lecture-6 chapter 23

23. Machining Processes used to Produce Round Shapes: Turning and Hole Making

2005 Pearson Education South Asia Pte Ltd

RECAP

• Turning operation and its arrangement• Various type of turning operations like facing,

turning, grooving, slotting etc etc.• Comparison between turning and other operations

and where turning stands w.r.t finish and accuracy.• Some basic mathematical relationship.



Lesson outcome course outcome

• Explain the machine tool for turning operations

• Identify the various features and functionalities

• Guidelines• Try to cover the details of

various operations that can be performed on lathe like screw thread, boring, drilling, reamer and tapping

• Explain the components of metal cutting and machine tools

• Can differentiate between various machining operations and relevant machine tool used

• Generate a tentative machining plan for a part

• Know about advance machine tools



Machine Tool

• The machine tool used for turning operation is called LATHE

• LATHE is used for turning , drilling , boring , facing and all the other allied operation.

• Identified by a chuck holding the work-piece and a stationary tool

• The chuck rotates and the tool stay still( only partially) though

• “ Does the tool really stay still”



23.3 Lathes and Lathe Operations

• Lathes generally are considered to be the oldest machine tools.

• The maximum spindle speed of lathes typically is around 4000 rpm but only may be about 200 rpm for large lathes.

• Although simple and versatile, an engine lathe requires a skilled machinist, because all controls are manipulated by hand.

• Consequently, it is inefficient for repetitive operations and for large production runs.

• The rest of this section will describe the various types of automation that usually are added to improve efficiency.



Typical LATHE



23.3.1 Lathes components

Bed• The bed supports all major components of the

lathe. • Beds have a large mass and are built rigidly,

usually from gray or nodular cast iron. • The top portion of the bed has two ways with

various cross-sections that are hardened and machined for wear resistance and dimensional accuracy during turning.




Carriage• The carriage or carriage assembly slides along

the ways and consists of an assembly of the cross-slide, tool post, and apron.

Headstock• The headstock is fixed to the bed and is equipped

with motors, pulleys, and V-belts that supply power to a spindle at various rotational speeds.




Tailstock• The tailstock, which can slide along the ways and

be clamped at any position, supports the other end of the workpiece.

• It is equipped with a center that may be fixed (dead center), or it may be free to rotate with the workpiece (live center).

• Drills and reamers can be mounted on the tailstock quill (a hollow cylindrical part with a tapered hole) to drill axial holes in the workpiece.




Feed rod and lead screw• The feed rod is powered by a set of gears through

the headstock. • The rod rotates during the lathe operation and

provides movement to the carriage and the cross-slide by means of gears, a friction clutch, and a keyway along the length of the rod.

• Closing a split nut around the lead screw engages it with the carriage; it is also used for cutting threads accurately.




Lathe specifications• A lathe generally is specified by:

1. Its swing, the maximum diameter of the workpiece that can be machined (Table 23.6).

2. The maximum distance between the headstock and tailstock centers.

3. The length of the bed.



23.3.2 Workholding devices and accessories

• Workholding devices are important, particularly in machine tools and machining operations, as they must hold the workpiece securely.

• A chuck usually is equipped with three or four jaws.

• Three-jaw chucks generally have a geared-scroll design that makes the jaws self-centering.

• The jaws in some types of chucks can be reversed to permit clamping of the workpieces either on the outside surfaces or on the inside surfaces of hollow workpieces, such as pipes and tubing.




A and b are draw-in type collet. The workpiece is placed in the collet hole, and the conical surfaces of the collet are forced inward by pulling it with a draw bar into the sleeve




• Chucks are available in various designs and sizes.

• Their selection depends on the type and speed of operation, workpiece size, production and dimensional accuracy requirements, and the jaw forces required.

• By controlling the magnitude of jaw forces, an operator can ensure that the part does not slip in the chuck during machining.




