and the other side will have more shear but with a strange appearance. The shear will look more like a rubbing action occurred.
What is actually happening is that the cutting action of the large gutting punch is forcing the strip to “squirt.” Strip growth occurred. A couple of stations later the part is formed down into a “U.” The height of the legs is not equal. The strip and the die are no longer aligned. An offcenter condition is the result.
The reason that it happens that way is that material is following the path of least resistance. It is easier for the strip to squirt away than to hold firm and take the hit from the gutting punch.
Fig. 1
Fig. 2 |
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发表于 2005-10-29 22:21:00
发表于 2005-10-30 23:47:00
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