An Intriduction To Fracture Modes , Welding Defects,Causes And Remedies

An Intriduction To Fracture Modes , Welding Defects,Causes And Remedies


Introduction :
Welding is one of the most important techniques in the fabrication industries to join metals in different geometries and sizes with cost-effective and reliable assembly. There are several types of welding processes used in the petrochemical industry that have been around for many decades and new methods developed in recent years. Basically, these processes vary in setup, essential variables and non-essential variables, such as shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW), etc. Each welding process has characteristics that affect its quality performance and the soundness of the weld. For example, a weld acceptable for one application, such as for a tank, may not meet the acceptance criteria for pressure vessels per applicable international codes. Welding imperfections such as cracks, porosity, lack of fusion, incomplete penetration, and spatter could be due to various causes, such as poor workmanship, design issues, incorrect material, improper weld procedure specifications, and/or an unfavorable environment. The impact of each defect varies from acceptable to not acceptable, and must be either repaired or cut-out.

Types of Welding 

1. Gas welding 

2. Arc welding 

3. Thermal and Chemical welding 

4. Resistance welding 

5. Newer welding techniques (electron beam,laser)

Definition of Welding Defects 

The defects are those unwanted irregularities or discontinuities produced during the welding of a metal which is beyond the accepted standardized limits and hence can eventually cause the metal to fail.It arises due to many factors like

1. Poor environmental conditions 

2. Incorrect welding parameters or applications 

3. Incorrect combination of the filler metal with the base metal 

It affects the weldability of the metals and hence decreases the chances of the metal to withstand a certain amount of load,hence not making the welded metal suitable for its satisfactory needs for which it was being designed

Classification of welding defects

Before analyzing each and every defect,these defects are classified into two groups

I) External or visual defects are those defects which occur externally at the surface of the weld and can be observed 

1. cracking 

2. porosity 

3. undercut 
4. underfill

5. overlap 

6. spatter 
7- weld decay

II) Internal or hiddendefects are those that occur internally and cannot be observed

1. slag inclusion 

2. lack of fusion

3. incomplete penetration 

4. shrinkage cavities

FIG 1

1- Cracking 

A.Cracking Probably one of the most problematic welding defects is cracking figure 2.Cracks may develop on the interior of a weld or at the surface and along many directions.It may also appear on the areas subjected to high temperatures.It destroys the shape and design of the weld and also makes it distorted. Its hard to observe when cracks develop internally and hence can make the weld less efficient.These defects cannot be overlooked and must be corrected as soon as possible.

FIG 2

Causes
1. Cracking generally takes place due to poor designing of the weld..
2. Prolonged exposure to contraction stresses.
3. Poor metallic properties of the base metal.
4. Due to unequal heating and during thermal shrinkage of the metals.
5. Electrodes having high hydrogen content.
Remedies
1. Avoiding instant cooling in order to prevent rapid shrinkage.
2. Equal and Pre-heating from time to time.
3. Electrodes having low hydrogen content.
4. Proper and symmetric designing of the material.
5. Keeping the surface clean before welding.
6. Reducing the gaps in order to prevent cracking during solidification.
7. Using materials having low impurity levels.

Cracks presents themselves in two major types:
Hot Cracks– These cracks occur during the welding or during crystallization where temperature can be as high as 10000-degrees Celsius.
Cold Cracks– Cold cracks occur after completion of the welding process or during the solidification process. They are normally visible after several hours or even several days after welding.

Cracks can be classified by shape figure 3 (longitudinal, transverse or branched) and position (HAZ, base metal, centreline, crater). An inspector will rarely classify a crack as a particular type
(i.e. fatigue, HICC or SCC) because in most cases it will not be possible to determine the precise cause of a crack until an examination is carried out. The shape and position are facts, anything else is supposition.