• Power chucks, actuated pneumatically or hydraulically, are used in automated equipment for high production rates, including the loading of parts using industrial robots.

• Face plates are used for clamping irregularly shaped workpieces. The plates are round and have several slots and holes through which the workpiece is bolted or clamped.




• Mandrels are placed inside hollow or tubular workpieces and are used to hold workpieces that require machining on both ends or on their cylindrical surfaces.




Accessories• Several devices are available as accessories and

attachments for lathes. Among these devices are the following:

1. Carriage and cross-slide stops, with various designs to stop the carriage at a predetermined distance along the bed.

2. Devices for turning parts with various tapers.

3. Milling, sawing, gear-cutting, and grinding attachments.

4. Various attachments for boring, drilling, and thread cutting.



23.3.3 Lathe operations

• The cutting tool is attached to the tool post, which is driven by the lead screw, and removes material by traveling along the bed.

• Form tools are used to produce various shapes on solid, round work-pieces by moving the tool radially inward while the part is rotating.

• As a rule– The formed length of the part should not be greater

than about 2.5 times the minimum diameter of the part,– the cutting speed should be set properly.– cutting fluids should be used.



23.3.3 Additional Lathe operations

• Boring on a lathe is similar to turning. It is performed inside hollow workpieces or in a hole made previously by drilling or other means.

• Drilling can be performed on a lathe by mounting the drill bit in a chuck in the tailstock quill.

• The tools for parting, grooving, thread cutting, and various other operations are specially shaped for their particular purpose or are available as inserts.



23.3.4 Types of Lathes

Bench lathes• As the name suggests, these lathes are placed

on a workbench or a table. • They have low power, are usually operated by

hand feed, and are used to machine small workpieces.

• Toolroom lathes have high precision, enabling the machining of parts to close dimensional tolerances.




Special-purposes lathes• These lathes are used for applications (such as

railroad wheels, gun barrels, and rolling-mill rolls) with workpiece sizes as large as 1.7 m in diameter by 8 m in length and capacities of 450 kW.




Tracer lathes• These lathes have special attachments that are

capable of turning parts with various contours. • Also called duplicating lathes or contouring

lathes, the cutting tool follows a path that duplicates the contour of the template, similar to a pencil following the shape of a plastic stencil.

• However, operations typically performed on a tracer lathe have been replaced largely by numerical-control lathes and turning centers.




Automatic lathes• Lathes have been automated increasingly over

the years; manual machine controls have been replaced by various mechanisms that enable machining operations to follow a certain prescribed sequence.

• In a fully automatic lathe, parts are fed and removed automatically, whereas in semiautomatic machines, these functions are performed by the operator.

• Automatic lathes may have a horizontal or vertical spindle and are suitable for medium- to high-volume production.” why not small production”




Turret lathes• Turret lathes (either bar type or chucking type)

are versatile, and the operations may be carried out either by hand, using the turnstile (capstan wheel), or automatically.

• Once set up properly, these machines do not require highly skilled operators.

• Vertical turret lathes also are available; they are more suitable for short, heavy workpieces with diameters as large as 1.2 m.




Turret lathes• Fig 23.9 shows the schematic illustration of the

components of a turret lathe. Note the two turrets: square and hexagonal (main).




Computer-controlled lathes• Fig 23.9 shows the (a) computer numerical-

control lathe. Note the two turrets on this machine.



Example 23.2 Typical parts made on CNC turning machine tools

The capabilities of CNC turning-machine tools Material and number of cutting tools used and machining times are indicated for each part. These parts also can be made on manual or turret lathes, although not as effectively or consistently.



