On-machine dry electric discharge truing of diamond wheels for micro-structured surfaces grinding
Introduction
The mechanical, optical, tribological, and fluidic characteristics, as well as many other properties, can be altered by fabricating microstructures on elements′ surface. Therefore, the elements with micro-structured surface are becoming more and more important in advanced industrial technologies, especially in optical industry which needs high form accuracies and surface finishes.
To achieve high quality of these micro-structured surfaces, precision grinding with diamond wheels gives a promising alternative [1], [2], [3]. The high concentration metal bonded diamond grinding wheels with precise profile can be used efficiently for the directly precision machining of optical glasses elements or molds preparing of tungsten carbide and ceramic mold inserts which are subsequently used in glass hot pressing operation [4], [5]. Duo to the combination of high abrasive concentration with metal bond, the grinding operation can be carried out with high profile constancy. On the other hand, however, this combination makes it difficult to true these diamond wheels efficiently, because of the remarkable resistance property and the geometrical limitation of small wheel profile.
Considering the electrical conductivity of the metal bond, the micro material removal processes based on the electrodischarge are often suggested as the truing method for these metal bonded diamond grinding wheels with high accuracy profile, such as wire electrodischarge truing (wire-EDT), dry electro-contact discharge truing (dry-ECDT) and micro electrodischarge truing (micro-EDT). In the Wire-EDT process, due to the high flexibility, the diamond wheels with user-defined profile can be obtained by programmed NC-code on the offline wire EDM machine [6]. The dry-ECDT is based on the mutual-wear between the fixed electrode and wheel. The efficiency of dry-ECDT has been proven for metal bonded V-shaped diamond wheels. The non-application of coolant not only reduced the pollution, but also was beneficial to obtain sharper V-tip [7]. But the efficiency of the process for wheels with other shape profile has not been presented. The micro-EDT of diamond wheels with precise micro profile can be done on-machine with different dielectric. Due to the constant electrode profile, multiple shape can be implemented on the diamond wheels by the combination of the electrode profile and programmed NC-code, such as arc shape [8], rectangular shape [9] and V shape [10], [11]. In summary, previous research works about precision truing of diamond wheels for micro-structured surfaces grinding mainly focus on realization of the truing process, truing mechanism and electric parameters optimization. However, the analysis of truing fundamental errors and the effects of non-electric factors, such as kinematic parameters of the truing process, on the truing accuracy have not been given yet, especially for the micro-EDT of diamond wheels.
In this paper, an on-machine dry-EDT method to precision shape and prepare diamond wheels with various profiles was proposed for micro-structured surface grinding. Firstly, the fundamental truing errors were analyzed based on the dry-EDT kinematics. And then the capabilities of dry-EDT truing for high abrasive concentration metal bonded diamond wheels with various shapes were presented. Next, the effects of kinematic parameters variables were investigated on trued wheel profile accuracy, and the morphology of trued diamond wheel was observed by a 3D optical microscope. Finally, the micro-structured surfaces on Silicon Carbide (SiC) and Tungsten Carbide (WC) were ground by these trued diamond wheels.
Section snippets
On-machine dry-EDT of diamond wheels
Fig. 1 shows the on-machine dry-EDT principle of diamond wheels. The rotary diamond wheel (positive pole) is driven with the grinding wheel speed N to slid over a rotary discal cathode along programmed truing paths with the feed rate f, and then a required wheel profile such as arc-shaped or V-shaped can be produced gradually with the truing depth per step a through the electric discharge remove of metal bonded material. The electric parameters of pulse current, pulse voltage, duty and the
Truing fundamental errors analysis
Truing errors of the diamond wheels must be controlled accurately, which are responsible for the final form accuracy of micro-structured surface [4]. Considering the dry-EDT kinematics, the effects of relative position errors between the grinding wheel and cathode on truing accuracy are shown in Fig. 2. For V-shaped wheels, the trued wheel profile is depended on the programmed truing paths. Therefore, the truing accuracy of V-shaped wheels is not affected by relative position errors between the
Experimental setup
Fig. 5 shows the experimental setup of on-machine dry-EDT. The machining system used in this experiment was a precision plane grinder MUGK7120×5 with a pulsed power source. The grinding wheel was connected with the anode of pulsed power source. And the cathode used was a copper disk with diameter of 70 mm and width of 10 mm, which was mounted on a precision spindle under the grinding wheel. Before truing, the position of cathode was adjusted precisely according to the errors analysis results
Radial run out of trued diamond wheels
Independent of initial champing deviation a low radial run out smaller than 4 μm was reached for both V-shaped wheels and arc-shaped wheels by dry-EDT. Fig. 7 shows the photo of trued arc-shaped diamond wheel (cleared by acetone) and the radial run out of wheel before and after dry-EDT with the parameters of 90 V voltage, 20 A current, 30% duty, 1700 rpm grinding wheel speed, 900 rpm cathode speed and 10 mm/min feed rate.
In order to reduce the run out of the trued wheel further, a smaller pulse
Conclusions
An on-machine dry electric discharge truing method to precision shape and prepare diamond wheels with various profiles was proposed for micro-structured surface grinding. The conclusions are as follows:
- (1)
The truing accuracy of V-shaped wheels are not affected by relative position errors between the grinding wheel and cathode, although the effective grinding depth of grinding wheel will be reduced due to Z-direction position deviation. For arc-shaped wheels, the angle errors have a significant
Acknowledgements
This work was supported by the China Postdoctoral Science Foundation funded project [2013M541361], the Fundamental Research Funds for the Central Universities [HIT. NSRIF. 2015056] and the Heilongliang Postdoctoral Fund [LBH-Z13104].
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