Automation of Groove-Grinding Machines

V. A. Bernarovich, A. P. Zaitsev

(Presented by the scientific seminar of the Electromechanical Faculty)

The seven-year plan for the development of the national economy of the USSR provides for the substantial development of machine building—the basis for the production of the means of production. In modern machine building, the relative share of grinding operations is steadily increasing, which characterizes the growth of production standards. Thus, in the bearing industry, grinding machines account for about 60% of all equipment. With the expanded use of precision casting, forging, stamping, and other progressive methods of manufacturing parts, grinding will occupy an even larger place in metalworking.

At the same time, existing grinding machines do not meet modern requirements either in productivity or in the level of automation. Most machines have only manual control; some machines operate as semiautomatic machines, and only individual enterprises have automatic grinding machines.

Alongside the design and creation of new, improved types of grinding machines, an important task is the modernization and automation of existing types of machines, aimed at increasing productivity, improving machining quality, and fully automating the processes.

For the bearing industry, of great importance is the automation of spherical-grinding machines that machine the grooves of ball-bearing rings. In these machines several motions are performed, some of which proceed in a strictly defined sequence, while another part of the motions is carried out simultaneously. At present these machines operate on a semiautomatic cycle with active size control by means of a contact sensor. All operations, except for the installation and removal of the ring being machined, the approach and withdrawal of the measuring head, and the starting of the machine, are automated. In this case one machine operator usually services 2–3 machines. However, there are possibilities for further increasing productivity and improving machining quality (especially with respect to grinding finish and dimensional scatter). To realize these possibilities, the automation of spherical-grinding machines should be carried out in two directions.

1. Conversion of the machines to automatic operation without human participation (with the exception of setup operations). In this direction it is necessary to equip the machines with a system having a simple and reliable auto-operator,

working together with transporting devices, which will make it possible to include groove-grinding machines in flow lines. The currently existing models of hydraulic automatic operators, developed at various plants of the bearing industry, are rather bulky, complicated, and unreliable in operation.

1. Modernization and automation of the electric drives of groove-grinding machines. In this direction it is necessary on all machines to use means of active control that act on the operation of the machine’s electric drives and ensure the optimum course of technological processes. Technological calculations and experiments carried out at the 4th State Bearing Plant and the 1st State Bearing Plant have shown that, on existing models of groove-grinding machines, machine time can be reduced in the following ways [1, 2, 3]:

a) Grinding at the maximum permissible constant linear speed, independent of wear of the grinding wheel. In this case an adjustable electric drive of the grinding wheel is used, the speed of which is increased as the wheel undergoes dimensional wear. The power of this electric drive is 2.5–3 kW, which requires the use of a rather powerful regulating device.

b) Increasing the linear speed of the grinding wheel from 35 to 50–60 m/sec. This method makes it possible to reduce machine time, increase the life of the grinding wheel, and improve the surface finish. However, the existing spindles of LZ-5 groove-grinding machines cannot withstand such high speeds; therefore the development of a new design of high-speed spindle is required. In addition, machining at a speed of 50–60 m/sec is possible only with special grinding wheels, which are as yet manufactured in insufficient quantity.

c) Regulating the speed of the transverse feed of groove-grinding machines according to a special law, with forcing of the cutting conditions at the beginning of machining (in order to shorten machine time) and with a reduction of the feed speed at the end of machining in order to obtain the specified qualitative indices of machining. During machining the feed speed must not exceed the limiting speed \((V_p)\), at which burning penetrates to the depth of the allowance being removed.

Regulation of the feed speed makes it possible to reduce machine time substantially (by 20–40%) while using a regulated electric drive of only 100–200 W [2].

A special feature of the use of systems with regulation of the transverse-feed speed is that the speed of the electric drive (the speed of the motor feed \(V_m\)) should be regulated as a function of the actual speed of allowance removal (the actual feed speed \(V_d\)), which differs from the motor speed by the speed of wheel wear \(V_k\) and by the magnitude of elastic deformations \(V_{\mathrm{el}}\):

\[ V_m = V_d + V_k + V_{\mathrm{el}}; \]

for displacements,

\[ S_m = S_d + S_k + S_{\mathrm{el}}. \]

This feature follows from the fact that the quality indices of grinding, with an approximately constant cutting ability of the grinding wheel, are determined by the speed of the actual feed and not by the speed of the electric drive. Therefore in the systems it is necessary to control either the speed of allowance removal or the grinding power, which also indirectly reflects the quality indices of machining.

