Control Systems for Machine Tools | Industrial Engineering

Control systems generate controlling movements for carrying out a machining process, like changing speeds and feeds, providing the working and auxiliary motions in a particular sequence as per the requirements for machining a component. Such systems should have high reliability, fast response, and high accuracy.

Speed and feed changing systems may be based on single lever or multiple levers. Single lever systems are more efficient and provide least fatigue to operator. However these are complicated and expensive.

It is practice now-a-days to minimise non-productive time in changing speeds and feeds. It is preferred to do jobs by pre-selective control system in which speed and feed are changed while an operation is in progress.

In manual control systems, the operator receives information about the operation by direct visual observation or from instruments and displays, processes the information in brain, decides upon a certain course of action and imparts the necessary motion to a control member. The response of machine tool to controlling instruction is again observed, processed and if necessary, new command is executed.

The ability of operator to above sequence is restricted by his body dimensions, capacity to apply force, ability to observe the process carefully and execute commands precisely. Accordingly the control members and display system should be designed keeping above limitations and requirements in view to cause least strain to operator.

Direction of Operation of Controls for Machine Tools:

Need for Standardization:

A machine operator uses a machine tool through its controls. After the operator has worked on any machine tool for a sufficient length of time, he becomes so familiar with its controls that the does not have to exercise special care or attention in manipulating these for the operation of the machine. At this stage, the operator’s efficiency on the machine attains the desired standard, and the operator cum-machine output becomes high.

In case, say, a turner is asked to work on a different make of lathe than the one he has been working on, his efficiency of production falls down, depending on how different this machine is from the former. The efficiency, nevertheless, improves as the turner learns to handle the controls more and more. Since it is desirable that a turner is able to handle a wide variety of lathes with equal dexterity, a strong need emerges for standardising the direction of operation of controls for all machine tools at the national level.

With the growing trade amongst the nations of the world, machine tools from all parts of the world find their way into a machine shop. In case, smooth working of the mixed set up with a variety of different makes of the same machine tool or different types of machine tools is aimed at, an international standard for the direction of operation of controls for machine tool becomes absolutely necessary.

Following advantages result in observing a uniformly accepted standard for the direction of operation of controls of machine tools:

1. Ease of handling different makes of the same cat­egory of machine tools by an operator.

2. Ease of handling different types of machine tools by an operator.

3. Greater safety to the operator because the opera­tion of controls results in anticipated movements of slides or functions.

4. Greater safety to the machine due to the same reason as explained in 3.

ISO recommendation R447 released in October 1965 concerned solely with the choice of the direction of operation of controls whose function is to produce movement of controlled machine tool components in one or the other of two opposing directions.

It lays general rules for: 

(a) The lever control

(b) The push-button control, and

(c) The wheel control.

The rules are applicable to all types of machine tools.

IS : 2987—1965 is also based on ISO recommendation R447.

The operator’s normal working position has a very important bearing on the direction of operation of controls of a machine tool.

In the case of push-buttons, the one nearer to the operator is for normal rotation and the further one for the reverse. In the event of the lever or the buttons being located in front of the operator, the right hand position is for the forward and the left hand for the reverse direction.

On machine tools (See Fig. 11.71) the push-buttons for the two directions of rotation of the spindle follow the above rule. See also Fig. 11.72 for the pole changing switch. It is customary to fix proper indicator plates to show the direction of rotation of the spindle for the different positions of the lever or for the push-buttons.

In the case of just on and off push-button, the RH button is for on and the LH for off. Red colour is selected for the OFF buttons and black or green for the ON buttons. In the case of vertical configuration, the top button is for ON and the bottom one for OFF.

According to IS : 2987, the direction of rotation is that obtained when viewing from the rear end of the spindle.

Lever Control:

The lever should be placed so that:

(а) For the control of rectilinear movement, the line joining the extreme positions of the handle on the either side of the neutral position be approximately parallel to the direction of the movement of the controlled component.

(b) For the control of the circular movement, the plane in which the lever arm rotates should be parallel to that of the controlled component. In either case the lever should cause the controlled part to move in the same direction as the lever is moved (Figs. 11.73 to 11.75).

Push-Button Control:

The line of push-buttons should be placed parallel to the movement of the controlled components and the operation of the right button, or the farthest, or the top button should produce a movement respectively to the right, or away, or upward (for an operator placed in the operating position).

In general, the practice followed by the machine tool industry all over the world is in line with the above rule.

Fig. 11.77 as such is confusing, but if shown as in Fig. 11.78, the rule becomes more clear.

The centre button is the stop button (coloured red) and buttons at either end are black, or sometimes green. Rapid movement button and inching button are situated close to the feed switches, but not governed by any rule.

When joy­stick controls are employed, the central position is for stop and other positions result in feed movement of the slides, etc., in the direction of the movement of the lever. Rapid traverse is normally a superimposed movement obtained by pressing a push-button on release of which rapid traverse disappears.

Wheel Control:

The clockwise rotation of the wheel should (for an operator facing that end of the shaft on which the hand wheel is mounted) produce the following controlled components:

(a) Either a rectilinear movement to the right, or away, or upwards (for an operator looking in a direction parallel to that of the operator placed in the operating position if the wheel axis is vertical, or facing the shaft of the wheel if it is horizontal). See Figs. 11.79 to 11.81.

(b) Or a clockwise rotation (for an operator facing that end of the shaft or spindle on which the controlled component is mounted) see Figs. 11.82 and 11.83.

(c) Or a movement towards the centre (clamping of chucks) see Fig. 11.84.

The case, as shown in Fig. 11.79 is universally adopted without exception. The rule, as shown in Fig. 11.80, is followed with the precondition that the clockwise rotation of the handle should result in bringing the job and the tool closer.

Thus in a cylindrical grinding machine clockwise movement of the hand wheel results in bringing the wheel slide towards the operator (and also towards the job). Almost all grinding machines have their controls built this way and as the tradition goes, there is no chance of any changeover in the future designs, since no manufacturer would risk to build machines the other way.

Similarly, rule as shown in Fig. 11.81 is not followed on grinding machines. Wheel slide of a surface grinding machine moving on a vertical column is fed down towards the job with the clockwise rotation of the hand wheel.

The rule shown in Fig. 11.84 is also used for clamping and unclamping of work pieces on a magnetic chuck. Clockwise movement is for clamping and anti-clockwise for unclamping.

Special Rules:

(i) The rules laid down under lever control and wheel control are not valid for cases in which the relative movement of workpieces and tool feed is considered. In these cases, the principle that the clockwise rotation of the lever or the hand wheel should result in the work piece and tool moving towards each other should be followed (See Fig. 11.85).

In order to appreciate the significance of this special rule, let us take the example of drilling machine head (Figs. 11.86 (a) and (b)). Applying rule of wheel control, the hand wheel rotation should have a different direction depending on whether the wheel is fitted on the RH or LH of the quill. Such a situation is avoided happily by the above exception.

(ii) If the direction (vertical, horizontal from the front, or horizontal from the end) of the movement of the control­led components is varied by a pre-adjustment independent of the controlled component considered, the rules laid down under lever control, push button control and wheel control apply to that of the direction which is most frequently used.

This clause indeed must again be read in conjunction with the special rule (i).

(iii) If the same lever is used for starting both the cutting movement and the feed movement of the tool, the rules laid down under lever, push-button and wheel controls apply to the feed movement.