Monitoring Devices Head Off Disaster

Whether you are a job shop running one-of-a-kind parts or a large manufacturing plant producing parts by the hundreds of thousands or millions, a machine tool crash can mean missed deliveries, lost production time and expensive repairs. The chances of such crashes are increasing as companies strive to get maximum utilization from their CNC machines by running them 24/7.

Crashes are an unavoidable part of life in the machine tool world. According to one source, every machine tool is involved in at least one crash over its working life, resulting in substantial down time and cost to replace damaged machine components, fixtures, tooling and so forth. Machining is a complex process that presents many opportunities for crashes: A part or tool can be in the wrong place at the wrong time; a tool can break . . . and even in shops with carefully thought out procedures in place designed to prevent accidents, an aberrant signal from a computer control can send a tool slamming into a workpiece or fixture.

Although there is no way to prevent such occurrences, technology is available that can greatly reduce their impact. For example, Brankamp Process Automation, Inc. (Cambridge, Massachusetts) offers process monitoring equipment that, among other things, can detect when something goes wrong with a machining operation and shut the machine down fast enough to prevent major damage to it, the workpiece or the tooling. The process monitoring equipment is analogous to an automobile airbag: it doesn't prevent the accident, but it reacts in time to minimize the injury.

The system is relatively simple. A high-resolution force sensor is mounted on one of the moving components of the machine tool. For example, the photo at left shows a sensor mounted on the turret housing of a CNC lathe. Preparing the machine to receive the force sensor involves simply drilling and tapping two holes in the turret housing. The installation is permanent, and the sensor can "read" all of the tools mounted in the turret one at a time.

The sensor is connected to and continuously monitored by the process control, in this case a Brankamp B400 monitor with up to eight monitoring channels. After a normal cutting signal is identified, an operating envelope or set of limits can be established. If a limit is exceeded—indicating some unexpected event—the process control can shut down the machine in 2 to 3 milliseconds (0.002 to 0.003 seconds), preventing major damage.

Process-monitoring systems can be installed on new machines or retrofitted to machines in the field. They require no machine disassembly or structural modification. Some do not even interface with the machine's CNC control. Systems range from one sensor and one controller for one machine, to central controllers monitoring banks of machine tools, each fitted with a sensor and an individual controller. Problems can be resolved as they arise by shop floor personnel, however the system also provides a window to the status of the operation for managers at a central location.

Companies that have installed collision-monitoring systems have been able to drastically reduce tool and die repair costs and machine downtime, which has led to significant savings. Engineers responsible for analyzing the effectiveness of such purchases on plant productivity will appreciate the fact that some controllers are equipped with a counter that counts every collision event. Those data can be used to calculate actual cost savings due to using the system.