workpiece diameter accuracy for machining centres is des- .... They are software solutions, hard- ... touch sensor makes the cutting tool itself a contact probe. Fig.
Int J Adv Manuf Technol (1999) 15:85–89 1999 Springer-Verlag London Limited
Repetitive Measurement and Compensation to Improve Workpiece Machining Accuracy Zhan-Qiang Liu Department of Manufacturing Engineering and Engineering Management, City University of Hong Kong, Kowloon, Hong Kong
Achieving workpiece high accuracy at low cost is one of the greatest challenges in the manufacturing industry. A repetitive error measurement and compensation scheme to improve the workpiece diameter accuracy for machining centres is described. The scheme entails an on-machine measurement and error compensation technology between machining processes. The workpiece diameters are measured along the workpiece length by using a fine touch sensor. The workpiece diameters in the compensation program are modified for implementation of next pass error correction. The technology is realised on a CNC turning centre. This method works well in hard machining and turned workpieces with large length–diameter ratios where the machining process induced errors are significantly greater than errors from other sources. It demonstrates that the workpiece can obtain maximum possible machining accuracy by this repetitive measurement and compensation technique. Keywords: CNC turning centre; Error compensation; Onmachine measurement; Workpiece accuracy
1. Introduction Computer numerical control (CNC) machine tools are being increasingly used by the manufacturing industry. Industry is looking for practical means to improve the precision of CNC machining processes. Methods for machining error compensation have been investigated to enhance the accuracy capabilities of CNC machine tools. Depending on how the compensation action is performed, error compensation can be divided into software and hardware error compensation. With the availability of low-cost computers, and high-accuracy sensors, software error compensatory control of machining accuracy has received increased attention. For many machining processes,
Correspondence and offprint requests to: Mr Liu Zhan-Qiang, Department of Manufacturing Engineering and Engineering Management, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong. E-mail: 95411413얀plink.cityu.edu.hk
error sources in machined workpieces can be classified into four categories (Fig. 1): 1. Machine tool geometric errors. 2. Deflections of the machining system which is composed of the machine, fixture, tool and workpiece (MFTW). 3. Thermal deformations of the MFTW system. 4. Wear errors of cutting tool and machine components. Much research has been devoted to the quasi-static error (geometric and thermal error) compensation of machine tools by software [1–7]. David McMurty says that the deflection of the machine owing to cutting forces dominates the error budget [5]. Previous research related to cutting force control has concentrated on adaptive control applications [8,9]. Most typical adaptive control applications described are for turning or milling on a CNC machine centre with a constant cutting force constraint. Maintaining a constant cutting force can be efficient for enhancing machining accuracy in milling operations. Since the resulting compliance between the tool and the workpiece varies along the workpiece length in turning operations, even if the cutting force is constant the deflection varies. This results in a machined workpiece with a non-uniform diameter. Kops et al. [10,11] described the relationship between the deflection of a workpiece and the depth of cut in turning. The relationship provided a method for error compensation through programming a lathe to a modified final diameter by subtracting twice the modified depth of cut from the diameter before cutting. However, most of the current error compensation research has not considered the errors that are a function of the machining process. The error sources attributable to the process can be listed as [12,13]: 1. Tool wear. 2. Tool deflection. 3. Workpiece deflection. They are independent of the machine tool itself so that they can be measured using the machine tool through an onmachine measurement technology [13]. This paper focuses on a repetitive measurement and compensation system to reduce workpiece diameter error induced by the machining process in CNC turning operations.
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Fig. 1. Machined workpiece dimensional error sources.
This paper first describes the fundamental structure of the repetitive measurement and compensation system. The onmachine workpiece profile measurement and compensation technique is described next. Finally, experimental results of turning a real cylindrical workpiece are presented. Through the use of this technique, the machined workpiece can have the maximum possible machining accuracy.
2. Fundamental Structure of the Repetitive Measurement and Compensation System To obtain the maximum possible accuracy for machined workpiece diameter in turning operations, a repetitive error measurement and compensation system has been developed. Figure 2 shows the fundamental structure of the proposed system. The precise workpiece will be turned in several passes using the same machining conditions. After each machining pass, the machined workpiece diameter is inspected along its length, with an on-machine measurement technology, that will be specified in the next section of this paper. The deviation between the expected and measured values is calculated and the diameters in the nominal tool path are modified before starting the next pass compensation machining. It is noted that the accuracy of the diameter of the machined workpiece, after only one correction pass, may still not be of
Fig. 2. Diagram of the repetitive error measurement and compensation system.
the maximum possible value. There may still be residual errors. A repetitive scheme to reduce workpiece diameter error is necessary. The diameters of the machined workpiece after the compensation can be inspected by the same measuring method and the tool path is then modified before starting a further compensation pass. This is repeated until the workpiece achieves its maximum possible turning accuracy.
