Q: I am the toolroom manager for a large electrical connector stamping firm. We have about 40 presses from 30 to 120 tons, running at speeds of 60 to 600 SPM. We have been working on continuous improvement; by far, our biggest problem is that when we have a misfeed, we break tooling. Not only is this costly, but it causes a lot of downtime for repairs, as well as missed customer deliveries.

We currently use misfeed sensors, feed sensors, and end-of-stock sensors, but we still get die wrecks from miss-hits. Any suggestions?

A: I would estimate that more than 90% of all tooling damage is caused by misfeeds or miss-hits—not only when the tool is running but also on lace-up of fresh coil stock. Only when you add up all the costs of replacement tooling, the toolmaker’s labor to facilitate repairs, diesetter labor pulling and resetting the dies, quality inspectors’ time repeating first-off inspections, and lost production while the tool is out—plus the intangible costs of unhappy customers who do not get their parts on time—do you begin to realize the magnitude of this issue.

A best-in-class stamper must do everything possible to eliminate miss-hits. You must get it under control to be competitive in today’s global market. Your goal should be nothing less than zero miss-hits. How does a stamper achieve this goal?

Most of what you need to do starts in the design phase. First, you must factor in how to eliminate half slugs. These typically will pull up and obstruct the stock in the feed cycle. Sometimes the part blank in forms can cause uneven pressure on the tooling, resulting in breakage. You also need to include a positive location for starting a new raw coil of stock. Guessing or relying on the diesetter’s ability to “eyeball” this point eventually results in half slugs and a miss-hit during lace-up, and that’s not reliable.

A bridge buckle sensor is a must. This bridge contains the raw stock from the feed rollers to the tool entrance. A gap in the top plate of the bridge is engineered so that the material will bow up if there is an obstruction in the feed cycle. A trip sensor just above the stock will pick this up the moment it starts to happen. This is only good if the material is obstructed at the beginning of the feed cycle, however. If the stock is obstructed at the very end of the feed cycle, the bow up will be minimal and might not be picked up by the trip sensor. And even if the sensor does pick it up, depending on the speed at which you are running the press, you might not be able to stop the press ram before a miss-hit.

Many standard sensors are available to incorporate in all your dies, such as pilot misfeed sensors, tonnage sensors, and micron sensors that detect when the die closes to the proper shut height (detecting double material and slug pulls in your die). The problem with all of these, however, is they will only stop the press after the fact.

When running a press at higher speeds, there is one way that works, and it will even stop a press running at 600 SPM when set up in the right conditions. (Again, this must be addressed during the tool design phase so the sensors can be built into the tool.): 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. Set up a prox sensor on the first hole in the strip so that it will turn on (to pick up solid material) when the strip advances about 30 degrees to cover the prox.

Assume the stock pierce hole is 0.125 in. dia. and the pitch is 0.5 in. Assume the prox sensors are 0.040 in. dia. Assume the press has a 1-in. stroke and a mechanical feed cycle that starts at 270 degrees and ends at 90 degrees. Since the stopping distance for the press ram will be different depending where you signal the ram to stop, let’s signal it at 90 degrees. Assume the ram travels 45 degrees from time of signal. And assume the point at which the tooling first makes contact is 145 degrees.

0.062 in. (1/2 hole dia.) + 0.02 in. (1/2 prox dia.) = 0.082 in.

At about 300 degrees, the first prox will be covered.

Do the same in reverse for the second prox, turning on and picking up the open hole in the strip (picking up no material) as the coil stock advances at the end of the feed cycle. In this case, you can sense stock has progressed and not just stood still by programming the sensors to turn on and off every cycle. You 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 will not make contact (at 145 degrees).

The feed rate is not proportional, as the feed typically accelerates and decelerates. In a real application, once the sensors are installed, you can cycle the press and take real numbers when the sensors turn on and off. Since most coil stock obstruction problems on running tools occur at the start of a feed cycle—such as when a punch breaks and gets stuck in the stock—use the first prox and signal the press to stop 30 degrees after the start of the feed cycle if it has not turned on. Use the second prox to signal the press to stop at 90 degrees if it does not see a clear through-hole in the stock.

In both of these cases, you will stop the press in time if the stock does not advance precisely and unobstructed. The beauty of this is that you can start running the press, slowly bring up the speed and activate the second sensor at the 90-degree ram position, and confirm the ram stops in time. As you speed up the press, the stopping distance will get longer, as it takes more time to stop the ram. When you get to the point where the ram cannot stop fast enough to prevent miss-hits, advance the feed cycle to start feeding before the 270-degree mark. With this earlier feed, you can retime your sensors and activate the second sensor earlier, allowing more time for the ram to stop before a misfeed.

This setup allows you to read the actual sensor timing and get real-life, press-specific confirmation by trial. The possibilities are endless!

One last word of advice: At least once a day, check that every sensor is working as intended.

See More by Thomas Vacca

Has a shop floor stamping or tool and die question stumped you? If so, send your questions to kateb@thefabricator.com to be answered by Thomas Vacca, director of engineering at Micro Co.

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