Simple Setup of Tonnage Monitors for Stamping Presses

May 26, 2020

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When properly installed and operating, tonnage monitors perform vital functions that help ensure production of quality parts, while helping to eliminate damage to tooling and presses. Tonnage monitors can extend the life of presses and dies by preventing excessive hard hits, and also can be used to determine the minimum amount of force needed to form a part. An ideal monitor enables the setting of tonnage capacity as well as high- and low-tonnage setpoints. Should the setpoints be crossed, the press will stop.

Most presses in today’s stamping plants employ some type of tonnage monitor to read press forces. Think of the last time that a slug buildup caused a die issue, or the die had a double-part condition. Did the tonnage monitor detect the hard hit, or let it pass? This article provides knowledge on how to activate a tonnage monitor in order that it becomes a more-effective stamping tool, and covers two main points:

1. Testing the units to see if they are bypassed

2. Activating the high-limit setpoints.

Looking at the big picture, suppose that dedicated tonnage monitors have been installed and calibrated. After calibrating, the technician leaves and the folks on the shop floor are left with limited working knowledge on how the monitors operate. As time passes, they notice that the monitors have not prevented hard hits.―The tonnage readings seem accurate from hit to hit, but the units do not stop the press for die crashes or for double parts in the die. Thus, the tonnage monitors have lost their credibility.

Typically, such conditions result in a technician returning to recalibrate the units. The technician recalibrates with load cells, but usually tweaks the values only by about 5 percent. Unfortunately, this recalibration fails to address the real problem, as the units still do not stop the press due to tonnage overloads.

Taking a closer look, stampers may find a simple solution.

Check the Stop Circuit

Initially, shops must test the tonnage monitor’s stop alarm. Ensure that the bypass switch is turned off; locate the switch inside of the tonnage monitor, though not all models have one. Then, carefully turn off the power on the unit. For many units, this requires pulling the fuse within the enclosure to cut the power. Other units may contain a power on/off switch. Perform this procedure with the press ram located at top dead center to eliminate the risk of stopping the press at the bottom of the stroke. Should the press still run, we’ve discovered part of the problem: the tonnage monitor is not wired into the stop circuit, or is jumped out. An electrician should be able to correct this without many complications by locating the terminal connections that open the circuit when power is lost or when the unit faults, and then tracing the wires back and connecting them to the top-stop circuit.

Activating the High-Alarm Setpoints

With the stop-circuit issues addressed, let’s discuss the high-alarm settings. Many units have a Setup mode that acts like a bypass switch—valuable when placing a die into the press. This allows the tonnage monitor to start a new job without triggering a nuisance alarm during setup.

Toledo Controls Bypass Switch However, a tonnage monitor unknowingly can be left in this mode and will not provide protection during light-tonnage jobs. For example, with the monitor in Set Up mode, a 1000-ton press will not stop on an overload until its force exceeds 1000 tons. The unit must operate in Run or Automatic mode to activate the high- and low-tonnage limits. A Learn function on a tonnage monitor provides great help in setting precise limits for the particular job being run. A 1000-ton press might be running a job at 200 tons. This Learn function, set at 20 percent, would “learn” the high limit of 240 tons instead of using the maximum of 1000 tons. Most tonnage monitors without a Learn function still offer a method to adjust the high- and low-tonnage limits. For example, an operator can dial it in manually at 240 tons to activate the high-alarm setpoint.

Changing Limits on PLC-Based Units

PLC-integrated tonnage monitoring offers expanded capabilities and enables automatic setting of limits for each die. Recently I worked at a plant with a highly skilled maintenance technician who had no idea how to change the limits on the PLC-based tonnage monitor. These limits were set at the tonnage capacity of the press. The PLC system still needed to be programmed for a specific job recipe, just as with die protection, programmable limit switches and shut height. Otherwise as mentioned, the values would be set to the tonnage capacity of the press. In this case, with a simple PLC program change, the tonnage parameters could be saved, recalled and loaded automatically with a job change.

Help Available for Tonnage Setting

Activating the alarm and limits makes tonnage monitors valuable tools in a stamping plant. A good supplier can provide experienced technicians to ensure proper settings. For its part, Toledo Integrated Systems has these technicians on staff, and also offers a simple ‘soft-calibration training’ process. Here, technicians reset the calibrated settings, address the stop circuit and train operators on how to activate alarm limits. Summarizing, there’s no better time than the present to use your tonnage monitors effectively. It’s easier than you think. MF