Crash Course

Tips and guidelines to properly set up the die and press to avoid crashes

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Setting up an efficient die protection program is a primary objective for stamping press operations, but it certainly isn’t uncommon for daily operations to sideline those efforts. Fortunately, resources are available to get a die protection program up and running without delay. Examples include virtual or in-person die protection clinics, such as those provided by Wintriss, a manufacturer of press automation, die protection and safety controls for the metal stamping industry.

With a proper program designed to provide die protection and in-die sensing information and training, die crashes can become a thing of the past. Ironically, a crash course in die protection basics will also help fabricators avoid die crashes. So, read on for tips and guidelines to properly set up the die and press and, in turn, run the most efficient press operations.

Product manager Jim Finnerty teaches a Wintriss die protection clinic where attendees can advance their press knowledge to avoid crashes and become more efficient operators overall.

Sensor setup

To begin, it may be helpful to review some of the most common mistakes made by both new and experienced sensor users when setting up sensors.

Using sensors without bench testing first. Companies embark on sensor implementation programs and have varying levels of success. Whether it’s a big-budget, plant-wide program involving a team of people or a one-person, “pay-as-you-save” ROI-based implementation, some programs succeed while others fail. All programs and approaches are a little different, but there is one ubiquitous step among the successful users: bench testing.

The best way to ensure that a sensor will work in the die is to try it out on the bench first. A well-stocked sensor lab will likely pay for itself the first time that one of those ideas that looked good on paper proves to be ineffective in practice. The worst – and most expensive – place to prove out sensors is in the press when the die should be in production.

The Wintriss DiPro 1500 die protection system helps to prevent costly repairs and press downtime.

Forgetting about the environment where the sensor will be installed. Try to anticipate all failure modes when you install sensors. All too often, sensors are destroyed by the very event they’re supposed to detect when that event occurs in an unanticipated fashion.

A good example surrounds the best practice of checking the stripper position at bottom dead center (BDC), which is usually employed to detect pulled slugs. The method is to install inductive proximity sensors in the bottom die to ensure that the stripper comes all the way down at BDC. Occasionally, a user will buck this trend and install the sensors in the upper die behind the stripper. The sensors are installed so that they will actuate at BDC if the stripper is too high by one material thickness. This method will be effective for detecting pulled slugs, but the sensors will be crushed when anything larger than one material thickness (like a piece of broken punch) ends up under the stripper.

Installing yellow sensors in the upper die. A “yellow” sensor is a sensor that is normally open and that only actuates when a problem exists. If you install a yellow sensor in the upper die, the wire to the sensor will eventually break due to repeated flexing. To a die protection control, a yellow sensor with a broken wire looks exactly like a yellow sensor not detecting a problem. A normally open sensor, which only actuates when there’s a problem, can often be converted, so to speak, to a normally closed “red” sensor, which is always actuated and turns off when there’s a problem. This is done by selecting the normally closed instead of the normally open output. The die protection control will immediately detect a broken wire on a red sensor and stop the press.

Note that when a sensor is installed in the upper die, its cable flexes every time the press cycles. This repeated flexing will cause the sensor cable to eventually fail. The expense of replacing the sensor can pale in comparison to the potential cost of die damage or even downtime caused by stopping in the middle of a run to replace the sensor.

Trying to use one type of sensor for every application. Standardization is good – to a point. The various types of electronic sensors have their own advantages and limitations, and not all are suited for every application. Try to avoid the natural inclination to stick with what has always worked rather than trying something new. Bench testing in a well-stocked sensor lab will help you to know when it’s time to try something new. If you can barely get something to work on the bench, it is unlikely that it will be reliable in a production environment.

Static normally open (yellow) sensors like this misfeed pilot should never be installed in the upper die. A normally open sensor can often be switched to a normally closed (red) sensor simply by connecting (or purchasing a sensor with) a normally closed output rather than a normally open output.