FIG 3

1-1 Hydrogen-induced cold cracking (HICC)

HICC may occur in the HAZ of all hardenable steels (i.e. C, C/Mn) or in the weld metal of high strength low alloy (HSLA) steels that are microalloyed with small amounts of titanium, vanadium or niobium (typically <0.05%).

hydrogen breaks down at increased temperatures into atomic hydrogen (which has a small atomic size) and escapes to the atmosphere through the steel microstructure. When the temperature reduces to below around 300 ⁰C the hydrogen starts reforming to the hydrogen element and will no longer

be able to escape from the material. As the H2 reforms it may build up an internal pressure stress within the material structure itself.

FIG 4

1-2 Solidification cracking

Solidification cracking figure 5 (also known as centreline cracking or hot cracking) is a fracture that occurs in the weld metal of ferritic steels with a high sulphur or phosphorus content or in joints with a large depth/width ratio.

FIG 5

1-3 Reheat cracking

Reheat cracking occurs primarily in the HAZ of thick-section high strength low alloy (HSLA) steels, 300 series stainless steels and nickel-based alloys. During PWHT or elevated service temperatures intergranular cracking can occur due to stress relaxation in coarse grained regions under high

restraint or residual stresses. The failure normally initiates at a stress concentration such as a notch or change in crosssection.

Reheat cracking can be avoided by:

. adequate preheating to reduce the stress levels in the HAZ;

. using joint designs that require less restraint during welding in thick sections;

. removing stress concentrations caused by sharp changes in cross-section, such as sharp undercut,        mechanical damage and poorly blended weld toes.

FIG 6

1-4 Lamellar tearing

Lamellar tearing occurs mainly in thick-section T-joints and closed corner joints in carbon and carbon manganese steels with a high sulphur content and/or high levels of restraint. It does not occur in cast or forged steels; only in rolled plate. It has the appearance of a ‘steplike’ crack (Fig. 7) and occurs in wrought (rolled) plates due to a combination of:

. contraction stresses from the cooling weld acting through the parent plate thickness plus

. poor through-thickness ductility due to impurities in the steel (such as sulphides, sulphur, microinclusions and small laminations).

FIG 7

Standard acceptability There is no acceptability for cracks in welding.

2-Porosity

Porosity It is that type of defect in which the welded area has small groups of voids or gas bubbles trapped inside.They may appear spherical in shape like a small colony of cavities.

Causes

. Loss of gas shield.

. Damp electrodes or fluxes.

. Arc length too large.

. Damaged electrode flux.

. Presence of excess moisture content, grease, and oil.

Remedies

1. Selecting electrodes with a proper coating.

2. Maintaining the arc length.

3. Proper cleaning and decreasing of the moisture content at the surface.

FIG 8

Standard acceptability

3-Undercut

UndercutDuring welding,the material has to go through high temperatures and residual stresses due to which it has a tendency to undergo a structural damage.Undercuts are those defects that develop at the welding zone when the base metal melts away forming a depression like groove or a notch and hence can be seen when there is a deep penetration. These defects reduce thestrength of the welding joint.It is mainly an arc welding defect.

Causes

. excessive amps/volts;
. excessive travel speed;
. incorrect electrode angle;
. incorrect welding technique;
. electrode too large.

. Accumulation of rust at the surface of the welding joint.

Remedies

1. Cleaning of the base material after welding.

2. Avoiding exposure to moisture and hence not allowing the metal to rest.

3. Proper arc and electrode settings.

4. Using a smaller electrode.

FIG 9

Standard acceptability Undercuts are not allowed more thanB 0.8mm diameter and must not exceed to more than 4-5% depth from the base material.

4-Underfill

Underfill (Fig. 10) is the term given to a joint that has not been completely filled to the parent metal surface but the edges of the joint have been fused. 

Causes 
. too small an electrode being used;
. too few weld runs;
. poor welder technique.

FIG 10

Standard acceptability

The fillet leg dimension shall not under run the momiral fQ.let size by more than 1/16 inch. For flange to web jolts the undersize condition may not be within two flange thicknesses of the weld end. 

Groove welds may be underfilled by 5 percent. or 1/32-inch, whichever is greater. 

