From Ask The Expert, appearing in the July/August 2010 issue of STAMPING JOURNAL®
August 4, 2010
Even with many styles of sensors on a progressive die, die wrecks continue to occur. Addressing potential miss hits during the design stage, and using proximity sensors in careful, deliberate ways, can help control the problem.
Q: I am a die setter running progressive dies at 150 strokes per minute (SPM). I currently use pilot misfeed sensors, double-material sensors for slug pulls, feed sensors, end-of-stock sensors, and others. Even with all these sensors, I still get die wrecks from miss-hits. Any suggestions?
A: If coil stock is obstructed from feeding freely through the tool, a bridge buckle sensor is a great way to pick it up. If the coil advance is obstructed close to the end of the feed cycle (perhaps the last 0.02 inch on a long pitch), though, a bridge buckle might not pick up enough material hump to prevent a miss-hit and “crunch.”
For long-term, best-in-class operations, be sure to address potential miss-hits during the design stage. Use strip start pins to determine exactly where to place the edge of the stock on lace-up. Accommodate the tool to prevent half-slugs that may pull or, worse, half-part blanks in forms that will cause uneven pressure on the tooling, resulting in breakage.
Let’s talk about how to avoid misfeeds on running tools. Let’s install two proximity sensors level in the die chase under the coil stock—one pitch apart and on-center to a pierced hole in the strip.
Assume the stock pierce hole is 0.125 in. in diameter and the pitch is 0.5 in. Assume the proximity sensors are 0.040 in. in diameter, and the press has a 1-in. stroke and a mechanical feed cycle that starts at 270+ degrees and ends at 90 degrees.
We need to know the stopping distance for the press ram. Let’s signal the press stop at 90 degrees. Assume the ram travels 45 degrees from time of signal.
Last, we need to know the point at which the tooling actually makes contact—before things crunch. Let’s assume that is 145 degrees.
Let’s wire in the proximity sensor on the first hole so that it will turn on to pick up solid material when the strip advances (about 30 degrees of press movement). With a 0.5-in. pitch and a feed rate that is proportional through the 180-degree feed cycle, this equals 0.0027 in. per degree.
The stock will need to move one-half the diameter of the hole so that the hole edge will be centered on the proximity sensor and then one-half the diameter of the proximity sensor to completely cover it. Calculate:
Stock moves 0.062 in. (1⁄2 hole dia.) + 0.02 in. (1⁄2 proximity sensor dia.) = 0.082 in.
The press will advance from the feed start position of 270+ degrees (0.082 in./0.0027 in. per degree = about 30 degrees). At 300 degrees, the first proximity sensor will be covered.
Do the same in reverse for the second proximity sensor, turning on and picking up the open hole as the coil stock advances at the end of the feed cycle. In this case, we now can sense stock has progressed and not just stood still by programming the sensors to turn on and off every cycle. We can signal a press stop at 90 degrees (the end of the feed cycle) if the coil stock is not on location. The ram will stop at 135 degrees, and the tooling has not made contact at 145 degrees.
Note that the feed rate is not proportional in real applications. Once the sensors are installed, you can cycle the press and take real numbers when they turn on and off.
Since most coil stock obstruction problems on running tools occur at the start of the feed cycle, let’s use the first proximity sensor and signal the press to stop 30 degrees after the start of the feed cycle if it has not turned on. Let’s move the sensors off the center of the stock holes and place them close to the edge of the holes so we can pick up a few thousandths of an inch of expected stock movement.
Let’s speed up the press and use the second proximity sensor to signal a press stop at 90 degrees and get real numbers as to how fast we can go before the stopping distance of the ram approaches the 145-degree crunch point. The earlier we feed, the earlier we can sense end of feed.
A servo feed will allow us to optimize when we start feeding, how many degrees we need to complete the feed, and feed speed. The goal is to get to end-of-stock sensing as soon as possible.
All these factors give you the ability to maximize your stamping speed and accommodate the longer ram stop distance that comes with faster press speeds.