Monday, December 17, 2007
Introduction
WHAT IS MILLING?
Milling is the process of cutting away material by feeding a workpiece past a rotating multiple tooth cutter. The cutting action of the many teeth around the milling cutter provides a fast method of machining. The machined surface may be flat,angular, or curved. The surface may also be milled to any combination of shapes. The machine for holding the workpiece, rotating the cutter, and feeding it is known as the milling machine
Milling is the process of cutting away material by feeding a workpiece past a rotating multiple tooth cutter. The cutting action of the many teeth around the milling cutter provides a fast method of machining. The machined surface may be flat,angular, or curved. The surface may also be milled to any combination of shapes. The machine for holding the workpiece, rotating the cutter, and feeding it is known as the milling machine
Methode Of Milling
1-Up Milling
Up milling is also referred to as conventional milling. The direction of the cutter rotation opposes the feed motion. For example, if the cutter rotates clockwise , the workpiece is fed to the right in up milling.
2-Down Milling
Down milling is also referred to as climb milling. The direction of cutter rotation is same as the feed motion. For example, if the cutter rotates counterclockwise , the workpiece is fed to the right in down milling.
The chip formation in down milling is opposite to the chip formation in up milling. The figure for down milling shows that the cutter tooth is almost parallel to the top surface of the workpiece. The cutter tooth begins to mill the full chip thickness. Then the chip thickness gradually decreases.
3-Other milling operations are shown in the figure
Up milling is also referred to as conventional milling. The direction of the cutter rotation opposes the feed motion. For example, if the cutter rotates clockwise , the workpiece is fed to the right in up milling.
2-Down Milling
Down milling is also referred to as climb milling. The direction of cutter rotation is same as the feed motion. For example, if the cutter rotates counterclockwise , the workpiece is fed to the right in down milling.
The chip formation in down milling is opposite to the chip formation in up milling. The figure for down milling shows that the cutter tooth is almost parallel to the top surface of the workpiece. The cutter tooth begins to mill the full chip thickness. Then the chip thickness gradually decreases.
3-Other milling operations are shown in the figure
Classification Of Milling
1-Peripheral Milling
In peripheral (or slab) milling, the milled surface is generated by teeth located on the periphery of the cutter body. The axis of cutter rotation is generally in a plane parallel to the workpiece surface to be machined.
In peripheral (or slab) milling, the milled surface is generated by teeth located on the periphery of the cutter body. The axis of cutter rotation is generally in a plane parallel to the workpiece surface to be machined.
2-Face Milling
In face milling, the cutter is mounted on a spindle having an axis of rotation perpendicular to the workpiece surface. The milled surface results from the action of cutting edges located on the periphery and face of the cutter.
3-End Milling
The cutter in end milling generally rotates on an axis vertical to the workpiece. It can be tilted to machine tapered surfaces. Cutting teeth are located on both the end face of the cutter and the periphery of the cutter body.
Thursday, November 15, 2007
Turning is the machining operation that produces cylindrical parts. In its basic form, it can be defined as the machining of an external surface:
1-With the workpiece rotating,
2-With a single-point cutting tool, and
3-With the cutting tool feeding parallel to the axis of the workpiece and at a distance that will remove the outer surface of the work.
Taper turning is practically the same, except that the cutter path is at an angle to the work axis. Similarly, in contour turning, the distance of the cutter from the work axis is varied to produce the desired shape.
Even though a single-point tool is specified, this does not exclude multiple-tool setups, which are often employed in turning. In such setups, each tool operates independently as a single-point cutter.
Adjustable cutting factors in turning:
The three primary factors in any basic turning operation are speed, feed, and depth of cut. Other factors such as kind of material and type of tool have a large influence, of course, but these three are the ones the operator can change by adjusting the controls, right at the machine.