Example 23.3 Machining of complex shapes



23.3.6 Design considerations and guidelines for turning operations

• When turning operations are necessary, the following general design guidelines should be followed:

1. Parts should compatible with work holding device

2. The dimensional accuracy and surface finish specified should match process capability.

3. If possible use near net shape rather then blank

4. Tool path optimization

5. The manufacturing technology must use standard tool and inserts

6. Machinability



23.3.6 Design considerations and guidelines for turning operations

Guidelines for turning operations• Possible cause of problem in turning



Additional Lathe operations

• Screw Thread• Boring• Drilling• Reamer• Tapping



23.3.8 Cutting screw threads

• A screw thread may be defined as a ridge of uniform cross-section that follows a helical or spiral path on the outside or inside of a cylindrical (straight thread) or tapered surface (tapered thread).

• Threads traditionally have been machined, but increasingly, they are formed by thread rolling.

• Threads can be machined externally or internally with a cutting tool with a process called thread cutting or threading. Internal threads also can be produced with a special threaded tool, called a tap, and the process is called tapping.



23.3.8 Cutting screw threads



23.4 Boring and Boring Machines

• Boring is performed to – enlarge a hole made previously by some other

process or – to produce circular internal profiles in hollow

workpieces.– The tool used is called boring bar with a turning

insert




• Boring operations on relatively small workpieces can be carried out on a lathe; large workpieces are machined on boring mills.

• In horizontal boring machines, the workpiece is mounted on a table that can move horizontally in both the axial and radial directions.

• A vertical boring mill (Fig. 23.18) is similar to a lathe, has a vertical axis of workpiece rotation, and can accommodate workpieces with diameters as much as 2.5 m.




Design considerations for boring• Guidelines for efficient and economical boring

operations are similar to those for turning – Additionally, the following factors should be

considered:– Whenever possible, through holes rather than

blind holes should be specified.



23.5 Drilling, Drills and Drilling Machines

• When inspecting various large or small products, note that the vast majority have several holes in them.

• Hole making is among the most important operations in manufacturing, and drilling is a major and common hole-making process.

• Drilling can be done using a drill machine but a Lathe can also be used for drilling

• In Lathe drill is held either in the tail stock or if multi tool turret is used drill can be held in tool post



23.5.1 Drills



23.5.1 Drills

• The capabilities of drilling and boring operations are shown in Table 23.10.



23.5.1 Drills

Spade drill• Spade drills have removable tips or bits and are

used to produce large-diameter and deep holes.• Fig 23.1 shows various types of drills.



23.5.1 Drills

Gun drilling• Developed originally for drilling gun barrels, gun

drilling is used for drilling deep holes and requires a special drill.

• Fig 23.22(a) shows the gun drill showing various features. (b) Schematic illustration of the gun-drilling operation.



23.5.1 Drills



23.5.1 Drills

Trepanning• In trepanning, the cutting tool produces a hole by

removing a disk-shaped piece (core), usually from flat plates.



23.5.1 Drills

Other types of drills• Fig 23.20 shows the Various types of drills and

drilling and reaming operations.



23.5.1 Drills

Other types of drills• A step drill produces holes with two or more

different diameters. • A core drill is used to make an existing hole

larger. • Counterboring and countersinking drills produce

depressions on the surface to accommodate the heads of screws and bolts below the workpiece surface.



23.5.2 Material-removal rate in drilling

• The material-removal rate (MRR) in drilling is the volume of material removed by per unit time. For a drill with a diameter D, the cross-sectional area of the drilled hole is πD2/4.

• The velocity of the drill perpendicular to the workpiece is the product of the feed, f (the distance the drill penetrates per unit revolution), and the rotational speed, N, where N=V/πD.

• Thus,



Example 23.4 Material-removal rate and torque in drilling

A hole is being drilled in a block of magnesium alloy with a 10-mm drill bit, at feed of 0.2 mm/rev, and with the spindle running at 800 rpm. Calculate the material-removal rate and the torque on the drill.



Solution The material-removal rate first is calculated from Eq. (23.3):

Referring to Table 21.2, let’s take an average unit power of 0.5 Ws/mm3 for magnesium alloys. The power required is then


s/mm210min/mm570,128002.04

10MRR 33

2

W1055.0210Power




Solution Power is the product of the torque on the drill and the rotational speed, which in this case is (800)(2π)/60 = 83.3 radians per second. Noting that W=J/s and J=Nm, we find that

mN25.18.83

105T



23.5.5 Drilling practice

• Drills and similar hole making tools usually are held in drill chucks, which may be tightened with or without keys.