3. Workpiece Diameter Measurement There are three modes for the measurement of machined workpiece dimensions. They are in-process measurement, postprocess measurement and process-intermittent measurement. In-process measurement takes place during the actual machining process without any interruption to the process itself [14,15]. It offers a practical solution for real-time measurement and quality control in manufacturing industry. The information produced by in-process measurement is provided continuously. However, it is difficult to use in turning operations where there may be a large amount of swarf and disturbance from coolant. Post-process measurement takes place on an independent inspection machine such as a CMM, after the machined workpiece is removed from the process [15]. The advantage of this method is that post-process measurement includes the effects of all error sources that affect the machined workpiece in a single set-up. When compared to in-process measurement, postprocess measurement is usually time-consuming and results in the risk of producing a number of defective workpieces before the inspection results are known. Process-intermittent measurement is also referred to as onmachine measurement or in-cycle measurement. Unlike inprocess measurement, the machining process must be stopped while the measurement takes place during the process cycle. The machined workpiece can be assessed either between the machining processes or after completion, prior to removal from the set-up [13,15]. On-machine measurement is the preferable process for improving the accuracy of the metal cutting process on a CNC turning centre. During turning operations, workpiece diameter errors caused by tool wear, tool deflection and workpiece deflection, that are independent of the machine tool itself, can be inspected using an on-machine measurement method. On-machine measurement uses a sensor-based machine toolmeasuring system. A wide variety of touch trigger probes have
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been applied to on-machine measurement on CNC machining centres. Probe-compatible software and hardware needs to be available for the machine tool to respond to a touch probe signal. However, many machines are normally fitted with nonprobe-compatible controllers. In this case, the probe cannot be simply applied to the machine tool. There are a number of techniques that can be used to achieve on-machine measurement with a probe system. They are software solutions, hardware solutions and remote microcomputer control solutions [16]. A novel on-machine workpiece measurement with a Qsetter and fine touch sensor is developed in this paper. 3.1 Q-setter and Fine Touch Sensor
This paper describes a new methodology for using a fine touch sensor to effect on-machine inspection for non-probecompatible CNC turning centres, equipped with Q-setters. The term Q-setter is an abbreviated form of “quick tool setter” that is mounted on the base of a CNC machine tool to facilitate cutting tool off-set measuring. During the cutting tool set-up, the Q-setter is opened to its working position, the tool tip is jogged into contact with it (Fig. 3). When the tool tip contacts with the sensor, the tool will automatically stop while the tool offset is recorded. After finishing tool set-up, the sensor is closed. The tool offset remains constant before making the tool tip contact with the sensor again. A fine touch sensor does not require an analogue mechanism but relies only on the switching principle. The fine touch sensor uses the fact that the MFWT system is an electromagnetic field generated by various electromagnetic sources such as servo motors as well as transformers. When a cutting tool is advanced to contact the workpiece, the electrically conductive MFWT system loop is closed and an electromagnetic current is generated around the loop (Fig. 4). The fine touch sensor has two connected parts: 1. An electric coil that surrounds the cutting tool holder. The coil, which acts as an electrical transformer, has the necessary number of turns and the cross-sectional area to output a current. 2. A separate amplifier circuit that produces a switching “ON” signal when it receives an output current from the coil while the tool is in contact in the workpiece. Thus, the fine touch sensor makes the cutting tool itself a contact probe.
Fig. 3. Tool set-up with Q-setter.
Fig. 4. On-machine measurement with fine touch sensor.
The fine touch sensor was calibrated using a laser interferometer measuring system. It was shown that the contact detection accuracy of the fine touch sensor is much better than 1 m. However, the automatic determination of the movement for the cutting tool is difficult since the machine tool does not react to the fine touch sensor switching “ON” or “OFF” signals. Therefore, the fine touch probe cannot be simply applied to workpiece inspections. Based on the measuring function of the Q-setter and the inspecting strategy of the fine touch sensor, a methodology for applying the fine touch sensor to on-machine workpiece measurement is proposed. 3.2 On-machine Workpiece Measurement Methodology
The machine used in this research is a CNC turning centre that is equipped with a Q-setter. The output of the separate circuit uses one place of the Q-setter output ports (Fig. 4). The method makes it possible to combine the functions of the Q-setter with that of the fine touch sensor to implement onmachine measurement of the workpiece. Before applying the on-machine workpiece inspection procedure, let the tool tip that is surrounded by the electric coil touch the centre of the opened Q-setter. Then the offset X of the current when the tool is being used is recorded. Afterwards, the Q-setter is closed and the tool will be used in its machining mode to turn the workpiece. After each turning pass, let the Q-setter open a little so as to be in its working position but not interfering with the workpiece that is clamped by the chuck, or with the movement of the cutting tool. Now, the cutting tool works in its onmachine measurement mode. The cutting tool approaches the workpiece, when the tool contacts the workpiece, a switching “ON” signal appears and the cutting tool stops automatically. A new “tool offset” XT is measured by the fine touch sensor. The deviation between the new “tool offset” and the offset during tool set-up arises because the measurement base changes: one base is that of the Q-setter and another is that of the workpiece. The on-machine measurement workpiece diameter Don-machine at the contact point is given by:
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Don-machine = 2 × L + 兩X兩 ⫺ 兩XT兩
