Development of Program Controlled Machine Tools | Industrial Engineering

In this article we will discuss about the development of prgram controlled machine tools.

All developments in Machine Tools have a single aim— increasing the profitability of machining operations. The demands on machine tool technology, to meet the ever- tightening accuracy and precision requirements of modern metal machining industry, have highlighted the utter incapability of conventional machine tools to perform the functions assigned to them at acceptable costs. This has motivated machine tools engineers to work on ways and means to build into the machine such features that would bring to the user the desired returns on his investment.

Highly skilled machinists are scarce and their services are expensive. A skilled machinist is able to produce on a conventional machine tool, the required precision by the application of his intimate knowledge of the response of the machine to his physical commands.

This necessarily implies special thinking and vigilance on the part of the machinist. In the case of medium and large batch sizes, this results in operator fatigue and reduces operational efficiency. Therefore, machine tool engineers have been trying to transfer the skill of the machinist to inanimate machine, so that, in batch and mass production operations, where the functions carried out by the machine are repetitive in nature, the operational efficiency is made independent of the operator’s skill and fatigue.

Another important aspect of machine tool development is to increase the proportion of time machine is actively engaged in profit-making metal removal to the total available machine time. All unproductive elements of machine operation, like the loading and clamping of work piece, selecting and setting of cutting parameters on the machine, positioning of the cutting tools for the cut to be taken, withdrawal of the tool from the work piece, inspection of the dimensions achieved, unclamping and unloading of the work piece have to be minimised in time and cost to increase the profitability of machining.

Economising Unproductive Machine Time:

The features that can be incorporated in machine tools to minimise unproductive machine time are discussed in brief in the following lines:

I. Loading and Clamping of the Work Piece in Preparation for the Cut:

i. Turning Machines:

Development of precision self- centering chucks with soft jaws and precision collets are instances of initial developments in work clamping to increase productivity. Hydraulically and pneumatically actuated chucks and collets, with electrical control and bar-feed attachments are logical improvements in this line.

ii. Milling Machines:

Development of special purpose fixtures with pneumatic or hydraulic actuators has considerably increased productivity.

iii. Grinding Machines:

Quick acting collets, magnetic chucks, automatic infeed of wheel head, etc. are some of the features available on modern machines. Magazine loading has also been successfully applied, particularly in centreless grinders.

II. Selection and Setting of Cutting Parameters on the Machine:

The cutting parameters referred to herein are the speed and feed. From the primitive belt changing; the development has gone through change gears to gear boxes with mechanical gear shifting of conventional machines. On Programme Controlled machines, use of electromagnetic clutches, eddy current clutches, hydraulic drives with D.C. motors has resulted in drastic reduction of unproductive time on the machines.

Selector switches and push buttons have replaced gear shifting levers making it possible to bring the controls to the most convenient position for the operator. Pre­-selection facilities afforded by the use of electro-magnetic clutches has also been of immense value.

III. Positioning of the Tool/Tools with Respect to the Cut to be Taken:

Incorporation of automatic rapid feeds in place of manual ones has cut done the approach and retraction times considerably.

Co-ordinate tables on drilling machines, indexing heads on milling machines, and automatic turret indexing on turret lathes, all point towards the efforts being made in minimising the time for positioning the tool with respect to the cut to be taken.

IV. Inspection of Dimensions:

The amount of inspection required is directly dependent on the certainty with which the man machine combination is able to maintain the dimensions desired. Any feature that helps in eliminating or reduction the human element will result in reduced inspection.

The first method of achieving the desired dimensions was by repeated trial and measurement on each work piece. Initial improvements came in the form of dials, scales-vernier scales, optical scales, fixed stops, indexable stops, etc. Digital readouts are the latest amongst visual indications of the sizes being achieved on the machines.

With all these innovations, Program Control machines reduce the need for inspection to the bare minimum thus producing parts with greater precision and repeatability.

V. Parameters that can be Programmed in Program Controlled Machine Tools:

One, or more than one, of the following parameters can be programmed in a Program Controlled Machines:

(a) Cutting Parameters: Speeds and Feeds.

