Unit-3

2. Turning:

2.1 Parts of a lathe:

A lathe may or may not have a stand (or legs), which sits on the floor and elevates the lathe bed to a working height. Some lathes are small and sit on a workbench or table, and do not have a stand.

All lathes have a "bed", which is (almost always) a horizontal beam (although some CNC lathes have a vertical beam for bed to ensure that swarf, or chips, falls free of the bed).

At one end of the bed (almost always the left, as the operator faces the lathe) is a "headstock". The headstock contains high-precision spinning bearings.

Rotating within the bearings is a horizontal axle, with an axis parallel to the bed, called the "spindle". Spindles are often hollow, and have exterior threads and/or an interior Morse taper on the "inboard" (i.e., facing to the right / towards the bed) by which accessories which hold the work-piece may be mounted to the spindle. Spindles may also have exterior threads and/or an interior taper at their "outboard" (i.e., facing away from the bed) end, and/or may have a hand-wheel or other accessory mechanism on their outboard end. Spindles are powered, and impart motion to the workpiece.

The spindle is driven, either by foot power from a treadle and flywheel or by a belt drive to a power source. In some modern lathes this power source is an integral electric motor, often either in the headstock, to the left of the headstock, or beneath the headstock, concealed in the stand.

The counterpoint to the headstock is the tailstock, sometimes referred to as the loose head, as it can be positioned at any convenient point on the bed, by undoing a locking nut, sliding it to the required area, and then relocking it. The tailstock contains a barrel which does not rotate, but can slide in and out parallel to the axis of the bed, and directly in line with the headstock spindle. The barrel is hollow, and usually contains a taper to facilitate the gripping of various type of tooling. Its most common uses are to hold a hardened steel centre, which is used to support long thin shafts while turning, or to hold drill bits for drilling axial holes in the work piece. Many other uses are possible.

Metalworking lathes have a "cross slide", which is a flat piece that sits crosswise on the bed, and can be cranked at right angles to the bed. Sitting atop the cross slide is a tool-post, which holds a cutting tool which removes material from the workpiece. There may or may not be a lead-screw, which moves the cross slide along the bed.

Woodturning and metal spinning lathes do not have cross slides, but have "banjos", which are flat pieces that sit crosswise on the bed. The position of a banjo can be adjusted by hand; no gearing is involved. Ascending vertically from the banjo is a tool post, at the top of which is a horizontal "tool rest". In woodturning, hand tools are braced against the tool rest and levered into the workpiece. In metal spinning, the further pin ascends vertically from the tool rest, and serves as a fulcrum against which tools may be levered into the workpiece.

2.2 ConstructionThe machine has been greatly modified for various applications however a familiarity with the basics shows the similarities between types. These machines consist of, at the least, a headstock, bed, carriage and tailstock. The better machines are solidly constructed with broad bearing surfaces (slides or ways) for stability and manufactured with great precision. This helps ensure the components manufactured on the machines can meet the required tolerances and repeatability.

Headstock:

Headstock with legend, numbers and text within the description refer to those in the image

The headstock (H1) houses the main spindle (H4), speed change mechanism (H2, H3), and change gears (H10). The headstock is required to be made as robust as possible due to the cutting forces involved, which can distort a lightly built housing, and induce harmonic vibrations that will transfer through to the workpiece, reducing the quality of the finished workpiece.

The main spindle is generally hollow to allow long bars to extend through to the work area; this reduces preparation and waste of material. The spindle then runs in precision bearings and is fitted with some means of attaching work holding devices such as chucks or faceplates. This end of the spindle will also have an included taper, usually mores, to allow the insertion of tapers and centers. On older machines the spindle was directly driven by a flat belt pulley with the lower speeds available by manipulating the bull gear, later machines use a gear box driven by a dedicated electric motor. The fully geared head allows the speed selection to be done entirely through the gearbox

Bed:The bed is a robust base that connects to the headstock and permits the carriage and tailstock to be aligned parallel with the axis of the spindle. This is facilitated by hardened and ground ways which restrain the carriage and tailstock in a set track. The carriage travels by means of a rack and pinion system, lead screw of accurate pitch, or feed screw.