In practice it is most convenient to monitor the speed of the actual feed, especially since machining on groove-grinding machines is now performed only with self-sharpening grinding wheels.

Systems for the automation of groove-grinding machines at 4 GPZ and 1 GPZ were implemented by creating an adjustable electric drive for transverse feeds. The 4 GPZ system [2] is a continuous-action reversible system; regulation is carried out as a function of the actual rate of removal of the allowance. The regulation law is specified as a function of the allowance. The system uses: a separately excited dc motor; electric-machine, magnetic, and semiconductor amplifiers; and an inductive size sensor used as an active-control element. The system is very difficult to adjust, since it has a high amplification factor in order to obtain high accuracy and several feedback loops to ensure stability. In addition to the difficulty of adjustment, the system is expensive.

The 1 GPZ system [3] is of relay action; feed-rate regulation is pulsed and is carried out as a function of the grinding power. The regulation law is specified as a function of the allowance. The system uses: a separately excited dc motor; a selenium rectifier; a number of intermediate relays that change the voltage at the motor armature; a relay measuring element for grinding power; a semiconductor amplifier; and a contact size sensor as an active-control element. The system is much simpler and cheaper than the preceding one, but has low reliability because of the large number of contacts (about 104,000 contact operations occur per day). In addition, in order to measure the grinding power it is necessary to take into account the power losses, which change during operation (the losses in the spindle of the grinding wheel increase especially strongly as the wheel wears). This entails additional complication of the system by introducing a memory device to account for the no-load power of the electric drive of the grinding wheel.

At the Department of Electric Drive and Industrial Electronics of the Tomsk Polytechnic Institute, an automation system for the LZ-5M model groove-grinding machine was developed for 5 GPZ, using an adjustable electric drive for the transverse feed. Regulation is carried out as a function of the allowance, with adjustment according to the speed of the actual feed. The regulation law for the speed of the actual feed is also specified as a function of the allowance. The system employs a new pulsed-control circuit using thyratrons, developed by the authors.

The following changes were made in the design of the LZ-5M machine.

1. The cam gearbox of the feed mechanism was converted to a conventional one with a gear ratio of 840/2450. One electromagnet is used to switch the gear ratio from 840 (used for rapid approach and withdrawal) to 2450 (used during grinding). The second electromagnet has been eliminated.

2. In place of the asynchronous motor, a separately excited dc motor of type MI-12F was installed on the feed box.

3. The contact size sensor was replaced by an inductive sensor with a sensitivity of 100 V/mm.

4. The time-relay block, the amplifier block of the contact sensor, and several intermediate relays were eliminated from the machine control circuit.

All other machine units remained unchanged. The electrical control circuit was changed only slightly; the devices already present in the machine were used in the circuit, and therefore the wiring diagrams are almost completely preserved. This will facilitate the re-equipment of the machines.

The principal and new unit in the machine automation system is the speed-control block for the feed motor, operating jointly with an inductive sensor for active dimensional control. At present the block is made as a separate attachment. The schematic diagram of the attachment block is shown in Fig. 1. The voltage from the transformer inductive sensor \(ID\), connected in a differential circuit, is fed to the input of the attachment block. The attachment block has a programming channel on the electron tube \(L_1\) and a trimming channel on \(L_2\) and \(L_3\). These two channels control the thyratron \(T\), regulating the supply of energy to the armature of electric motor \(D\)

Fig. 1.

in the form of pulses. The thyratron operates in a half-wave rectifier circuit with rigid negative voltage feedback from the motor armature. This makes it possible to obtain low, stable speeds and improves the dynamic properties of the system. The thyratron is controlled according to the “open—closed” switching principle. If the feed speed is low, the thyratron is “open,” current passes through the motor armature, and the motor speed increases. If the feed speed is high, the thyratron “locks,” and the motor speed decreases. This process is repeated at a high frequency, so that the feed speed remains approximately at the specified level.