Trying to use sensors that do not meet the electrical requirements of the control. There are many output options available when you’re ordering electronic sensors, and not all outputs are compatible with all controls. If you are unsure if a particular sensor is compatible with your die protection system, consult your manufacturer or supplier.

Not planning the wiring before installing sensors. More sensors are replaced because of cable damage than any other reason. This can be avoided by using die-mounted junction boxes and carefully planning the sensor wiring so that the cables are protected. Often, the most difficult part of installing a sensor is figuring out how to get the wiring from the sensor to the junction box. If you wait until the die is assembled and the sensor is installed, it is probably too late. All new die designs should have the sensor wiring on the CAD drawing.

Press stopping time

The faster the press stops, the more time a die protection system has to detect and react to problems. In addition to regular brake maintenance, the following elements and processes can improve press stopping time.

The SmartPAC Pro from Wintriss makes setups easy with its large touchscreen controller. Seen here is the SmartPac Pro counterbalance programming menu.
  • Quick dump valves on the clutch/brake. A quick dump valve is a low-cost, pneumatically controlled valve that, as the name implies, is designed to quickly remove the downstream air when the supply air is shut off. A quick dump valve installed on the clutch/brake assembly of a press will quickly exhaust the air from the clutch when the clutch/brake valve opens. Many newer air clutches have quick dump valves built in. Older machines can easily be retrofitted with a quick dump valve.
  • Properly set counterbalance air pressure. The air counterbalance system on a press is designed to counterbalance the weight of the ram and upper die. Heavier upper dies need a higher counterbalance pressure. If the counterbalance pressure is set too low, it can cause, among other things, an increased stopping time on the downstroke of the press. If it is set too high, the press will take longer to stop on the upstroke. Optimum stopping time is achieved when the counterbalance pressure is correctly set for each upper die. For example, the RamPac module for the Wintriss SmartPac press controller determines this setting automatically.
  • Properly set clutch air pressure. Most press manufacturers recommend that the clutch air pressure be set to around 60 psi. Unfortunately, many users think that if 60 psi is good, 80 psi must be even better. However, there is no benefit to using a pressure higher than the clutch’s design limit. Using a higher pressure increases the volume of air that must be exhausted from the clutch at disengagement. More air takes more time to get through the valve and increases the press’s stopping time.
  • Clean or replace the muffler on the clutch/brake valve. For the machine to stop, the pressurized air must be removed from the system. The last possible impediment to air evacuation is the muffler on the valve. If it’s not there already, this item should be added to the press maintenance schedule.

Know the critical angle

The critical angle is the last point in the crankshaft’s rotation where a stop can be signaled so that the ram stops before the die closes. Because the whole goal of die protection is to stop the press before a bad hit can occur, knowing the critical angle for every die is, well, critical.

If your press is equipped with a Wintriss SmartPac controller, you can easily determine the critical angle by following some straightforward steps.

The SmartPac brake monitor screen shown with the stop angle highlighted.
  1. Load the die in the press and inch the press to observe and record the die closure angle.
  2. Disable the sensors, and with no material present, run the die at its normal operating speed. If your press is equipped with a tonnage monitor, you’ll have to recalculate the setpoints before you start in order for it to let the press run empty.
  3. Select “Brake Monitor” from the SmartPac’s run mode.
  4. Press the cursor “up” key to perform a 90-degree stop time test. This will signal the press to stop halfway through the downstroke where the brake has to work the hardest to stop the machine. This will give you the “worst case” stopping performance.
  5. When the press stops, record the “Stopping Angle” shown on the “Stop Time Status” screen (highlighted in the photo above).
  6. Subtract the “Stop Angle Value” (from Step 5) from the die closure angle (observed in Step 1) to get the critical angle.

Note that the ready signals for your cyclic (green) sensors should end before the critical angle to ensure that the press will stop before it makes a bad hit.

There are many other considerations that will help to avoid die crashes. Follow the guidelines presented to gain a strong foundation for die protection best practices.

Wintriss Controls Group

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