Lathe Machine (Machining)

To Study the different parts of lathe machine and to understand tool, workpiece and chip contact in turning operation and to

study the different operations on lathe machine

Machine:A machine is a tool containing one or more parts that uses energy to perform an intended action. Machines are usually powered by mechanical, chemical, thermal, or electrical means, and are often motorized. Historically, a power tool also required moving parts to classify as a machine. However, the advent of electronics has led to the development of power tools without moving parts that are considered machines.

LATHE MACHINE

What is Lathe Machine?A lathe is a machine tool which rotates the work piece on its axis to perform various operations such as cutting, sanding, knurling, drilling, or deformation, facing, turning, with tools that are applied to the work piece to create an object which has symmetry about an axis of rotation. Examples of objects that can be produced on a lathe include candlestick holders, gun barrels, cue sticks, table legs, bowls, baseball bats, musical instruments (especially woodwind instruments), crankshafts, and camshafts.

History:Lathes were developed as early as the 15th century and were known as "bow" lathes. The operator rotated the work piece by drawing a bow back and forth, either by hand or with the use of a foot treadle. Next come Bassoons lathe in 1568, which was driven by a cord passing over a pulley above the machine. This in turn drove two other pulleys on the same shaft which rotated the work piece and a crude, wooden lead screw, which in turn allowed the operator to remove metal from the piece being machined. The screw cutting lathe originates in the 17th century. Development and advancements have continued and today we have sophisticated computerized controlled lathes.

Types of Lathe:

woodworking lathe:Woodworking lathes are the oldest variety. All other varieties are descended from these simple lathes. An adjustable horizontal metal rail - the tool rest - between the material and the operator accommodates the positioning of shaping tools, which are usually hand-held. With wood, it is common practice to press and slide sandpaper against the still-spinning object after shaping to smooth the surface made with the metal shaping tools.

Most woodworking lathes are designed to be operated at a rate of between 200 and 1,400 revolutions per minute, with a rate slightly over 1,000 rpms being considered optimal for most such work, and with larger work pieces requiring slower rates.

CNC metalworking lathe:In a metalworking lathe, metal is removed from the work piece using a hardened cutting tool, which is usually fixed to a solid moveable mounting, either a tool-post or a turret, which is then moved against the work piece using hand wheels and/or computer controlled motors. These (cutting) tools come in a wide range of sizes and shapes depending upon their application. Some common styles are diamond, round, square and triangular.

Some lathes may be operated under control of a computer for mass production of parts Manually controlled metalworking lathes are commonly provided with a variable ratio gear train to drive the main lead-screw. This enables different thread pitches to be cut. While more modern or expensive manually controlled lathes have a quick change box to provide commonly used ratios by the operation of a lever.

Turret Lathe:Turret lathes are used in production machine shops where several sequential operations are needed on single work piece. It is costly and time consuming to remove a work piece from one machine and hold it in another. Removing and reholding a work piece also introduces errors in work alignment and machining accuracy.

Cue lathes:Cue lathes function similar to turning and spinning lathes allowing for a perfectly radially-symmetrical cut for billiard cues. They can also be used to refinish cues that have been worn over the years.

Glass-working lathes:Glass-working lathes are similar in design to other lathes, but differ markedly in how the work piece is modified. Glass-working lathes slowly rotate a hollow glass vessel over a fixed or variable temperature flame. The source of the flame may be either hand-held, or mounted to a banjo/cross slide that can be moved along the lathe bed.

The flame serves to soften the glass being worked, so that the glass in a specific area of the work piece becomes ductile, and subject to forming either by inflation ("glassblowing"), or by deformation with a heat resistant tool. Such lathes usually have two head-stocks with chucks holding the work, arranged so that they both rotate together. Air can be introduced through the headstock chuck spindle for glassblowing. The tools to deform the glass and tubes to blow (inflate) the glass are usually handheld.

Metal spinning lathes:In metal spinning, a disk of sheet metal is held perpendicularly to the main axis of the lathe, and tools with polished tips (spoons) or roller tips are hand held, but levered by hand against fixed posts, to develop pressure that deforms the spinning sheet of metal.

Metal spinning lathes are almost as simple as wood turning lathes, and usually are. Typically, metal spinning requires a mandrel, usually made of wood, which serves as the template onto which the work piece is formed (asymmetric shapes can be made, but it is a very advanced technique). For example, to make a sheet metal bowl a solid block of wood in the shape of the bowl is required; similarly to make a vase a solid template of the vase is required.

Rotary lathes:A lathe in which softwood, like spruce or pine, or hardwood, like birch, logs are turned against a very sharp blade and peeled off in one continuous or semi-continuous roll. Invented by Immanuel Nobel. The product is called wood veneer and it is used for making plywood and as a cosmetic surface veneer on some grades of chipboard.