(b) Dimensional Parameters: Positioning of tool/tools with respect to work ; length/lengths of cut.

(c) Auxiliary Parameters: Starting of spindle/cycle; loading/ unloading of work piece; clamping/unclamping of work piece; starting/stopping of coolant; indexing tool post or changing tool; stopping and braking of spindle; rapid approach and return; indexing of stops.

VI. Description of Program Control Methods:

The following three basic methods of program control are feasible in machine tools:

(i) Mechanical Program Control:

By the use of cams, a certain amount of program control by mechanical means has been achieved in the case of single spindle and multi-spindle automatics. The cams serve both as actuators and programming devices.

Thus, in effect, each cam ‘commands’ certain response from the corresponding element according to the shape and angular disposition of the lobes. The moving element is actuated directly by the ‘programming’ through the cam follower.

However, the limitations of a mechanical program control are:

(a) Accuracy obtainable by cam setting or other mechanical means is not very high.

(b) Setting the program is difficult and time consuming.

(c) Special cams are required for each program.

(d) Number of parts subjected to wear and tear is larger, causing expensive breakdowns.

(ii) Electro-Mechanical Program Control:

Electro-Mechanically program controlled machines can have the following features:

(a) Main Drives:

Speed selection will be by electro­magnetic clutches in the gear box as against conventional mechanical gear shifting. Electro-hydraulic gear shifting is also amenable to electrical programs. Variable speed D.C. motors and multi-speed A.C. motors have also been used in programmed main drives. Pre-selection facilities are also normally available.

(b) Feed Drives:

Electrically controlled variable speed drives, like eddy current clutches, D.C. motors, pulse motors, gear boxes with electro-magnetic clutches are amongst the various drives which can be programmed.

Some of these main drives and feed drives used on HMT machines, and how they assist in programming, will now be discussed.

VII. Description of Electro-Mechanical Program Control Method:

HMT has developed a number of machines incorporating electromechanical program control facilities.

Among them, E2 Milling Machine, ‘Chucker’ (Short Piece Turning Machine) has gained considerable popularity.

The Main and Feed Drives in the E2 Milling Machines have electro-magnetic clutches. The main drive, or the spindle speed of a milling machine is usually kept constant depending upon the cutter diameter. For this reason, the spindle speed in E2 has not been provided with program facilities. Nevertheless, the spindle speed can be readily selected by rotating dial mounted in front of the operator; the feed drives can be programmed by means of stop dogs and limit switches.

To explain the background of such a control system, let us take an example as shown in Fig. 34.22. In Fig. 34.22 (a), an electrical circuit diagram has been shown and the actual position of the electrical elements on the machine tools has been shown in Fig. 34.20. b₁ is the push button for starting the table movement; It is a limit switch which can be operated by the stop dog; C₁ is an electro-magnetic clutch mounted on the gear shaft.

The ‘electromagnetic’ clutch when energised engages the driven gear shaft with the driving shaft and thus the rotary motion is transmitted. In this example, the driven shaft will give a longitudinal movement to the table. The operator pre-sets the stop dog on the machine at a position where he wants to stop the longitudinal movement of the table, so that at that point the stop dog presses the limit switch.

The limit switch, on the other hand, is mounted in the limit switch box, which is clamped on the body of the machine. Let us now assume that the table is at a position as shown in Fig. 34.20.

Under this condition, the operator presses the start button b₁ and holds it there. The table starts moving in the desired direction and after traversing through the desired length, the stop presses the limit switch Ir which opens out the contacts. This will de-energise the electromagnetic clutch C₁ although the operator is pressing the start button; thus the table movement stops.

A further development is shown in Fig. 34.23 in which the operator need not keep the start button b₁ in a depressed condition. As soon as the operator presses the start button, a relay is actuated, which closes the contacts 1d₁ and 2d₁. Relay d₁ will now get the supply voltage through 2d₁ closed contact, and will be in energised condition, although the operator removes his finger from the start button.