5- Overlap 

Overlap This kind of defect usually occurs due to poor welding procedures and parameters.Overlapping or the over flow of the welded metal takes place beyond its welded joint or toe.

FIG 11

Causes

1. Improper angles of electrode and wrong travel velocities of the arc.

2. Poor welding techniques,procedures and applications.

Remedies

1. Grinding off the excess metal spill can make the welded joint smooth and repair the overlapping.

2. Using proper and correct welding techniques.

Standard acceptability Overlapping of the welded joint cannot be accepted.

6- Spatter

Spatter is molten globules of consumable electrode that are ejected from the weld and quench quickly wherever they land on the weldment. They can therefore cause cracking on susceptible materials so they should be removed and then the area tested with PT or MT. Other problems caused by spatter

include prevention of UT (because UT needs a smooth surface for the probes), unwanted retention of penetrant during PT and problems with paint retention. 

Causes

. excessive current;

. damp electrodes;

. surface contamination from oil, paint, moisture or grease;

. incorrect wire feed speed during MAG welding.

. Large arc length during arc welding process.

Remedies

1. Scrapping or cleaning of the product by hair brush or by
    washing after welding.
2. Proper use of welding equipment.
3. Using correct welding current and arc length.

FIG 12

Standard acceptability spatters are not accepted in welded products as spatters are easier to remove as well.

7-Weld decay

Weld decay is a form of intergranular corrosion that occurs in the HAZ of unstabilised stainless steels. Within the temperature range of around 600—850 ⁰C chromium comes out of solution  to join with free carbon and form chromium carbides. The chromium was in the grain to help

prevent corrosion so corrosion can now occur where it has been depleted. Once this chromium depletion occurs the depleted area is said to be sensitised (meaning it is susceptible to corrosion) and will corrode in the presence of an electrolyte. The critical region is usually in the HAZ parallel to the weld toes (Fig.13) and once the area is sensitised, corrosion can lead to rapid failure.

Weld decay can be avoided by:

. Using low carbon grade stainless steels.

. Using stabilised stainless steels instead of unstabilised grades.

. Quench cooling.

. Keeping heat inputs and interpass temperatures low.

. Solution heat treatment after welding.

FIG 13

8- Slag inclusion

Slag inclusion When impurities,fluxes or many other particles and droplets which can be metallic or sand are entrapped inside the welded zone, inclusions occur which makes the welded metal

brittle.It may be present internally, on the surface and across turns.This defect greatly affects the structural design of the metal and affects its weldability and toughness thereby making it more

susceptible to fractures

Causes

. inadequate cleaning of slag originating from the welding flux;

. inadequate removal of silica inclusions in ferritic steels during MAG or TIG welding;

. touching the tungsten to the weld pool during TIG welding;

. the melting of the copper contact tube into the weld pool during MIG/MAG welding.

. Improper cleaning of the surface.

Remedies

1. Reducing rapid cooling and solidification

2. Maintaining welding speed.

3. Grinding and cleaning of the weld.

Standard acceptability Inclusions should exceed more than 3.2mm in diameter approximately after every 5 inches of weld.

FIG 14

9- Lack of fusion

Lack of fusion (Fig. 15) is weld metal not correctly fused to the parent material or the previous weld bead. 

Causes 

. incorrect joint preparation (narrow root gap, large root face);

. incorrect welding parameters (current too low);

. poor welder technique (incorrect electrode tilt or slope angles);

. magnetic arc blow;

. poor surface cleaning.

Remedies

1. Proper cleaning and positioning of the bead.

2. Maintaining the speed and welding current.

3. Proper cleaning and procedures of welding.

4. Re-welding can also avoid such type of errors.

Standard acceptability Incomplete fusion is not accepted and hence re-welding must be done.