Speed: always refers to the spindle and the workpiece. When it is stated in revolutions per minute (rpm) it tells their rotating speed. But the important figure for a particular turning operation is the surface speed, or the speed at which the workpeece material is moving past the cutting tool. It is simply the product of the rotating speed times the circumference (in feet) of the workpiece before the cut is started. It is expressed in surface feet per minute (sfpm), and it refers only to the workpiece. Every different diameter on a workpiece will have a different cutting speed, even though the rotating speed remains the same.
Feed: always refers to the cutting tool, and it is the rate at which the tool advances along its cutting path. On most power-fed lathes, the feed rate is directly related to the spindle speed and is expressed in inches (of tool advance) per revolution ( of the spindle), or ipr. The figure, by the way, is usually much less than an inch and is shown as decimal amount.
Depth of Cut: is practically self explanatory. It is the thickness of the layer being removed from the workpiece or the distance from the uncut surface of the work to the cut surface, expressed in inches. It is important to note, though, that the diameter of the workpiece is reduced by two times the depth of cut because this layer is being removed from both sides of the work.
1-With the workpiece rotating,
2-With a single-point cutting tool, and
3-With the cutting tool feeding parallel to the axis of the workpiece and at a distance that will remove the outer surface of the work.
Taper turning is practically the same, except that the cutter path is at an angle to the work axis. Similarly, in contour turning, the distance of the cutter from the work axis is varied to produce the desired shape.
Even though a single-point tool is specified, this does not exclude multiple-tool setups, which are often employed in turning. In such setups, each tool operates independently as a single-point cutter.
Adjustable cutting factors in turning:
The three primary factors in any basic turning operation are speed, feed, and depth of cut. Other factors such as kind of material and type of tool have a large influence, of course, but these three are the ones the operator can change by adjusting the controls, right at the machine.
Speed: always refers to the spindle and the workpiece. When it is stated in revolutions per minute (rpm) it tells their rotating speed. But the important figure for a particular turning operation is the surface speed, or the speed at which the workpeece material is moving past the cutting tool. It is simply the product of the rotating speed times the circumference (in feet) of the workpiece before the cut is started. It is expressed in surface feet per minute (sfpm), and it refers only to the workpiece. Every different diameter on a workpiece will have a different cutting speed, even though the rotating speed remains the same.
Feed: always refers to the cutting tool, and it is the rate at which the tool advances along its cutting path. On most power-fed lathes, the feed rate is directly related to the spindle speed and is expressed in inches (of tool advance) per revolution ( of the spindle), or ipr. The figure, by the way, is usually much less than an inch and is shown as decimal amount.
Depth of Cut: is practically self explanatory. It is the thickness of the layer being removed from the workpiece or the distance from the uncut surface of the work to the cut surface, expressed in inches. It is important to note, though, that the diameter of the workpiece is reduced by two times the depth of cut because this layer is being removed from both sides of the work.
Wednesday, November 14, 2007
The turning machines are, of course, every kinds of lathes. Lathes used in manufacturing can be classified as engine, turret, automatics, and numerical control etc.
1-Centre Lathe:
The term Centre Lathe is derived from the fact that in its operation the lathe holds a piece of material between two rigid supports called centres, or by some other device such as a chuck or faceplate which revolves about the centre line of the lathe.
The lathe shown above is a typical example. This machine is usually used in a jobbing (one off) situation or for small batch work where it would be too expensive to specially 'tool up' for just a few items.
The lathe on which you will work is a machine used to cut metal. The spindle carrying the work is rotated whilst a cutting tool, which is supported in a tool post, is made to travel in a certain direction depending on the form of surface required. If the tool moves parallel to the axis of the rotation of the work a cylindrical surface is produced as in Fig 2 (a) , whilst if it moves at right angles to this axis it produces a flat surface as in Fig 2 (b).
The lathe can also be used for the purposes shown in Fig 2c, 2d, 2e and 2f.
2-Turret Lathes:
In a turret lathe, a longitudinally feedable, hexagon turret replaces the tailstock.
