GB2038672A

GB2038672A - Feed drive system

Info

Publication number: GB2038672A
Application number: GB7942534A
Authority: GB
Original assignee: HERBERT Ltd ALFRED
Current assignee: HERBERT Ltd ALFRED
Priority date: 1978-12-12
Filing date: 1979-12-10
Publication date: 1980-07-30
Also published as: GB2038672B

Abstract

A feed drive system for a movable member, such as a slide 1, of a machine tool, employs a driving force applied to the slide by a ram having a piston 5 slidable in a cylinder 4 formed in the slide, such force being applied against the mechanical constraint of a screw 12 connected to the member, the screw 12 being rotatable by a stepper motor 15 so as to control the movement of the member in response to the driving force. <IMAGE>

Description

SPECIFICATION Feed drive system This invention relates to a feed drive system for a movable member of a machine tool, such as a slide thereof, the system including a fluid pressureoperated ram and means for controlling the drive imparted by the ram.

In one type of conventional system employing a ram, a valve mechanism is employed to control the flow of fluid, usually oil, to the ram. A valve mechanism capable of providing the necessary accuracy of control is expensive, leading to a correspondingly expensive system incorporating such a valve mechanism, as well as requiring time-consuming adjustments, as for example to set the point of changeover from fast to feed motion and to set end of travel limits.

Other systems employ stepper motors, either to drive a member directly via a screw orto drive a hydraulic servo valve in turn driving a ram or hydraulic motor and screw. The direct drive arrangement is of limited capacity because of the relatively small torque available from a stepper motor, and the servo valve type of system is expensive because of the high cost of the valves.

An object of the invention is to provide an improved feed drive system for a movable member of a machine tool, such as a slide thereof, which is of simple and relatively inexpensive construction and flexible and convenient in operation.

According to the present invention, a feed drive system for a movable member of a machine tool comprises a fluid pressure-operated ram arranged to apply driving force to the member against the mechanical constraint of screw means operatively connected to the member, and drive means operable to drive the screw means in a direction such as to permit movement of the member in a controlled manner in response to said driving force.

In one convenient arrangement, the drive means is a stepper motor.

The invention will now be described, by way of example, with reference to the single accompanying drawing which is a diagrammatic representation of one form of the feed drive system of the invention.

Referring to the drawing, this shows a feed drive system for a movable member shown as a slide 1 of a machine tool, the slide being mounted for reciprocal movement along a slideway 2 on a fixed part 3 of the machine tool structure. In the particular arrangement illustrated, a cylinder 4 is formed within the slide and receives a piston 5 rigidly secured to the fixed part 3. A drilling 6 enables hydraulic fluid to be supplied through a fixed part 3 into a chamber 7 of the cylinder 4, the fluid being supplied from a reservoir 8 by a pump 9 via a valve 10, the latter being, for example of the spool type and being operable to connect the cylinder 4 either to the pump 9 or to the reservoir 8. A pressure relief valve 11 protects the pump against overloading in conventional manner.

The ram is capable of imparting a force to the slide in excess of the maximum cutting requirements of the cutting tool used, and the pumped volume of oil available is in excess of that required to move the slide at its fastest feed rate.

Mounted for rotation in the fixed part 3 is a screw 12 supported in and constrained against axial movement by a thrust bearing 13. The screw 12 is threadably engaged with the slide 1 or with means fast with the latter, illustrated as a lug 14. Rotation of the screw is effected in controlled manner by means of a stepper motor 15 via a suitable transmission, shown in this case as gears 16 and 17.