Feed and lead screws:The feed-screw (H8) is a long driveshaft that allows a series of gears to drive the carriage mechanisms. These gears are located in the apron of the carriage. Both the feed-screw and lead screw (H9) are driven by either the change gears (on the quadrant) or an intermediate gearbox known as a quick change gearbox (H6) or Norton gearbox. These intermediate gears allow the correct ratio and direction to be set for cutting threads or worm gears. Tumbler gears (operated by H5) are provided between the spindle and gear train along with a quadrant plate that enables a gear train of the correct ratio and direction to be introduced. This provides a constant relationship between the numbers of turns the spindle makes, to the number of turns the lead-screw makes. This ratio allows screw-threads to be cut on the workpiece without the aid of a die.

The lead-screw will be manufactured to either imperial or metric standards and will require a conversion ratio to be introduced to create thread forms from a different family. To accurately convert from one thread form to the other requires a 127-tooth gear, or on lathes not large enough to mount one, an approximation may be used. Multiples of 3 and 7 giving a ratio of 63:1 can be used to cut fairly loose threads. This conversion ratio is often built into the quick change gearboxes.

Carriage:

Carriage with legend, numbers and text within the description refer to those in the image

In its simplest form the carriage holds the tool bit and moves it longitudinally (turning) or perpendicularly (facing) under the control of the operator. The operator moves the carriage manually via the hand-wheel (5a) or automatically by engaging the feed-screw with the carriage feed mechanism (5c), this provides some relief for the operator as the movement of the carriage becomes power assisted. The hand-wheels (2a, 3b, 5a) on the carriage and its related slides are usually calibrated, both for ease of use and to assist in making reproducible cuts.

Cross-slide:The cross-slide stands atop the carriage and has a lead-screw that travels perpendicular to the main spindle axis, this remits facing operations to be performed. This lead-screw can be engaged with the feed-screw (mentioned previously) to provide automated movement to the cross-slide; only one direction can be engaged at a time as an interlock mechanism will shut out the second gear train.

Compound restThe compound rest (or top slide) is the part of the machine where the tool post is mounted. It provides a smaller amount of movement along its axis via another lead-screw. The compound rest axis can be adjusted independently of the carriage or cross-slide. It is utilized when turning tapers, when screw cutting or to obtain finer feeds than the lead-screw normally permits.

Tool post:The tool bit is mounted in the tool-post which may be of the American lantern style, traditional 4 sided square style, or in a quick change style such as the multifix arrangement pictured. The advantage of a quick change set-up is to allow an unlimited number of tools to be used (up to the number of holders available) rather than being limited to 1 tool with the lantern style, or 3 to 4 tools with the 4 sided type. Interchangeable tool holders allow the all the tools to be preset to a center height that will not change, even if the holder is removed from the machine.

Tailstock:

Tailstock with legend, numbers and text within the description refer to those in the image

The tailstock is a tool-holder directly mounted on the spindle axis, opposite the headstock. The spindle (T5) does not rotate but does travel longitudinally under the action of a lead-screw and hand-wheel (T1). The spindle includes a taper to hold drill bits, centers and other tooling. The tailstock can be positioned along the bed and clamped (T6) in position as required. There is also provision to offset the tailstock (T4) from the spindles axis; this is useful for turning small tapers.

The image shows a reduction gear box (T2) between the hand-wheel and spindle, this is a feature found only in the larger center lathes, where large drills may necessitate the extra leverage.

2.3 Types of metal lathes:There are many variants of lathes within the metalworking field. Some variations are not all that obvious, and others are more a niche area. For example, a centering lathe is a dual head machine where the work remains fixed and the heads move towards the workpiece and machine a center drill hole into each end. The resulting workpiece may then be used "between centers" in another operation. The usage of the term metal lathe may also be considered somewhat outdated these days, plastics and other composite materials are in wide use and with appropriate modifications, the same principles and techniques may be applied to their machining as that used for metal.

Center lathe / engine lathe / bench lathe:

Two-speed back gears in a cone-head lathe.

A typical center lathe:The terms center lathe, engine lathe, and bench lathe all refer to a basic type of lathe that may be considered the archetypical class of metalworking lathe most often used by the general machinist or machining hobbyist. The name bench lathe implies a version of this class small enough to be mounted on a workbench (but still full-featured, and larger than mini-lathes or micro-lathes). The construction of a center lathe is detailed above, but depending on the year of manufacture, size, price range, or desired features, even these lathes can vary widely between models.

