Recollections Of My First Die Cart

Approximately 25 years ago the boss said to me “I’m putting a lot of faith in you.” That was the start of a kick-off meeting I happened to be invited to. He continued “You’re going to design a die cart.” ?The task was, by far, larger than anything I had done to date. What followed was weeks on end of careful guidance, diligent work, and blunt criticisms from a leader who really knew what he was doing. This is an attempt to pen a few thoughts before memory fades.

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??????????????? The kick-off meeting outlined a simple die cart that would carry two dies weighing 40,000 pounds each. One die would be pre-staged on the cart and the other die would be in the press making parts. When change-over time came, the task at hand was to pull the 40,000 pound die from the press onto the cart, move the cart to position the new die in front of the press, then push that new die into the press. Non value-added motion included retracting the chain, engaging/disengaging a shot pin, and moving the die cart to and from a parked position. Value-added or not, it all had to happen comfortably under 10 minutes. The equipment list was previously determined to establish cost. In our world, 40,000 pounds is easy to move. It’s routine, in fact. Established during the sales process, this particular die set was to be moved by a single 60PSC Chain, one of our medium duty platforms. In an effort to reduce unnecessary complexity and cost, the Rigid Chain System – eh, hem… that is: Serapid’s Rigid Chain System [sic] - was to be stationary so that one system could do the work of two. The stationary Push/Pull, as we call them internally, also provided an opportunity to house a stationary shot pin to precisely locate the cart. Among the data collected was bolster height, die weight, die dimensions, scrap chute details, press dimensions where relevant, available space, and power. With all these details having been handed off, from sales to engineering (me), I began.

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??????????????? So now what? Where do I start? This was my first die cart. I saw a few of them. I even installed a couple. You see, my tenure at Serapid started on the manufacturing floor. I began my day walking in the back door before sunrise. When this opportunity came along, however, I had accumulated experience in building, designing, servicing, installing, and supporting rigid chain systems that may well number in the thousands. This experience highlighted the importance of alignment. I’ll share a few thoughts on that. First, I needed a cart so, of course, I started there. Our office worked in 2D CAD at that time. With that, I drew up an idea, crashed and burned, revised it again and again, and continued until I got it right. There would have been no lessons learned without perseverance. Bring that perseverance and I’ll share a few highlights.

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Bolster height and die weight play a big role. When the rolling path is low and a two station die cart is on the purchase order the challenge may lie in stacking up the necessary components without overshooting the target elevation. I started this particular stack-up with 8” diameter wheels, chosen somewhat arbitrarily, intending to build upward from there. The simplest design is best, yes? With this in mind, I decided a gearbox would drive two wheels directly by mounting a hollow-shaft gearmotor right on the axle with the wheels. There would be two of these assemblies for a total of four wheels, all driven. Having decided on the wheel diameter, I put off specifying the gearmotor so I could continue on good momentum, picking bearings and sketching a vast welded frame atop a half-vast driveline. I say half-vast because when I circled back to the drive motor the next day I found a suitable gearbox would not fit because the axle was too close to the floor. I needed bigger wheels or adapt with a chain drive or external gears. While it may seem unlikely, bigger wheels was the easy choice despite the re-work it required. It changed the height of the frame. Bigger wheels also affected the gearbox. There was some back and forth before arriving at the right combination. Needless to say my much anticipated, first (and failed) approach to the frame weldment was doomed.

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After some frame rework, I was advised to spec bearings that could take the load as if the entire cart, fully laden, were supported on only three wheels. Although I was green this made sense to me as three points define a plane. It is safe to assume a less than perfect floor/rail installation. Even if the laden frame absorbs the variations via deflection, how much force is required to produce that deflection? In my mind, two wheels will certainly see that as additional load, the third wheel maintaining something resembling balance on those two wheels because the whole of the system will be somewhat level. Needless to say, the bearings required were pretty big. That meant the frame weldment was again subject to revision. ($%&@#!)