FIG 15

10-Incomplete root penetration

Incomplete penetration usually occurs when the molten metal does not extend to the appropriate depth of the root hence leaving behind an incompletely filled groove.It may also lead to the propagation of cracks

Causes

1. Improper filling of the root gap.

2. When welding metal deposition is low.

3. When electrode diameter is large.

4. Welding done at low temperatures.

Remedies

1. Proper filling of the molten metal.

2. Proper supply of heat.

3. Appropriate functioning and proper diameter of the electrodes.

Standard acceptability Weld metal should not have any type of penetration.

FIG 16

11-Shrinkage cavities

Shrinkage cavities The last and final defect for this paper. Shrinkage cavities are formed during solidification of the welding metal.The welding metal shrinks during solidification.Allowances

must be provided for this defect.

Causes

 It occurs many times during solidification of themetal.

Remedies

. Deposition of filler metal must be done that can compensate foVr the loss of metal during shrinking.

Standard acceptability Shrinking of the metals must be compensated as it may not be able to provide a proper fitting or joining.

12-Root concavity

Root concavity (Fig. 17) is a groove in the root of a butt weld, but with both edges correctly fused. It is sometimes referred to as ‘suck back’. 

Causes

. the root face or root gap too large;

. excessive purge pressure being applied when welding using the TIG process;

. excessive root bead grinding before the application of the second weld pass.

Remedies

. Increase heat input. Reduce root gap. 

. Reduce backing gas pressure. 

. Reduce current for overhead welds.

Standard acceptability some welding codes limit root concavity to 1/32" or about .8mm
others allow for 1/16" 1.3mm

FIG 17

13- Excessive root penetration

A protruding penetration bead is classed as excess penetration because it is excess to requirements and does not contribute to the weld strength. If the level of root penetration is in excess of the design code acceptance criteria it is then classed as excessive penetration (Fig. 18). Do not confuse excess penetration (which may be acceptable to code requirements) with excessive penetration (which by definition is not acceptable to code requirements). 

Causes 

• Incorrect welding parameters. 

• Weld energy input too high. 

• Travel speed too low. 

• Size and type of electrode and welding position. 

• Incorrect assembly or preparation e.g. edge preparation too thin to support the weld underbead,           excessive root gaps, gaps in the abutments of a backing bar system, irregularities in the weld                bead  support system when “single-sided” mechanised welding is used. 

• Poor welder technique.

Remedies

• Verification of weld parameters by qualified testing/welder. 

• Adjusting the welding conditions. 

• Better weld preparation at the root of the joint. 

• Use of permanent or temporary backing bars. 

• Attention to the fit-up of backing systems in single-sided welding.

Standard acceptability When viewing radiographs and there is excessive penetration and the acceptance criteria says maximum of 2mm protrusion.

FIG 18

14- Arc strike

An arc strike (or stray flash) is accidental arcing on to the parent material. This can lead to cracking on crack-sensitive materials due to the fast quenching of the arc strike, causing localised hardened regions. These hardened regions are susceptible to brittle fracture. They can also cause stress

concentrations leading to in-service failures such as fatigue fractures. Arc strikes caused by poorly insulated cables or loose earth clamps may introduce copper or other dissimilar materials into the weldment, causing liquation cracking or other contamination problems. Arc strikes on susceptible

materials require removal and PT or MT to ensure no cracking is present.

FIG 19

15-Magnetic arc blow

Magnetic arc blow is an uncontrolled deflection of the welding arc due to magnetism. This causes defects such as lack of root fusion or lack of sidewall fusion. 

Causes :

. deflection of the arc by the Earth’s magnetic field (can occur in pipelines);

. poor position of the current return cable (the magnetic field surrounding the welding arc interacts        with the current flow in the material to the current return cable and is sufficient to deflect the arc);

. residual magnetism in the material causing distortion of the magnetic field produced by the arc            current.

Remedies

. welding towards or away from the clamp;

. using a.c. instead of d.c.;

. demagnetising the steel before welding.