The helix angle of the screw 12 is chosen, taking into account the frictional characteristics of the screw and nut materials and other factors, to lie within a range such that the screw, when not rotated, provides a mechanical constraint preventing movement of the slide 1 under the action of fluid pressure supplied to the chamber 7. Movement of the slide is permitted by operating the stepper motor 15 to rotate the screw at a rate such as to produce the desired slide feed rate. Since the screw is, in effect, releasing the load applied by the hydraulic ram, it will be seen that, because of the chosen helix angle of the screw, the effort required to rotate the screw can be made relatively small compared with the ram load being released.The use of suitable eonectronic driving equipment can enable the stepper motor to release the screw at any rate from fast motion to a desired feed motion, and by means of pulse counting techniques applicable to the stepper motor, such factors as points of changeover, final stop position and duration of dwell, as well as the feed rates themselves, can be accurately controlled by an electronic control system, making the feed drive system ofthe invention particularly useful in automatic machine tools.

With the arrangement illustrated, the unloaded slide can be driven forwards at a fast rate by means of the stepper motor alone to the point where the feed rate is to commence, oil being drawn into the cylinder 4 directly from the reservoir under atmospheric pressure. At this point, the solenoid valve 10 is operated by an electrical signal from the control system to enable oil pressure to be applied to the cylinder from the pump, and to cut offthe direct connection to the reservoir. When the slide has reached its desired forward position, as for example, at the end of a machining operation by a tool mounted on the slide, the valve 10 can be actuated again automatically to exhaust the cylinder to the reservoir and the unloaded slide is then returned to its initial position at a fast rate by reversal of the stepper motor drive.It will be appreciated that with this arrangement the oil volume demanded from the pump is limited to the requirements of the feed rate only and the size of pump is chosen according to this need.

When the system is used with a heavy slide the torque available from the stepper motor may not along be sufficient to provide the force necessary to move the slide at a fast rate. In such a case the ram would be made double acting with an appropriate change-over valve and the stepper motor would control the release of force for the whole of the forward and return strokes.

The pump then would have to be of a larder capacity in order to cope with the oil demand at the fast approach and return rates.

The invention will be seen to provide a feed control system of simple and therefore relatively inexpensive construction and which is extremely flexible and convenient to use. Particular advantages of this system are that the pressure supply can be constant and the hydraulic side of the system requires only a simple solenoid-operated changeover valve. The system is consequently unaffected by changes of oil viscosity. Another major advantage of the system is that movement of the slide or other movable member can be remotely controlled by a central control system without recourse to setting such mechanical devices as dogs or dead stops. In some circumstances, the fluid used could be compressed air rather than hydraulic fluid.

As an alternative to the stepper motor, a suitable D.C. servo motor in conjunction with a resolver could be used to provide a controlled release of the hydraulically applied force.

Claims

1. A feed drive system for a movable member of a machine tool, comprising a fluid pressureoperating ram arranged to apply driving force to the member against the mechanical constraint of screw means operatively connected to the member, and drive means operable t drive the screw means in a direction such as to permit movement of the member in a controlled manner in response to said driving force.

2. A feed drive system according to Claim 1 wherein the screw means is rotatably mounted in a support which is fixed relative to the movable member and threadedly engaged with the member or with means secured thereto.

3. A feed drive system wherein drive is imparted to the screw means from the stepper motor via a geared tranmission.

4. A feed drive system according to any one of the preceding claims wherein the ram is single acting and controlled by a valve operated to depressurise the ram at the end of the stroke, whereby the member may perform a return stroke under the action of the stepper motor.

5. A feed drive system according to any one of Claims 1 to 3 wherein the ram is single acting and the member is spring-urged in a direction opposing action of the ram. whereby the stepper motor controls release of force to the member in two opposed directions.

6. A feed drive system according to any one of Claims 1 to 3 wherein the ram is double acting and the stepper motor controls release of force to the memb in two opposed directions.

7. A feed drive system according to any one of the preceding claims wherein the drive means is a stepper motor.

8. A feed drive system according to any one of Claims 1 to 6 wherein the drive means is a D.C. servo motor in conjunction with a resolver.

9. A feed drive system for a movable member of a machine tool substantially as hereinbefore described with reference to the accompanying drawings.

10. A machine tool incorporating a feed drive system according to any one of the preceding claims arranged to control the movement of a member in the form of a slide of the machine tool.