(1)
where, X is the tool offset when the cutting tool is in contact with the Q-setter. XT is the “tool offset” when the cutting tool is in contact with the workpiece. L is the distance from the centre of the Q-setter to the centreline of the spindle in the X-axis direction and is provided by the machine tool manufacturer. Both X and XT are measured in the workpiece diameter direction. Different points are measured in the same way to obtain the error distribution along the machined workpiece length. The deviations between the on-machine measurement workpiece diameters and the desired diameters are the machining errors ␦ caused by the turning process: ␦ = Don-machine ⫺ Ddesired
(2)
Before the cutting tool returns to its normal machining mode from its measuring mode, the tool offset is reset to its initial value X and the Q-setter is closed. This on-machine inspection approach has the following advantages: 1. It is carried out while the spindle is stopped and while the machined workpiece is still on the machine. It extends the function of a conventional machine tool as a measuring machine and reduces other measuring equipment requirements. 2. It combines the functions of the Q-setter sensor with the fine touch sensor so that it can be applied to a machine tool that is equipped with a Q-setter. 3. It is very economical because it uses a fine touch sensor instead of an expensive touch trigger probe. 4. It eliminates the need for changing the tool for a probe for workpiece measurement since it uses the cutting tool itself as a contact probe. In this case, the cutting tool has two modes of operation: one is the machining mode and another is the measuring mode. However, the error sources that arise from the machine tool’s inherent geometric errors and thermal errors are undetected by on-machine measurement. Therefore, the method is mainly applied to a precise machine or a machine where the machining process induced errors are significantly greater than the geometric and thermal errors such as for hard machining or for turning a workpiece with a large length–diameter ratio.
This means the actual depth of cut in the compensation turning process is the sum of the nominal depth of cut and the workpiece diameter errors. Since, in practice, the machined dimensions for turning are introduced through the desired diameters in the workpiece program, the diameters in the compensation program Dprog can be achieved through programming the turning centre for modified final diameters by subtracting twice the modified depth of cut from the diameters before cutting (Fig. 5): Dprog = Ddesired ⫺ 2(ap + .␦)
5. Experimental Results and Discussion Measurement and compensation tests were conducted in a precise CNC turning centre to verify the measurement and compensation methodology. The workpiece material was mild steel. A cemented carbide cutting insert of the following geometry was used: ␥n = 10 °, ␣n = 8 °, n = 10 °, r = 75 °, and nose radius r⑀ = 0.4 mm. The cutting conditions were: cutting speed v = 240 m min⫺1, feed rate f = 0.1 mm rev⫺1, and depth of cut ap = 0.5 mm. The workpiece was slender with a length of 100 mm and diameter of 30 mm. During the tool set-up, the tool offset for the tool being used was measured by the Q-setter and recorded. The turning tests were made for a stable thermal state of the machine tool so that the variations in workpiece errors owing to thermal deformations of the machine can be discounted. The major sources of inaccuracy in the diameter were the machining process errors such as tool wear, tool deflection and workpiece deflection, owing to the cutting force. The machined workpiece was measured with the fine touch sensor after the first turning pass. Then, the workpiece diameter machining errors were calculated and the nominal diameters for the next turning pass were modified. The next compensation turning pass was implemented using the modified program. The experimental results with and without compensation are shown in Fig. 6. The horizontal axis shows the length of the workpiece and the vertical axis shows the workpiece diameter machining errors. It can be seen from Fig. 6 that after the first compensation pass, the diameter accuracy for the machined
4. Workpiece Diameter Machining Error Compensation Once the workpiece diameter error is determined, some means of compensating for the error must be developed in order to improve the accuracy of the machined workpiece. A common and easy way to compensate for errors in CNC machining operations is by workpiece program modification. To eliminate the workpiece diameter machining errors, the nominal depth of cut in the next compensation pass is modified.
(3)
Fig. 5. The programmed diameter in compensation machining.
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the machine tool’s inherent geometric errors and thermal errors and to reduce the number of compensation passes to one.
References
Fig. 6. Distribution of diameter machining errors along the length of the workpiece.
workpiece is not satisfied. Further compensation passes are needed to reduce the workpiece diameter errors by repeating the above measurement and compensation system. It was found that the workpiece can achieve its maximum possible machining accuracy with one more compensation pass. An attempt to continue reducing the errors in the diameter through additional compensation passes failed. Similar results were obtained from a number of experiments under the same machining conditions.
6. Conclusion This paper describes the development of a repetitive error measurement and compensation system for CNC turning operations to improve the accuracy of the diameter of the machined workpiece. The on-machine workpiece measurement, error compensation and implementation have been integrated. An on-machine workpiece measurement technique was described. A method of modifying the nominal diameters in a CNC machining program to implement error compensation was proposed. The errors attributable to the machining process such as tool wear, tool deflection and workpiece deflection, etc. that are independent of the machine tool were measured and corrected. The experimental results show that the maximum error in the diamater was reduced by 65% after the first compensation pass and 90% after the second compensation pass, compared to no compensation. The repetitive measurement and compensation approach required at least two passes for the diameter of the machined workpiece to achieve maximum possible accuracy. Further research is required to account for
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