The following methods are employed for achieving the quick motion in backward stroke and slow motion in forward stroke of the planer table: 1. D.C. Reversible Motor 2. Fast and Loose Pulleys 3. Hydraulic System.
1. D.C. Reversible Motor:
In this method, a special motor which can change its speed according to the field current applied, is used. The motor speed can be changed very quickly from full speed forward to speed in reverse direction and the reversals nearly instantaneous.
At the end of the stroke, the trip changes over the supply to the motor and it moves accordingly. Power from the motor is conveyed to the table rack by a reduction gearing and a worm in mesh with the table rack.
This method is most commonly used on modern planers because it gives a wide range of table speeds and more responsive control.
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Ward-Leonard Drive for Planing Machine:
The modern planing machines employ not only high speed, but to an increasing extent tungsten carbide steel tools also. To get the most out of carbide steel tools higher cutting speeds and power are necessary. Due to shorter working cycle, the number of hourly reversals of the table is, however, considerably increased and the drive must be capable of withstanding these conditions for years without breakdown.
The Ward-Leonard system owes its reputation as a reversible drive capable of changing direction millions of times, with its almost legendary reliability, flexibility and ease of control, unequalled by any other form of electrical drive.
Operating Requirements of Table Drives:
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i. Infinitely variable speed control over a wide speed range:
To enable a planing machine to be employed efficiently and to afford variable machining conditions for workpiece of different materials using suitable tools, it is essential for the speed of the table drive to be infinitely variable.
ii. Speed stabilization:
Since any undue variation in speed during the cutting process will impair both the tool and the accuracy of planing, the adjusted cutting speed must not vary unduly, even if the load were to fluctuate.
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iii. Rapid, smooth and accurate reversal of table:
Short deceleration and acceleration paths represent time gained and thus increased production. The reversing time must, however, be long enough to permit, for instance, the work to be advanced before the cutting stroke begins and the tools to be lifted prior to the return of the table. To reduce wear and tear of the planer, the table and the other moving components must be reversed smoothly without shock.
Table drive should incorporate a device, which ensures that reversal always occurs accurately at the same point, provides maximum safety while machining up to shoulders and teaches the operator always to allow the minimum overrun even when not machining up to the shoulder and thus gain time.
iv. Frequent working cycles:
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The drive must be capable of bringing about several thousand table reversals per hour without break down.
v. Simple control and long life of drive:
Because of the widely differing workpieces, which have to be machined, it must be possible to set up the work tool and speed of the drive easily and rapidly. The large number of work cycles per hour necessitates machines and apparatus which will withstand arduous service and millions of operations.
vi. Rapid braking and jogging of table:
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The device should be capable of stopping the work table suddenly for checking the dimensions of workpiece. It should to rapid to gain time during frequent stops.
Most of the castings have hard parts, which if machined at normal cutting speeds, would damage the tool. By depressing a separate inch button just before the tool reaches such a part, the table should be intermediately and smoothly retarded and the hard part should be cut at a slower rate.
The mode of operation of the Ward-Leonard drive and the selection of the most suitable type of planer work table. The feature which typifies this form of drive is that the variable speed D.C. motor armature is permanently connected to the variable voltage generator.
The D.C. generator will be driven by an A.C. motor and these two machines together are known as the Ward-Leonard Convertor. The electrical connection diagram of the reversing drive for the planer is given in Fig. 15.3, from which the fundamental design of the Ward-Leonard controlled drive will be clear.
The speed of the D.C. motor can be varied by two means:
(a) By varying the armature voltage supply (Armature or straight Ward-Leonard control).
(b) By varying the excitation of the D.C. motor.
These two methods of speed control for D.C. motors result in different torque-speed and output speed characteristics. With armature voltage regulation, the motor can deliver a constant torque over the entire speed range so that the motor output will rise in direct proportion to the speed. In the case of field regulation, however, the motor output remains constant, with the result the torque drops off with hyperbolic characteristics as the speed increases.
There is a third method of speed control which is the combination of both the above-mentioned means.
Before we choose which one of the above-mentioned three methods should be used for an electrical drive of a planer, it would be interesting to analyse the power required by the work table of a planer.
The power required by the table in kW is given by:
P = 100. F. D. S / K. E.
where F: Feed in cm/stroke, D: depth to cut in cm
S: Cut stroke speed in metre/min, E: planer efficiency
N: Number of heads cutting under the same condition
K: Metal removal factor in terms of cm3/min/kW input assuming an efficiency of 100%.
This factor depends on the type of tool used and the type of workpiece to be planed.
Each planing operation is designed for the removal of a particular operation volume of a particular type of metal per minute by using a given type of tool. Maximum cut can be taken only at a particular maximum speed.