The turret, on which six tools can be mounted, can be rotated about a vertical axis to bring each tool into operating position, and the entire unit can be moved longitudinally, either annually or by power, to provide feed for the tools. When the turret assembly is backed away from the spindle by means of a capstan wheel, the turret indexes automatically at the end of its movement thus bringing each of the six tools into operating position. The square turret on the cross slide can be rotated manually about a vertical axis to bring each of the four tools into operating position. On most machines, the turret can be moved transversely, either manually or by power, by means of the cross slide, and longitudinally through power or manual operation of the carriage. In most cased, a fixed tool holder also is added to the back end of the cross slide; this often carries a parting tool.
Through these basic features of a turret lathe, a number of tools can be set on the machine and then quickly be brought successively into working position so that a complete part can be machined without the necessity for further adjusting, changing tools, or making measurements.
Single-Spindle Automatic Screw Machines:
There are two common types of single-spindle screw machines, One, an American development and commonly called the turret type (Brown & Sharp), is shown in the
following figure. The other is of Swiss origin and is referred to as the swiss type. The Brown & Sharp screw machine is essentially a small automatic turret lathe, designed for bar stock, with the main turret mounted on the cross slide. All motions of the turret, cross slide, spindle, chuck, and stock-feed mechanism are controlled by cams. The turret cam is essentially a program that defines the movement of the turret during a cycle. These machines usually are equipped with an automatic rod feeding magazine that feeds a new length of bar stock into the collect as soon as one rod is completely used.
3-CNC Machines:
Nowadays, more and more Computer Numerical Controlled (CNC) machines are being used in every kinds of manufacturing processes. In a CNC machine, functions like program storage, tool offset and tool compensation, program-editing capability, various degree of computation, and the ability to send and receive data from a variety of sources, including remote locations can be easily realized through on board computer. The computer can store multiple-part programs, recalling them as needed for different parts. A CNC turret lathe in Michigan Technological University is shown in the following picture.
1-Centre Lathe:
The term Centre Lathe is derived from the fact that in its operation the lathe holds a piece of material between two rigid supports called centres, or by some other device such as a chuck or faceplate which revolves about the centre line of the lathe.
The lathe shown above is a typical example. This machine is usually used in a jobbing (one off) situation or for small batch work where it would be too expensive to specially 'tool up' for just a few items.
The lathe on which you will work is a machine used to cut metal. The spindle carrying the work is rotated whilst a cutting tool, which is supported in a tool post, is made to travel in a certain direction depending on the form of surface required. If the tool moves parallel to the axis of the rotation of the work a cylindrical surface is produced as in Fig 2 (a) , whilst if it moves at right angles to this axis it produces a flat surface as in Fig 2 (b).
The lathe can also be used for the purposes shown in Fig 2c, 2d, 2e and 2f.
2-Turret Lathes:
In a turret lathe, a longitudinally feedable, hexagon turret replaces the tailstock.
The turret, on which six tools can be mounted, can be rotated about a vertical axis to bring each tool into operating position, and the entire unit can be moved longitudinally, either annually or by power, to provide feed for the tools. When the turret assembly is backed away from the spindle by means of a capstan wheel, the turret indexes automatically at the end of its movement thus bringing each of the six tools into operating position. The square turret on the cross slide can be rotated manually about a vertical axis to bring each of the four tools into operating position. On most machines, the turret can be moved transversely, either manually or by power, by means of the cross slide, and longitudinally through power or manual operation of the carriage. In most cased, a fixed tool holder also is added to the back end of the cross slide; this often carries a parting tool.Through these basic features of a turret lathe, a number of tools can be set on the machine and then quickly be brought successively into working position so that a complete part can be machined without the necessity for further adjusting, changing tools, or making measurements.
