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CHAPTER CONTENTS
93
6 MACHINING OPERATIONS
6.1 Turning
6.2 Milling
6.3 Drilling and Reaming
6.4 Planing, Shaping and Broaching
6.5 Boring
6.6 Gear Manufacturing
Introduction
Turning is a machining process to produce parts round in shape by a single point tool on lathes. The tool
is fed either linearly in the direction parallel or perpendicular to the axis of rotation of the workpiece, or
along a specified path to produce complex rotational shapes. The primary motion of cutting in turning is
the rotation of the workpiece, and the secondary motion of cutting is the feed motion.
Cutting conditions in turning
Cutting speed in turning V in m/s is related to the rotational speed of the workpiece by the equation:
V = πDN
where D is the diameter of the workpiece, m; N is the rotational speed of the workpiece, rev/s.
direction of feed motion
tool
workpiece
machined surface
work surface
transient surface
chip
direction of primary motion
f
d
D
D
o
f
Turning operation
6.1 TURNING
94 Turning Valery Marinov, Manufacturing Technology
One should remember that cutting speed V is always a linear vector. In the process planning of a
turning operation, cutting speed V is first selected from appropriate reference sources or calculated
as discussed in Section 5.10 Selection of Cutting Conditions, and the rotational speed N is calculated
taking into account the workpiece diameter D. Rotational speed, not cutting speed, is then used to
adjust lathe setting levers.
Feed in turning is generally expressed in mm tr-1 (millimetres per revolution).
The turning operation reduces the diameter of the workpiece from the initial diameter Do to the final
diameter Df. The change in diameter is actually two times depth of cut, d:
2d = Do - Df
The volumetric rate of material removal (so-called material removal rate, mrr) is defined by
mrr = Vfd
When using this equation, care must be exercised to assure that the units for V are consistent with
those for f and d.
Operations in turning
Turning is not a single process but class of many and different operations performed on a lathe.
Turning of cylindrical surfaces
The lathe can be used to reduce the diameter of a part to a desired dimension. The resulting machined
surface is cylindrical.
V
f
V
f f
V
facing tube turning parting (cutting-off)
Turning of flat surfaces
A lathe can be used to create a smooth, flat face very accurately perpendicular to the axis of a cylindrical
part. Tool is fed radially or axially to create a flat machined surface.
V
f
V
f
straight turning plunge turning
Threading
Different possibilities are available to produce a thread on a lathe. Threads are cut using lathes by
advancing the cutting tool at a feed exactly equal to the thread pitch. The single-point cutting tool cuts
in a helical band, which is actually a thread. The procedure calls for correct settings of the machine,
and also that the helix be restarted at the same location each time if multiple passes are required to
cut the entire depth of thread. The tool point must be ground so that it has the same profile as the
thread to be cut.
Valery Marinov, Manufacturing Technology Turning 95
Another possibility is to cut threads by means of a thread die (external threads), or a tap (internal
threads). These operations are generally performed manually for smal thread diameters.
f
V
threading die threading tap threading
Form turning
Cutting tool has a shape that is imparted to the workpiece by plunging the tool into the workpiece.
In form turning, cutting tool is complex and expensive but feed is linear and does not require special
machine tools or devices.
V
f
V
f
V
f
forming plunge grooving face grooving
Contour turning (profiling)
Cutting tool has a simple shape, but the feed motion is complex; cutting tool is fed along a contour thus
creating a contoured shape on the workpiece. For profiling, special lathes or devices are required.
V
f
V
f
contour turning (profiling) taper turning
Producing tapers on a lathe is a specific task and contour turning is just one of the possible solutions.
Some other methods for turning tapers are discussed later.
96 Turning Valery Marinov, Manufacturing Technology
Lathes
A lathe is a machine tool that rotates the workpiece against a tool whose position it controls. The spindle
(see picture in the next page) is the part of the lathe that rotates. Various work holding attachments
such as three jaw chucks, collets, and centers can be held in the spindle. The spindle is driven by an
electric motor through a system of belt drives and gear trains. Spindle rotational speed is controlled by
varying the geometry of the drive train.
The tailstock can be used to support the end of the workpiece with a center, or to hold tools for drilling,
reaming, threading, or cutting tapers. It can be adjusted in position along the ways to accommodate
different length workpieces. The tailstock barrel can be fed along the axis of rotation with the tailstock
hand wheel.
The carriage controls and supports the cutting tool. It consists of:
v a saddle that slides along the ways;
v an apron that controls the feed mechanisms;
v a cross slide that controls transverse motion of the tool (toward or away from the
operator);
v a tool compound that adjusts to permit angular tool movement;
v a tool post that holds the cutting tools.
There are a number of different lathe designs, and some of the most popular are discussed here.
Miscellaneous operations
Some other operations, which do not use the single-point cutting tool can be performed on a lathe,
making turning one of the most versatile machining processes.
V
f
V V
f f
drilling internal grooving boring
Knurling
This is not a machining operation at all, because it does not involve material removal. Instead, it is a
metal forming operation used to produce a regular crosshatched pattern in the work surface.
(Left) Knurling operation; (Right) Knurling tool and knurling
wheel. Wheels with different patterns are easily available.
Valery Marinov, Manufacturing Technology Turning 97
Engine lathes
The basic, simplest and most versatile lathe. This machine tool is manually operated that is why it
requires skilled operators. Suitable for low and medium production, and for repair works.