Engine lathe is the name applied to a traditional late-19th-century or 20th-century lathe. It is assumed that the word engine was added to the description to separate them from foot-powered and hand-powered lathes. The word engine would refer to a steam engine, which was the standard industrial power source for many years. The works would have one large steam engine which would provide power to all the machines via a line shaft system of belts. Therefore early engine lathes were generally 'cone heads', in that the spindle usually had attached to it a multi-step pulley called a cone pulley designed to accept a flat belt. Different spindle speeds could be obtained by moving the flat belt to different steps on the cone pulley. Cone-head lathes usually had a countershaft (lay-shaft) on the back side of the cone which could be engaged to provide a lower set of speeds than was obtainable by direct belt drive. These gears were called back gears. Larger lathes sometimes had two-speed back gears which could be shifted to provide a still lower set of speeds.

When electric motors started to become common in the early 20th century, many cone-head lathes were converted to electric power. At the same time the state of the art in gear and bearing practice was advancing to the point that manufacturers began to make fully geared headstocks, using gearboxes analogous to automobile transmissions to obtain various spindle speeds and feed rates while transmitting the higher amounts of power needed to take full advantage of high speed steel tools.

The inexpensive availability of electronics has again changed the way speed control may be applied by allowing continuously variable motor speed from the maximum down to almost zero RPM. (This had been tried in the late 19th century but was not found satisfactory at the time. Subsequent improvements have made it viable again.)

Tool room latheA tool-room lathe is a lathe optimized for tool room work. It is essentially just a top-of-the-line center lathe, with all of the best optional features that may be omitted from less expensive models, such as a collet closer, taper attachment, and others. There has also been an implication over the years of selective assembly and extra fitting, with every care taken in the building of a toolroom model to make it the smoothest-running, most-accurate version of the machine that can be built. However, within one brand, the quality difference between a regular model and its corresponding toolroom model depends on the builder and in some cases has been partly marketing psychology. For name-brand machine tool builders who made only high-quality tools, there wasn't necessarily any lack of quality in the base-model product for the "luxury model" to improve upon. In other cases, especially when comparing different brands, the quality differential between (1) an entry-level center lathe built to compete on price, and (2) a toolroom lathe meant to compete only on quality and not on price, can be objectively demonstrated by measuring TIR, vibration, etc. In any case, because of their fully-ticked-off option list and (real or implied) higher quality, toolroom lathes are more expensive than entry-level center lathes.

Turret lathe and capstan latheTurret lathes and capstan lathes are members of a class of lathes that is used for repetitive production of duplicate parts (which by the nature of their cutting process are interchangeable). It evolved from earlier lathes with the addition of the turret, which is an index able toolholder that allows multiple cutting operations to be performed, each with a different cutting tool, in easy, rapid succession, with no need for the operator to perform setup tasks in between (such as installing or uninstalling tools) nor to control the tool path. (The latter is due to the tool paths being controlled by the machine, either in jig-like fashion [via the mechanical limits placed on it by the turret's slide and stops] or via IT-directed servomechanisms [on CNC lathes].)

There is a tremendous variety of turret lathe and capstan lathe designs, reflecting the variety of work that they do.

Gang-tool lathe:A gang-tool lathe is one that has a row of tools set up on its cross-slide, which is long and flat and is similar to a milling machine table. The idea is essentially the same as with turret lathes: to set up multiple tools and then easily index between them for each part-cutting cycle. Instead of being rotary like a turret, the index able tool group is linear.

AccessoriesUnless a workpiece has a taper machined onto it which perfectly matches the internal taper in the spindle, or has threads which perfectly match the external threads on the spindle (two things which almost never happen), an accessory must be used to mount a workpiece to the spindle.

A workpiece may be bolted or screwed to a faceplate, a large flat disk that mounts to the spindle. Alternatively faceplate dogs may be used to secure the work to the faceplate.

A workpiece may be clamped in a three- or four-jaw chuck, which mounts directly to the spindle or mounted on a mandrel.

In precision work (and in some classes of repetition work), cylindrical work-pieces are invariably held in a collet inserted into the spindle and secured either by a drawbar, or by a collet closing cap on the spindle. Suitable collets may also be used to mount square or hexagonal work pieces. In precision tool making work such collets are usually of the draw in variety, where as collet is tightened the workpiece moves slightly back into the headstock, whereas for most repetition work the dead length variety is preferred as this ensures that the position of the workpiece does not move as the collet is tightened, so the workpiece can be set in the lathe to a fixed position and it will not move on tightening the collet.

A soft workpiece (wooden) may be pinched between centers by using a spur drive at the headstock, which bites into the wood and imparts torque to it.

