Press tonnage is one of few variables that can be collected in the 1/50th of a second it takes to make a forging. This is an important tool in the measurement and monitoring of stresses during forging cycles, and it will help you see how stresses affect the performance and life of your mechanical press.

Floor view of a mechanical press fitted with a tonnage-monitoring system.


My last article was about tonnage monitoring for hydraulic presses, and although this one is on the same topic as it relates to mechanical presses, some basic business principles remain the same. For example, good business still starts with smart investments, and the smartest investments protect mission-critical equipment that makes it possible to ship forgings out the door. You can replace a forklift or a MIG welder in less than a day. You can’t replace or perform a big repair on your mechanical press in just one day.

Mechanical presses make closed-die forgings faster and more efficiently than hydraulic presses, but they are very unforgiving pieces of equipment. A mechanical press, by its very nature, will attempt to complete every forging cycle, whether it is being overloaded or not. In fact, it will destroy itself to make forgings for you. Mechanical presses will do what you ask of them, but they will not forgive you for overloading them forging cycle after forging cycle.

This characteristic of mechanical forging presses is what makes the use of tonnage monitors critical. Equating press tonnage with press uptime and the cost of maintaining the press is a worthwhile pursuit. You want to produce forgings. You want as much uptime as possible while minimizing maintenance. You want the press to last for many years, and you want to make as much profit from your forging press investment as possible. Well, monitoring the tonnage can improve your chances of success on all of these fronts.

It is a known fact that when making forgings on a mechanical press it may not take all the tonnage the press is generating to produce a geometrically acceptable forging. When running a higher tonnage than required, press uptime and maintenance costs come under assault.

Measuring Press Tonnage

In the 1/50th of a second that it takes to make a forging, there are very few process variables that can be measured and collected that relate directly to the condition of the process. Press tonnage is one of these few variables that can be collected the instant a forging is made. This can be viewed as a snapshot of the forging as the two dies responsible for the finished forging size are as close to one another as the process permits. In fact, this is equivalent to measuring the forging in the closed-die cavity under load.

Press tonnage is a dependent variable whose magnitude is derived from the independent variables of the process. The independent process variables that affect the tonnage required to produce a forging are numerous and always in flux. Some of these independent variables are billet temperature, temperature profile within the billet, billet size and weight, forging die wear, forging die/bolster temperature, elapsed time from furnace to finish die, die-lubrication mix consistency, and die-lubrication application/spraying consistency.

Figure 1. Diagram of surface mount hardware on a press frame

Die-Bed Force Distribution

The distribution of forces within the die bed is another benefit of having a tonnage monitor on your press. If all four columns of the press are monitored, the force values are isolated column by column. Thus, it can be determined where the forces are within the die bed during a forging cycle. It is always best to have the distribution of forces equal when forging near the machine’s rated capacity. It is possible to optimize your process by trying to get the forces equal for each column during these demanding strokes. Moving dies in the bolster is always an unpopular subject, but we don’t do it just for adventure. We do it to optimize the distribution of forces within the die bed.

The sensors used to measure press deflection are called Linear Variable Differential Transformers (LVDTs). These have the unique ability to measure displacement and strain without the need to be recalibrated. The LVDT system measures strain in press tie rods or in solid cast machine housings. The strain measured is converted into tonnage incurred during each forging cycle. Figure 1 is a diagram of the surface-mount hardware; Figure 2 is a diagram of the in-tie-rod hardware. For superior accuracy, the LVDTs measure the strain over a distance of 50 inches or more.

Figure 2. Diagram of tie-rod strain gauge detail

Calibration of Tonnage Monitors

After the LVDTs and supporting hardware are installed on a forging press, the system requires a calibration procedure, which uses hydraulic rams to simulate a load on the forging press. To calibrate, hydraulic rams are placed in the die area. The press is jogged forward until the press ram is located at bottom dead center. This can be accomplished with a dial indicator on the press bottom hard plate. The hydraulic pressure in the rams is gently raised in increments of 500 psi up to 10,000 psi. Twenty data points of column strain versus tonnage is collected for each of the four channels. The strain data is then used to determine the calibration parameter for each channel. The calibration parameters are entered into the operating program as a multiplier to reliably report the tonnage for each forging stroke.

With a tonnage monitor installed and calibrated and with the knowledge of which process variables are important to your forging process, it is now possible to take the next step – the collection and analysis of process data.

Figure 3. Production run setup window

Data-Acquisition Systems

Data acquisition is a great complement to tonnage-monitoring systems on mechanical forging presses. The data-acquisition system takes tonnage and alarm data from the tonnage monitor and organizes the data into convenient spreadsheet files that can be accessed at any time during or after the production run. The files contain much information that corresponds to the specific production run.

The system also uses a pyrometer to record the temperature of the billet sitting on the buster dies. The temperature snapshot is recorded in the spreadsheet when the brake is disengaged to start a forging cycle. The pyrometer signal is also used as a digital input to inform the data-acquisition system that a bust forging cycle is about to commence.

The system sorts all the forging cycles – bust, block and finish, for example – into separate worksheets within the spreadsheet workbook file. It will also create a worksheet for all workpieces that are started in the forging process and never make it to the finished dies. In other words, if the workpiece is busted and blocked but never finished, the tonnage and temperature data will go into an incomplete-forgings worksheet.

Each stroke of the press will receive a time stamp in the data file to further identify when certain events took place. The system creates a separate spreadsheet file for each production run and places that file into a folder named after the production run number.

At the top of each spreadsheet, there are 27 different parameters used to identify and track critical forge production data. Some of these include actual piece count, production run number, job number, die serial number, customer, lubrication number/type, material heat code, production run date/start time, column alarms tally, total alarms tally and many more.

The system also permits the creation of a descriptive document that can be opened from the data-acquisition user interface and added to the file to make comments of any kind during the production run. This is a handy tool to identify unexpected downtime or other important events.

When the production crew is preparing to make a production run, the crew leader will open the production-run window and enter in all of the critical production-run data. Figure 3 is the production-run setup window for a 4,000-ton Erie press.

The data-acquisition system runs on a common PC and can be used as a host to monitor multiple presses. Figure 4 is the main press selection screen. There are three presses connected to this particular host PC. The PC plugs into the existing Ethernet backbone, which is common in most forge shops. Files created on the PC can be viewed or off-loaded at any time with the company Ethernet system.

Data acquisition is a great way to collect, organize and store all production-run information. Data files collected can be interrogated at any time in the future. Data acquisition is another tool available to help get a handle on the forging process.

Figure 4. Main press selection screen showing three presses connected to host PC.

Conclusion

Just as you would not drive a sports car without a tachometer nor fly an airplane without a gyroscope, you should not operate a mechanical forging press without a tonnage monitor. There are ways to get more production hours out of every mechanical forging press. Armed with the proper tools, you can learn more about process variables and how they relate to press tonnage.

Author Steven F. Rasmussen is president and engineer at Angstrom Corporation, Twinsburg, Ohio. He may be reached at 330-405-0524, 1-888-73-GAUGE, or at stever@neohio.twcbc.com