Work is held in the lathe with a number of methods,
v Between two centres. The workpiece is driven by a device called a dog; The method is
suitable for parts with high length-to-diameter ratio.
v A 3 jaw self-centering chuck is used for most operations on cylindrical workparts. For
parts with high length-to-diameter ratio the part is supported by center on the other end.
v Collet consists of tubular bushing with longitudinal slits. Collets are used to grasp and
hold barstock. A collet of exact diameter is required to match any barstock diameter.
v A face plate is a device used to grasp parts with irregular shapes:
There are two tool feed mechanism in the engine lathes. These cause the cutting tool to move when
engaged.
ΠThe lead screw will cause the apron and cutting tool to advance quickly. This is used for
cutting threads, and for moving the tool quickly.
The feed rod will move the apron and cutting tool slowly forward. This is largely used
for most of the turning operations.
Four work holding methods used in lathes: (a) mounting the work between centers using a dog, (b) three-jaw chuck, (c)
collet, and (d) face plate for noncylindrical workparts.
The principal components of an engine lathe
98 Turning Valery Marinov, Manufacturing Technology
Turning tapers on engine lathes
A taper is a conical shape. Tapers can be cut with lathes quite easily. There are some common methods
for turning tapers on an engine lathe,
v Using a form tool: This type of tool is specifically designed for one cut, at a certain
taper angle. The tool is plunged at one location, and never moved along the lathe slides.
v Compound Slide Method: The compound slide is set to travel at half of the taper
angle. The tool is then fed across the work by hand, cutting the taper as it goes.
v Off-Set Tail Stock: In this method the normal rotating part of the lathe still drives
the workpiece (mounted between centres), but the centre at the tailstock is offset
towards/away from the cutting tool. Then, as the cutting tool passes over, the part is
cut in a conical shape. This method is limited to small tapers over long lengths.
The tailstock offset h is defined by
h = Lsinα
where L is the length of workpiece, and α is the half of the taper angle.
Three methods for turning tapers on an engine lathe: (a) using a form tool, (b) the
compound slide method, and (c) offsetting tailstock.
(a) (b) (c)
(Left) Turret lathe; (Right) Close-up view of a turret lathe showing a set of three octagonal turrets with a
total number of 24 different cutting tools, and the bar workpiece held in a collet.
Turret lathes
These machines are capable of carrying out multiple cutting operations on the same workpiece. Several
cutting tools are mounted on a tetra-, penta-, or hexagonal turret, which replaces the tailstock. These
tools can be rapidly brought into action against the workpiece one by one by indexing the turret. In
some machines four additional tools are mounted in a square turret on the cross slide, or two or three
more tools are mounted in tool posts on several cross slides. Turret lathes are used for high-production
work. The up-to-date lathes are numerically controlled as discussed later.
Valery Marinov, Manufacturing Technology Turning 99
Single-spindle and multi-spindle bar machines
In these machines, instead of a chuck, a collet is used, which permits long bar stock to be fed through
the headstock into position. At the end of each machining cycle, a cutoff operation separates the new
part. Owing to the high level of automation, the term automatic bar machine is often used for these
machines. Bar machines can be classified as single spindle or multiple spindle. The single-spindle bar
machine is sometimes referred to as swiss automatics.
The single-spindle bar machine has up to six upper cross slides and two horizontal cross slides with cutting
tools, which move radially inwards. All operations on the machine are controlled by appropriately
shaped cams. The machine is usually equipped with three-spindle drilling/threading turret, or with a
multi-position turret. More recent machines are numerically but not cam controlled.
Schematics showing the principal components of a single-spindle
bar machine with two cross slides, one horizontal cross slide and a
hexagonal turret (cutting tools are only shown on the upper cross
slides)
A close-up view of a single spindle bar machine. Typical parts produced on a single spindle bar
machine.
100 Turning Valery Marinov, Manufacturing Technology
To increase production rate, multiple-spindle bar machines are available. A spindle carrier in which four
to eight spindles feed and rotate as many bars replaces the headstock of the lathe. A tetra-, hexa-, or
octagonal axial tool slide on which tool holders are mounted replaces the tailstock. Additional tools
are engaged radially, mounted on lower cross slides. So, multiple parts are machined simultaneously by
multiple tools. At the end of each machining cycle, the spindles are indexed to the next set of cutting
tools. A single part is completed at each indexing of the spindle carrier.
The principle components of a six-spindle bar machine CNC controlled multiple-spindle bar machine
Computer-controlled lathes (CNC lathes)
Computer-controlled (numerically controlled, NC, CNC) lathes incorporate a computer system to control
the movements of machine components by directly inserted coded instructions in the form of numerical
data. A CNC lathe is especially useful in contour turning operations and precise machining. There are
also not chuck but bar modifications. A CNC lathe is essentially a turret lathe. The major advantage
of these machines is in their versatility - to adjust the CNC lathe for a different part to be machined
requires a simple change in the computer program and, in some cases, a new set of cutting tools.
CNC chuck lathe Ten-position turret of a CNC lathe
Valery Marinov, Manufacturing Technology Turning 101
Cutting tools
The geometry and nomenclature of cutting tools used in turning is standardized by ISO 3002/1-1982:
Cutting edges, surfaces and angles on the cutting part of a turning tool
The figure shows only the most important geometrical features of a turning cutting tool. Recommendations
for proper selection of the cutting tool geometry are available in the reference materials.
