Practical tool condition monitoring

Manufacturing Engineering, Jan 1998 by Meredith, Don

Lasers and sensors that detect vibration, acoustic emission, horsepower, torque, and force are the keys to successful in-process sensing.

Acceleration or vibration sensors measure the vibration of the tool through the machine structure and back to some point located on the part side of the machine. When a tool breaks or becomes worn, a large amount of energy passes through the machine structure. These sensors pick up that change, either as a burst of energy or an increase in amplitude of vibration over a short time. Because the frequency of the measurement is the same as the frequency generated by bad bearings, a bad bearing will hide a bad tool on the spindle side, so they should be mounted on the part side.

Acoustic emission (AE) sensors measure the burst of energy that indicates a broken tool on the surface. They are used primarily to detect breaks in small carbide tools, which usually travel at high speed and emit only high-frequency energy when they shatter.

The workhorses of the sensor toolkit, the horsepower sensors, mostly of the Hall-effect, currentvoltage type, can handle 70% of the monitoring challenges we face. They are the cheapest, the easiest to install (it takes about 20 minutes and 3 wires), and least intrusive. Placed in the line with the spindle motor, the HP sensor measures current and voltage through that line.

Sensing HP is not a universal solution, however. Measurement lags real time because it comes from the spindle motor driving the tool. The HP sensor cannot detect a broken tool on a machining center moving at 20,000-30,000 rpm in time for the operator to stop the machine. In tapping operations, where you tap down, reverse the motor, and tap back out, reversing the motor causes a power spike, and that's the point at which a tap typically breaks. HP doesn't work well here because it's hard for the operator using HP to see the break during that power peak. Torque sensors are best here.

Multispindle drilling operations also challenge HP. Though some vendors have used HP sensors on 8-10 heads, four heads are probably the maximum number that HP can handle effectively. A new system is easy to operate and inexpensive. It uses a camera and shadows to monitor individual spindles in a multispindle head. The user supplies a light source and aims a camera mounted on the multispindle head in the general direction of the multispindle. The camera will fine-tune itself, record the head in a teach cycle, and store that target image. When actual machining begins, the system compares the contours of the target to those of the image it detects, registering a missing or broken individual drill, even a tool with a missing tip or an improperly clamped drill. In fact, the system actually measures the pixels in the shadow cast by the tooltips. If they decrease by a certain percentage, the system reports a broken or chipped tool.

To monitor tapping operations, use torque sensors. These sensors have one element on the shaft and one on the body of some fixed part of the machine like the spindle body. As the rotating part or source twists, the fixed part of the sensor monitors the twisting action. There is a strain gage inside the tool; the stator portion of the device, like a motor, feeds current into the tool and measures the physical strain on the device as it twists. As a tool breaks, the point where it contacts the part being machined will slow down, while the other end of the tool maintains the same speed. If an earlier drilling or casting operation created a hole of the wrong depth, torque sensors will detect it before the tap runs into the bottom of the hole.