Single-Spindle Automatic Screw Machines:
There are two common types of single-spindle screw machines, One, an American development and commonly called the turret type (Brown & Sharp), is shown in the
following figure. The other is of Swiss origin and is referred to as the swiss type. The Brown & Sharp screw machine is essentially a small automatic turret lathe, designed for bar stock, with the main turret mounted on the cross slide. All motions of the turret, cross slide, spindle, chuck, and stock-feed mechanism are controlled by cams. The turret cam is essentially a program that defines the movement of the turret during a cycle. These machines usually are equipped with an automatic rod feeding magazine that feeds a new length of bar stock into the collect as soon as one rod is completely used. 3-CNC Machines:
Nowadays, more and more Computer Numerical Controlled (CNC) machines are being used in every kinds of manufacturing processes. In a CNC machine, functions like program storage, tool offset and tool compensation, program-editing capability, various degree of computation, and the ability to send and receive data from a variety of sources, including remote locations can be easily realized through on board computer. The computer can store multiple-part programs, recalling them as needed for different parts. A CNC turret lathe in Michigan Technological University is shown in the following picture.
Monday, November 12, 2007
Accessories
The devices employed for holding and supporting the work and the tool on the lathe are called accerssries. They incluedethe device like chucks, driving plat ,dogs,toolholder,andposts,centers,mandrels,jigs and fixtures ,etc the selection of the accessories is governed by the type of the job to be made.
1-Workpiece holding accessories:
A) Chucks:
1-Three-jaw:
A three-jaw chuck is a rotating clamp which uses three or 'jaws', usually interconnected via a scroll gear (scroll plate), to hold onto a tool or work piece. Three-jaw chucks are usually self-centering (as a result of the jaws' meshing with the scroll plate) and are best suited to grip circular or hexagonal cross sections when very fast, reasonably accurate (±.005" TIR) centering is desired. Independent-jaw versions can be obtained.
The image shows a three-jaw chuck and key with one jaw removed and inverted showing the teeth that engage in the scroll plate. The scroll plate is rotated within the chuck body by the key, the scroll engages the teeth on the underside of the jaws which moves the three jaws in unison, to tighten or release the workpiece.
2-Four-jaw:
A four-jaw chuck is similar to a three-jaw chuck, but with four jaws, each of which can be moved independently. This makes them ideal for (a) gripping non-circular cross sections and (b) gripping circular cross sections with extreme precision (when the last few hundredths of a millimeter [or thousandths of an inch] of runout must be manually eliminated). The non-self-centering action of the independent jaws makes centering highly controllable (for an experienced user), but at the expense of speed and ease. Four-jaw chucks are almost never used for tool holding. Four-jaw chucks can be found on lathes and indexing heads.
The image shows a four-jaw chuck with the jaws independently set. The key is used to adjust each jaw separately.
3-Multi jaw:
For special purposes, and also the holding of fragile materials, chucks are available with six or eight jaws. These are invariably of the self-centering design, and are built to very high standards of accuracy.
Two jaw chucks are available and can be used with soft jaws (typically an aluminum alloy) that can be machined to conform to a particular workpiece. Many chucks have removable jaws, which allows the user to replace them with new jaws, specialized jaws, or soft jaws.
4-Self-centering four jaw:
A four jaw chuck with a mechanism for centering the work piece. Sometimes used to refer to chucks where the jaws are moved in interconnected pairs.
5-Magnetic:
Used for holding ferromagnetic work pieces, a magnetic chuck consists of an accurately centered permanent magent face. Electromagnets or permanent magnets are brought into contact with fixed ferrous plates, or 'pole pieces', contained within a housing. These pole pieces are usually flush with the housing surface. The part or 'work piece' to be held forms the closing of the magnetic loop or path, onto those fixed plates, providing a secure anchor for the work piece.
6-Face plates:
For irregular shapes, a face plate can be used. The face plate shown in Fig.2 has racially placed slots which allow the workpiece to clamp to it by means of bolts.
B) lathe centers:
For accurate turning operation, or in cases where the work surface is not truly cylindrical, the workpiece can be turned between centers. This form of work holding is show in fig.3 initially the workpiece has a conical hole drilled at each end to provide location for the lathe centers.
