The aim of the test series is to investigate the influence of the conditioning of vitrified bonded diamond grinding wheels on the grinding wheel wear, respectively, the specific grinding power. Therefore, the influences of the process parameters and the conditioning tool specification on the grinding tool topography are investigated.
The investigations are carried out on a Wendt WAC 715 Centro cutting insert grinding machine. This machine is equipped with a RotoDress conditioning system. In contrast to the usual dressing systems, which operate with diamond or SiC discs, this conditioning system uses cup dressing rolls made of vitrified bonded white corundum. The conditioning system allows both, the conditioning in the non-productive time and in-process conditioning during grinding (Figure 1, left). The rotational movement is performed by a hydraulic motor with a rigid structure. The rotational speed can be varied in the range nd = 220 - 600, which corresponds to a circumferential speed of the conditioning tool of vrd = 1.5 - 3.5 m/s.
When dressing in the non-productive time, there is no contact between the workpiece and the grinding tool. The grinding tool rotates at the rotational speed ns and the conditioning tool at the rotational speed nd. The conditioning tool is fed by an axial feed axis at a feed frequency fd by the depth of cut per conditioning stroke aed. On each conditioning stroke, the conditioning tool is rapidly moved against the grinding layer by depth of cut between aed = 0.5 – 1.5 µm. Hence, the average axial infeed rate is the product of aed and fd the strokes are performed. During in-process conditioning, the grinding tool engages the workpiece and the conditioning tool simultaneously. The workpiece is moved at the axial feed rate vfa against the abrasive layer normal to the flat abrasive layer surface. Contrary to common dressing processes with cup dressing tools, the aim is to achieve an areal contact between the dresser and the grinding layer (Figure 1, right). The infeed movement of the cup conditioning tool is directed normally to the surface of the grinding layer. The planar alignment of the faces of the grinding and conditioning tool ensures the areal contact and thus a self-compensating axial run-out of both tools. Therefore, there are two engagement zones between the grinding layer surface and the cup conditioning tool. As a result of the two engagement zones, the direction of vrd is crossed in relation to the direction of vs. Thus, a corundum grain of the conditioning tool engages in the first engagement zone at the left edge of the abrasive layer and in the second engagement zone at the right edge of the abrasive layer.
To investigate the influence of circumferential speed, corundum grain size, and cut depth in conditioning, two series of tests are performed (Figure 2). Input variables in test series 1 are topography parameters of the grinding surface. In all test series, the grinding layer is conditioned with the same parameters to create an unworn state. Thus, constant starting conditions are provided for the investigation. For this purpose, 100 strokes with an infeed aed = 1 µm each are performed each using a vitrified white corundum cup dresser with a grain size of dgd = 48 µm.
In the first test series, the influence of the conditioning process on the grinding layer topography is investigated without the influence of the grinding process. To generate the same worn initial state, the grinding layer is worn by grinding a thick film PcBN insert with vc = 20 m/s, vfa = 4 mm/min and aed = 4 · 100 µm. The cemented carbide support of the cutting insert causes welds on the grinding tool topography. In this series of tests, the abrasive layer is conditioned by varying the grain size of the dresser dgd and the circumferential speed of the dressing tool vrd. The conditioning infeed is kept constant at aed = 1 µm and 30 strokes are performed. At the different stages, 3D profiles of the worn topography of the abrasive layer are generated using a 3D white-light microscope InfiniteFocus G5 from Alicona (Figure 3, bottom). The topography of the abrasive layer is described by the topography parameters of the Abbott-Firestone curve as depicted in Figure 3. The areal parameters reduced peak height Spk and the reduced valley depth Svk are compared with the surface characteristics of the previous condition. A decrease in the reduced peak height with otherwise identical surface characteristics indicates a flattening of the diamond grain. A decrease in the reduced valley depth at the same core roughness depth is an indication of clogging of the grinding layer.
Furthermore, the PcBN generates a circumferential notch with a depth of h1 = 10 µm (Figure 4, top). The generated topography of the abrasive layer is described using the envelope of the 3D profile (Figure 4, bottom), in order to evaluate the resulting change in the height of the notch Δh = h2 - h1. Consequently, the specific conditioning material removal rate Q'sd is calculable according to Equation 1.
