"Pilot" Hydraulic Copying Lathe | Machine Tools | Industrial Engineering

Here, we will consider hydraulic devices only. The introduction of the technique of copying on production lathes and other machine tools represents an important stage in the development of automation of machine tools. There has been a tremendous increase in the saving of labour required for the production of certain components by the copying method.

Everyone knows that the setting and control of machines by means of hydraulic, pneumatic, electric and combined systems of cycle automations are designed for high versatility and a wide range of adjustments. The dynamic features, space and weight reduction, high accuracy, remote control facilities, low price, etc., are added advantages of hydro-copying systems. We get a very high power-to-weight ratio and thus reduced construction and installation costs.

Consequently, the hydraulic control copying machines have conquered the field of small and medium size machines with particularly high rates of feeds. As regards the work, incomparable setting simplicity, higher dimensional stability of products coupled with higher chip flows, better surface finish and, in particular, fidelity of shape are the significant features obtained by the use of hydro-copying machines.

A hydraulic system consists of pumps delivering oil under pressure to operate cylinders, and serves mainly to obtain reciprocating motions. Control devices set up the magnitude and direction of travel of the various units, as well as the sequence of their operations within the working cycle.

As distinct from mechanical systems, hydraulic devices do not have rigid transmission ratios between the driven and driving elements. Mineral lubricating oil is used for the movements of tables in copying units, for self-clamping devices and indexing units. Here we will consider the hydro-copying methods alone.

Hydro-copying units are extensively used in automatic lathes to turn the contours on a work piece to the profiles of a template. The tracer (or stylus) of the copying unit operates a hydraulic valve in a hydraulic system.

In combination with the longitudinal movement of the saddle, effected by mechanical or hydraulic means, the stylus follows the profile of the template and operates the control valve that admits or exhausts oil under pressure on either end of a piston, which transmits its motion to the tool slide. The advantage of these systems over mechanical type includes the magnitude of the force (very small) acting upon the tracing stylus which controls the valve.

Inspite of the number of advantages, the hydraulic feeding system has a few drawbacks of its own. Frictional losses in the fluid, internal and external leakages, the variation, in the supply of liquid, and the varying speed of the mechanisms due to fluctuations of temperature, viscosity of the fluid and other factors, like the compressibility of the fluid, leakage of air into the system, heating of the hydraulic fluid, the in- flammability of the oils, etc., are some of the shortcomings. Except for the fluctuation in the feed rates, all other defects can easily be avoided or compensated by a discriminating selection of designs of hydraulic circuits.

The fluctuations in feed rates of a hydraulically driven feed device are eliminated by the mechanically controlled piloting system, thus eliminating the defects of instability and irregularity. When turning with copying lathe, the spindle speed and the rate of feed must constantly be adjusted so that both the steadiness of the cutting speed and the constant feed increment per revolution of the chuck are maintained, regardless of the tool position with relation to the centre of rotation of the work piece, i.e. if the hydraulic feed is constant and independent of the spindle speed, the feed per revolution can suffer considerable changes in machining stepped parts and hence it should be adjusted accordingly.

Hydraulic System of S-Pilot Lathes:

The hydraulic system consists of a motor pump unit and a reservoir, a cylinder set in the headstock body in which moves a piston to which is attached the saddle, a valve set in a block piece of the machine base, a relief valve and short rigid connections between pump, valve and cylinder.

The rapid advance or rapid return are obtained by the movements of the piston according to the difference in oil thrust experienced by it due to the difference in area of pistons on either side. The relief valve opens only when the slide comes up against a fixed part of the machine.

The mechanical portion of the piloting system consists of:

i. A pilot lead screw, free to rotate and to move axially, carrying the pilot valve and supported at each end by two smooth bearings.

ii. A hydraulic spring (a constant force by differential piston) tending to push the pilot lead screw and the valve towards the headstock.

iii. A helical gear carried in the saddle meshing with the pilot lead screw.

iv. A male member of a cone clutch fixed to an irreversible worm carries a hand wheel, the other member of the cone clutch being fixed to the helical gear.

v. A feed box driven from the spindle by a vee belt, and driving the lead screw through a pair of gears and clutch/ brake.

vi. An adjustable stop fixed on the pilot lead screw.

(a) Rapid Return (Refer Fig. 33.9):

Under the action of a solenoid, the friction cone J of the cone clutch is freed from the cone M. Now the helical gear I, meshing with the pilot lead screw G, is set free to rotate. Thus, consequently, the pilot lead screw is set free to move axially. The hydraulic spring H, pushes the pilot screw, which is no longer controlled by the free-to-rotate gear, which in turn, pushes in the control valve E.

By pushing in the valve, the outlet orifice is closed and inlet orifice is opened, making the pressures in both the chambers equal. The effective areas of cross-section of the small and big chambers B and C, are in the ratio 1 : 2.