At speeds higher than this, volume of material removed will be lesser to get good surface finish. From the above equation it will be clear that till the base speed, the table will be essentially requiring a drive with constant torque and after the base speed, the table requires drive with constant H.P. characteristics.
The characteristics of the planer work table and the characteristics of three drives should be compared and best one chosen.
Reversing of Planer:
A majority of planing machines only cut in one direction. In consequence, the reciprocating motion of the table consists of alternate cutting and return strokes. The speed of the return stroke is always adjusted to a maximum value allowed by the mechanical parameters of the machine.
The forward and return strokes do not represent a full working cycle in themselves, as a short time is required to reverse the table at the end of the forward and return strokes. The reversing time repeated twice per work cycle, represents the loss of time and must, therefore, be kept as short as possible.
In this respect, however, there are mechanical and electrical limits which, from the point of view of reliability, must be considered. It is worthwhile considering what takes place from mechanical and electrical point of view during this reversal process.
The relation between the total moment of inertia and motor torque is a measure of time required for reversal, i.e., time necessary to accelerate or decelerate the table, work, etc.
If it is assumed that the decelerating and accelerating torques are constant, and effects of friction during acceleration and deceleration can be ignored, the time required for deceleration can be given by:
t = ((2.61.GD2tot.n.10-3) / Mb)
where Mb is accelerating or decelerating torque.
GDfot is total moment of inertia of the system.
n is speed at which it starts decelerating.
The reversing of the D.C. motor is done by reversing the field of the generator. The time reversal of motor is dependent on the accelerating torque, inertia of the system, speed and time constant of the generator field.
The accelerating or decelerating torques depend, in turn, on the generated and back e.m.f. which in turn depends on the time constant of the field of the generator and speed of the motor. All these parameters are interdependent and the proper matching of these parameters bring about satisfactory reversal.
Advantages of Ward-Leonard Drive:
An inherent property of the Ward-Leonard equipment, which makes it eminently suitable for reversible drives is that the motor is reversed without any additional losses while kinetic energy stored in the moving part is recuperated during the deceleration process.
No other electrical or mechanical reversing drive exhibits this advantage and this is particularly appreciated when reversal takes place in rapid succession. Under such conditions, even a rapid reversing drive may be highly stressed so that one can imagine what is happening to the electrical and mechanical reversing drives which have no possibility of recuperating the kinetic energy and with which any change in speed during the period of deceleration will cause heavy additional losses in the form of heat.
The electrical braking of Ward-Leonard system brings other advantages with respect to the construction and efficiency of machine tools. Electrical braking replaces the mechanical braking system in almost every case and, in consequence, the design of the machine tool becomes simpler, cheaper and more reliable.
A further important feature of the Ward-Leonard system is that the process of reversal is unaffected by load conditions, allowing large or small moving masses to be accelerated or retarded with equal rapidity. This is because the armature current, and with it the motor torque, varies in direction proportion to momentary load. The accuracy of reversal is, therefore, unimpaired.
Constructional Features of the Electrical Equipment:
The machine constituting the drive should be so chosen or adjusted that a single control suffices to ensure the required functions, namely change of table direction, speed deceleration, stopping of motor and speed stabilisation during load fluctuations. These functions should be carried out with the same degree of accuracy and rapidity even after years of service.
The control and protective gear should be of an extremely simple design. All apparatus and contacts, considered not absolutely essential, should be dispensed with to avoid potential sources of trouble and thus ensure reliability.
The part of the apparatus which is subjected to the heaviest duty, such as reversing contactors, reversing switches for automatically reversing the table drive, and the stop switches to initiate the accelerated motion, should be designed to withstand at least 10 million operations and to function reliably and accurately.
2. Fast and Loose Pulleys:
The planers of old type use this type of drive. This makes use of 3 pulley drives with one belt.
In this mechanism belt shifting fork is connected to dogs fitted at the end of stroke in the planer bed. These automatically shift the belt from pulley R, to pulley K after the end of every stroke. When belt is on pulley R, it constitutes forward stroke. Motion from pulley is transmitted to the rack from gear A to gear B, gear C, gear D and is thus very much reduced speed.
When belt is on pulley K then motion is transmitted to rack pinon through cast gear E with pulley if and the gear D. In this case as motion is transmitted without any reduction, therefore, it constitutes return stroke. The direction of reversing may be followed from arrows shown in Fig. 15.4. When no motion is required, the belt is on loose pulley.
3. Hydraulic System:
Hydraulic drive for shaper and planer is exactly similar and has already been discussed in the chapter of shaper. Nearly all the modern planers use the hydraulic system because of the various advantages associated with it. In this system also the trip dogs fastened to the table are used to regulate the stroke length and to change the position of the table.
