Avoid pitfalls by thinking ahead
February 8, 2005
Upgrading any piece of machinery can produce anxious headaches. Often we cause our own headaches by failing truly to justify the reasons to upgrade, choosing instead simply to answer the question, "Can we afford to spend the money?"
Answering that question is really easy. Either the money's available or it's not. The complexity begins and increases when we try to answer why we need to upgrade.
Aside from saving the expense of buying a new machine, reasons to upgrade are to:
Each of these reasons carries oblique issues that must be addressed. You have to keep in mind the possible pitfalls in upgrading and how to overcome them.
Sometimes the decision to upgrade is simple, such as when safety is compromised. But you must think about the technical aspects involved. Are light curtains required? Are special safety locks required? Will production rates be affected?
An example of a hydraulic press upgrade might be a feature that monitors positions of valves to show if a safety problem is occurring. Pressure valves also could be monitored to check for magnitudes needed to create proper holding forces. Actions then could be tied to these pieces of information to create warnings or stoppages.
If your company has been cited for a safety violation, have an open and detailed conversation with the inspector to determine the exact technical solutions. Then contact a company that specializes in machine tool safety. Those companies know the rules and revisions, and they understand how to integrate without interference.
If your concern is too much maintenance or too little production, the complexity of the decision rises substantially.
The answer lies in understanding how the machine operates. Are the mechanical forces radial or axial? Are materials causing too much wear? Is the process causing the high maintenance?
For example, if a die is not centrally located in relation to the applied forces, or if software is used to create hydraulic valve sequences that cause pressure spikes that are then applied in a manner that is thought to shift the valves faster, the process becomes high-maintenance. Both of these scenarios may produce immediate decreases in part production, but the final result is an overall decrease in productivity.
Applied Forces. You need to keep in mind that mechanical operations are governed by the laws of physics. Because of this, increasing productivity is not just a matter of speeding up the machine.
For instance, look at the forces required to move something. The law F = MA states that the forces (F) are generated when a mass (M) is accelerated (A). Acceleration is simply the time given to change from one velocity to another. If you want the change to occur more quickly, you need higher forces. And the greater the mass, the more force is required to produce the change. More force requires more power, which might be costly.
Hydraulic Valve Sequences. Valves used with fluid power come in all sizes. Some can fit in your palm, while others cannot be lifted without help. The valves can be configured with the force taken from whatever pressure is required to move the downstream load. This can vary instantaneously and multiple times in a single machine cycle, leading to instability in valve shifting and creating ill-timed events or loss of control.
Sometimes a second pressure source must be installed for the sole purpose of providing the force to shift the valves. The flow is small, but the force is large, and the expense is not as high as might be expected.
Another common reason for upgrading is that a component is no longer supported. A lot of older machine tools are not flexible. They were designed as rigid tools that perform the same process minute after minute. When your jobs change, the old tools might not be capable of making the new part. An upgrade can make the machine more flexible, allowing it to be used on a changed production process.
When the objective of upgrading is to improve part quality, payback can be very fast. Variable slide speeds, tonnages, dwell, active measurement, and other features added to an old machine can go a long way toward improving the quality of parts.
One common problem with hydraulic presses is programmable logic controllers with slow scan times. Scan time is the time it takes the controller to travel through its entire program one time. If the time between scans is long, say, 50 to 100 milliseconds, an event might occur after the scan has passed and be captured by the next scan. Timing problems arise between process actions. New controllers have very fast scan times, plus they are readily available and becoming less costly.
For machine tools, one of the most misunderstood technical concepts is closed-loop control. In closed-loop control, a target is given, but automatic adjustments are made to help ensure the target is obtained. Closed-loop is programmable and ensures that the displayed results are equal to the target.
In closed-loop control, some kind of feedback device transforms process variables into pieces of voltage feeding a programmable logic controller, personal computer, or motion card. Then the controller, PC, or motion card collects the pieces of voltage, transforms them into units that make the process understandable, and makes decisions about where the current value is compared to the projected value. The controller, PC, or motion card then issues new signals based on the magnitude of the differential.
A control system is not closed-loop simply because it has some type of servo device, programmable logic controller, or PC-based human-machine interface. There is a clear difference between the results. With a closed-loop system, the accuracy in achieving the target is increased as much as tenfold.
When you're looking for an upgrade, be sure to develop a written list of what you want. This list should include general requirements such as "Must store recipes." Then leave it up to the vendor to develop a solution for that requirement. Without a list, it is easy to rely on the vendor to give you something, and it might not be exactly what you want. Take the time to help everyone understand your needs.
Just because the upgrade is complete and acceptable doesn't mean you're done. It's still a machine tool, and it will need maintenance. Make sure you have an operator's manual filled with pictures of the components.
In addition, your maintenance personnel and operators will need to be trained so they know what to do with the upgrade. Their comfort with it will affect your production rate.