Grinding at a speed of 70 m/s with vitrified-bond wheels
Abstract Generated abstract
This paper examines the feasibility of cylindrical grinding at 70 m/s using composite vitrified-bond wheels made from standard wheels intended for 35 m/s operation. The authors tested an abrasive ring bonded to a metal ring after balancing, dressing, and strength testing, measuring wheel wear, metal removal, power consumption, and cutting force while grinding hardened 9Kh2MF steel on a modified cylindrical grinding machine. The experiments indicate that the composite wheel design can operate at increased speed, although limited cooling, machine rigidity, and wheel loading affected surface quality and force measurements. High-speed grinding increased cutting productivity, while specific metal removal did not show a substantial change in the short tests, and the authors recommend substantially more powerful motors for machines designed or modernized for high-speed grinding.
Full Text
GRINDING AT A SPEED OF 70 m/s WITH VITRIFIED BOND WHEELS
V. A. Govorukhin, B. F. Sherkunov, N. V. Yaremenko
(Presented by the Joint Scientific Seminar of the Department of Machine Tools and Metal Cutting and the Department of Mechanical-Engineering Technology)
Recently, industry in the USSR and in foreign countries has begun to introduce grinding at speeds of 60–100 m/s. However, the widespread use of high-speed grinding is limited by a number of technical reasons. First, there are no designs and bonds for grinding wheels that would allow the latter to be produced in a broad assortment. As a rule, high-speed wheels are used only on organic bonds, which make it possible, during manufacture of the wheel, to reinforce it in order to increase its strength with various interlayers made of strong fabric—nylon, kapron, or glass mesh. In the manufacturing process, vitrified-bond grinding wheels, in order to obtain their working properties, are subjected to heat treatment at a considerable temperature, as a result of which the reinforcing elements burn out. This does not make it possible to create a strong wheel for high grinding speeds. In the FRG, the USA, and Sweden, patents have appeared for designs of high-speed wheels on a vitrified bond, the strength of which is achieved by reinforcing the wheel during its manufacture with a metal mesh or a metal ring. In the first case, moreover, the wheel design becomes rather complicated. All this is the reason for the small number of studies of the operation of high-speed vitrified-bond grinding wheels.
Second, there is an almost complete absence of grinding machines and machines for testing the strength of high-speed grinding wheels. In addition, installations must be created for supplying, at a high flow rate and under considerable pressure, the lubricating-cooling fluid.
In this connection, the aim of the present work was to test the possibility of grinding at a speed of 70 m/s with composite grinding wheels [1] made from standard vitrified-bond grinding wheels intended for operation at a speed of 35 m/s.
The composite grinding wheel was an abrasive ring with an outside diameter of 300 mm, an inside diameter of 248 mm, and a height of 40 mm, cut from a grinding wheel 39A16ST1K and glued to a metal ring with an inside diameter of 63 mm. Before operation, the composite grinding wheel was subjected to static balancing, to a strength test at a peripheral speed of 105 m/s,
dressing on the machine with a diamond pencil and secondary balancing, followed by a strength test.
Grinding was carried out on a model 312M cylindrical grinding machine, modernized for high-speed grinding. The spindle of the grinding wheel was mounted on rolling bearings. The rotational speed of the grinding wheel was changed by means of interchangeable pulleys. The parts being machined were bushings 65 mm in diameter made of 9Kh2MF steel, hardened to a hardness of HRC 63–64. The bushings were mounted on a mandrel in the fixed centers of the machine. The coolant used was weakly emulsified water with a 5% content of calcined soda. The coolant was supplied to the cutting zone by a pump with a capacity of 20 l/min.
During grinding, the wear of the wheel, the volume of material removed, and the power expended on grinding were measured. The wear of the grinding wheel was measured with the aid of the simplest device, consisting of a dial indicator with a scale division value of 0.01 mm, fixed on a bar. Before the start of grinding, the distance was measured from the periphery of the wheel, on which the bar was placed, to the cylindrical rim present on the metal disk of the assembled grinding wheel. As the abrasive ring wore, the measured distance decreased, and the difference between two measurements made it possible to determine the wear of the grinding wheel in the radial direction.
The power consumed by the main-motion motor was measured with a wattmeter. The difference between the wattmeter readings before grinding and during grinding made it possible, with some error, to estimate the power and the principal component of the cutting force \(P_z\). The inaccuracy of the measurement was due to the fact that there were no data on the change in the efficiency of the electric motor and of the machine spindle as a function of the transmitted power.