Cutting tool are available in different brazed or clamped designs for different operations. Some of the
clamped tools are shown in the figures:
Cutting tool for straigth turning Cutting tool for grooving
Cutting tool for profiling Cutting tool for threading
102 Turning Valery Marinov, Manufacturing Technology
Process capabilities and process planning in turning
The general steps when turning external workpart hold in a chuck should follow the next sequence,
ΠFirst rough cuts are applied on all surfaces, starting with the cylindrical surfaces (largest
diameters first) and then proceeding with all faces;
Special operations such as knurling and grooving (if any) are applied;
Ž Diameters are finished first, then the faces. The maximum surface finish if turning steel
is Ra ~ 1.6 μm. If higher surface finish is required, grinding should follow machining.
Grinding and other finishing operations are discussed in Chapter 7;
External threads (if any) are cut;
Deburring is applied, if necessary.
If the part is to be mounted between centres, plan should precede by,
ΠThe workpiece is hold in a chuck, and the face is squired;
A centre hole is drilled using a center drill (Section 6.3);
Ž The workpiece is reversed in the chuck. Steps Œ and are repeated for the other face;
The workpiece is mounted between centres and the general plan is followed.
If the workpart has a central hole, the hole is drilled starting with a centre drill, and increasing drill
diameters gradually. Finally, boring is applied (Section 6.5) to achieve the final diameter of the hole.
Machining of the internal features is scheduled after rough cuts and before special operations (after
step Πin the general plan).
93
6 MACHINING OPERATIONS
6.1 Turning
6.2 Milling
6.3 Drilling and Reaming
6.4 Planing, Shaping and Broaching
6.5 Boring
6.6 Gear Manufacturing
Introduction
Turning is a machining process to produce parts round in shape by a single point tool on lathes. The tool
is fed either linearly in the direction parallel or perpendicular to the axis of rotation of the workpiece, or
along a specified path to produce complex rotational shapes. The primary motion of cutting in turning is
the rotation of the workpiece, and the secondary motion of cutting is the feed motion.
Cutting conditions in turning
Cutting speed in turning V in m/s is related to the rotational speed of the workpiece by the equation:
V = πDN
where D is the diameter of the workpiece, m; N is the rotational speed of the workpiece, rev/s.
direction of feed motion
tool
workpiece
machined surface
work surface
transient surface
chip
direction of primary motion
f
d
D
D
o
f
Turning operation
6.1 TURNING
94 Turning Valery Marinov, Manufacturing Technology
One should remember that cutting speed V is always a linear vector. In the process planning of a
turning operation, cutting speed V is first selected from appropriate reference sources or calculated
as discussed in Section 5.10 Selection of Cutting Conditions, and the rotational speed N is calculated
taking into account the workpiece diameter D. Rotational speed, not cutting speed, is then used to
adjust lathe setting levers.
Feed in turning is generally expressed in mm tr-1 (millimetres per revolution).
The turning operation reduces the diameter of the workpiece from the initial diameter Do to the final
diameter Df. The change in diameter is actually two times depth of cut, d:
2d = Do - Df
The volumetric rate of material removal (so-called material removal rate, mrr) is defined by
mrr = Vfd
When using this equation, care must be exercised to assure that the units for V are consistent with
those for f and d.
Operations in turning
Turning is not a single process but class of many and different operations performed on a lathe.
Turning of cylindrical surfaces
The lathe can be used to reduce the diameter of a part to a desired dimension. The resulting machined
surface is cylindrical.
V
f
V
f f
V
facing tube turning parting (cutting-off)
Turning of flat surfaces
A lathe can be used to create a smooth, flat face very accurately perpendicular to the axis of a cylindrical
part. Tool is fed radially or axially to create a flat machined surface.
V
f
V
f
straight turning plunge turning
Threading
Different possibilities are available to produce a thread on a lathe. Threads are cut using lathes by
advancing the cutting tool at a feed exactly equal to the thread pitch. The single-point cutting tool cuts
in a helical band, which is actually a thread. The procedure calls for correct settings of the machine,
and also that the helix be restarted at the same location each time if multiple passes are required to
cut the entire depth of thread. The tool point must be ground so that it has the same profile as the
thread to be cut.
Valery Marinov, Manufacturing Technology Turning 95
Another possibility is to cut threads by means of a thread die (external threads), or a tap (internal
threads). These operations are generally performed manually for smal thread diameters.
f
V
threading die threading tap threading
Form turning
Cutting tool has a shape that is imparted to the workpiece by plunging the tool into the workpiece.
In form turning, cutting tool is complex and expensive but feed is linear and does not require special
machine tools or devices.
V
f
V
f
V
f
forming plunge grooving face grooving
Contour turning (profiling)
Cutting tool has a simple shape, but the feed motion is complex; cutting tool is fed along a contour thus
creating a contoured shape on the workpiece. For profiling, special lathes or devices are required.
V
f
V
f
contour turning (profiling) taper turning
Producing tapers on a lathe is a specific task and contour turning is just one of the possible solutions.
Some other methods for turning tapers are discussed later.
