11.5.1 Introduction
Primary motion between tool and workpiece characterizes the grinding process, but also supporting processes and systems are of major importance. In this chapter, basically the monitoring of the conditioning process and the coolant supply will be discussed.
11.5.2 Sensors for Monitoring of the Conditioning Process
The condition of the grinding wheel is a very decisive factor for satisfactory grinding results. Thus, the grinding wheel has to be prepared for the grinding by using a suitable conditioning technology. The major problem in any conditioning operation is the possible difference between nominal and real conditioning infeed. There are four main reasons for these deviations. The unknown radial grinding wheel wear after removal of a specific workpiece material volume must be regarded as a significant factor. Also, the changing relative position of grinding wheel and conditioning tool because of thermal expansion of machine components is relevant. As a third reason, infeed errors related to friction of the guide-ways or control accuracy have to be considered, although their influence is declining in modern grinding machines. The last reason to mention is the wear of the
conditioning tool, which is, of course, dependent on the individual type of tool. The first wear effects for rotating dressers may be noticable only after regular use for several weeks.
Because of the immense importance of the grinding wheel topography, the monitoring of the conditioning operation has been the subject for research for many decades. In the early 1980s it was first used as an AE-based system for the monitoring of the dressing operation. At that time, the work was concentrated on dressing of conventional grinding wheels with a static single-point diamond dresser. It was possible to detect first contact of the dresser and the grinding wheel and the AE intensity could be used to determine the real dressing infeed in dependence of dressing feedrate and grinding — wheel speed. The dressing feed speed could be identified by the AE signal [Inasaki 1985b]. In addition, it was stated that the AE signal reacts significantly faster to the first contact of the dressing tool and grinding wheel compared to monitoring by means of the spindle power.
The limitation to straight cylindrical profiles was overcome by Meyen [1991] who developed a system capable of detecting dressing errors on any complex grinding wheel profile (Figure 11.25). The strategy comprises the determination of a sliding average value with static and dynamic thresholds for every single dressing stroke. The different geometry elements are identified and the currently measured AE signal is compared to the reference curve, which has to be defined in advance. With the calculation of further statistical quantities like standard deviation or mean signal inclination, it is possible to identify the typical dressing errors in case of exceeding the thresholds.
As a consequent next step, AE systems were tested for conditioning operations of superabrasives such as CBN [e. g., Heuer 1992, Wakuda et al. 1993]. The high hardness and wear resistance of these grinding wheels require a different conditioning strategy and monitoring accuracy compared to conventional abrasives.
The conditioning intervals due to the superior wear resistance can amount up to several hours. The dressing infeed should be limited to a range between 0.5 pm and 5 pm instead of 20 pm to 100 pm for conventional wheels in order to save wheel costs. Especially for vitreous-bonded CBN grinding wheels, it was proposed to use very small dressing infeeds more frequently in order to avoid an additional sharpening. This strategy known as “touch dressing” revealed the strong demand to establish reliable contact detection and a monitoring system for dressing of superabrasives. In most of the cases, rotating dressing tools are used. The schematic setup of a conditioning system with a rotary cup wheel, which is often used on internal grinding machines, is shown in Figure 11.26.
The conditioning cycle consists of four stages: fast approach, contact detection, defined infeed, and new initiation. Besides AE techniques, other methods have also been tested. Heuer [1992] additionally investigated the possibility of using either the required power of the dressing tool spindle or a piezoelectric force measurement for monitoring. The latter technique was available because a piezoelectric actuator was installed as a high precision positioning system for the infeed of the dressing tool.
A further technique for contact detection was introduced by Tonshoff, Falkenburg, and Mohlfeld [1995]. The measurement of the rotational speed change of the high frequency dressing spindle, which gives a maximum number of revolutions of 60,000 min-1, was used to determine not only the first contact, but also the whole dressing process. After contact detection of any of the mentioned systems, the conditioning program is continued until the desired number of strokes and infeed is reached. Depending on the type of system, it is possible to monitor the course of the signal over the whole width of the grinding wheel.
The use of AE sensors for contact detection of the conditioning, respectively, dressing operation can be regarded as state of the art. Many different systems are available. New grinding machine tools with self-rotating conditioning tools are usually equipped with an AE system already in the delivery state.