Table 1
| Experiment No. | \(V_k\), m/sec | \(n_d\), rpm | \(V_d\), m/min | \(S_{\mathrm{pr}}\), mm/min | \(q\) | \(P\) | \(^{3}Z\), mm/mm·sec | \(N_e\), kW | \(P_z\), kg |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 35 | 300 | 42.4—47 | 3600 | 44.7—49.5 | 33.4 | 1.06 | 2.71 | 7.82 |
| 2 | 35 | 250 | 41—44 | 3000 | 47.7—51.2 | 25.4 | 0.977 | 2.56 | 7.4 |
| 3 | 35 | 150 | 23.50—24.6 | 1800 | 85.5—89.5 | 30.3 | 0.567 | 1.21 | 3.5 |
| 4 | 70 | 300 | 46.4—48.2 | 3600 | 87—90.5 | 2.47 | 0.162 | 4.82 | 7.2 |
| 5 | 70 | 400 | 61—68.5 | 4800 | 61.5—65.5 | 3.47 | 1.53 | 6.2 | 8.9 |
| 6 | 70 | 500 | 70.8—77.4 | 6000 | 54.5—60 | 21 | 1.73 | 6.2 | 8.9 |
The experiments were conducted at grinding-wheel speeds of 35 m/sec and 70 m/sec, with a longitudinal feed equal to 0.3 of the wheel height and a grinding depth of 0.005 mm per table stroke.
The average power, measured with a wattmeter, does not remain constant during the grinding process, but gradually increases until the wheel is completely loaded; this is illustrated in Fig. 1, which shows the change in power over time for experiment No. 6. At the beginning of the grinding process the power drops sharply over 2–4 table strokes, and after that begins to increase with periodic drops as a result of
Fig. 1. Dependence of power on the operating time of the grinding wheel
\((V_k = 70\ \text{m/sec},\ t = 0.005\ \text{mm/stroke},\ S_{\text{pr}} = 0.4B,\ q = 54.5—60,\) steel 9Kh2MF, 99A16ST1K)
renewal of a certain number of cutting grains. After the grinding wheel had operated for 211 sec, the machine was switched off in order to detect defects on the machined surface, and after this it was switched on again. In the figure, this moment corresponds to a sharp increase in grinding power to 11 kW, then its drop to 4.7 kW and a further increase in power. Such a change in grinding power is explained by insufficient rigidity of the machine. The short operating time of the grinding wheel in this and other experiments is explained by the mismatch between the wheel specification and the grinding conditions, and by an insufficient quantity of cutting fluid.
The data in Table 1 show that, at a constant rotational speed of the grinding wheel, increasing the speed of the workpiece, with all other parameters constant, leads to an increase in the grinding power and the force \(P_z\). This is caused by an increase in the chip thickness \(a\) falling on each cutting grain of the grinding wheel [2]:
\[ a=\frac{2\cdot\sqrt{t_{\phi}}}{q\cdot i_p}\cdot\sqrt{\frac{D+d}{D\cdot d}}, \tag{1} \]
where \(q\) is a parameter characterizing the ratio of the wheel rotational speed to the workpiece rotational speed,
\[ q=\frac{V_k}{V_d}; \]
\(t_{\phi}\) is the actual depth of cut;
\(i_p\) is the number of cutting grains per unit length of the wheel;
\(D\) and \(d\) are the diameters of the grinding wheel and the workpiece.
Comparison of experiments 3 and 4 shows that at a grinding speed of 70 m/sec the cutting force \(P_z\) exceeds that at a speed of 35 m/sec,
The increase in cutting force with an increase in cutting speed in these experiments is explained by insufficient cooling and loading of the wheel at a speed of 70 m/s, which led to the appearance of burns and waviness of the machined surface.
No substantial change in the specific metal removal \(P\) under conventional and high-speed grinding was detected either, owing to the short duration of the experiments and the very small wear of the wheel.
\[ P=\frac{Q_{\mathrm{M}}}{Q_{\mathrm{A}}}\, \frac{\text{volume of ground-off material}}{\text{volume of worn material}}. \]
However, the cutting productivity, understood as the amount of material removed in one second by one millimeter of the height of the grinding wheel, is higher in high-speed grinding:
\[ Z=\frac{Q_{\mathrm{M}}}{B \cdot t}\,\frac{\mathrm{mm}^{3}}{\mathrm{mm}\cdot \mathrm{s}}. \]
As a result of the experiments carried out, it was established that the design of the composite grinding wheel makes it possible to operate at increased speeds; at the same time, a certain complication of the wheel is compensated by a reduction in the consumption of abrasive material, since the abrasive ring is used completely.
Newly designed and modernized machines for high-speed grinding should be equipped with electric motors at least twice as powerful.
References
-
V. A. Govorukhin et al. Experimental verification of the strength of composite grinding wheels. In: Proceedings of the Scientific and Technical Conference “Technology of Mechanical Engineering.” Part III, Tomsk, 1970.
-
G. M. Ippolitov. Abrasive-Diamond Machining. Mashinostroenie, Moscow, 1969.