1- Dead center:
A dead center (one that does not turn freely, ie: - dead) may be used to support the workpiece at either the fixed or rotating end of the machine. When used in the fixed position, a dead center produces friction between the workpiece and center, due to the rotation of the workpiece. Lubrication is therefore required between the center and workpiece to prevent friction welding from occurring. Additionally the tip of the center may have an insert of carbide which will reduce the friction slightly and allow for faster speeds. Dead centers may also be fully hardened to prevent damage to the important mating surfaces taper of the taper and to preserve the 60 ° nose taper.
2-Soft center:
Soft centers are identical to dead centers except the nose is deliberately left soft (unhardened) so that it may be readily machined to the correct angle prior to usage. This operation is performed on the headstock center to ensure that the centers axis is aligned with the spindles axis.
3-Live or revolving center:
A live center or revolving center is constructed so that the 60 ° center runs in its own bearings and is used at the non driven or tailstock end of a machine. It allows higher turning speeds without the need for separate lubrication, and also greater clamping pressures. They are used almost exclusively in CNC lathes as well as for general machining operations.
The term live center may also refer to a dead center when mounted in the spindle of the machine, where it is considered to be live by virtue of the spindle bearings rather than its own bearings.
4-Pipe center:
A pipe center has a larger diameter at the 60 ° taper end. This allows the center to be used in the bore of a pipe (or similar workpiece). While a pipe center ensures the workpiece remains concentric, its main advantage is that it supports the workpiece securely. Thin walled material such as pipes easily collapse if excessive pressures are used at the chuck end.
5-Cup center:
The cup center is a variation of the live center and is used in woodworking to support the softer material around the actual center and prevent the material splitting.
6-Drive center:
A drive center is used in the driving end of the machine (headstock). It consists of a dead center surrounded by hardened teeth. These teeth bite into the softer workpiece allowing the workpiece to be driven directly by the center. This allows the full diameter of the workpiece to be machined in a single operation, this contrasts with the usual requirement where a carrier is attached to the workpiece at the driven end. They are often used in woodworking or where softer materials are machined
C) Mandrel:
It is a device for holding and rotation a hollow piece of work that has been previously drilled or bored. The work revolves with the mandrel which is mounted between two centers. The mandrel should be true with accurate centre holes for machining outer surface of the workpiece, concentric with the bore. To avoid distortion and wear, it is made of high carbon steel. For different sizes of holes in workpiece, different mandrels.
D) rests:
When very long job is to be turned between centers on lather, due to its own weight it
1-Workpiece holding accessories:
A) Chucks:
1-Three-jaw:
A three-jaw chuck is a rotating clamp which uses three or 'jaws', usually interconnected via a scroll gear (scroll plate), to hold onto a tool or work piece. Three-jaw chucks are usually self-centering (as a result of the jaws' meshing with the scroll plate) and are best suited to grip circular or hexagonal cross sections when very fast, reasonably accurate (±.005" TIR) centering is desired. Independent-jaw versions can be obtained.
The image shows a three-jaw chuck and key with one jaw removed and inverted showing the teeth that engage in the scroll plate. The scroll plate is rotated within the chuck body by the key, the scroll engages the teeth on the underside of the jaws which moves the three jaws in unison, to tighten or release the workpiece.
2-Four-jaw:
A four-jaw chuck is similar to a three-jaw chuck, but with four jaws, each of which can be moved independently. This makes them ideal for (a) gripping non-circular cross sections and (b) gripping circular cross sections with extreme precision (when the last few hundredths of a millimeter [or thousandths of an inch] of runout must be manually eliminated). The non-self-centering action of the independent jaws makes centering highly controllable (for an experienced user), but at the expense of speed and ease. Four-jaw chucks are almost never used for tool holding. Four-jaw chucks can be found on lathes and indexing heads.
The image shows a four-jaw chuck with the jaws independently set. The key is used to adjust each jaw separately.
3-Multi jaw:
For special purposes, and also the holding of fragile materials, chucks are available with six or eight jaws. These are invariably of the self-centering design, and are built to very high standards of accuracy.
