The following points highlight the classification of machine tool drives. The types are: 1. Standard Main Drives 2. Standard Feed Drives.
Type # 1. Standard Main Drives:
Main drives are constant power drives providing a range of speeds to sit the workpiece and/or tool conditions. The physical built up of main drives is shown in Fig. 11.65. The main drive consists mainly of power input, speed transformer and output.
The power input can either be through a foot mounted or a flange mounted electric motor, either directly coupled to the speed transformer or driving the speed transformer though a pair of spur gears or belts as the case may be. The output side consists of a main spindle and intermediate drive to attain suitable main spindle speed ranges. Depending on the configuration of the machine tool the main spindle will have to be designed.
Apart from the input and output stages, the speed transformer as such can be standardised. The speed transformer will hitherto be referred as main drive. The two parameters, namely, the power and the speed range of a main drive, need standardisation.
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Horse power range classification of the main drive can be based on the economical design of structures and the main spindle. It is no use designing an extra rigid structure for a low Horse Power Machine and vice versa. HMT has classified the H.P. ranges into three groups which would satisfy most of the limiting conditions.
Group 1 6.3 — 10 H.P.
Group 2 12.5 — 20 H.P.
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Group 3 25 — 40 H.P.
The speed range is derived from the type and size of milling cutters in the case of milling machine and from the size of workpieces in the case of centre lathes. In the case of boring mills a combination of milling, drilling and other operations has to be considered.
A speed range of 1 : 50 in 18 steps with a progression ratio of 1.25 should catre to most of needs economically in a machine tool.
The grouping of speed ranges and H.P. can be done as under:
Each group of main drives can be designed for one speed range and with the help of change gears other ranges can be provided. Fig. 11.66 shows a Main drive of 6.31 H.P. developed for one of the machine tools.
In this unit all the driving elements of the main drive are housed in a barrel-shaped casting. The main drive barrel with a motor of 7.5 H.P. forms a unit and is inserted into one of HMT milling machine column for driving the main spindle. The barrel provides 18 speeds in a progression ratio of 1.25 having a speed range of 31.4—1500. A higher of lower speed range can be provided with the help of change gears.
Type # 2. Standard Feed Drives:
Machine tools can be divided into two categories, namely machines with feed drives which are advantageously, kinematically linked with the main drive, and machines which do not require the feed drive kinematically bound to the main drive.
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Whatever be the kinematic relationship to the main drive. It has been found that the range of feeds, be it in terms of mm/rev., or mm/rain, remains restricted within a certain range. As regards other parameters in the drive, namely force, with these categories, the entire range of requirements of various types of machine tools can be covered.
It may be pointed out that, in contrast of the main drives, feed drives are designed to transmit a constant torque over the entire range of speeds. The feed thrust is the sum total of feed force required for machining and the force required to move the dead weight of the feeding elements.
The feed drives in a machine tool could generally be divided into 3 components, namely power input, feed unit, power output (Refer Fig. 11.67). As already indicated the input could either be a drive from the main drive or by an independent motor. This part of the feed drive is not easily amenable to standardisation.
The most widely used variants on the output side are rack and pinion drives, screw and nut or worm and worm rack drives. Since there is a basic requirement of non-return ability, suitable design elements have to be incorporated in the rack and pinion drives.
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Further attention has to be paid to the efficiency of the output components so that a common feed unit can be used at will, with either rack and pinion or a screw and nut as output side has been standardised kinematically as shown in Fig. 11.67.Actual design will have to be tailored to suit the individual machines in accordance with the configuration and direction of movements required by them.
The feed drive has been foreseen in three sizes to achieve net feed thrust as follows:
Size : 18 Tons, Size—2 : 4 Tons, Size—3 : 2 Tons
The second parameter, i.e., feeds have been standardised so that a feed range of 1 : 200 in 24 steps with a progression ratio of 1.25 can be obtained. This wide range of feeds should satisfy most of the machine tool configurations.
