The principle of working of the sliding head type single spindle automatic machine is explained by the sectional view of old type of machine as shown in Fig. 34.2. However some refinements have been carried out in it, but the working principle remains the same.
This machine has been developed for manufacturing quantities of small precision components, and is often referred to as Swiss Auto. This machine is best suited where work is long and slender and where accuracy, finish and concentricity are important.
Fig. 34.2 shows the general layout of sliding head type single spindle automatic machine. The bar stock is fed through a sliding headstock and held by a collet chuck. It has a sliding headstock which is traversed by a bell or plate type cam. A tool bracket, supporting five tool slides, each slide, operated by a separate cam, is mounted in the centre. Tool bracket also contains a guide or steady bush for the bar stock.
Cams are held on a front cam shaft. These control the tool slides and headstock movements. In place of turret slide is mounted a feed base having single or multi-spindle for operations like drilling, reaming, screwing, tapping, etc. The side view of the tool bracket is shown in Fig. 34.3.
The main drive is from an electric motor at the bottom left hand of the machine which drives a gear box housed in the cabinet base. From gear box the drive is transmitted to the spindle by two roller chains which pass up through the base of bed.
Thus the spindle speed can be varied from 60 to 200 R.P.M. in steps. For turning operations the reverse direction of spindle is employed by use of left hand chain and the slow speed for purpose of screwing and remaining operations is obtained by using right hand chain drive.
Spindle reversal is effected by the operation of the clutch on the spindle and for both the left hand and right hand chains, fast and slow speed are possible through clutches in the gear box. Thus two forward and two reverse speeds are possible for each combination of pick-off gears.
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The operation of these and the spindle clutch takes place from the back shaft and thus the speeds could be changed or reversed automatically also. The spindle is mounted in preloaded radial roller bearings which prevent excessive clearance in the spindle over a prolonged period of operation.
On the base of the machine is mounted a bed which carries the main elements of the machine as listed below:
(1) Work spindle
(2) Back shaft (auxiliary control shaft)
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(3) Front shaft (cam shaft)
(4) Turret and its slide
(5) Cross-slides (front and rear).
The back shaft and front shaft are mounted, along the bed and carry most of the cams and clutches for the operation of the movements. The back shaft is belt driven at constant speed from the main motor. It carries four sets of clutches which set in motion the movements of the machine elements that are independent of any tool action or cutting.
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The various movements achieved by these clutches are:
(1) Spindle reversal
(2) Fast and slow speed
(3) Feed and chucking
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(4) Turret indexing.
The clutches are of the toothed dog clutch pattern, slide on the back shaft and when engaged cause their mating members to rotate with the shaft. They are actuated by levers which extend across the machine and are pivoted at the centre with their opposite end actuated by trips attached to drums rotating on the front shaft. The drive to cam shaft is transmitted through back shaft via a cycle time change gears. One revolution of the cam shaft produces one component.
The headstock moves the rotating bar past the tools fitted longitudinally on tool bracket. All these tools cam move independently in or out transversely. The tools could either move in the ratio of 1 : 1 with the cam throw or in different ratio through adjustable levers.
Referring to Fig. 34.3, it will be seen that tools 1 and 2 are mounted on the front and rear of a rocker operated by a single cam and thus these can’t be used together. Tool No. 1 is recommended for plain turning only, the other tools being used for parting off, forming etc. The tool bracket holds a guide bush through which the bar passes giving a high degree of stability and steadiness during machining, and also permitting bar to be mounted as cantilever for machining operations.
The tools mounted on feed base can be given rotary motion. Thus, though bar may be rotating at constant speed for turning but still operations like screwing can be done at a slow speed by differential principle of machining because relative cutting speed is difference of speed of bar and speed of taps or die head in feed base.
It is also possible to carry out, drilling at fast speed by rotating drill in opposite direction in the feed base. Right hand threads are cut by revolving die head faster than work spindle and left hand threads by revolving die head slower than work spindle.
On multi-spindle feed bases, each spindle can be indexed in line with the work axis when required. In this type of machine, no special bar stop is required because the headstock itself traverser. The parting off tool is left in the forward position after cutting through the bar in previous operation.
The chuck opens and the headstock moves back. The bar being preloaded moves forward against the parting off tool. When the headstock has reached its initial position, the chuck grips the bar. The parting off tool retracts and the work cycle commences.