FIG 20

An Introduction To Lathe types , Parts ,Uses ,Operations And Calculations

An Introduction To Lathe types , Parts ,Uses ,Operations  And Calculations


Introduction 
The lathe is a machine tool used principally for shaping articles of metal (and sometimes wood or other materials) by causing the workpiece to be held and rotated by the lathe while a tool bit is advanced into the work causing the cutting action. The basic lathe that was designed to cut cylindrical metal stock has been developed further to produce screw threads. tapered work. drilled holes. knurled surfaces, and crankshafts. The typical lathe provides a variety of rotating speeds and a means to manually and automatically move the cutting tool into the workpiece. Machinists and maintenance shop personnel must be thoroughly familiar with the lathe and its operations to accomplish the repair and fabrication of needed parts.

Types of Lathes.
Lathes can be conveniently classified as engine lathes, turret lathes, and special purpose lathes. All engine lathes and most turret and special purpose lathes have horizontal spindles and, for that reason, are sometimes referred to as horizontal lathes. The smaller lathes in all classes may be classified as bench lathes or floor or pedestal lathes, the reference in this case being to the means of support

A- Engine Lathes.
The engine lathe is intended for general purpose lathe work and is the usual lathe found in the machine shop. The engine lathe may be bench or floor mounted; it may be referred to as a tool room-type lathe, or a sliding-gap or extension-type lathe. The engine lathe consists mainly of a head stock, a tail stock, a carriage, and a bed upon which the tail stock and carriage move. Most engine lathes are back-geared and high torque, which is required for machining large diameter workpieces and taking heavy cuts. The usual engine lathe has longitudinal power and cross feeds for moving the carriage. It has a lead screw with gears to provide various controlled feeds for cutting threads.

FIG 1

 (1) Bench-Type Engine Lathe.
The bench-type engine lathe (figure 2 ) is generally powered by an electric motor, mounted to the bench behind the lathe head stock, and is driven by means of a flat leather belt. Some bench lathes use an underneath motor drive where the drive belt passes through a hole in the bench. This arrangement is convenient where space in the shop is limited. The bench type engine lathe is generally equipped with the necessary tools, chucks, lathe dogs, and centers for normal operation. The lathe may have a quick change gearbox for rapid change of threading feeds, or gears may have to be installed singly or in combination to achieve the proper threading feeds.  The bench lathe may or may not have a power-operated cross feed drive.

FIG 2

(2) Floor-Mounted Engine Lathe
The floor-mounted engine lathe (figure 3) or pedestal-type engine lathe, is inherently more rigid than the bench-type lathe and may have a swing as great as 16 or 20 inches and a bed length as great as 12 feet, with 105 inches between centers. The drive motor is located in the pedestal beneath the lathe head stock. A tension release mechanism for loosening the drive belt is usually provided so that the drive belt may be quickly changed to different pulley combinations for speed changes. The head stock spindle is back-geared to provide slow spindle speeds, and a quick-change gearbox for controlling the lead screw is installed on all currently manufactured floor-mounted lathes. The floor-mounted engine lathe usually has a power-operated cross feed mechanism.

FIG 3

(3) Toolroom Lathe.
The toolroom lathe (figure 4 ) is an engine lathe equipped with more precision accessories and built to greater standards of accuracy than standard engine lathes. It may be either floor-mounted or a bench-mounted.
The toolroom-type lathe is usually supplied with a very precise lead screw for threading operations. It comes equipped with precision accessories such as a collet, chuck attachment, a taper attachment, and a micrometer stop. Therefore, work of a better class and of a more complete nature may be accomplished on a toolroom-type engine lathe

FIG 4

(4) Sliding Gap-Type Floor-Mounted Engine Lathe. 
The sliding gap-type floor-mounted lathe or extension gap lathe contains two lathe beds, the top bed or sliding bed, and the bottom bed (figure 5 ). The sliding bed mounts the carriage and the tail stock and can be moved outward, away from the head stock as desired. By extending the sliding bed, material up to 28 inches in diameter may be swung on this lathe. The sliding bed may also be extended to accept between centers workpieces that would not normally fit in a standard lathe of the same size

FIG 5

B- Turret Lathes.
 The turret lathe (figure 6) is a lathe used extensively for the high speed production of duplicate parts. The turret lathe is so named because it has a hexagonal turret, or multiple tool holder, in place of the tail stock found on the engine lathe. Most turret lathes are equipped with a pump and basin for the automatic application of a coolant or cutting oil to the work piece.