96 Turning Valery Marinov, Manufacturing Technology
Lathes
A lathe is a machine tool that rotates the workpiece against a tool whose position it controls. The spindle
(see picture in the next page) is the part of the lathe that rotates. Various work holding attachments
such as three jaw chucks, collets, and centers can be held in the spindle. The spindle is driven by an
electric motor through a system of belt drives and gear trains. Spindle rotational speed is controlled by
varying the geometry of the drive train.
The tailstock can be used to support the end of the workpiece with a center, or to hold tools for drilling,
reaming, threading, or cutting tapers. It can be adjusted in position along the ways to accommodate
different length workpieces. The tailstock barrel can be fed along the axis of rotation with the tailstock
hand wheel.
The carriage controls and supports the cutting tool. It consists of:
v a saddle that slides along the ways;
v an apron that controls the feed mechanisms;
v a cross slide that controls transverse motion of the tool (toward or away from the
operator);
v a tool compound that adjusts to permit angular tool movement;
v a tool post that holds the cutting tools.
There are a number of different lathe designs, and some of the most popular are discussed here.
Miscellaneous operations
Some other operations, which do not use the single-point cutting tool can be performed on a lathe,
making turning one of the most versatile machining processes.
V
f
V V
f f
drilling internal grooving boring
Knurling
This is not a machining operation at all, because it does not involve material removal. Instead, it is a
metal forming operation used to produce a regular crosshatched pattern in the work surface.
(Left) Knurling operation; (Right) Knurling tool and knurling
wheel. Wheels with different patterns are easily available.
Valery Marinov, Manufacturing Technology Turning 97
Engine lathes
The basic, simplest and most versatile lathe. This machine tool is manually operated that is why it
requires skilled operators. Suitable for low and medium production, and for repair works.
Work is held in the lathe with a number of methods,
v Between two centres. The workpiece is driven by a device called a dog; The method is
suitable for parts with high length-to-diameter ratio.
v A 3 jaw self-centering chuck is used for most operations on cylindrical workparts. For
parts with high length-to-diameter ratio the part is supported by center on the other end.
v Collet consists of tubular bushing with longitudinal slits. Collets are used to grasp and
hold barstock. A collet of exact diameter is required to match any barstock diameter.
v A face plate is a device used to grasp parts with irregular shapes:
There are two tool feed mechanism in the engine lathes. These cause the cutting tool to move when
engaged.
ΠThe lead screw will cause the apron and cutting tool to advance quickly. This is used for
cutting threads, and for moving the tool quickly.
The feed rod will move the apron and cutting tool slowly forward. This is largely used
for most of the turning operations.
Four work holding methods used in lathes: (a) mounting the work between centers using a dog, (b) three-jaw chuck, (c)
collet, and (d) face plate for noncylindrical workparts.
The principal components of an engine lathe
98 Turning Valery Marinov, Manufacturing Technology
Turning tapers on engine lathes
A taper is a conical shape. Tapers can be cut with lathes quite easily. There are some common methods
for turning tapers on an engine lathe,
v Using a form tool: This type of tool is specifically designed for one cut, at a certain
taper angle. The tool is plunged at one location, and never moved along the lathe slides.
v Compound Slide Method: The compound slide is set to travel at half of the taper
angle. The tool is then fed across the work by hand, cutting the taper as it goes.
v Off-Set Tail Stock: In this method the normal rotating part of the lathe still drives
the workpiece (mounted between centres), but the centre at the tailstock is offset
towards/away from the cutting tool. Then, as the cutting tool passes over, the part is
cut in a conical shape. This method is limited to small tapers over long lengths.
The tailstock offset h is defined by
h = Lsinα
where L is the length of workpiece, and α is the half of the taper angle.
Three methods for turning tapers on an engine lathe: (a) using a form tool, (b) the
compound slide method, and (c) offsetting tailstock.
(a) (b) (c)
(Left) Turret lathe; (Right) Close-up view of a turret lathe showing a set of three octagonal turrets with a
total number of 24 different cutting tools, and the bar workpiece held in a collet.
Turret lathes
These machines are capable of carrying out multiple cutting operations on the same workpiece. Several
cutting tools are mounted on a tetra-, penta-, or hexagonal turret, which replaces the tailstock. These
tools can be rapidly brought into action against the workpiece one by one by indexing the turret. In
some machines four additional tools are mounted in a square turret on the cross slide, or two or three
more tools are mounted in tool posts on several cross slides. Turret lathes are used for high-production
work. The up-to-date lathes are numerically controlled as discussed later.
Valery Marinov, Manufacturing Technology Turning 99
Single-spindle and multi-spindle bar machines
In these machines, instead of a chuck, a collet is used, which permits long bar stock to be fed through
the headstock into position. At the end of each machining cycle, a cutoff operation separates the new
part. Owing to the high level of automation, the term automatic bar machine is often used for these
machines. Bar machines can be classified as single spindle or multiple spindle. The single-spindle bar
machine is sometimes referred to as swiss automatics.
The single-spindle bar machine has up to six upper cross slides and two horizontal cross slides with cutting
tools, which move radially inwards. All operations on the machine are controlled by appropriately
shaped cams. The machine is usually equipped with three-spindle drilling/threading turret, or with a
multi-position turret. More recent machines are numerically but not cam controlled.
