Keywords

1 Introductions

Precision machining plays an essential role in modern and sustainable manufacturing in the automotive, aerospace, micro-machining, medical, robotics, and IT industries, including cutting, forming, and non-conventional processes. In machining, one of the challenges lies in re-clamping and repositioning the part after it has been removed and brought back to continue processing. Accurate re-clamping is a difficult task, which may require a long time to reposition and adjust the gripping force to avoid deflections and deformations. Furthermore, often the workpiece must be held with small and accurate clamping forces to avoid deformation. However, the forces should be high enough to avoid or limit movement or deflection during machining. This paper describes the development and features of a precision pneumatic clamping solution for machining with controlled clamping forces combined with a very accurate visual position measurement device. In addition to improved quality, the new sustainable system provides shorter set-up times, simplicity in positioning and re-clamping, better control and selection of machining conditions, and fewer defective products and waste. The developed system allows for clamping force adjustment along the process chain, considering the machining conditions, thus improving quality, saving time, and reducing cost.

1.1 State of the Art

A high-precision method for machining flexible and accurate components using a pneumatic chuck is presented in [1], including the development of a unique tilting device to control inclination. Monitoring and controlling the clamping forces can influence positioning accuracy, workpiece deformation, thus improving the production outcome [2, 3]. In some cases, hybrid micro-machining techniques are implemented to achieve better performance [4]. A combination of work holding devices with sensing technologies and actuation systems is described in [5]. Different types of sensors, such as foils, piezo-electric devices, and strain gauges, can be used to measure and control the clamping forces [6]. In the current investigation, the calibration and experiments were carried out with strain gauges and piezo-electric sensors on a high-precision pneumatic chuck with three jaws for milling, produced by Henri Azaria, PAL, Israel [7]. The utilized machine vision technology is similar to the two CCDs used to measure tool positioning errors and compensation [8]. The limitations of position repeatability for workpiece re-clamping are based on geometrical tolerances, deformations, and clamping elements [9].

1.2 Definition of Goals

This investigation aims to define, simulate, develop, and analyze a clamping solution with controlled forces and a high-precision positioning device, especially for small and/or thin-walled workpieces. The new adaptive clamping system uses a pneumatic chuck with force-controlled jaws corresponding to the air-controlled pressure, combined with a visual pattern recognition device enabling repositioning of the workpiece to ensure high positioning repeatability when re-clamping and during machining operations. The developed visual method for position recognition includes special and unique algorithms for position assignment and controlled motion of the machine table to provide accuracy and repeatability of ±1 µm. The x and y motions are combined with table rotation and can use the method and inclination device for the z axis positioning [1] and z motion of the cutting tool.

2 The Developed Position and Force-Controlled Clamping System

2.1 Features and Functions

The high-precision system is designed to enable clamping and re-clamping with very accurate positioning of up to ±1 µm. It is essential to adjust the clamping force to avoid deflections and deformations of the workpiece. The workpiece position after the first clamping is identified and measured using a high-precision vision device with a CCD camera. Force and position data are collected and analyzed for calculating the repositioning and the motion of the machine’s x and y axes. Once the workpiece has been removed and re-clamped with the defined forces, the vision device identifies the new workpiece position. The difference between the measured position after re-clamping and the reference position is calculated, yielding the motion command to the x, y, and rotation axes.

Fig. 1.
figure 1

The high-precision clamping system, (a) Experimental set-up for force measurement and force control, (b) Pneumatic chuck with three jaws equipped with strain gauges, (c) Clamping system on the machine tool with the vision set-up.

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Figure 1 shows the experimental set-up of the newly developed system with the pneumatic chuck, the vision set-up for position recognition, and the jaws with strain gauges for clamping forces measurement. The device is mounted onto a high-accuracy rotating table for angular positioning, which in turn is mounted on the highly accurate CNC machine tool table (Fig. 1 (c)).

