List of Machine Tools used for Cutting Metals!
1. Centre Lathe:
The lathe is the father of all machine tools. It is a universal machine tool which can be used—with certain attachment—for many processes. For its development to the form in which we now know it, we owe much to Henry Maudlsey, who developed the sliding carriage, and in 1800 built a screw cutting lathe on which he turned screws having from 6 to 40 threads per centimeter and which were the best screws that had been made up to that time.
About 1830 Maudlsey constructed a lathe, with 3 metre face plate, which was used to bore large cylinders and turn flywheels.
In its operation the lathe holds a piece of material between two rigid supports called centers or by some other device such as a chuck or a face plate. The spindle carrying the work is rotated whilst a cutting tool supported in a tool post is caused to travel in a certain direction, depending upon the form of the surface required. If the tool moves parallel to the axis of rotation of the work a cylindrical surface is produced whilst if it moves perpendicular to this axis it produces a flat surface.
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Figure 39.2 shows a simple centre-lathe or general purpose lathe showing its main components. The components are as follows.
(i) Bed:
The bed of the lathe is made of a good quality gray iron casting or alloy and is supported on legs or a bench. It has accurately machined hardened and ground flat or inverted Vee-shaped inner and outer surfaces to guide the carriage head stock and tail-stock and to ensure accurate alignment.
(ii) Head Stock:
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It is the powered end of a lathe which is securely fixed on the inner ways of the bed. It is always on the left side of the operator. It houses the speed changing gears and the driving spindle. Normally a built in electric motor supplies power to the spindle. The centre of the spindle is hollow so that long bars can be put through and be held by chucks for machining. It is made of cast iron.
(iii) Tail Stock:
It is supported on the inner ways of the bed and is located on the right side of the operator. The main purpose of the tail-stock is to support the free end of the work piece when it is machined between centers. It is also used to hold tools when operations such as drilling, reaming, tapping etc. are performed on a lathe. (Ref. Fig. 39.3).
(iv) Carriage:
It consists of three main parts—a saddle, a cross slide and apron. It is used for moving the,-cutting tool along the lathe bed.
The saddle, an H-shaped casting, is bridged across the lathe bed and carries a cross-slide, a compound rest, a top slide and a tool post. The cross slide can be moved at right angles to the lathe bed either by hand or by power. The compound rest permits the swiveling of the tool to the required angle.
The apron is fastened to the saddle and contains the gears and clutches for transmitting motion from the feed rod to the carriage, and also contains the split nut (or half nut) which engages with the lead screw while cutting threads.
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The Compound Slide:
On most lathes a compound slide is interposed between the Tool post and the cross-slide. This slide may be swiveled to any angle and its use enables the tool to be moved in directions other than those permitted by the carriage and cross-slide. The compound slide is useful for turning and boring short tapers, chamfers and other jobs requiring an angular movement of the tool.
(v) The tool post is located on the top of the compound-rest. The purpose of tool post is to hold the tool and to enable it to be adjusted to a convenient working position.
(vi) Chucks may be of the 4-jaw independent, or a 3-jaw self-centring patterns.
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Each jaw of the 4-jaw chuck is operated by a separate square-threaded screw, whilst in the 3-jaw type all the jaws close in together, actuated by the scroll, which is a spiral groove cut on the face of a flat disc. The principle is similar to the movement of a nut by a screw except that the screw is cut on the face of a disc.
A 3-jaw Chuck is easier to operate, but the gripping efficiency is much less than that of the 4-jaw. The chuck is adapted to the nose of the lathe by being screwed to a back-plate which screws on to the machine nose. A Magnetic Chuck is available in several designs, most common being a rotary type.
An Air or Hydraulic Chuck is often used on lathes and other turning machines engaged in a mass production work. It is quick acting and grips the work strongly. The chuck can be worked by compressed air or by a fluid.
Lathes are manufactured in such a large number of types and sizes, and for such variety of purposes that it is quite impossible to attempt within the scope of a volume like this, to illustrate completely the different classes of lathes.
Some of the different types of lathes are as follows:
1. Speed Lathe:
Normal lathe 1200-3600 rpm. These lathes are used for polishing, metal spinning and wood turning Hand-operated tool.