Schematics showing the principal components of a single-spindle
bar machine with two cross slides, one horizontal cross slide and a
hexagonal turret (cutting tools are only shown on the upper cross
slides)
A close-up view of a single spindle bar machine. Typical parts produced on a single spindle bar
machine.
100 Turning Valery Marinov, Manufacturing Technology
To increase production rate, multiple-spindle bar machines are available. A spindle carrier in which four
to eight spindles feed and rotate as many bars replaces the headstock of the lathe. A tetra-, hexa-, or
octagonal axial tool slide on which tool holders are mounted replaces the tailstock. Additional tools
are engaged radially, mounted on lower cross slides. So, multiple parts are machined simultaneously by
multiple tools. At the end of each machining cycle, the spindles are indexed to the next set of cutting
tools. A single part is completed at each indexing of the spindle carrier.
The principle components of a six-spindle bar machine CNC controlled multiple-spindle bar machine
Computer-controlled lathes (CNC lathes)
Computer-controlled (numerically controlled, NC, CNC) lathes incorporate a computer system to control
the movements of machine components by directly inserted coded instructions in the form of numerical
data. A CNC lathe is especially useful in contour turning operations and precise machining. There are
also not chuck but bar modifications. A CNC lathe is essentially a turret lathe. The major advantage
of these machines is in their versatility - to adjust the CNC lathe for a different part to be machined
requires a simple change in the computer program and, in some cases, a new set of cutting tools.
CNC chuck lathe Ten-position turret of a CNC lathe
Valery Marinov, Manufacturing Technology Turning 101
Cutting tools
The geometry and nomenclature of cutting tools used in turning is standardized by ISO 3002/1-1982:
Cutting edges, surfaces and angles on the cutting part of a turning tool
The figure shows only the most important geometrical features of a turning cutting tool. Recommendations
for proper selection of the cutting tool geometry are available in the reference materials.
Cutting tool are available in different brazed or clamped designs for different operations. Some of the
clamped tools are shown in the figures:
Cutting tool for straigth turning Cutting tool for grooving
Cutting tool for profiling Cutting tool for threading
102 Turning Valery Marinov, Manufacturing Technology
Process capabilities and process planning in turning
The general steps when turning external workpart hold in a chuck should follow the next sequence,
ΠFirst rough cuts are applied on all surfaces, starting with the cylindrical surfaces (largest
diameters first) and then proceeding with all faces;
Special operations such as knurling and grooving (if any) are applied;
Ž Diameters are finished first, then the faces. The maximum surface finish if turning steel
is Ra ~ 1.6 μm. If higher surface finish is required, grinding should follow machining.
Grinding and other finishing operations are discussed in Chapter 7;
External threads (if any) are cut;
Deburring is applied, if necessary.
If the part is to be mounted between centres, plan should precede by,
ΠThe workpiece is hold in a chuck, and the face is squired;
A centre hole is drilled using a center drill (Section 6.3);
Ž The workpiece is reversed in the chuck. Steps Œ and are repeated for the other face;
The workpiece is mounted between centres and the general plan is followed.
If the workpart has a central hole, the hole is drilled starting with a centre drill, and increasing drill
diameters gradually. Finally, boring is applied (Section 6.5) to achieve the final diameter of the hole.
Machining of the internal features is scheduled after rough cuts and before special operations (after
step Πin the general plan).
MILLING PROCESS-
Milling is the machining process of using rotary cutters to remove material[1] from a workpiece advancing (or feeding) in a direction at an angle with the axis of the tool.[2][3] It covers a wide variety of different operations and machines, on scales from small individual parts to large, heavy-duty gang milling operations. It is one of the most commonly used processes in industry and machine shops today for machining parts to precise sizes and shapes.
Milling is the machining process of using rotary cutters to remove material[1] from a workpiece advancing (or feeding) in a direction at an angle with the axis of the tool.[2][3] It covers a wide variety of different operations and machines, on scales from small individual parts to large, heavy-duty gang milling operations. It is one of the most commonly used processes in industry and machine shops today for machining parts to precise sizes and shapes.
Milling can be done with a wide range of machine tools. The original class of machine tools for milling was the milling machine (often called a mill). After the advent of computer numerical control (CNC), milling machines evolved into machining centers (milling machines with automatic tool changers, tool magazines or carousels, CNC control, coolant systems, and enclosures), generally classified as vertical machining centers (VMCs) and horizontal machining centers (HMCs). The integration of milling into turningenvironments and of turning into milling environments, begun with live tooling for lathes and the occasional use of mills for turning operations, led to a new class of machine tools, multitasking machines (MTMs), which are purpose-built to provide for a default machining strategy of using any combination of milling and turning within the same work envelope.
Module
4
General Purpose Machine Tools
Version 2 ME, IIT Kharagpur
Lesson
25
Estimation of machining time
Instructional objectives
At the end of this lesson, the students will be able to
(i) Realize the necessity of evaluating the machining time requirement
(ii) Identify the factors that govern the machining time.
(iii) Estimate or evaluate the time required for specific;
(a) turning operation
(b) drilling and boring operations
(c) shaping and planing operations
(d) milling operation.
(i) Necessity Of Estimation Or Determination Of Machining Time Requirement For Particular Operations.