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A note on the frame. In the vast majority of cases a die will roll on a set of rollers arranged in two rows. With wide tools comes a wide rolling path. This particular cart was to have capacity for two 40,000 pound dies with the long side of the dies arranged inline, side by side, along the cart’s length. The cart rolls on rails embedded in concrete while the dies roll across the cart perpendicular to the cart and rails. The frame loading was, essentially, four points of equal load across a beam resting on two axles, the axles being centered between each load set. That structure of beams, the frame weldment, concentrated the weight of each die onto the axle it rode upon. As each die was 40,000 pounds, deflection was inevitable. How much deflection is acceptable and what is the nature of that deflection? Does that beam deflect downward at each supporting row of rollers - meaning the cart sags in the middle and at both ends? Be careful here (and look again). In the house of Serapid, alignment is king. The frame must deflect less than that required for a smooth die transfer. That deflection must also land within the alignment tolerance of our Push/Pull system. What is acceptable to the push/pull may well be less than the elastic limits of the frame. Stated another way: Analysis might say the frame is adequate but a Push/Pull may require more rigidity. By the way, don’t count on the middle of the cart deflecting under load. If that deflection is concentrated at the ends of the cart, you’ve compromised alignment at the expense of longevity.

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I’ve seen any number of creative mechanisms to move heavy loads into and out of a press. While bias may seem apparent, I challenge anyone to come up with a better means than Serapid’s Rigid Chain systems. As with anything, there are “tricks of the trade” to maximize durability. Stroke path alignment is an obvious starting point. I say obvious with a hint of tongue-in-cheek because it seems often overlooked. In a die cart, vertical alignment is defined by deflection while horizontal alignment is defined by cart positioning accuracy. Deflection must be less than the vertical alignment tolerance. When designing the shot-pin, alignment of the chain guides is the criteria by which we tolerance the shot pin and it’s receiver. If one is tempted by economy to “weld at assembly”, alignment is a good reason to reject that idea.

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??????????????? Vertical alignment versus deflection covered, accurately positioning a fully laden die cart that weighs nearly 100,000 pounds comes next. The Rigid Chain appropriate for this application came with a horizontal alignment tolerance of half a millimeter. For this reason, I recall, it was decided an electric shot pin operating horizontally would be the solution. A vertical shot pin, an alternative that was explored, required a receiver embedded in a concrete floor. That receiver – basically a steel lined hole in the floor - would certainly gather chips and debris over time. When sufficiently full, this would interfere with the shot pin’s operation. Another detraction was locating that hole in the floor. Could it be done accurately enough before the cart is even installed? If not, how do you dial it in after the concrete is poured? Debris intrusion aside, onboard vertical shot pins might be an answer but would require either two assemblies or two adjustable holes. If two shot pin assemblies are the choice, each shot pin assembly would have to be slot-mounted to adjust for location. If two holes are chosen, good luck adjusting the hole locations. Can it be done? Sure, but I don’t see that as being fiscally responsible. The reasoned choice was to create a horizontal shot pin that gathers no chips and could accommodate adjustment by moving a slot mounted receiver. For some reason I called that receiver a Bezel and I mounted two of them on the long side of the cart. Each Bezel defining the location of one of the dies. The shot pin assembly was mounted on the floor beside the cart in the same housing as the Push/Pull. That housing was about knee high, affectionately referred to as the washing machine.

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At risk of repetition, the guides for this Rigid Chain are required to be aligned within “twenty thou” or half a millimeter. To achieve this level of accuracy, clearance on the shot pin receiver must be less than that. This aligned (snicker) with our decision to run with an electric shot pin as it can be fashioned to pull the cart gracefully into a more precise location, assuming a sufficiently tapered nose. We created a bezel with .010” total clearance. This bezel was machined out of a tough steel that interacted with a case hardened shot pin. That shot pin – 3” in diameter - was driven by a gear rack, a motor sufficient to move the cart, and included a tapered nose that could accommodate the cart being ?” off location either way.

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There’s certainly more to it than that but I’ll couch any omissions in the need for brevity rather than losses to the passage of time. Ultimately, I did not get everything right the first time around but perseverance carried me thru. I had the pleasure of installing that die cart. As the installation was neared completion, a tall Asian gentleman in a suit and hard hat came to have a look. I was disguised in dirt and grime so he had no idea I was the designer. With just a hint of accent, he commented “That die cart is over designed. It’s TOO good.” That kind of gratification doesn’t happen every day. Alas, I am proud to say that 25 years later, that die cart is still in operation. Lest I sound boastful remember I was the newbie under the best of guidance, freed by a sales force wise enough to build adequate budget, and supported by an assembly team of endless talent. With such support there is little opportunity to go wrong. All I brought was the perseverance. With that, 25 years later, I end my day walking out the front door before sunset.