Two jaw chucks are available and can be used with soft jaws (typically an aluminum alloy) that can be machined to conform to a particular workpiece. Many chucks have removable jaws, which allows the user to replace them with new jaws, specialized jaws, or soft jaws.
4-Self-centering four jaw:
A four jaw chuck with a mechanism for centering the work piece. Sometimes used to refer to chucks where the jaws are moved in interconnected pairs.
5-Magnetic:
Used for holding ferromagnetic work pieces, a magnetic chuck consists of an accurately centered permanent magent face. Electromagnets or permanent magnets are brought into contact with fixed ferrous plates, or 'pole pieces', contained within a housing. These pole pieces are usually flush with the housing surface. The part or 'work piece' to be held forms the closing of the magnetic loop or path, onto those fixed plates, providing a secure anchor for the work piece.
6-Face plates:
For irregular shapes, a face plate can be used. The face plate shown in Fig.2 has racially placed slots which allow the workpiece to clamp to it by means of bolts.
B) lathe centers:
For accurate turning operation, or in cases where the work surface is not truly cylindrical, the workpiece can be turned between centers. This form of work holding is show in fig.3 initially the workpiece has a conical hole drilled at each end to provide location for the lathe centers.
1- Dead center:
A dead center (one that does not turn freely, ie: - dead) may be used to support the workpiece at either the fixed or rotating end of the machine. When used in the fixed position, a dead center produces friction between the workpiece and center, due to the rotation of the workpiece. Lubrication is therefore required between the center and workpiece to prevent friction welding from occurring. Additionally the tip of the center may have an insert of carbide which will reduce the friction slightly and allow for faster speeds. Dead centers may also be fully hardened to prevent damage to the important mating surfaces taper of the taper and to preserve the 60 ° nose taper.
2-Soft center:
Soft centers are identical to dead centers except the nose is deliberately left soft (unhardened) so that it may be readily machined to the correct angle prior to usage. This operation is performed on the headstock center to ensure that the centers axis is aligned with the spindles axis.
3-Live or revolving center:
A live center or revolving center is constructed so that the 60 ° center runs in its own bearings and is used at the non driven or tailstock end of a machine. It allows higher turning speeds without the need for separate lubrication, and also greater clamping pressures. They are used almost exclusively in CNC lathes as well as for general machining operations.
The term live center may also refer to a dead center when mounted in the spindle of the machine, where it is considered to be live by virtue of the spindle bearings rather than its own bearings.
4-Pipe center:
A pipe center has a larger diameter at the 60 ° taper end. This allows the center to be used in the bore of a pipe (or similar workpiece). While a pipe center ensures the workpiece remains concentric, its main advantage is that it supports the workpiece securely. Thin walled material such as pipes easily collapse if excessive pressures are used at the chuck end.
5-Cup center:
The cup center is a variation of the live center and is used in woodworking to support the softer material around the actual center and prevent the material splitting.
6-Drive center:
A drive center is used in the driving end of the machine (headstock). It consists of a dead center surrounded by hardened teeth. These teeth bite into the softer workpiece allowing the workpiece to be driven directly by the center. This allows the full diameter of the workpiece to be machined in a single operation, this contrasts with the usual requirement where a carrier is attached to the workpiece at the driven end. They are often used in woodworking or where softer materials are machined
C) Mandrel:
It is a device for holding and rotation a hollow piece of work that has been previously drilled or bored. The work revolves with the mandrel which is mounted between two centers. The mandrel should be true with accurate centre holes for machining outer surface of the workpiece, concentric with the bore. To avoid distortion and wear, it is made of high carbon steel. For different sizes of holes in workpiece, different mandrels.
D) rests:
When very long job is to be turned between centers on lather, due to its own weight it
provides a springing action and carries a lot of bending moment. The result is that turning tool is spoiled very soon, and may even break sometimes. To avoid this, such jobs are always supported on an attachment known as "steady rest" .this prevent the deflection of the job and at the same time enables the operator to take heavy cuts.fig.5 shows the steady rest and follower rests.
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