The major aim and objectives in machining industries generally are;
• reduction of total manufacturing time, T
• increase in MRR, i.e., productivity
• reduction in machining cost without sacrificing product quality
• increase in profit or profit rate, i.e., profitability.
All those objectives are commonly and substantially governed by the total machining time per piece, Tp, where again, CpiCLTTTTTCTT=++ (4.9.1)
where, Ti = idle time per piece, min
TC= actual cutting time per piece
TL= Tool life
TCT= average tool change time per piece.
Ti and TCT could have been spectacularly reduced by development and application of modern mechanisation or automation.
The tool life, TL has been substantially enhanced by remarkable developments in the cutting tool materials.
Therefore, the actual cutting or machining time TC remains to be controlled as far as possible for achieving the objectives and meeting the growing demands.
Hence, it becomes extremely necessary to determine the actual machining time, TC required to produce a job mainly for,
• assessment of productivity
• evaluation of machining cost
• measurement of labour cost component
• assessment of relative performance or capability of any machine tool, cutting tool, cutting fluid or any special or new techniques in terms of saving in machining time.
The machining time, TC required for a particular operation can be determined
ο roughly by calculation i.e., estimation
ο precisely, if required, by measurement.
Measurement definitely gives more accurate result and in detail but is tedious and expensive. Whereas, estimation by simple calculations, though may not be that accurate, is simple, quick and inexpensive.
Hence, determination of machining time, specially by simple calculations using suitable equations is essentially done regularly for various purposes.
(ii) Major Factors That Govern Machining Time
The factors that govern machining time will be understood from a simple case of machining. A steel rod has to be reduced in diameter from D1 to D2 over a length L by straight turning in a centre lathe as indicated in Fig. 4.9.1.
Fig. 4.9.1 Estimation of machining time in turning.
Here, CCpoLTxNs= (4.9.2)
where, LC = actual length of cut
= L + A + O
A, O = approach and over run as shown
N = spindle speed, rpm
so = feed (tool), mm/rev
np = number of passes required
Speed, N, is determined from cutting velocity, VC /min1000CDNVmπ= (4.9.3)
where, D = diameter of the job before cut
Therefore, 1000CVND=π (4.9.4)
The number of passes, np is mathematically determined from, 12pDDnt−= (4.9.5)
where, t = depth of cut in one pass, mm.
But practically the value of t and hence np is decided by the machining allowance kept or left in the preformed blanks. Usually, for saving time and material, very less machining allowance is left, if not almost eliminated by near – net – shape principle.
Hence, number of passes used is generally one or maximum two : one for roughing and one for finishing.
However, combining equations 4.9.2, 4.9.4 and 4.9.5, one gets, ()122000CCCoDLDDTVstπ−= (4.9.6)
or 1000CCCoDLTVsπ= for single pass turning (4.9.7)
Equation 4.9.7 clearly indicates that in turning to a given diameter and length, the cutting time, TC is governed mainly by the selection of the values of cutting velocity, VC and feed, so. This is true more or less in all machining operations being done in different machine tools.
A number of factors are essentially considered while selecting or deciding the values of VC and so for any machining work.
The major factors considered for selecting VC are :
• Nature of the cut;
o Continuous cut like turning, boring, drilling etc. are done at higher VC
o Shock initiated cuts in shaping machine, planing machine, slotting machine etc. are conducted at lower VC
o Intermittent cuts, as in milling, hobbing etc. are done at quite lower speed for dynamic loading
• Work material (type, strength, hardness, heat resistance, toughness, chemical reactivity etc.) For instance;
o Harder, stronger, heat resistant and work hardenable materials are machined at lower VC
o Soft, non-sticky and thermally conductive materials can be machined at relatively higher cutting velocity
• Cutting tool material (type, strength, hardness, heat and wear resistance, toughness, chemical stability, thermal conductivity etc.); For instance;
o HSS tools are used at within 40 m/min only in turning mild steel whereas for the same work cemented carbide tools can be used at VC, 80 to 300 m/min
o High performance ceramic tools and cBN tools are used at very high speed in machining steels of different strength and hardness.
o Diamond tools can be used in machining various materials (excepting Fe-base) at VC beyond 500 m/min
• Cutting fluid application; for instance,
o Proper selection and application of cutting fluid may allow increase in VC by 20 to 50%
• Purpose of machining; for instance,
o Rough machining with large MRR is usually done at relatively low or moderate velocity
o Finish machining with small feed and depth of cut is usually done at high VC
• Kind of machining operation;
o Unlike turning, boring etc. the operation like threading, reaming etc. are carried out at much lower (20 to 50%) cutting velocity for achieving quality finish
• Capacity of the machine tool
o powerful, strong, rigid and stable machine tools allow much higher VC, if required and permissible
• Condition of the machine tool
o Cutting velocity is kept lower than its normal value stipulated for a given tool – work material pair , if the machine tool is pretty old and / or having limitations due to wear and tear, backlash, misalignment, unstability etc.
The factors that are considered during selecting the value of feed, so are,
• Work material (type, strength, hardness etc.)
• Capacity of the machine tool (power, rigidity etc.)
• Cutting tool; material, geometry and configuration
• Cutting fluid application
• Surface finish desired
• Type of operation, for instance threading operation needs large feed according to the lead of the thread.