FIG 6

(1) Floor-Mounted Horizontal Turret Lathe.
The floor-mounted horizontal turret lathe (figure 7) is intended for quick turning of bar stock and chucked workpieces with a minimum amount of adjustments between operations. The lathe uses a collet chuck and a hollow headstock spindle for feeding bar stock into the machine, or may use a universal scroll chuck for swinging the workpiece.

FIG 7

C. Special Purpose Lathes
Some lathes have characteristics that enable them to do certain work well. Some of these lathes are of the heavy-production type where large numbers of identical parts must be produced to make the
operation more economical. Other special purpose lathes are specialized for machining specific items and cannot be adapted to the common types of lathe operations.

FIG 8

(1) Bench-type Jeweler's Lathe. 
The bench-type jeweler's lathe (figure 9) is actually a miniature engine lathe designed for the precision machining of small parts. The usual jeweler's lathe contains a collet-type chuck, lead screw, change gears for threading operations, and a precise manual crossfeed. Controls and feeds are calibrated in smaller increments than with the engine lathe and, as a result, workpieces of small dimensions can
be machined to a great degree of accuracy. The jeweler's lathe is belt driven by an independent motor which can be mounted above or behind the lathe.

FIG 9

(2) Other Special Purpose Lathes.
Other special purpose lathes (figures 10,11) include the production lathe, the automatic lathe, the automatic screw machine, the brakedrum lathe, the crankshaft lathe, the duplicating lathe, the multispindle lathe, and lathes designed for turning car axles or forming sheet metal.

FIG 10

FIG 11

Lathe parts
Figure 12  provides a general illustration of the parts normally found on a lathe.

FIG 12

1. Bed
It is the main body of the machine. All main components are bolted on it. It is usually made by cast iron due to its high compressive strength and high lubrication quality. It is made by casting process and bolted on floor space.

2. Tool post
It is bolted on the carriage. It is used to hold the tool at correct position. Tool holder mounted on it.
3. Chuck
Chuck is used to hold the workspace. It is bolted on the spindle which rotates the chuck and work piece. It is four jaw and three jaw according to the requirement of machine.
4. Head stock
Head stock is the main body parts which are placed at left side of bed. It is serve as holding device for the gear chain, spindle, driving pulley etc. It is also made by cast iron.
5. Tail stock
Tail stock situated on bed. It is placed at right hand side of the bed. The main function of tail stock to support the job when required. It is also used to perform drilling operation.
6. Lead screw
Lead screw is situated at the bottom side of bed which is used to move the carriage automatically during thread cutting.
7. Legs
Legs are used to carry all the loads of the machine. They are bolted on the floor which prevents vibration.
8. Carriage
It is situated between the head stock and tail stock. It is used to hold and move the tool post on the bed vertically and horizontally. It slides on the guide ways. Carriage is made by cast iron.
9. Apron
It is situated on the carriage. It consist all controlling and moving mechanism of carriage.
10. Chips pan
Chips pan is placed lower side of bed. The main function of it to carries all chips removed by the work piece.
11. Guide ways
Guide ways take care of movement of tail stock and carriage on bed.
12. Speed controller
Speed controller switch is situated on head stock which controls the speed of spindle.
13. Spindle
It is the main part of lathe which holds and rotates the chuck.

SPECIFICATION OF LATHE

The size of a lathe is generally specified by the following means:

(a) Swing or maximum diameter that can be rotated over the bed ways

(b) Maximum length of the job that can be held between head stock and tail stock centres

(c) Bed length, which may include head stock length also

(d) Maximum diameter of the bar that can pass through spindle or collect chuck of capstan lathe.

Figure 13.    illustrates the elements involved in specifications of a lathe. The following data also contributes to specify a common lathe machine.