Fig: Live center (top) Dead center (bottom)

A soft dead center is used in the headstock spindle as the work rotates with the centre. Because the centre is soft it can be trued in place before use. The included angle is 60 degrees. Traditionally a hard dead center is used together with suitable lubricant in the tailstock to support the workpiece. In modern practice the dead center is frequently replaced by a live center or (revolving center) as it turns freely with the workpiece usually on ball bearings, reducing the frictional heat, which is especially important at high RPM. A lathe carrier or lathe dog may also be employed when turning between two centers.

In woodturning, one subtype of a live center is a cup center, which is a cone of metal surrounded by an annular ring of metal that decreases the chances of the workpiece splitting.

A circular metal plate with even spaced holes around the periphery, mounted to the spindle, is called an "index plate". It can be used to rotate the spindle a precise number of degrees, and then lock it in place, facilitating repeated auxiliary operations done to the workpiece.

3.Shaping:The shaping machine is used to machine flat metal surfaces especially where a large amount of metal has to be removed. Other machines such as milling machines are much more expensive and are more suited to removing smaller amounts of metal, very accurately.

The reciprocating motion of the mechanism inside the shaping machine can be seen in the diagram. As the disc rotates the top of the machine moves forwards and backwards, pushing a cutting tool. The cutting tool removes the metal from work which is carefully bolted down.

A shaping machine is used to machine surfaces. It can cut curves, angles and many other shapes. It is a popular machine in a workshop because its movement is very simple although it can produce a variety of work.

Shaping machines come in a range of sizes but the most common size is seen opposite.

The main parts are indicated below:

The tool feed handle can be turned to slowly feed the cutting tool into the material as the 'ram' moves forwards and backwards. The strong machine vice holds the material securely. A small vice would not be suitable as the work could quite easily be pulled out of position and be damaged. The vice rests on a steel table which can be adjusted so that it ca be moved up and down and then locked in position. Pulling back on the clutch handle starts the 'ram' moving forwards and backwards.

The tool post and the tool slide can be angled as seen below. This allows the shaper to be used for different types of work

DIA A:

The tool post has been turned at an angle so that side of the material can be machined

DIA B:

The tool post is not angled so that the tool can be used to level a surface

DIA C:

The top slide is slowly feed into the material so that a rack can be machined for a rack and pinion gear system.

A quick return mechanism such as the one seen opposite is used where there is a need to convert rotary motion into reciprocating motion. As the disc rotates the black slide moves forwards and backwards. Many machines have this type of mechanism and in the school workshop the best example is the shaping machine.

3.1 CLASSIFICATION:

According to the type of mechanism used for giving reciprocating motion to the ram.

1. Crank type

2. Geared type

3. Hydraulic type

According to the position and travel of ram

1. Horizontal type

2. Vertical type

3. Traveling head type

According to the type of design of the table

1. Standard shaper

2. Universal shaper

According to the type of cutting stroke

1. Push type

2. Draw type

3.2 SHAPER SPECIFICATIONS:

Length of stroke.

Maximum vertical travel of table.

Maximum horizontal travel of table.

Maximum distance from table to ram.

Size of side tabletop.

Size of side table.

Power of motor.

Maximum vertical travel of tool slide.

Ram cycles per minute or strokes per minute.

Approximate net weight.

Floor space required.

4. PLANER

4.1 PLANING:

Planning is a machining process in which the metal is removed by relative linear movement of the tool over the surface of the work. In addition to producing the flat straight surfaces on large work pieces like machine tool beds and slides, Planing is also used to produce contours and a variety of irregular configurations like deep slots on large rotors, helical grooves on large rolls, internal guide surfaces in large walls, etc. The planning operation is also employed for machining smaller parts, for e.g. jig frames, weldments, etc.

4.2 PRINCIPLE PARTS OF THE PLANER: Bed

Table or platen

Tool head

Cross rail

Housing or column upright

Driving and feed mechanism

4.3 CLASSIFICATION OF PLANING MACHINE:

Double housing planer

Open side planer

Pit planer

Edge or plate planer

Divided table planer

4.4 SPECIFICATIONS OF A PLANER:

The planer is specified as follows:

1. Largest rectangular solid that can reciprocate, the size of the largest solid is known by horizontal distance between two vertical housings.

2. Table size.

3. Height of the cross rail from the top of the table.

4. Stoke length.

5. Power of the machine.

6. Number of feeds and speeds available.

7. Maximum width of the job that can be machined or planing width.

8. Type of drive.

9. Floor space required.

4.5 PLANER OPERATION:

The common operations performed in planner are:

Planing flat horizontal surfaces

Planing vertical surfaces

Planing at an angle and machining dovetails

Planing curved surfaces

Planing slots and grooves