Machine Monitoring Enables Control of Cutting Forces

Figure 1. Cutting forces in machining can be measured indirectly with high accuracy, applying piezoelectric strain sensors such as the 9232A from Kistler. (Image: Kistler Group)

High productivity, low manufacturing costs, and high workpiece quality: these are the key factors that deliver sustainability, profitability, and competitive edge for industrial manufacturers. Reliable machine monitoring yields valuable real-time insights into ongoing processes; it is the basis for dependable, productive, and reproducible manufacturing and it helps machine operators to reach well-founded decisions on both short- and long-term improvements. This technology can even capture anomalies in highly dynamic machining processes, so users can respond instantly to ensure high productivity, decrease scrap rates, and prolong tool lifetimes. Thanks to all these advantages, continuous machine and process monitoring based on suitable sensor technology is a critical success factor in today’s manufacturing industry.

Machine Monitoring with PE Strain Sensors

Machine monitoring solutions from Kistler Group, Winterthur, Switzerland, feature sensors that can be integrated into a machine’s mechanical and electrical environment. However, only those sensors that are actually necessary will be installed at defined positions in the machine. This approach minimizes both the complexity and cost of the machine monitoring system, although integration of the sensors into the machine’s mechanical and electrical structures can still be challenging.

Sensor type and position are critical success factors in capturing highly dynamic processes with maximum accuracy. The sensor must be placed close to the cutting point in such a way that it does not interfere with the machining process, but at the same time, it should not be subject to any influences other than the actual cutting. Monitoring of cutting forces plays a key part in ensuring the stable machining operations that are needed to produce tight-tolerance components. Users who opt for machine monitoring solutions of this sort can also advance towards autonomous machining and reduce their total machining costs as they do so.

One way to capture the cutting forces is by measuring the deformation forces acting on the machine tool structure. This can be achieved with piezoelectric surface strain sensors attached to, or integrated into, the machine structure. In a recent project with MINGANTI GLOBAL AB, Lidköping, Sweden, vertical lathes were equipped with an integrated Kistler PE surface strain sensor 9232A, which offers the high sensitivity needed to measure both tensile and compressive strains on a surface thanks to a measuring range of ±600 microstrain (με).

Indirect Cutting Force Measurement Close to The Cutting Edge

Figure 2. The ram of the machine where the cutting tool holders are clamped. (Image: Kistler Group)

This machine monitoring system is intended to detect high tool wear and tool breakage and to stop the machine in case of crashes. Also, once force control is established on the basis of precise measurement data, it can be used to monitor and improve profile accuracy. The large radial forces that occur in hard turning cause elastic deformation of the ram that carries the cutting tool holders . The deformation is very small, and it can be partially compensated by the machine control unit. But even so, it is still large enough to be captured by a strain sensor with good accuracy located very close to the cutting edge.

Machine monitoring can also be based on machine signals, but in this application, the quality depends both on the machine type and the state of wear. Machine signals sometimes show deviations that are unrelated to the cutting process, so they are not sufficient for a machine control system targeting tool wear or product quality.

Minganti and Kistler decided to install a second sensor to complete the machine monitoring solution. This provides a certain redundancy, and also takes account of the high cost of parts. For this purpose, an 8274A single-axis piezoelectric accelerometer from Kistler was additionally integrated into the ram. It complements the force (strain) measurements by supplying higher-frequency measurements so that sudden changes in the process can be detected. Both sensor inputs are processed with a digital charge amplifier, which is connected to the machine control via PROFINET. One of the major advantages of this charge amplifier is that its users can automatically perform low- and high-pass filtering, making it easy to focus on the highly dynamic part of the signal.

Achieving High Surface Profile Accuracy – With No Extra Effort

Figure 3. The diagram shows good correlation between the force (strain) signal from the Kistler strain sensor (upper plot) and the surface profile measurement after cutting (lower plot). (Image: Kistler Group)

The extended machine monitoring system is used to monitor tool wear, detect tool breakage, and to detect and control machine crashes. It automatically learns the regular signal levels and then monitors changes to detect deviations. This allows operators to initiate necessary actions such as tool changes, machine stops, and others. Tool failures (breakages, edge chipping, etc.) can be caused by various reasons including incorrect cutting data loaded in the numerical control, low cutting-edge quality, wrong coolant pressure, or rings that are too oval, leading to major changes in cutting depth with each rotation. When such failures occur, their effect is clearly visible in both the force and vibration signals.

One of the most important quality parameters for bearing components is the profile accuracy (‘straightness’) of the surfaces. This parameter affects the friction between the rolling elements and the rings in a bearing. Maintaining tight tolerances (3 - 6 μm) on a large-size ring (1 to 2 meters in diameter) is a challenging aspect of the hard turning process, which is why many bearing producers perform grinding or honing as an extra step. The Kistler machine monitoring system can indirectly measure the strain (deflection) of the tool, so the profile accuracy (form deviation) is clearly visible on the strain (indirect force) signal. The force signal input can be used directly during the turning operation to change cutting conditions with the help of closed-loop control, so deflections can be compensated.

The Importance of Accurate Machine Monitoring for Accurate Machining

Although monitoring machine signals can provide information about operating conditions, it might indicate issues that are not directly related to the accuracy of the actual machining. As we described in this article, however, Kistler has methods, such as strain gauges and accelerometers that directly monitor the cutting process. Monitoring of cutting forces plays a key part in ensuring the stable machining operations that are needed to produce tight-tolerance components. Accurate measurement of cutting forces is an important step in maintaining profile accuracy, said Marcus Caldana, President of Minganti Global AB.

This article was written by Buelent Tasdelen, Business Development Manager at Kistler. For more information, go here .