As the 1d₁ contact is also closed, the electro-magnetic clutch C₁ in X-direction has been energised and the table starts moving in X-direction. After the pre-set traverse of the table, the stop dog presses the limit switch lr, thus contact lr₁ opens out and contact lr₂ closes, resulting in de-energising the clutch C₁ and energising the clutch C₂.

Clutch C₂ which is in Y-direction, will now be engaged and the table starts moving in Y-direction. Thus, the movement from X-direction to in­direction has been transferred through the control circuit.

When the table moves to the desired position in Y-direction; contact of the limit switch 2r will be opened out because of the stop dog. Thus, the supply to the clutch C₂ will be cut-off and the table comes to a standstill condition. Making use of this principle, the control circuit for the E₂ milling machine has been developed.

Table movement of E₂ can be programmed in any 2 axes out of the 3 axes. By setting the stop dogs mounted on the vertical and horizontal channels of the machine the table movement and feed can be programmed to a completely automatic cycle as shown in Fig. 34.24.

In the actual machine, there will be a number of limit switches and stop dogs, each set designated with a certain specific function. A simplified control block diagram has been shown in Fig. 34.23 (c). Here the limit switches, in association with the stop dogs, send position information to the electrical control unit which, in turn, sends the commands to the individual electromagnetic clutches.

The HMT ‘Chucker’ which is a short piece turning machine, works on a similar principle as discussed above. This machine has got automatic chucking features in addition to the programmable slides (top slide and bottom slide). The programme is set by means of stop dogs and limit switches. Fig. 34.25 shows one typical cycle of operation which can be programmed on the Chucker.

(iii) Electro-Hydraulic Program Control:

On hydraulically operated machines, program control can be conveniently achieved by the introduction of electrically actuated hydraulic elements, like direction control valves, flow control valves, etc., in the hydraulic system. A simplified circuit of hydraulic diagram involving a cylinder, a direction control valve and an electrically controlled flow control valve has been shown in Fig. 34.26.

C is a hydraulic cylinder which is connected through a spring centred three position direction controlled valve D to a pressure source. The return oil goes back to the tank through an electrically controlled flow control valve F. When both solenoids X and Y are de-energised, the direction control valve is in the middle position (position 1).

In this position all the connections from the direction control valve are blocked and, therefore, no motion of the piston in the hydraulic cylinder takes place. When solenoid X is energised, the oil port P is connected to connection A of cylinder, and connection B of cylinder is connected to connection R of flow control valve F.

The piston will now start moving to the right at a traverse rate depending on the electrical setting of the flow control valve. When solenoid Y is energised, the direction control valve assumes position 3, connecting pressure oil P to connection B of cylinder, and connection A of cylinder to connection R of flow control valve.

This results in a leftward movement of the piston. By introducing limit switches in the circuits of the solenoids X and Y and actuating the limit switches by stop dogs attached to the driven element, it will be possible to programme the movement of the piston and hence the driven element.

This type of electro-hydraulic programming has been used on HMT’s Minichucker, which is a short piece turning machine.

The Minichucker has two slides (front and rear) hydraulically operated. The programme channel on one of the slides is shown in Fig. 34.27.

The operational sequence is as below:

(a) When starting while the slides are in extreme retracted positions, dog D_3a should be pressing the limit switch L_3a on both the slides.

(b) When the cycle is started with the push button, the slides will move forward rapidly until the dog D₁ operates the limit switch L₂, and then it will move with the set feed rate. Thus rapid approach length is adjustable by setting the position of the dog D₁.

(c) When the limit switch L₂ is operated by the dog D₂, the other side is brought into operation and performs the cycle similar to the primary slide. When the cycle selector switch is set to have only one slide in operation, the limit switch L₂ is ineffective.

(d) When the limit switch L₃ is operated by the dog D₃ the slide reverses the movement rapidly and comes back to the starting point.

(e) When both the slides have returned to the starting point and when both the limit switches L₃ are pressed, the spindle stops, thus completing the cycle.

(f) The cycle is identical for both the slides.

On the Minichucker, the electro-hydraulic control has been used for work clamping in hydraulically actuated collets. Also, the program permits braking of the spindle at the end of the cycle.