• Nature of cut; continuous, shock initiated type, and intermittent
Feed, which raises cutting forces proportionally, is kept low in
shock and intermittent type cuts
Apart from the total volume of material to be removed, permissible values of cutting velocity, feed and depth of cut and cutting fluid application, there are few more factors which also play role on machining time.
Those additional factors include :
ο Quick return ratio in operations like shaping, planing, slotting, gear shaping etc.
ο Jobs of odd size and shape and irregular and harder surfaces like large castings are essentially machined much slowly with lower cutting velocity
ο Some special techniques like hot machining and cryomachining enables faster machining of some exotic materials and even some common metals like steels at higher VC and so.
(iii) Estimation Of Machining Time By Calculations
(a) In case of turning in lathes
Fig. 4.9.1 and equations like Equation 4.9.7 enable determination of the amount of time required for straight turning in lathes following the given procedural steps :
• Determine the length of cut by proper selection of amount of approach, A (2 ~ 5 mm) and over run, O (1 to 3 mm), if required
• Select the approximate values of VC and so based on the tool – work materials and other factors previously mentioned [depth of cut is decided based on the machining allowance available and the final diameter desired]
• Determine the spindle speed, N using equation 4.9.4 and then fix N as well as so from the chart giving the lists of N and so available in that lathe
• Finally determine TC using equation 4.9.7. ()1000wCCoDLAOTVsπ++=
Example
For, D = 100 mm, Lw = 200 mm, A = O = 5 mm, VC = 120 m/min and so=0.2 mm/rev, ()10020055)10001200.2CxxTxxπ+= min
= 2.75 min
The machining time for facing, grooving, taper turning, threading, parting etc. in lathes can also be determined or estimated following the same principle and method.
(b) In case of drilling and boring
The basic principle and procedure of estimation of machining time in drilling and boring are almost same as that of turning operations. Fig. 4.9.2 shows making through hole by drilling and boring.
Fig. 4.9.2 Drilling and boring operations.
For drilling a through hole (Fig. 4.9.2),
The machining time, TC is estimated from, 'CCoLTNs= (4.9.8)
where, LC’ = Lh + A + O + C
A, O = approach and over run
and C = cot2Dρ
D = diameter of the hole, i.e., drill
ρ = half of the drill point angle.
Speed, N and feed so are selected in the same way as it is done in case of turning.
Therefore, the drilling time can be determined from, ()1000hCCoDLAOCTVsπ+++= (4.9.9)
In the same way TC is determined or estimated in boring also. Only the portion ‘C’ is not included.
For blind hole, only over run, ‘O’ is excluded.
Example
For D = 25 mm, ρ = 60o, VC = 44 m/min
L = 60 mm, so = 0.25 mm/rev
A = O = 2 mm
TC = πx25{60 +2 +2 + (25/2)cot600} / (1000x44x0.25)
= 0.5 min.
(c) Machining time in shaping and planing
Machining time in shaping can be estimated using the scheme given in Fig. 4.9.3 which shows the length of tool – work travels required to remove a layer of material from the top flat surface of a block in a shaping machine. top view front view
Fig. 4.9.3 Surfacing in shaping machine.
Using Fig. 4.9.3, the total machining time, TC can be determined form the expression, 0wCsLTNs= min (4.9.10)
where, Lw = total length of travel of the job
= W + A’ + O’
w = width of the job
A’, O’ = approach and over run
Ns = number of strokes per min
so = feed of the job, mm/stroke
Ns has to be determined from, ()11000sCCNVL⎡=⎣ m/min (4.9.11)
where, VC = cutting velocity, m/min
LC = stroke length, mm
= Lw + A’ + O’
Lw = length of the workpiece
A’, O’= approach and over run
and Q = quick return ratio
= time of return stroke ÷ time of cutting stroke
Therefore, ()()1000/1)sCCNVLQ⎡=⎣ (4.9.12)
Practically the speed that is available nearest to this calculated value is to be taken taken up.
The values of VC and so are to be selected or decided considering the relevant factors already mentioned in case of turning.
Example
For Lw = 100 mm, A = 5, O = 5, W = 60, A’ = O’ = 2
Q = 2/3 , VC = 40 m/min and so = 0.2 mm/stroke
Ns= (1000x40)/[(100+5+5)(1+2/3)] = 200
Then, TC = (60+2+2)/(0.2x200) = 1.6 min
Machining times of planing operations in planing machine are also determined in the same way, because the only difference is that in planing machine, cutting strokes and feed travels are imparted to the job and the tool respectively, just opposite to that of shaping machine. Besides that, though both shaping and planing are reciprocating type, planing machine may allow higher VC.
(d) Machining time in Milling operations
There are different types of milling operations done by different types of milling cutters;
ο Plain milling by slab milling cutter mounted on arbour
ο End milling by solid but small end mill cutters being mounted in the spindle through collet
ο Face milling by large face milling cutters being directly fitted in the spindle.
Fig. 4.9.4 shows the scheme of plain milling by a plain or slab milling cutter and indicates how the machining time is to be calculated.
Fig. 4.9.4 Plain milling operation.
Following the Fig. 4.9.4, the machining time, TC for plain milling a flat surface can be determined as,
TC = LC / sm (for job width < cutter length) (4.9.13)
Where, LC = total length of travel of the job
= Lw + A + O + Dc/2
Lw = length of the workpiece
A, O = approach and over run (5 to 10 mm)
DC= diameter of the cutter, mm
Sm= table feed, mm/min
= soZCN
where, so = feed per tooth, mm/tooth
ZC= number of teeth of the cutter
N = cutter speed, rpm.
