Broken Tool Detection In Machining And Turning Centers

Broken tools can create headaches for shops. Scrap, re-work and machine downtime, in addition to the time involved in identifying where and why the tool was damaged, can lead to considerable lost productivity and increased costs. As shops increasingly look to lightly tended and untended machining to improve productivity, these issues become even more significant. Without an operator present, the machine is more likely to produce a large amount of bad parts or even crash before the problem is detected. Broken-tool detection systems are helping shops to overcome these issues and keep machines more productive.

For turning centers, traditional, contact-based tool measurement systems typically are used to identify broken tools. According to Dave Bozich, product manager at Renishaw Inc., this method is still the most effective for turning operations because the tool remains stationary. His company offers three different configurations, virtually all of which are sold to OEMs, who implement the tool setting arm at the factory when they build the machines.

“We’ve redesigned these systems through the years, but they’re still, essentially, variations of what the original products were,” Mr. Bozich says of the contact-based sensors. Describing the three different methods of operation, he explains, “There could be a plug-in arm that the operator loads into the machine when he wants to set tools, or the arm could reside inside the machine, with either manual pull-down functionality or motorized operation.” For all three, the tool setting probe and arm mechanism is moved out of the work envelope during machining and is only put into place for tool setting or detection.

For machining centers, Renishaw offers the TRS2 non-contact system designed exclusively for use in broken-tool detection. According to Mr. Bozich, this single-sided system can provide significant cost savings compared with traditional tool measurement systems (often relegated to use strictly for tool detection) and two-sided tool detection systems. “Traditional laser systems have a transmitter and a receiver that require maintaining alignment between the two,” he explains. “The TRS2, being only a single unit, requires less hardware for installation. Plus, because the unit is not doing any measurement—instead, only looking for the presence of a tool—alignment and rigidity are not as critical.” The 3.27-inch by 1.50-inch by 2.87-inch unit is installed outside the workzone with a simple, U-shaped bracket that attaches directly to the machine guarding. The operator points the laser toward the tool, initially aligning it by sight. When the five-tiered LED signal-strength meter on the face of the unit indicates the optimum reading, the unit is tightened in that position. Control of the non-contact system from the part program requires only a few lines of macro code versus the hundreds of lines of code needed to control a measurement laser.

Capable of detecting tools as small as 0.008 inch (0.2 mm), the tool detection system works by sensing the reflective properties of the tool. The laser is projected from the device, and as the tool rotates, the detector that is housed inside the sensor (within the same unit that holds the laser) examines the reflective pattern that comes back. A series of internal microprocessors are dedicated to detect tools at different velocities (5,000, 1,000 and 200 rpm), depending on the operations being performed and the tools being used. Mr. Bozich says, “You’re not going to run a large gun drill at 5,000 rpm or even 1,000 rpm because it will whip around. For these, the system will read at 200 rpm. On the other hand, if it’s on a small machine running at fast velocities, the operator may not want to drop the spindle speed all the way down to 1,000 rpm, so the reading can be done at 5,000 rpm.”

The system is designed to run when the tool is not in the cut. The operator includes a broken tool detection check in the program, either before or after a machining operation. For example, a particular feature on a part may require three or four different tools. If the first tool, maybe a piloting twist drill, breaks during machining, every tool after that could pile up inside the hole. In this case, the feature would be machined, and on the way back to the tool change to get the subsequent tool, the twist drill is cycled into the laser beam. If the system sees that the tool is there, it knows it is OK to move on to the next operation. Otherwise, a redundant tool can be called up, or the machine can go into program stop mode.

During the detection process, as the tool makes a complete revolution, TRS2 takes a series of snapshots. The snapshots are generated by the reflecting light pattern representing light and dark pulses. System electronics compare the light and dark pulses looking for a synchronous pattern. A repeating synchronous pattern represents a good tool. A random pattern represents a broken tool. While the system can distinguish between the tool and coolant or chips under real machining conditions, Mr. Bozich recommends shutting down the coolant during the detection operation. With a clear view of the tool, the TRS2 can detect a repeating pattern on the tool in about a second.

This highly focused system, as part of the programmed machining sequence, allows quick detection of broken tools in machining centers. With in-process detection, machine downtime can be kept to a minimum for higher productivity.