Ask the Stamping Expert: How can we predict our tool service needs?

Q: We manufacture many types of clips, connectors, and small battery caps using about 60 stamping presses from 10 to 60 tons. Our problem is we cannot accurately predict our tool service needs. For the most part, we leave the stamping dies in the presses until the quality department rejects the parts for dimensions or excessive burrs. This causes the work load in the maintenance toolroom to vary greatly; sometimes we have no work, and other times presses are waiting for days before we can have the die serviced and back in the press. How would you address this problem?

A: The goal of your tooling maintenance program should be zero unplanned downtime. Of course, there is no way to eliminate service downtime altogether, but imagine your world if you could very specifically predict when and how long your die would be out of service.

If you cannot predict—within 10 to 15 percent—the number of parts you can make per service because it can vary so greatly, you need to address this problem first. If this is the case, I understand why you leave the die in the press until quality shuts it down: You don’t want to pull a die after making 100,000 pieces knowing you could have gotten 200,000 pieces. But if you try to implement a service plan under these conditions, you will fail. To achieve zero unplanned downtime, you would have to set your service limits so low that your labor costs and downtime would put you out of business. I call this chasing the tail when you want to catch the dog! You need to maximize the number of parts you can make after a service to maximize your profits.

Before you proceed, make sure your lubrication systems, stock feeds, and presses are in good shape. You can have the best tool in the world, but put it in a sloppy, worn-out press and your production will never be repeatable.

If your tools are unreliable and do not repeat, look at the die shoe guide pins. Measure and confirm that they are not worn out and are of sufficient size and quantity. In general, a 1-foot-long tool has four 1-inch-diameter precision ball guide pins, one in all four corners; a 3-ft.-long tool uses six ball guide pins at 1.5 in. dia.; and a 6-ft.-long tool uses eight at 2.5 in. dia.

How thick is the die shoe? Most tend to be too thin and flexible. As a rule, start with a 2-in.-thick bottom shoe for tools up to 1 ft. long and add 1 in. of thickness per foot length of tool, topping off at 5 in. of die shoe thickness for a tool up to 6 ft. long. The top shoe is not as important if the press ram totally backs it up. Build in oversize downstops in the tool. If you can adjust the part dimensions by moving the ram and overhitting the die, then something in the tooling is giving.

Once you have established a stable, reliable, and repeatable process, you can begin to determine a standard number of parts to make before pulling the tool for service—and thus plan your labor needs. For larger-volume tools, you might want to have cutting and forming inserts you can change out to get the tool back to the press quickly. In addition to the benefit of quick turnaround, they also allow you to use the dull cutting and forming inserts as a work load buffer when there are not enough production tools to keep the toolroom busy.

The forming inserts on complex parts usually take much longer to wear than the cutting tooling, and stampers tend to wait until there is a problem before changing them. But if you do this, you will likely find that even after installing a new insert, the stamping form is still out of specification. It can sometimes take days to get back to spec. You can’t just look back to when the die was new and producing good parts, because there may have been a dozen factors that together resulted in a good formed part. No one factor can be singled out.

So you need to rotate out your forms on a regular basis, before they wear out. Not only does this prevent steady degradation, but it will give you additional assurance that the form tooling has been tried and proven.