FIG 13

(i) Maximum swing over bed

(ii) Maximum swing over carriage

(iii) Height of centers over bed

(iv) Maximum distance between centers

(v) Length of bed

(vi) Width of bed

(vii) Morse taper of center

(viii) Diameter of hole through spindle

(ix) Face plate diameter

(x) Size of tool post

(xi) Number of spindle speeds

(xii) Lead screw diameter and number of threads per cm.

(xiii) Size of electrical motor

(xiv) Pitch range of metric and inch threads etc.

LATHE OPERATIONS

Operations, which can be performed in a lathe either by holding the workpiece between centers or by a chuck are :

FIG 14

1. Straight turning  2. Shoulder turning 3. Taper turning 

4. Chamfering 5. Eccentric turning  6. Thread cutting 7. Facing 

8. Forming 9. Filing  10. Polishing 11. Grooving  12. Knurling

13. Spinning  14. Spring winding

FIG 15

Operations  (figure 15 ) which are performed by holding the work by a chuck or a face plate or an angle plate are:

1. Undercutting 2. Parting-off 3. Internal thread cutting 4. Drilling

5. Reaming 6. Boring 7. Counter boring 8. Taper boring 9. Tapping

FIG 16

Operations which are performed by using special lathe attachments are:

1. Milling 2. Grinding

Turning

Turning is a machining process in which a cutting tool, typically a non-rotary tool bit, describes a helix toolpath by moving more or less linearly while the workpiece rotates.
Tapering
Tapering is to cut the metal to nearly a cone shape with the help of the compound slide. This is something in between the parallel turning and facing off. If one is willing to change the angle then they can adjust the compound slide as they like.

Chamfering

Chamfering is a cut on the edge or corner of something that makes it slope slightly rather than being perfectly square

Parting
The part is removed so that it faces the ends. For this the parting tool is involved in slowly to make perform the operation. For to make the cut deeper the parting tool is pulled out and transferred to the side for the cut and to prevent the tool from breaking.
Eccentric turning
Eccentric turning is one when a work is turned not on the normal center axis, instead it is done at an offset (as per the requirement). An engine crankshaft is the immediate example we can think of. Usually, general purpose lathe will have a three jaw chuck - which will take
care regular turning operations.

Drilling
For producing holes in jobs on lathe, the job is held in a chuck or on a face plate. The drill is held in t
he position of tailstock and which is brought nearer the job by moving the tailstock along the guide
ways, the thus drill is fed against the rotating job
Thread cutting
Thread cutting on the lathe is a process that produces a helical ridge of uniform section on the workpiece. This is performed by taking successive cuts with a threading toolbit the same shape as
the thread form required.
Facing
Facing is the process of removing metal from the end of a workpiece to produce a flat surface. Most often, the workpiece is cylindrical, but using a 4-jaw chuck you can face rectangular or odd-shaped
work to form cubes and other non-cylindrical shapes.
Forming
The forming is an operation that produces a convex, concave or any irregular profile on the workpiece.s
Filing
Filing is a material removal process in manufacturing. Similar, depending on use, to both sawing and grinding in effect, it is functionally versatile, but used mostly for finishing operations, namely in deburring operations. Filing operations can be used on a wide range of materials as a finishing operation. Filing helps achieve workpiece function by removing some excess material and deburring
the surface. Sandpaper may be used as a filing tool for other materials, such as wood.
Knurling:
The knurling is a process of embossing (impressing) a diamond-shaped or straight-line pattern into the surface of workpiece. Knurling is essentially a roughening of the surface and is done to provide a
better gripping surface.
Reaming:
Reaming: The holes that are produced by drilling are rarely straight and cylindrical in form. The reaming operation finishes and sizes the hole already drilled into the workpiece.
Boring:
Boring: The boring operation is the process of enlarging a hole already produced by drilling.
Polishing
Polishing is finishing processes for smoothing a workpiece's surface using an abrasive and a work wheel or a leather strop. Technically polishing refers to processes that use an abrasive that is glued to the work wheel, while buffing uses a loose abrasive applied to the work wheel. Polishing is a more aggressive process while buffing is less harsh, which leads to a smoother, brighter finish. A common misconception is that a polished surface has a mirror bright finish, however most mirror bright finishes are actually buffed.
Grooving
The term grooving usually applies to a process of forming a narrow cavity of a certain depth, on a cylinder, cone, or a face of the part. The groove shape, or at least a significant part of it, will be in the
shape of the cutting tool. Grooving tools are also used for a variety of special machining operations.
Spinning
Metal spinning is a form of symmetrical metalworking where a flat circle, or circular, piece of metal is fitted into a hand lathe or CNC lathe. Held in place by a pressure pad, the metal disk is spun at an appropriate speed. A localized force utilizing a variety of rollers or tools is applied either by hand or by machine to gradually form the metal over a “chuck” or mandrel. In complex spinnings, multiple chucks may be used to accomplish a specific shape. Chucks are made from hardened metals or wood

