It's happened again. Just a few weeks ago, I was told by a company executive that he did not need electronic sensors for die protection--he had tonnage monitors on his presses. He felt that sensors in the die would be redundant. "Isn't the whole point of tonnage monitoring to have a quick form of die protection?" he asked.
Well, no! In my opinion, true die protection is when you have a mechanism that can detect a malfunction in the stamping process before the die closes. If, for example, the part has not been ejected properly an electronic sensor would have caught this malfunction at the end of the part ejection window and the die protection unit would have sent a stop signal to the press before the die would have had a chance to close. Of course a tonnage monitor, with strain gages mounted on the frame of the press, properly calibrated and adjusted, probably would have caught the double hit as well. But not before the die closed! One needs what is called a "bad hit" for that doubling up to be detected.
I firmly believe that press tonnage monitors are crucial to the metalforming process, but not as die protection so much as press protection systems. Should the part-out sensor have failed, been disconnected or bypassed (which happens all too often) or some failure within the die protection monitor itself occurred--the tonnage monitor probably would have stopped the press as the double hit would have tripped it. This stoppage is important for the protection of the press and the tooling.
To put it another way, press monitored tonnage should not be seen as the primary line of defense for die protection. In that context, tonnage monitoring should be perceived as the policeman of last resorts. When the in-die process monitoring sensors fail to do their job (which is extremely rare if proper mounting, adjustment and respect are accorded to them), then the press monitoring tonnage system kicks in and sends a signal to stop the pressÑbut not before the die has closed on top of a mess.
I also have heard from those with the power to sign purchase orders who said they did not need tonnage monitors on their presses--they had electronic sensors for die protection. This is a totally wrong perception. For one, as you can see above, the role of press tonnage monitoring is a very important backup for press protection. Press-mounted tonnage monitors also serve a very important set of functions during setup and process monitoring conditions.
They can, if properly set and the sensitivity is available, act as material property monitors. As the hardness of the strip fluctuates, it is possible in some applications to have the press-mounted tonnage monitor stop the process if the material hardness tolerances are exceeded. Again, this is very dependent on the sensitivity properties of that particular setup. It is unfortunate that in some shops this is seen as a handicap. They complain that their presses are stopped by the tonnage monitors every time the material gets too hard for the press to handle it! Amazing comment--but I have heard it myself.
Press tonnage monitoring also is excellent as an aid in the setup of tooling. A good setter will compare the tonnage being displayed with the setup sheets to verify that the die-to-press-to-material relationship has been properly set. Repeatable setups are not the only goal here--press-to-tooling problems, such as poor parallelism, bolster plate buckling, nitrogen cylinder failures, spring failures, press component failures and other issues, can be detected in some applications by the press tonnage monitors.
A relatively recent approach to press tonnage monitoring offers an excellent improvement on this time-tested and proven technology--the capture and analysis of the tonnage during the entire cycle of the process. By acquiring and analyzing the tonnage fluctuations over time, the science of what is called tonnage signature analysis has propelled shops into a new level of force monitoring. Not only does the unit detect the maximum force exerted by the die when it bottoms out--but much, much more.
As the die begins to close, staggered punches exert some preliminary forces--they are monitored. These punches are followed by the bulk of the piercing, blanking and forming operations and they are monitored as well. As the die opens, the snap-through or reverse tonnage also is captured. All of this happens in real time and all within a specific time line. In other words, the "cardiogram" of the forces is captured and analyzed in real time.
An exciting development is the placement of force monitoring sensors inside the die to capture the forces within individual stations in the tool. Strategic placement of these sensors allows for detection of subtle changes in say, a pierce-and-draw operation--to the exclusion of forces being exerted in other parts of the die. Couple this ability to a signature analyzer and one now can capture and analyze in real time the performance of individual stations within the tooling. To accomplish this with a press-mounted tonnage monitoring process, with the sensors not in the die but on the frame of the press, would present a nearly impossible challenge.
A further refinement of the signature analysis concept was demonstrated at METALFORM'97 by Signature Technologies. They were capturing and analyzing the signatures not only of the force monitoring sensors, but the die protection sensors as well. This is an interesting and worthwhile development that merits attention. As microprocessors become more and more powerful, and creative engineers and applications specialists take on more and more challenges, we can expect brilliant new approaches to the protection of both the tooling and the press within one monitoring unit.
I am convinced that a good metalforming operation will have a tonnage monitoring system AND a die protection system working in partnership with one another. The dies run with less maintenance, near-zero die crashes, unattended and faster in many cases. Who could possibly be against such performance? Yes, investments in equipment, training and discipline are required. But who can afford not to do these things as our customers demand zero or near-zero defects and on-time deliveries?
I heard an interesting comment about tonnage monitoring that I think should be shared with you. How would you react to the following: "I am not fond of tonnage monitoring, for you see, every time I have a problem, my tonnage monitor shuts me down and it's usually production that gets yelled at."
Some of us are so obsessed with part counts that we will do anything it takes to fill up the boxes at the expense of crashed dies and/or damaged presses. Please don't blame a properly functioning tonnage monitoring system for detecting a poor setup, bad incoming strips or press malfunctions. Don't kill one of the finest process monitors you can have--your tonnage monitor--for doing its job!
MF