The resultant hydraulic thrust due to the difference in the piston area pushes the piston towards the tail stock side. This effects the rapid return of the copying saddle. The oil leaving the small chamber joins the oil coming from the pump and fills the large chamber at a rapid rate.

(b) Rapid Approach and Skip Feed:

The friction cone is disengaged and thus the helical gear is set free as in the previous case. But a solenoid pulls the pilot screw towards the tailstock, bringing with it the control valve. Now the outlet orifice is opened to the large chamber, and the inlet orifice communicating with the two chambers is closed. All the oil from the pump feeds the small chamber and at the same time the pressure in the large chamber drops. This effects the rapid approach (rapid forward traverse).

(c) Arrest and Equilibrium:

The saddle on rapid return comes to rest against the stop O fixed on the pilot screw. The saddle thus tends to push the stop along with it in the same direction. The stop then tends to pull the pilot lead screw, which in turn, effects in pulling out the control valve. But as soon as the valve allows the outlet orifice to open; the pressure in the large chamber drops and the hydraulic thrust tends to force the piston in the opposite direction.

The hydraulic spring on the pilot screw, tends to push the valve in, and thus effects rapid return. Hence, there are a series of oscillating valve movements before the opposing forces on the piston become equal, and it comes to equilibrium position. The same result is obtained if, during rapid return, the friction cone is used to arrest the rotation of the gear I.

Piloting:

Mechanical piloting is a corrective action for the drawback of the hydraulic feeding device.

The function of the piloting mechanism is as follows:

The saddle is in equilibrium, and the gear / is locked by the friction cone. The pilot lead screw is driven by the main spindle through the feed box. Now the gear acts as a nut, and the lead screw begins to move, axially when it is rotated, bringing with it the valve. As soon as the outlet orifice is increased, the pressure in the large chamber drops and the piston starts moving, which carries the saddle along with it.

The principle underlying this is, if the saddle is connected to one groove of the thread in the pilot screw when it is in rotation, it will feel a force which will try to move the pilot screw axially, and will act on the pilot valve.

This valve will change the pressures inside the longitudinal cylinder on either side of the piston so that the force between the screw and the saddle is removed, and a synchronism between the saddle displacement and the number of pilot screw revolutions multiplied by the pitch of the screw is rigorously established.

After establishing of synchronism, all changes of motion of saddle, either reducing or increasing the speed of movement or stopping, will find immediately corrective movement of the pilot screw and pilot valve, which will alter the pressure. Hence the pulling force acts on the saddle to re-establish the synchronism.

The corrective movement of very small value (0.02— 0.04 mm) takes place in fraction of a second.

This first function, called piloting of hydraulic feeds, is meant for guaranteeing uniform feed per revolution of the main spindle which gives a smooth and accurate finish to the work piece. This, in addition, gives automatic safety, wider variation of feed valves, etc.

The second function is of getting pure hydraulic feed. We get feed motions, rapid traverses, smooth and wide range of feeds with a relatively simple and automatic control.

A feed box, between the spindle drive and the lead screw, is used to effect feed changes (by pick-off gears), automatic feed reduction and reversal.

The feed box, output is driven through a clutch, which effects, by stopping the rotation of the pilot screw, the arrest of the saddle feed.

A sensitive hand control, set on the saddle, is used for longitudinal displacements of the saddle during setting. The gear I is made solid with the worm wheel K by the friction cone J. By turning the hand wheel L, the gear I is made to rotate and the pilot lead screw is moved axially by about 0.2 mm, altering the valve orifice. This results in a change of pressures of oil on either side of the piston, so that a force acts on the piston, tending to move the saddle in the same direction as required by hand control.

The gear I rolls over the threads of the pilot screw, like a pinion meshing with a rack. The amount of saddle displacement will be equal to the developed length effected by gear I-provided that the pilot lead screw remains stationary.

If the lead screw rotates the feed effected will depend upon the relative movement of the screw and gear. The helical gear is irreversible, i.e. the rotation of gear cannot rotate the screw but the rotation of screw will rotate the gear.

Advantages of Piloting:

Some advantages of piloting:

(i) We set purely hydraulic feed, but in mm per revolution.

(ii) The feed stops as soon as the spindle is stopped.

(iii) The feed may not be engaged unless the spindle rotates.

(iv) The feed per revolution is absolutely constant, irrespective of the cutting forces, the viscosity or temperature of the oil, or the spindle speed.

(v) Mechanical control with virtually no wear or effort, completely unaffected by cutting forces. No wasted power; all the power being retained at the head-stock for metal removal.

(vi) Effortless, accurate and fine adjustment of the saddle position by the hand-wheel control assisted by hydraulic power.