The swift and reliable detection of broken tools in machining centres is essential. A broken tool can lead to scrap and rework, and can have costly implications if left undetected.
Philip Smith, technical sales manager with Renishaw (Canada) Ltd., Mississauga, ON notes that, “Conventional contact broken tool detection systems have a number of weaknesses and are often unsuitable
for smaller delicate tools.”
The emergence of compact laser technology in recent years has enabled the development of non-contact, broken tool detection systems and the ability to measure increasingly small tools safely.
However, using a ‘beam block’ system for broken tool detection has its weaknesses, as it can not distinguish between a tool and contaminants such as coolant and chips-potentially leading to unreliable results and the possibility of the system indicating that a broken tool is good.
Currently, contact systems are the most common method of broken tool detection. The two major types are the button system and the rotating wire system. The button system involves bringing a tool into contact with the button or anvil, thus triggering the device which confirms that the tool is present and not broken.
The rotating wire system consists of an actuator, which rotates a thin wire wand or whisker until it comes into contact with a tool. Failure to contact the tool therefore leads to the conclusion that it is broken.
Conventional non-contact tool setting systems use a beam of laser light passing between a transmitter and a receiver, located either on the table of the machine, or on each side of it so that the beam passes through the
working volume.
The presence of a tool in the beam causes an eclipse of the beam and a reduction in light seen at the receiver and a trigger signal is generated. If there is no reduction in the light received, it is assumed that the tool has failed to obstruct the beam and therefore must be broken.
Another problem is that contact with the tool can actually break or damage small delicate, coated tools. Consequently, only tools over a certain diameter can be safely tested.
Even so, larger tools or those with a delicate surface coatings can also be at risk. Furthermore, using contact methods for broken tool detection is slow and adds significantly to production cycle times as the tool must contact slowly to avoid damage.
Often, contact systems must also be mounted within the working environment, taking up valuable table space and creating a danger of collision. Those with actuators are prone to coolant ingress and can jam, resulting in poor reliability. Smith points out that “Using a conventional non-contact laser system for tool breakage detection can present a number of unique problems due to the system’s inability to distinguish between a tool and contaminants such as coolant or chips.”
The challenge has been to design a reliable system that can consistently distinguish between a good tool and a broken one and do it in the worst environmental
conditions.
Renishaw’s response has been to develop the TRS1,  a new, unique device dedicated to broken tool detection. It uses a laser beam, but dispenses with the beam block method of tool detection.
Instead, the TRS1 relies on the beam being reflected back to the receiver, which is contained in the same housing as the transmitter. The unique tool recognition system (TRS) electronics then establish whether a tool is present – and hence is good, or if not present, broken. “A single sided system has always been our goal . We’ve been pushed by our customers to develop a reliable and fast means to be able to check the tool status. The only way is by using a compact single laser device and although there were some challenging design hurdles, we’ve finally achieved it.”
The TRS1 works by directing a laser beam towards a point where the tool detection is to be conducted. The tool is then positioned so that the laser beam shines onto the tip of the tool – typically 3 mm from the end of the tool. The tool is rotated at 1000 rpm, and the laser is reflected off the tool and back to the TRS1 receiver. Due to the tool’s rotation, the reflected light level varies resulting in a repeating pattern. This repeating pattern is recognized by the micro controller within the TRS1 and the output relay is triggered, rapidly signifying a good tool and allowing the machining cycle to continue. As the repeating light pattern can only occur when a tool is present, the TRS1 cannot be fooled by contaminants such as chips or coolant. If no tool is identified, at the end of a given period the application software issues a broken tool alarm.
The TRS1 is designed for safe and reliable operation. It relies on reflected light to identify the tool-the amount of light reflected depending on a number of factors, such as tool size, surface finish, tool shape, operating range and machining environment. If the tool cannot be rapidly recognized, the user can vary the amount of time allowed before an alarm is generated.
Typically, the TRS1 requires around 1 second to identify a good tool, but in cases where the reflected light level is low, or the repeating pattern is obscured, the detection cycle may last longer than this. This time is only then required for certain specific circumstances, not for every tool detection cycle.
The TRS1 can operate over a range of up to 2 m and  can detect tools as small as 0.5 mm diameter. High feed rates can be used, resulting in short cycle times. Unlike existing non-contact methods of tool detection, the TRS1 can not mistake coolant or chips for the tool and so it becomes virtually impossible for a broken tool to pass undetected.

This article was written by Philip L. Smith, technical sales manager with Renishaw (Canada) Ltd., Mississauga, ON.
renishaw.com