Transcription or Recording lathes:Transcription or Recording lathes are used to make grooves on a surface for recording sounds. These were used in creating sound grooves on wax cylinders and then on flat recording discs. Originally the cutting lathes were driven by sound vibrations through a

horn and then later driven by electric current when microphones were used in recording. Many of these were professional models, but there were some used for home recording and were popular before the advent of home tape recording.

Parts of Lathe Machine:

Description of Parts:

Bed:Usually made of cast iron Provides a heavy rigid frame on which all the main components are mounted.

Ways:

Inner and outer guide rails that are precision machined parallel to assure accuracy of movement.

Headstock:

Mounted in a fixed position on the inner ways, usually at the left end. Using a chuck, it rotates the work.

Gearbox:

Inside the headstock, providing multiple speeds with a geometric ratio by moving levers.

Spindle:

Hole through the headstock to which bar stock can be fed, which allows shafts that are up to 2 times the length between lathe centers to be worked on one end at a time.

Chuck:

3-jaw (self-centering) or 4-jaw (independent) to clamp part being machined.it allows the mounting of difficult work pieces that are not round, square or triangular.

Tailstock:

Fits on the inner ways of the bed and can slide towards any position the headstock to fit the length of the work piece. An optional taper turning attachment would be mounted to it.

Tailstock Quill:

Has a Morse taper to hold a lathe center, drill bit or other tool.

Carriage:

Moves on the outer ways. Used for mounting and moving most the cutting tools.

Cross Slide:

Mounted on the traverse slide of the carriage, and uses a hand wheel to feed tools into the work piece.

Tool Post:

To mount tool holders in which the cutting bits are clamped.

Compound Rest:

Mounted to the cross slide, it pivots around the tool post.

Apron:

Attached to the front of the carriage, it has the mechanism and controls for moving the carriage and cross slide.

Feed Rod:

Has a keyway, with two reversing pinion gears, either of which can be meshed with the mating bevel gear to forward or reverse the carriage using a clutch.

Lead Screw:

For cutting threads

Split Nut: When closed around the lead screw, the carriage is driven along by direct drive without using a clutch.

Quick Change Gearbox:

Control the movement of the carriage using levers.

Operations:

Facing:

Facing is done to decrease the length a single point tool is used for this purpose

Straight Turning:

Straight turning is done to decrease the diameter of a cylindrical work piece.

Taper turning:

Taper turning is done to produce a taper on a cylindrical body at a certain angle.

Grooving:

It is done to produce external grooves on a work piece.

Cut off:

It is done for parting of the work piece at any position.

Threading:

It is done to produce threading on a work piece.

Tracer turning:

it is done to produce a certain shape by cutting the work piece at the external periphery.

Drilling:

It is done to drill holes into the work piece.

Boring:

It is used to increase the internal diameter of an already drilled hole.

Knurling:

It is not a pure cutting operation it is used to produce a cross hash tag for strong gripping.

Chip Contact of Tool and Workpiece in Turning:

Contact of tool with workpiece should be such that how much tool is above and below the center line. It is known as tool centering

In turning angle made by rake surface and horizontal tangential line between the workpiece and cutting tool is called inclination angle.

If a rake surface is down from the centering line then inclination angle is positive, if along the centering line then angle is zero and if above then it is negative.

Angle between rake surface and horizontal surface plane is known as normal rake angle.

Angle between flank surface and vertical tangential line is known as clearance angle.

Flank surface of the tool is attached with the machined surface Chip formed moves over the rake surface of the tool Chip removal is easy when there is less friction between the chip formed and

rake surface of the tool In machining chip cut thickness is greater than uncut chip thickness due to chip

contact with the tool.

Shank:It is main body of tool. The shank used to grip in tool holder.

Flank:The surface or surface below the adjacent of the cutting edge is called flank of the tool.

Base:It is actually a bearing surface of the tool when it is held in tool holder or clamped

directly in a tool post.

Heel:It is the intersection of the flank & base of the tool. It is curved portion at the bottom of

the tool.

Cutting edge:It is the edge on face of the tool which removes the material from work piece. The

cutting edges are side cutting edge (major cutting edge) & end cutting edge (minor

cutting edge)

 TOOL ANGLES:

1. Side cutting edge angle (lead angle):  It is the angle between side cutting edge & side of tool flank

2. End cutting edge angle:This is the angle between end cutting edge & line normal to tool shank. It satisfactory value is 80 to 150. 

3.  Top rake angle:It is the angle between face of tool & plane parallel to base. 

4. Side rake angle:It is angle between face of tool & the shank of the tool

5. Side clearance:This formed by the side surface of the tool with a plane normal to the base of the tool.It avoid rubbing between flank & work piece when tool is fed longitudinally

6. Nose radius:Nose Radius makes the finish of the cut smoother as it can overlap the previous cut and

eliminate the peaks and valleys that a pointed tool produces. Having a radius also

strengthens the tip, a sharp point being quite fragile.