2.2 Force and Pressure Control of Clamping Forces

Figure 2 shows a block diagram depicting the architecture of the force and pressure-controlled pneumatic chuck. The high-precision clamping system enables adaptive workpiece holding and repositioning with force-controlled clamping jaws. The air pressure is supplied from an external source while controlling the pressure regulator and flow direction using a 5/2 valve (Fig. 1 (a)). The three jaws are equipped with strain gauges to control the clamping forces (Fig. 1 (b)). The force signals are transferred to a USB data acquisition system controlled by a PC with LabView Software. Based on the principle of identical pressure and clamping forces on all three jaws, it is sufficient to control the force on only one jaw in industrial applications. If the clamping force values are directly proportional to the air pressure and can be used in a wider range, force control can be maintained.

Fig. 2.
figure 2

Block diagram of the pneumatic chuck with controlled clamping forces and air pressure.

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The clamping forces for the three jaws with strain gauges are measured and controlled during the experiments. The jaws’ design, material, and features should correspond with the range of the clamping forces, the workpiece properties, and the machining conditions. Different types and structures of jaws are designed and calibrated on the device shown in Fig. 3 (a). The deformation under variation of clamping forces is simulated using FEA (Fig. 3 (c) and 3 (d)). The jaws are equipped with force sensors to control the applied forces during clamping motion to secure minimum holding forces, avoid overloading, and secure clamping forces under machining conditions. The strain gauges are glued onto the zone with maximum deformation to obtain precise results and high resolution (Fig. 3 (b)). After preliminary tests with various materials (plastic, aluminum, steel), jaws made of steel are selected for testing the pneumatic chuck.

Fig. 3.
figure 3

(a) The force calibration device, (b) Steel jaws, with glued strain gauge, (c), Deformation of a closed profile jaw using FEA, (d) Deformation of an opened profile jaw using FEA.

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The required gripping force range is F < 300 N, evaluated using FEA of thin-walled tubes and low to medium cutting forces. The calculated and measured deformation with closed profiles is too low compared to the opened profile. The tests are evaluated with 800 N and a maximum strain of 600 µ.

2.3 FEA of Thin-Walled Workpiece

The developed system is designed to eliminate overloads to limit or control the deformation of the workpiece. Strains and stresses over a certain value may result in plastic deformation and inaccuracies of the finished product. Machining when the component is under elastic deformations will result in inaccuracies of the finished component. Therefore, it is essential to minimize and control the clamping force. Figure 4 presents an example of an FEA model, calculating the maximum deformation of a 0.2 mm thin-walled aluminum tube. When using 300 N clamping force on each jaw, the maximum calculated deformation is 0.5 mm.

Fig. 4.
figure 4

Deformation of a cylindrical tube using FEA, (a) Clamping forces of the three jaws pneumatic chuck, (b) Calculated deformation of an aluminum tube using FEA.

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2.4 Combination of Force Control, Vision Device and Motion Control Algorithm

The block diagram in Fig. 5 describes the workpiece movement procedure to the reference position, controlled by the vision device for position identification and by the clamping forces, using the high-precision CNC machine tool and an adjusted algorithm developed during this investigation.

Fig. 5.
figure 5

Block diagram depicting the algorithm for visual position identification, moving the workpiece to the required position, and controlling clamping forces using the air pressure.

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The clamping system combines the pneumatic chuck for holding the workpiece within a defined holding force range and the visual position identification system for identifying the workpiece position and providing data to move and/or rotate the workpiece with the chuck in the x-y plane.

3 Test Procedure and Test Results

The selected jaws are calibrated with strain gauges at Braude College in Israel and with a force cell using a piezo-electric device at Fraunhofer IWU in Germany. The visual identification method is tested on the high-precision CNC machine tool with HV indentation marks. Investigations are carried out regarding the accuracy and repeatability of workpiece position, combined with the vision system, and the correlation between air pressure and clamping forces.

3.1 The Vision Device for Position Recognition and Repositioning

The vision device has a very high resolution of 0.002 µm in the x and y direction, 0.05 µm in the z direction, and a resulting repeatability of 0.1 µm.

The digital CCD image of the three indentations on the clamped demonstrator identified and presented the pattern location in x, y, and z coordinates with a pixel resolution of 1 \(\mathrm{\mu m}\). A subpixel resolution of the pattern recognition uses the light intensity with 8-bit information for a position accuracy of 0.1 µm. The camera for pattern identification of the HV pattern center point uses image contrast information, depending on the illumination and surface reflection in confocal microscopy.