On the basis of technological studies by 4 GPZ [2] and experience in operating LZ-5 machines, we selected the law for regulating the actual feed speed as a function of the allowance, shown in Fig. 2.

The maximum constant speed for removing the allowance was selected for economic reasons. The change in speed over the last third of the allowance being removed was chosen so that at any point the feed speed would be lower than the limiting speed \(V_{\mathrm{p}}\) by a factor of \(1.25 \div 1.5\), on the condition that burn be absent. Machining is completed at a speed

\[ V_0 \leqslant 0.1 \ \frac{\mathrm{mm}}{\mathrm{min}} \]

from the condition of stable achievement of the 8th class of surface finish. Dwell (grinding without feed) is absent, which ensures a very small dimensional scatter of the ground parts.

To implement the adopted law for regulating the feed speed, the system has two channels.

Fig. 2.

The programming channel (lamp 1) converts the voltage of the induction sensor, directly proportional to the stock being removed,

\[ U_{\mathrm{c}x}=kS_x \]

into a command voltage varying according to a trapezoidal law as a function of the stock. By selecting the parameters, the form of the command voltage is made to approximate the adopted law of regulation of the feed speed.

The command voltage is taken from resistor \(R_1\) of the anode load \(L_1\) and from resistor \(R_2\) of the trimming channel.

The command voltage from resistor \(R_1\) is fed into the grid circuit of the thyratron in phase with the anode voltage of the thyratron. As a result, the motor speed is regulated approximately according to the specified law. The error here arises because feedback is effected by the voltage on the motor armature, and not by the motor speed. It is not expedient to seek an increase in the accuracy of regulation of the motor speed, since, as was already stated above, it is necessary to control the speed of the actual feed.

Thus, the programming channel serves to obtain command voltages that vary according to the adopted law of regulation of the actual feed speed as a function of the stock, and for coarse regulation of the motor-feed speed as a function of the stock.

For more accurate regulation, the trimming channel on \(L_2\) and \(L_3\) is used. At the input of this channel, the command voltage \(U_{R2}\) and the voltage proportional to the actual feed speed, \(U_{Vg}\), are supplied in phase opposition; the latter is taken from resistor \(R_3\) after rectification, smoothing, and differentiation of the voltage \(U_{\mathrm{c}g}\).

\[ U_{Vg}=\frac{T\dfrac{dU_{\mathrm{c}g}}{dt}}{1+T\dfrac{dU_{Vg}}{dt}} \simeq T\frac{dU_{\mathrm{c}g}}{dt} = k_1T\frac{dS_g}{dt} = k_2V_g . \]

As a result, an error voltage acts on the grid of \(L_2\),

\[ \Delta U = U_{R2} - U_{Vg}. \]

This voltage is amplified by a two-stage amplifier using \(L_2\) and \(L_3\), and is developed across the resistance \(R_4\) of the anode load of \(L_3\), entering the grid circuit of the thyratron.

Thus, in the grid circuit of the thyratron there appears a trimming signal that will act on the speed of the feed motor, reducing the error between the specified and the actual feed rate to zero.

The circuit contains a variable resistor \(R_5\)—“zero adjustment.” By adjusting this resistance, the “rest” currents of the electron tubes \(L_1\) and \(L_3\) are balanced. With the selected circuit, the trimming signal can act both in the direction of increasing and in the direction of decreasing the speed of the motor feed.

To record the completion of machining when the specified dimension is obtained, a zero-indicator circuit is used, responding to a change in the phase of the voltage taken from the induction sensor. The zero-indicator circuit provides the necessary accuracy; it contains an amplifier on a semiconductor triode P2B and an RP-5 relay, acting directly on the machine-control circuit. In addition, the feed electric-drive system itself is phase-sensitive. In the event of passing beyond the specified dimension as a result of any malfunction, the feed electric drive automatically stops operating.

Power for the entire attachment unit is supplied from an alternating-current mains at a frequency of 50 Hz. In this case, the electron tubes themselves rectify the current.

The developed control system for the electric motor of the feed mechanism has good energy and regulating characteristics; it is very simple, reliable, and inexpensive. This has been achieved through the use of a new pulse-control circuit employing a thyratron.