It describes the speed with which the grinding layer profile is restored. Q’sd is calculated by determining the difference in notch depth Δh in the abrasive layer between worn profile h1 and conditioned profile h2. This height difference is multiplied by the ring area of the abrasive layer and divided by the width of cut of the conditioning tool apd and the total time of the conditioning process td. In the present case, apd corresponds to the width of the abrasive layer bs. Q’sd is varied under the same process parameters in test series 2. Hereby, the influence on the specific cutting power is investigated. The optimum value Q'*sd lies in the minimum of the volume-specific cutting power P'''c.
In the second series of tests, the influence of the grinding layers topography and its interaction with the process parameters of the grinding process is determined. For these investigations, in-process conditioning is applied on the grinding process of PcBN inserts. The parameter variation of the conditioning process is performed ceteris paribus the first test series (Table 1) and, in addition, the infeed of the conditioning tool per stroke aed is varied as shown in Table 2. In the process, the time course of the power consumption of the grinding spindle is determined using the Tyrolit Toolscope in process measurement technology according to [13]. This data recorder enables the recording of the spindle power and position of the axes of the machine tool. Therefore, the specific material removal V’w is calculated from the position of the x-axis. P’s is calculated by dividing Ps by the width of cut ap. The maximum cutting power per tool engagement P’c is calculated by subtracting the idle power of the machine tool spindle from the maximum spindle power Ps. The rise of the specific cutting power with increasing V'w, of example P'''c, is an indicator for the microscopic wear rate of the grinding tool (Figure 5).
P''' c is the slope of a linear regression of P'c to V'w. As shown by [13], the specific spindle power is highly correlated with the specific cutting energy, which depends not only on the chip formation but also on sliding in a grinding process. Both, flattening of the diamond grains as well as grain dulling and weld build-up, lead to an increased coefficient of friction between grinding tool and workpiece. This increased coefficient of friction leads to an increase of the spindle power. Accordingly, a grinding process with the lowest possible volume-specific spindle power, e.g. a static level of P'c, is the objective of the investigations.
In each of the experiments, face-centered test plans with randomized test points are used. Both test series are performed with three different conditioning tools with varied grain sizes dgd = 30 µm (#500 mesh), 48 µm (#320 mesh) and 125 µm (#120 mesh) of white corundum (Table 1). These are typical grain sizes that can be provided by the dressing tool manufacturer Saint-Gobain Abrasives GmbH. The circumferential speed of the conditioning tool is varied in three steps between vrd = 1.5 m/s and 3 m/s In the first series of tests, 30 strokes of the conditioning tool are performed consecutively. In the second series of tests, continuous dressing is performed. The dressing depths of cut per tool stroke is varied between aed = 0.5, 1.0 and 1.5 µm. The grinding tool is a cup grinding wheel with an outer diameter ds = 400 mm. The abrasive layer has a width of bs = 15 mm and consists of vitrified diamond grain of grain size D15A, which corresponds to an average grain diameter dg = 12 µm. An abrasive concentration C100 is used, which corresponds to a volume fraction of the diamond grain C = 25%.
Table 1
Factor levels of test series 2: Determining the influence of the dressing parameters on the process parameters
Test series
|
Parameter
|
Test point
|
−1
|
0
|
+1
|
1, 2
|
dgd [µm]
|
30
|
48
|
125
|
1, 2
|
vrd [m/s]
|
1.5
|
2.2
|
3.0
|
2
|
aed [µm]
|
0.5
|
1.0
|
1.5
|
Table 2
Specification of ground PcBN workpieces
Specification
|
Binder
|
Concentration of cBN [%]
|
cBN Particle size [µm]
|
Hardness [HV0.2]
|
A
|
TiN
|
55
|
2
|
2,561±92
|
B
|
TiN/TiC
|
75
|
2
|
3,315±78
|
In test series 2 two different specifications of thick film PcBN inserts with cemented carbide support are ground (Table 2): Specification A has a cBN content of 55%. The hardness of the material is determined at 2,561±92 HV0.2. Specification B has a cBN content of 75% and a hardness of 3,315±78 HV0.2. The average particle size of cBN of both specifications is dcBN = 2 µm. The workpieces are ground four times on each flank face with an infeed ae = 50 µm. Therefore, 16 engagements to a total material removal V’w = 10.4 mm3/mm are performed.
In all tests, a mineral oil is used as cooling lubricant with a flash point of 165°C. The cooling lubricant is applied to the engagement zone between the workpiece and the grinding tool using needle nozzles with a flow rate of Q = 35 l/min.