Preliminary testing of the vision recognition device, its accuracy, and repeatability are carried out on a demonstrator equipped with three HV (Hardness Vickers) indentations. Figure 6 shows the patterns of the three HV indented marks, the reference position A and the new position B after re-clamping (∆x and ∆y). The position identification after re-clamping is used to calculate the required machine tool movement relative to the reference point. After repositioning the reference point, the workpiece with the chuck can rotate to ensure the repositioning of the additional two points. The center points of the indentation patterns in the optical coordinates are transferred to the coordinate corrective values in the x and y directions and to the motion axes drives. Finally, the pattern center point of the new position is moved to the reference point using the x and y axes of the machine coordinate system. The complex position identification is characterized by a very high repeatability of less than 1 µm for pattern identification.

Fig. 6.
figure 6

(a) Motion of the demonstrator from the reference (blue lines) position to the new position, (b) Identifying the new and reference positions with three HV indentations.

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3.2 Accuracy and Repeatability of the CNC Machine Tool

Tests are carried out on the CNC machine tool without re-clamping the workpiece to define the accuracy and repeatability of the new clamping device. Figure 7 (a) shows the tested high-precision machine developed at Fraunhofer IWU with the pneumatic chuck and the CCD camera. Figure 7 (b) presents an example of the accuracy and repeatability of two points (1, 2). The machine tool is moved a few times from the (0:0) position, measured with the vision system, and back to the reference position. The accuracy values are high, mostly <1 µm, with few exceptions, and the repeatability values are very high, <1 µm.

Fig. 7.
figure 7

(a) The high-precision CNC machine with the pneumatic chuck, (b) Accuracy and repeatability results.

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3.3 Force Measurements with a Kistler Device as a Function of Air Pressure

The investigations are carried out on the pneumatic clamping chuck with simultaneously moving jaws. The tests checked the correlation between the air pressure and the clamping force. In this case, it is sufficient to measure the clamping force on one of the jaws. Figure 8 (b) shows the measurement set-up using a Kistler 9317B. The air supply inlet pressure varies from 1.0 to 7.0 bar.

Three repetitions are performed for every pressure step showing a constant increase with the pressure (Fig. 8 (a)). The blue line A is measured with permanent pressure without stopping the pump; the red line B illustrates force values measured after stopping the pump but without releasing the pressure, which means that the clamping forces are reduced by approx. 10%. When decreasing the inlet air pressure, the clamping forces will not drop immediately due to the chuck’s design and friction conditions.

Fig. 8.
figure 8

(a) Forces as a function of air pressure level (*identical to manufacturer’s specification). (b) Force measurement set-up for a single jaw. (c) The pneumatic chuck with central piston and wedge acting on the three jaws.

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Figure 8 (c) demonstrates the area between the wedge and the three clamping jaws influencing the friction forces; thus, the clamping forces are also influenced. After clamping, these friction forces eliminate the immediate release of the jaws with decreasing air pressure. Therefore, the air inlet pressure can only be used as a clamping force sensor during increasing air pressure but is limited as a parameter during pressure release. Using a force measurement on one jaw can provide higher accuracy and should be preferred.

4 Conclusions and Further Developments

This paper presents a new adaptable clamping system for controlled high-precision positioning and repositioning of a workpiece in the x-y plane with visual pattern recognition, adjustment, and controlled clamping forces. Reproducible position correction and clamping of workpieces with a pneumatic chuck were investigated, evaluated, and combined, forming the new adaptive workpiece clamping device to ensure high positioning repeatability during re-clamping operations, and different machining processes.

The clamping device’s position corrections and force control are based on a vision system and accurate force control. The design of the associated jaws is based on FEA to ensure workpiece holding, limited forces, and limited deformations. The visual pattern recognition is tested with a pneumatic clamping chuck on a high-precision CNC machine tool, providing very high accuracy and excellent repeatability. The authors are currently developing a new device with three piezo-electric motors for micro-machining with lower forces and very high accuracy.