– Special chucks and collets with various quick-change features

– To keep the drill more centered, the point angles of the spot drill and of the drill are matched.

– minimizing walking of the drill bit are to use a centering punch




Drilling Recommendations• The feed in drilling is the distance the drill travels

into the workpiece per revolution.• Chip removal during drilling can be difficult,

especially for deep holes in soft and ductile workpiece materials.




Drilling reconditioning• Drills are reconditioned by grinding them either

manually or using special fixtures.

Measuring drill life• Drill life, as well as tap life, usually is measured by the

number of holes drilled before they become dull.



23.5.6 Drilling on machines other than LATHE

• Drilling machines are used for drilling holes, tapping, reaming, and small-diameter boring operations.

• The types of drilling machines range from simple

– bench-type drills used to drill small-diameter holes

– large radial drills which can accommodate large workpieces.

– Hand held drill but not included in machining category



23.5.6 Drilling machines

a) schematic illustration of the components of a vertical drill press. b) A radial drilling machine.



23.5.6 Multi tool or CNC Drilling machines

A three-axis computer numerical-control drilling machine. The turret holds many as eight different tools, such as drills, taps, and reamers.



23.5.6 Drilling machines

• Drilling machines with multiple spindles (gang drilling) are used for high production- rate operations.

• Workholding devices for drilling are essential to ensure that the workpiece is located properly.



23.5.7 Design considerations for drilling

• The basic design guidelines for drilling are as follows:

1. Designs should allow holes to be drilled on flat surfaces and perpendicular to the drill motion.

2. Interrupted hole surfaces should be avoided or minimized for improved dimensional accuracy, drill life, and to avoid vibrations.

3. Hole bottoms should match, if possible, standard drill-point angles; flat bottoms or odd shapes should be avoided.



23.6 Reaming and Reamers

• Reaming is an operation used to (a) make an existing hole dimensionally more accurate than can be obtained by drilling alone, and (b) improve its surface finish.

• The most accurate holes in workpieces generally are produced by the following sequence of operations:

1. Centering

2. Drilling

3. Boring

4. Reaming




• For even better accuracy and surface finish, holes may be burnished or internally ground and honed.

• A reamer is a multiple-cutting-edge tool with straight or helically fluted edges that remove very little material.



23.7 Tapping and Taps

• Internal threads in workpieces can be produced by tapping. A tap is a chip-producing threading tool with multiple cutting teeth.

• Fig 23.27(a) shows the terminology for a tap. (b) Tapping of steel nuts in production.




• Tapered taps are designed to reduce the torque required for the tapping of through holes.

• Bottoming taps are for tapping blind holes to their full depth.

• Collapsible taps are used in large-diameter holes; after tapping has been completed, the tap is collapsed mechanically and is removed from the hole without rotation.

• Chip removal can be a significant problem during tapping because of the small clearances involved.




• Tapping may be done by hand or with machines such as (a) drilling machines, (b) lathes, (c) automatic screw machines, and (d) vertical CNC milling machines combining the correct relative rotation and the longitudinal feed.

• Tap life can be determined with the same technique used to measure drill life.

• Self-reversing tapping systems also have been improved significantly and now are in use with modern computer-controlled machine tools.




• Chipless tapping is a process of internal thread rolling using a forming tap.



• Assignment Number 3– Write a paragraph on an advance machining

method which can produce holes without any physical contact between tool and workpiece and can be there any technique which produce holes without any tool

Submission: before next class Monday 12-12-2011



• Assignment number 4– Chapter 23

• 23.28• 23.34• 23.35• 23.37

Submission :15-12-2011

lecture-6 chapter 23

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