The basic function of many a machine tools is to attain the desired shape of a component by the process of machining, the material being removed from the blank in the form of chips or swarf. With the introduction of carbide tools and rapid development of machine tools for higher rate of metal removal, large quantities of chips are produced in a short time.
If the chips are not cleared off in time, further flow of chips from the tool is obstructed causing newly formed chips to get entangled around the elements surrounding the tool and posing a hazard to the operator and the machine.
It may also affect the quality of the work-piece. The seemingly unimportant question of chip removal is in fact a practical problem; particularly so in automatic machines where the rate of stock removal is high and the operator’s attention is missing. Designing suitable methods of chip disposal calls for a study of the various forms of chips, their specific volumes, the minimum inclination of the chip tray and the chip handling methods in a plant.
Chip Forms:
The various forms of chips are shown in Fig. 11.88. The form of a chip is dependent on many variables like feed, depth of cut, material, tool geometry and type of operation. Compound forms of chips may also occur in practice which are combinations of one or more of the indicated forms.
Volume of Chips:
The specific volume of chips is indicated in Table 11.1.
The ratio K of the volume of chips to the volume of solid material removed has an important bearing on the design of a suitable chip collecting and disposal arrangement in machine tools. The volume of chips accumulated during a given time can be calculated if the form of chips is known.
For instance, if in the case of a gear blank which is turned from steel, the volume of raw material—the volume of finished component = volume of material removed = 510 c.c. (say); and if 6 pieces are turned per hour, then the volume of metal removed per hour = 510 x 6 = 3,060 c.c.
The operations involved are turning, facing and drilling on a turret lathe. Continuous helical chips (Fig. 11.87) of different lengths are produced. From Table 11.1, K = 28.
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Hence the volume of chips accumulated = 3050 x 28 = 85,680 c.c. per hour.
Inclination of Chip Tray:
Another important factor in design is the inclination of the surface of the chip tray or guard. Minimum inclination of the surface is dependent on the form of chips, the height from which the chips fall, and the velocity of impact of chips on the surface. Table 11.2 indicates the values of minimum inclination for the common type of chips.
Chip Collection:
The device provided for collecting chips in a machine tool is dependent on the type of machine, operation, rate of stock removal and form of chips, Following are some of the arrangements found in different machine tools.
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In Centre Lathes and Turret Lathes the hollow space between the end leg and cabinet leg below the guide ways is utilised to collect the chips, by providing a tray which may be a separate element as in Fig. 11.90 or integral with bed as in Fig. 11.90. The chips fall through the hollow pockets in the bed and collect in the tray which are normally cleared from the front side after stopping the machine for safety.
Heavy duty lathes usually have a chip collecting place at the rear of the machine either on the ground level or below. The bed of the machine is designed to facilitate this arrangement as shown in Fig. 11.91.
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Copying lathes, which are meant for mass production of components, usually have specially designed beds for continuous disposal of chips without disturbing the operation of the machine. One such typical arrangement is indicated in Fig. 11.92, which the inclined configuration of the guide ways facilitates easy flow of chips on to a portable trolley, which can be handled from the rear of the machine.
Horizontal boring machines normally have inclined pockets in the bed, opening out to the sides of the machine. The chips have to be cleared from the floor.
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Planning machines, which normally handle castings, produce fragmental chips spreading over the entire length of the machine. Some manufacturers provide small pockets at the end of the table T-slots to collect the chips that tend to fall over the guide ways. Most of the chips scatter around the machine on the floor.
Milling machines are provided with trough type of trays on the base to collect all chips that are pushed over from the table saddle and knee of the machine. Drilling machines do not normally have any arrangement for chip collection since the quantity of swarf involved is not substantial.
Manual Handling and Disposal:
In any metal- working shop, the handling and disposal arrangement employed is linked with the quantity of chips produced.
In a plant having medium and light machine tools, chips are collected from the machines manually and loaded into a trolley which transports it to a collection yard. From then onwards, they are handled in bulk by trailers and transported to a salvage yard, which is normally located on the outskirts of the plant area for further disposal.
On the other hand, if the plant is employing heavy machine tools like planers, borers, heavy duty millers, etc., which normally handle castings and do not have separate chip trays, and as such, chips are collected from the floor and dumped into containers kept below the floor level and covered.
The containers are transported to a salvage yard at the end of the day. This scrap is ultimately consumed by foundries and steel plants. If the plant runs its own foundry, the chips can be routed accordingly. Non-ferrous chips are handled separately in order to salvage the high material cost.
Some of the tools normally used for manual handling of chips are illustrated in Fig. 11.93.
Mechanised Handling and Disposal System:
With the introduction of numerically controlled machines as well as with automatics, continual chip disposal is necessary with mechanised disposal system to keep the machine in good condition. The common type of machanised handling is self- contained conveyorised system.
In mass production plants like those manufacturing automobiles, tractors etc., where the rate of production of chips is high continuous disposal of chips is absolutely essential. In such cases, a fully mechanised system covering the whole plant is the best answer. Machines are installed in line so that a conveyor line collects the chips from all the machines and dumps them at the end of the line into portable trolleys. Apron and drag chain conveyors are suitable for this purpose.
Chips of light metals like aluminium are cleared with the help of coolant. The coolant is reclaimed at the end of the line.
Disposal Problems and their Possible Solutions:
i. Non-Ferrous Metals at High Speeds:
While machining non-ferrous metals like aluminium, bronze, etc., chips scatter around the machine over a wide area because of high cutting speeds employed, rendering the collection of valuable chips difficult.
In turning machines, it is necessary to have a sufficiently high rear chip guard with a proper slope at the top and a front chip guard properly bent to divert the chips towards the trays. The front guard should be provide with an unbreakable transparent sheet for viewing. The problem is more complex in milling machines.
ii. Non-Ferrous Metals Mixing with Coolants:
When non-ferrous metals are machined, small fragments and dust of the valuable material escape through perforation in the chip tray meant for collecting the coolant. Since, normally materials like aluminium, bronze, etc., are machined dry, it is a good practice to cover the perforated area properly.
Change of metals from non-ferrous to ferrous. The change of material loaded on the machine from ferrous to non-ferrous and vice versa, requires complete clearing of chips from the machine, to avoid non-ferrous chips mixing with the other.
This process of expensive cleaning involves stoppage of the machine for a considerable period. A detachable collection tray placed over the normal chip tray is an advantage.
iii. Oil Recovery:
In a plant having a large number of machines, like broachers, gear cutters, bar automatics, a considerable amount of valuable cutting oil is carried away with the chips. In such a case, it is advantageous to employ an oil recovery system before the chips are finally transported to the salvage yard.
The oil swarf is continuously fed from the hopper to a chip wringer, whose centrifuge recovers 99% of the oil present, and discharges the swarf dry and ready for transportation. Dirty reclaimed oil is transferred to a storage tank through another centrifuge for purification.
iv. High Specific Volume of Chips:
Steel chips, because of their high specific volume, pose handling and disposal problems. Conversion of chips either to brackets or small broken fragments by appropriate machinery solves the problem provided the quantity of chips handled justifies the economy of the employment of such machinery.