FIG 17

Lathe Cutting Tools
A lathe is a machine that rotates the workpiece about an axis of rotation to perform various operations such as turning, undercutting, knurling, drilling, facing, boring and cutting, with lathe cutting tools that are applied to the workpiece to create an object with symmetry about that axis.
For general purpose work, the tool used in is a single point tool, but for special operations, multipoint tools may use.

FIG 18

In a lathe machine work, different operations require different types of lathe cutting tools, which are as follow,
1- According to the method of using the tool
    1- Turning tool.
    2- Chamfering tool.
    3- Thread cutting tool.
    4- Internal thread cutting tool.
    5- Facing tool.
    6- Grooving tool.
    7- Forming tool.
    8- Boring tool.
    9- Parting-off tool.
   10- Counter boring tool
   11- Undercutting tool

FIG 19

2- According to the method of applying feed

A Left-hand turning tool

B Round-nose turning tool

C Right-hand turning tool

D Left-hand facing tool

E Threading tool

F Right-hand facing tool

G Cutoff tool

FIG 20

FIG 21

FIG 22

Applications for the cutting tools are shown in Fig 23.  and include turning, facing, threading, cutoff, boring, and inside threading.

FIG 23

HOW TO CENTRE THE CUTTING TOOL
Before any turning takes place it is common practice to check that the point of the lathe tool is centred. This means that the lathe tool point should be the same height as the tip of the tailstock centre. If this is not done and the tool point is either above or below the centre point - usually the finish to the steel will be poor. Also, a significant amount of vibration could take place during turning.

FIG 24

Turning operation calculations

Cutting speed. Cutting speed is given in surface feet per minute (sfpm) and is the speed of the workpiece in relation to the stationary tool bit at the cutting point surface. The cutting speed is given by the simple relation

FIG 25

and

where 

S = cutting speed, sfpm or m/min

df = diameter of work, in or mm

rpm = revolutions per minute of the workpiece

When the cutting speed (sfpm) is given for the material, the revolutions per minute (rpm) of the workpiece or lathe spindle can be found from

and

Lathe cutting time. The time required to make any particular cut on a lathe may be found using two methods. When the cutting speed is given, the following simple relation may be used:

and

where 

T = time for the cut, min

df = diameter of work, in or mm

L = length of cut, in or mm

F = feed, ipr (inches per revolution) or mmpr (millimeters per revolution)

S = cutting speed, sfpm (surface feet per minute) or m/min

Volume of metal removed. The volume of metal removed during a lathe cutting operation can be calculated as follows:

and

where 

Vr = volume of metal removed, in³ or cm³

Cd = depth of cut, in or mm

F = feed, ipr or mmpr

S =  cutting speed, sfpm or m/min

Machine power requirements

The following formula is for approximating machine power requirements for making a particular cut:

hp = dfSC

where 

hp = required machine horsepower

d = depth of cut, in

f = feed, ipr

S = cutting speed, sfpm

C = power constant for the particular material