Again, N has to be determined from VC as 1000CCDNVπ= m/min
VC and so have to be selected in the usual way considering the factors stated previously. Since milling is an intermittent cutting process, VC should be taken lower (20 ~ 40%) of that recommended for continuous machining like turning. So should be taken reasonably low (within 0.10 to 0.5 mm) depending upon the tooth – size, work material and surface finish desired.
Example :
Determine TC for plain milling a rectangular surface of length 100 mm and width 50 mm by a helical fluted plain HSS milling cutter of diameter 60 mm, length 75 mm and 6 teeth. Assume A = O = 5 mm, VC = 40 m/min and so = 0.1 mm/tooth
Solution: min100553014020.161000100040200600.26200120/minCCmCCwmoCCCmLTsDLLAOmmssZNxxNVxNrpmDxsxxmm==+++=+++=====≅ππ==
where,
So, CCmLTs= 1401.17min.120==
In the same method, TC can be determined for end milling and face milling by proper selection of speed and feed depending upon the tool – work materials and other relevant factors.
Exercise – 4.9
1. How much machining time will be required to reduce the diameter of a cast iron rod from 120 mm to 116 mm over a length of 100 mm by turning using a carbide insert. Reasonably select values of VC and so.
2. Determine the time that will be required to drill a blind hole of diameter 25 mm and depth 40 mm in a mild steel solid block by a HSS drill of 1180 cone angle. Assume suitable values of VC and so.
3. In a mild steel block, a flat surface of length 100 mm and width 60 mm has to be finished in a shaping machine in a single pass. How much machining time will be required if Ns = 80, so = 0.2 mm/stroke, A = O = 5 mm, QRR = 0.5.
4. Estimate the machining time that will be required to finish a vertical flat surface of length 100 mm and depth 20 mm by an 8 teeth HSS end mill cutter of 32 mm diameter and 60 mm length in a milling machine. Assume, VC = 30 m/min, so = 0.12 mm/toot
Solution of the Problems in Exercise – 4.9
Problem – 1
Solution : LCTCNso= for single pass
LC = 100 + 5 + 5 = 110 mm 1000VCND=π
For turning C.I. by carbide insert, VC is taken as 100 m/min and so = 0.2 mm/rev 1000100250.120xNr ∴
Nearest standard speed, N = 225 1102.5min2250.2CTx∴ Ans.
Problem – 2
Solution :
Assumed for the given condition, VC = 25 m/min and so = 0.16 mm/rev
'CCoLTNs= LC’ = Lh + A + O + C
= 40 + 5 +0.0 +25/2cot59o = 50 mm 100010002532025CVxNrDx==≅ππ
Nearest standard speed, N = 315 rpm 501.0min3150.16CTx∴ Ans.
Problem – 3 top view front view
Solution : 0wCsLTNs=; Lw = W + A’ + O’ = 60 + 5 + 2.5 = 67.5 mm
VC = NsLC(1+Q) mm/min
For the given condition, let VC = 20 m/min, so = 0.12 mm/stroke
Also assume Q = 0.6
Then 20x1000 = Ns x(100+10+10)(1 + 0.6)
∴ Ns ≅ 100
Nearest (lower side) standard speed, Ns = 90
Then, 67.56.25min900.12CT Ans
Or 6055704.4min800.216wCoLT Ans
Problem – 4
Solution :
TC = LC / sm ; LC = 100 + 2 +2 + 16 = 120 mm
sm = soZCN = 0.12x8xN 100010003030032CCVxNrDx==≅ππ
Then sm = 0.12x8x320 = 320 mm/min 1200.40min300CT∴ Ans.
Thursday, 21 January 2016
Sunday, 8 November 2015
Sunday, 8 November 2015
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Shell mold casting or shell molding is a metal casting process in manufacturing industry in which the mold is a thin hardened shell of sand and thermosetting resin binder, backed up by some other material. Shell molding was developed as a manufacturing process during the mid-20th century in Germane.
- Pattern creation - A two-piece metal pattern is created in the shape of the desired part, typically from iron or steel. Other materials are sometimes used, such as aluminum for low volume production or graphite for casting reactive materials.
- Mold creation - First, each pattern half is heated to 175-370°C (350-700°F) and coated with a lubricant to facilitate removal. Next, the heated pattern is clamped to a dump box, which contains a mixture of sand and a resin binder. The dump box is inverted, allowing this sand-resin mixture to coat the pattern. The heated pattern partially cures the mixture, which now forms a shell around the pattern. Each pattern half and surrounding shell is cured to completion in an oven and then the shell is ejected from the pattern.
- Mold assembly - The two shell halves are joined together and securely clamped to form the complete shell mold. If any cores are required, they are inserted prior to closing the mold. The shell mold is then placed into a flask and supported by a backing material.
- Pouring - The mold is securely clamped together while the molten metal is poured from a ladle into the gating system and fills the mold cavity.
- Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting.
- Casting removal - After the molten metal has cooled, the mold can be broken and the casting removed. Trimming and cleaning processes are required to remove any excess metal from the feed system and any sand from the mold.
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