2. A bench Lathe:
A small lathe which can be mounted on a work bench. All operation carried on centre lathe can be there with this lathe.
3. A Precision Lathe:
It is a bench-type lathe capable of giving very accurate work.
4. A Centre Lathe:
It is a general purpose lathe.
5. A Tool Room or Tool Maker’s Lathe:
Similar in appearance to centre lathe, but is very accurate. Used for precision work on tools, jigs, dies etc.
6. A Production Lathe:
Simple lathe for medium run production work. No lead screw, no compound rest.
7. A Single Spindle Automatic Lathe:
It is used to produce components in large quantities.
8. A Gap Bed Lathe:
It has a bed with a removable section adjacent to the head-stock so that a large work can be swung.
9. Special Purpose Lathe:
It has such characteristics that they are meant for only certain types of work.
The operations that can be performed on a lathe are known as turning operations. The word turning usually refers to machining external surfaces while machining internal surfaces is termed boring.
The operations carried out on a lathe can be grouped into the following categories:
(1) Plain turning between the centres
(2) Facing in chucks
(3) Taper turning
(4) Screw cutting
(5) Eccentric turning
(6) Drilling and boring
(7) Knurling
(8) Form turning
2. Drilling Machines:
Drilling is a manufacturing process by means of which cylindrical holes are produced in the work piece. The cutting tool used for this purpose is called a Drill.
Drills are of two types:
(1) Flat Drill:
The flat drill, which is the forerunner of the twist drill. This drill is easily forged and ground to any required size, but the results it gives are not comparable with those obtained from a twist drill; this is mainly because the twist-drill point is backed up and kept true by the body of the drill following behind, while the point of the flat drill has no such influence to guide it.
The principal present day use of the flat drill is for boring very long holes, in which process a cutter in the form of a flat drill is carried at the end of a long bar. The arrangement gives the best results when the work rotates, the drill and bar being held stationary and fed into the hole (as in lathe). Generally the bar which carries the flat cutter has tube sunk into it for conveying cutting lubricant to flush away the swarf and avoid choking up of the drill.
(2) Twist Drill:
The twist drill is the most commonly used variety of drill, and twist drills are specified according to the end by which they are held, called the Shank. Drills may be taper shank, parallel shank or jobber’s drills .
The drills are held in the machine by special chucks. Drills may be made of either carbon steel or high speed steel.
The three principal types of power machine used for drilling are briefed as below:
(a) The Sensitive Machine:
This is a high form of machine used for holes upto about 1/2″ (12 mm) diameter.
(b) The Upright or Pillar Machine:
The machines ‘are in sizes upto that which can drill holes up to 2-3 in. diameter (50-75 mm).
The main components of this drilling machine are:
(i) A column fixed at the base which is resting on floor.
(ii) A table with job/work fixing arrangement and which can be moved up and down.
(iii) Drill chuck and feeding mechanism.
(iv) A set up belt and pulleys for changing the sped of drill.
(v) Base frame on which machine is mounted.
Working:
Work piece is held on the table by means of fixtures. Table can be moved so that drilling can be possible by rotating the hand wheel connected for feeding the drill.
The speed of the drill can be changed from 470-1440 rpm. This variation in speed can be achieved by providing an electrical pole changing motor.
(c) The Radial Drilling Machine:
In this machine the drill head is on an arm and may be swung or moved over the area of the machine table or base.
Radial drilling machine may be of the sensitive or heavy types.
(d) Hand-Drilling Machine:
The fitter will have occasion to use various hand-drilling appliances for holes where it is not possible to bring the work up to a machine. The breast-drill 39.16 may be used for holes upto about 1/2 inch. (12 mm) diameter. Pressure is applied to the drill by pressing the body on the shaped end plate, whilst the drill is rotated by the handle.
The hand-drill for drills upto 1/4″ (6 mm) is similar but smaller and has a handle instead of a breast plate. Both breast and hand drills are fitted with a chuck for holding straight shank jobber’s drills.
(e) Electric Hand Drills:
For a quickness of operation, the electric drill has a great advantage over the breast drill, for not only does it leave both hands free to guide and feed the drill, but also rotates the drill with more uniformity and possesses that extra amount of weight necessary to give improved balance and manipulation.
