A New Mechmism of Grinding Wheel Loading Phenomenon Evaluated by Machining a Magnesium Alloy

NGUYEN Tien Dong, Koji MATSUMARU, Masakazu TAKATSU and Kozo ISHIZAKI

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AZojomo (ISSN 1833-122X) Volume 6 December 2010

Topics Covered

Abstract

Grinding-wheel loading can be defined as ground material either adhere to abrasive grains or become embedded in voids on a grinding-wheel surface. This phenomenon deteriorates grinding-wheels, and consequently the surface integrity of ground materials, such as surface roughness of the ground materials due to excessive friction and heat. Newly developed cup-type diamond-grinding-wheels with hexagonal pattern were used to grind a magnesium alloy AZ31B in the present work.

The hexagon edges of 1 or 2 mm width contain diamond grains and, the inside of hexagons contains porous green carborundum without diamond grains. The direction of hexagonal edges can be located roughly either parallel or perpendicular to wheel- rotating direction, i. e., wheel-circumferential direction or wheel-radial direction, respectively. Wheel loadings appear in the hexagonal edges along circumferential direction on hexagonal wheel surfaces, but do not appear in those along radial direction, while appear equally at any positions on conventional wheel surface. Previous researches considered that the mechanism of the loading was based on rather microscopic phenomenon, i. e., caused by one grain or neighboring two abrasive grains. In this paper, a new phenomenological mechanism of wheel loading is presented in which the mechanism of wheel loading is totally different to those proposed by the previous researches. Hexagonal structure shows the advantage to reduce wheel loading phenomenon in grinding light metals such as magnesium alloys.

Keywords

Wheel Loading, Magnesium Alloys, Hexagonal Structure, Surface Roughness, Grinding

Introduction

Recently, light metals, e. g., aluminum, magnesium and their alloys, offer the greatest potential for weight reduction of all metallic materials, which have been used in automobile, aerospace, agricultural, chemical and construction industries as well as electrical appliances. The main concern in machining these light materials is chip adhesion on cutting tool surfaces. In grinding process, this phenomenon is called “(wheel) loading”, which can be defined as the state of a grinding wheel when particles of a work-piece material either adhere to the grits or become embedded in the spaces between abrasive grains on grinding wheels. The loaded grinding wheel becomes dull very fast, raising grinding forces and temperature thus reducing its grinding ability. The wheel loading phenomenon is to be regarded as one of the most disturbing factors in grinding process, and it is about 15% of decisive criterion for redressing of grinding wheels [1]. Magnesium alloys have been used for a broad range of industrial applications from aerospace to transportation and more due to their high specific strength and the lowest density of all metallic structural materials. Magnesium alloy AZ31B (ASTM – American Society for Testing & Materials) was selected to represent a light metals in the present work.

There have been many researches on various aspects of wheel loading in grinding various materials such as Ti alloys, stainless steel or aluminum oxide by straight-type conventional grinding wheels. These researchers dealt with investigating the loading phenomenon under different grinding parameters such as wheel speed, cutting depth, and work-piece feeding speed [2-5]. They reported that any noticeable loading was not found for low cutting depth [2], or the loading tendency increased with the greater removal rate, i. e., the greater cutting depth and/or the greater feeding speed [3]. Srivastava et al. contradictorily reported that feeding speed has no significant effect on wheel loading [4]. Koshima et al. presented that the loading of CBN wheels in up-cut grinding was less than that in down-cut grinding [5].

Other researches aimed to explain the mechanism of wheel loading by giving some aspects on chip formation processes. Schmaltz and Konig reported that the mechanical interlocking of the chips with the wheel surface and the chemical affinity between the work-piece and grits materials might be the possible causes of loading [6]. Komanduri and Shaw suggested that the loading over the wheel surface may be due to pressure welding of ground chips on abrasive grits [7]. Yossifon and Rubenstein also revealed the same phenomenon, but added a possibility that wheel loading is determined by the presence of protecting oxide layer on the work-piece during grinding in air [8].

The previous researchers clearly indicated that the mechanism of wheel loading was the adhesion between abrasive grains and the work-piece material during grinding process. They proposed a mechanism of wheel loading by one abrasive grain or two neighboring abrasive grains. In other words, the loading phenomenon is rather microscopic phenomenon at most 100 μm, i. e., the dimension of the distance between abrasive grains. In this work, newly developed cup-type diamond grinding wheels were used to grind a magnesium alloys. The effects of hexagonal structure on ground surface and the integrity of grinding wheel were evaluated. This paper presents a new mechanism of wheel loading, and proposes a new method to enhance the performance of grinding light metals such as magnesium alloys.

Experimental Procedure

Diamond Grinding Wheels with Hexagonal Structure

Diamond grinding wheels are newly developed with hexagonal pattern, which contain abrasive diamond grains on hexagon edges and green carborundum porous material without diamond grains in inside of the hexagons [9]. These wheels are characterized by hexagonal geometrical factors: size of hexagons, x, and width of hexagon edges containing diamond grains, w as shown in Fig. 1 (a). The direction of hexagonal edges can be located roughly either parallel or perpendicular to wheel-rotating direction in Fig. 1 (b). Grinding stone ratio, R is defined as the ratio between the hexagonal edge area containing abrasive grains and the total area of the wheel surface. Four newly developed diamond grinding wheels with different R (13%, 19%, 25% and 36%) and a conventional wheel (R 100%) (diamond grain size: #200, grain concentration 0.48, vitrified bond 20 mass %, porosity 32%, NanoTEM Co. Ltd., Nagaoka, Japan) were used as shown in Table 1.

Figure 1. Hexagonal diamond grinding wheel schematically shown by (a), and photographically shown by (b). Hexagonal structural is characterized by hexagonal geometrical factors: hexagon size, x, and an edge width, w. Diamond abrasive grains are placed only within the edge width, which is order of mm.

Table 1. Grinding stone ratios, R for hexagonal wheels and a conventional wheel.

Grinding Procedures

A regulated-force-feeding (RFF) grinding system, which was developed by Kim et al., was employed in the present work [10, 11]. The table feeding force was kept constant at 3.8 N during process. Samples were magnesium alloys AZ31B (Al 3%, Zn 1%, Mn 0.45%) (Kamado Laboratory, Nagaoka University of Technology, Nagaoka, Japan) with dimensions of 50 x 40 x 1.5 mm, and was placed on vacuum vise. Grinding wheel rotation speeds, n were 500, 1500 and 3000 rpm, cutting depth was 10 μm per pass, and total cutting depth was 200 μm. The wheels were re-dressed before each grinding experiment. The coolant water was sprayed into the contact zone between grinding wheel and sample during truing, dressing, and grinding process with 0.5 l/min flow rate. Grinding conditions are listed in Table 2.

Results

Wheel loading did not occur for n at 500 and 1500 rpm, but for n 3000 rpm, and appeared equally at any positions on conventional wheel surfaces as shown in Fig. 2(a), while appeared only in the hexagonal edges along circumferential, i. e., parallel to the wheel-rotating direction on the hexagonal wheel surfaces, but not in the edges along radial direction as shown in Figs. 2 (b) (c) (d) and (e).

Figure 2. Grinding wheel surface after grinding experiments. White areas correspond to loaded magnesium alloy. (a) conventional wheel with R = 100%. (b), (c), (d) and (e) are for hexagonal grinding wheels with R = 13%, 19%, 25%, and 36%, respectively.

Table 2. Specifications of grinding wheels, grinding conditions and sample.

Discussion

Previous researchers considered that all wheel loading phenomena were started with chip adherence on the rake face of the same cutting grain and continued to grow. In a recent paper, Gift el al. suggested a mechanism of wheel loading in six stages, and proposed a mechanism of wheel loading by neighboring abrasive grains [12]. In other words, the loading phenomenon is rather microscopic phenomenon, i. e., the dimension of the neighboring distance between abrasive grains. There may be a small difference in the loading mechanisms proposed till now. But all of them consisted on the loading initiates by only one abrasive grain or at most two neighboring grits, and focused rather microscopic adhesion, between abrasive grains and work-piece materials.

The present research shows the phenomena are based on rather macroscopic effects, and on cooperative work of many grits. These results reveal that the mechanism of wheel loading is not microscopic phenomenon, i.e., loading can be initiated by only one grit or at most two neighboring grits as proposed previously, but rather a phenomenon which is caused by macroscopic, i. e., order of hexagon length or 10 mm length. The phenomena can be explained by cooperative work of many grits. The lengths of hexagonal edges are order of 10 mm.

The average neighbor distance between diamond grains on the wheel surface is about 70 μm. The edge width of hexagons is 1 mm or 2 mm. Therefore there are about 14 or 30 grains in the hexagon edges in the transverse direction on the grinding surface. But there were no loadings in the edges along the radial direction of the wheels. The loadings can be found only in the edges located along the circumferential direction of the wheels. The longitudinal length of the edges is in the order of 10 mm as seen in Figs. 2 (b) (c) (d) and (e). Along this direction of edges, there about 300 grits. If the mechanisms proposed previously would be right, there must be loadings on all edges of hexagons, because there are more than enough diamond grains in all edges. The loading is not caused by one or two grits, but caused by many grits.

The loading mechanism proposed here is as the following. To initiate loading hot ground chips must grow a critical size to touch hot abrasive grains. The ground chips may be molten metal or solid metal at temperature close to melting point to form critical size of droplets to initiate loading. Hot ground chips move and join together to form large droplets of hot metal, and hit hot diamond grains to adhere them. About 100 ground chips need to form rather large molten metal droplets to start loading for the present grinding conditions. This is the critical size of droplets for loading. Ground chips along circumferential edges of length of about 10 mm may face 300 grits and about the same number of ground hot chips at the end of hexagonal edge along the circumferential movement. Therefore the loadings start at the end of the hexagonal edge along the circumferential motion. This may be the reason why we can find the formation of loadings at the downstream side of hexagonal circumferential edges to form a shape close to letter “Y”, as seen in Figs. 2 (b) and (c) for hexagon edge width of 1 mm. For thicker hexagonal edges of 2 mm width, this initial point of loading may occur at any positions in the edges as seen in Figs. 2 (d) and (e) since there are more chances for chips to join in any positions of hexagon edges along circumferential direction.

In hexagonal grinding wheels, there is difference in height of hexagonal edges and inner green carborundum parts after dressing. The difference is about 30 to 40 μm. Hot chips ground by the radial edges pass and join to form a small droplet through moving 14 to 30 grits, and fall into practically empty space between lower green carborundum and work-piece. They are too small to start loading. While moving this green carborundum space, chips will be cooled down. The cold chips or droplets formed by a few chips hit hot diamond grains which are located in the next hexagonal edge along radial direction, can not join to form larger droplets because of low temperature, and do not react with the diamond grains because of low temperature and too small droplet size. This is the reason why loadings can not be found hexagon edges along radial direction.

Therefore, wheel loading can be found at any positions in conventional wheels. For slow grinding speed of n = 500 and 1500 rpm, cooling speed was fast enough to prevent to form critical size of droplets for all grinding wheels. If the grinding conditions would be severe, loadings could be found in more possible locations. In this case a few large ground chips must be enough to form a critical size droplet. The ideal design of grinding wheel to prevent loading consist only radial direction of abrasive grains with long interval volume of non abrasive grains.

Conclusions

In this work, a magnesium alloy is ground by a conventional and newly developed hexagonal diamond grinding wheels. Hexagonal wheels contain abrasive diamond grains on hexagon edges, and the inside potion of hexagons is filled up by porous green carborundum without diamond grains. Depending on the direction of hexagon edges on the wheel surfaces, these edges can be roughly located either parallel or perpendicular to the wheel-rotating direction during grinding process. The following can be concluded:

1. The wheel loading mechanism is not related to local positioning of one or two abrasive grains, but related macroscopic positions of several active grains. In other word, the characteristic distance of loading is not order of μm, but in order of mm distance.

2. The mechanism of loading phenomenon appears equally at any positions on conventional wheel surface. However, the hexagonal wheels show that loading occurs on the hexagonal edges parallel to the wheel-rotating direction than the edges perpendicular to it.

3. It is possible to reduce and prevent loading phenomenon in grinding light metals such as magnesium alloys by designing a grinding wheel with proper abrasive grain configuration.

Acknowledgement

The authors wish to express their gratitude to the Japanese government for partially supporting this work through the 21^st Century Center of Excellency (COE) Program and City Area Nagaoka of Promotion of Science and Technology in Regional Areas of the Ministry of Education, Culture, Sports, Sciences and Technology.

References

Contact Details

Tien Dong Nguyen, Koji Matsumaru and Kozo Ishizaki
Department of Mechanical Engineering
Nagaoka University of Technology
Nagaoka, Niigata 940-2188, Japan

Masakazu Takatsu
Nano-TEM Co., Ltd.
Shimogejo 1-485, Nagaoka, Niigata 940-0012, Japan

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 11[2] (2009) 37-42.