A switch is incorporated convenient for the hand so that the drill may be positioned before it is started-up. Electric drills of the breast type may be obtained to take drills upto about 5/8″ (16 mm) in diameter.
Where extremely high speeds are necessary, small hand drills driven by a compressed air motor may be used.
Different Drilling Operations:
Other than drilling, following operations can be performed on the drilling machine:
1. Reaming:
A drill cannot be relied upon to produce a hole having sufficiently good qualities of finish and accuracy for many purposes, and when accurate holes are required a reamer is a must for finishing to size. The reamer does not originate the hole in the same way as the drill but merely imparts to the previously drilled hole the necessary smoothness, parallelism, roundness and accuracy in size. Reamers may be made of cast steel, case- hardened mild steel.
2. Countersinking:
Countersunk head screws and wood screw require a 90° chamfer cut round their hole as a seating for the underside of the head. This is cut by means of a countersinking cutter as shown.
3. Counter Boring:
The preparation of holes for certain purposes involves increasing the diameter of the hole for a certain distance down. This is called Counter-boring, and is done with a cutter of the type shown.
3. Grinding Machine:
Introduction:
Grinding is a process of metal cutting or metal removal, to obtain better finish by a rotating abrasive wheel called grinding wheel.
It removes very little quantity of metal and we get a very good surface finish.
1. To sharpen the cutting tools
2. To grind the gears
3. For thread grinding
4. To obtain better surface finish
5. For removing any excess material to get the required dimensions in very close tolerances.
Operation done by Grinding Machine:
1. Excess metal removing
2. Finishing
3. Buffering
A typical grinding wheel is shown in Fig. 39.21.
(A) Rough Grinding:
When the surface finish and accuracy are not of prime importance, then this method is used. In this the job is pressed against the rotating grinding wheel (or vice-versa), so as to remove excess metal.
Following are the Rough Grinding machines:
(1) Stand Grinder:
Figure 39.22 shows a stand grinding machine. In this case on the cast stand motor and grinding wheel are mounted. Grinding wheel will be rotating and the job in pressed as shown to remove excess metal.
(2) Portable Grinding Machines. Figure 39.23 shows a portable grinding machine (The figure is self-explanatory).
(3) Swing Frame Grinders:
It has a horizontal arm AOB. The length of the arm will be around 2-3 m. O in the centre of gravity where it is used. It can move within the specified area for grinding purpose. For grinding we have to apply the force at end B, so that the rotating grinding wheel is pressed against the job and excess metal is removed.
(B) Precision or Fine Grinding:
Whenever surface finish and accuracy are of prime importance then this method is used to obtain finished products.
Classification Precision Grinding according to the type of surface:
(i) External cylindrical grinding
(ii) Internal cylindrical grinding
(iii) External taper grinding
(iv) Surface grinding
(v) Face grinding
4. Power Saw:
It is a machine, which is used to cut the metal bars or work pieces to the required lengths.
(i) Reciprocating Power Saw.
(ii) Circular Power Saw.
(iii) Band Power Saw.
Figure 39.30 shows the horizontal power saw. In this case the work piece will be held by means of the clamps in the correct position. As the ram with blade reciprocates the job will be cut.
In this case a coolant is generally used for cooling purpose since it increases the blade life. The machine does cutting during forward stroke only and return stroke will be idle. During return stroke a crank mechanism in provided to raise the blade to avoid damage of blade. In this case continuous attention is not required during cutting because the machine automatically shuts off after cutting.
It is to be noted that a dash pot mechanism consisting of piston and cylinder filled with oil is provided on the machine. It produces reciprocating motion of the ram and blade when the power is supplied.
The average thickness of cut of power saw is about 3 mm during each forward stroke.
5. CNC Machines:
Computer Numerically Controlled machines (CNC machines) are extensively used these days for precision machining. There is hardly a facet of manufacturing that is not in some way touched by what these innovative machine tools can do. It incorporates a dedicated computer or microprocessor. In the CNC machines a computer receives information from a tape directly or from computer storage and controls the machine.
In the NC machines the physical components are connected through hardware whereas in the CNC the physical components are soft wired. There is lot of flexibility in the CNC machines as compared to the NC machines.
The block diagram of a CNC and DNC machine along with that of NC machine is given below:
