Lathe Work : Lathe Work Types of Lathe : Types of Lathe 1. Speed lathe:
a. Wood working b. Polishing c. Spinning
2. Engine Lathe or Centre lathe
a. Belt drive engine lathe.
b. Gear head lathe.
3. Bench lathe
4. Tool-room lathe
5. Capstan and Turret lathe
6. Automatic lathe
7. Special purpose lathe
a. Missile lathe b. Gap bed lathe c. T- lathe d. Wheel lathe
e. Duplicating (or) Copying lathe. Specifications of Lathe : Specifications of Lathe 1. The length of bed :
It indicates the approximate floor space occupied by the lathe.
2. The length between centres :
It is the maximum length of work that can be mounted between the lathe centres. Specifications of Lathe : Specifications of Lathe 3. The height of centres from the bed : It is the distance between top surface of the bed and the imaginary center line passing through live centre and dead centre.
4. The maximum bar diameter : It is the maximum diameter of work that will pass through the hole of the head stock spindle.
5. The swing diameter of work over bed : It is the largest diameter of work that will revolve without touching the bed. It is twice the height of the centres from the bed.
6. The swing diameter of work over the carriage : It is the largest diameter of work that will revolve over the lathe saddle. It is smaller than the swing diameter over bed. Lathe : Lathe General view of a typical lathe, showing various components. Parts of Lathe : Parts of Lathe Slide 7: The lathe bed forms the base of the machine.
The headstock is mounted on the left end, the carriage in the middle and the tailstock at the right end of bed. The carriage and the tailstock moves over the bed.
On the top of the bed, there are two guide ways namely inner and outer guide ways. Carriage slides on the outer guide ways where as tail stock slides on the inner ways.
The bed should be very strong to resist the cutting forces and vibrations.
The bed has ribbed construction. The guide ways are very accurate for getting accuracy in jobs. The bed is made of cast iron alloyed with nickel and chromium.
The guide ways of the bed may be flat and inverted ‘V’ having an included angle of 90°. Bed Slide 8: Bed
Made of Cast Iron
Backbone of Lathe Ways
Hardened sliding surfaces.
Do not damage the ways! Bed Cross Braces : Bed Cross Braces Bed cross-section Slide 10: The headstock is the powered end and is mounted on the inner ways at the left end of the bed.
It has a hollow spindle made of carbon or nickel chrome steel. The front end of the hole is tapered for holding centres, collets and other tools having a standard tapered shanks. There are two types of spindle nose namely threaded nose and flanged nose. The threaded nose is commonly used which carries chucks and face plates.
In a lathe it is necessary to vary the speed of the work to suit different machining conditions. The usual methods to vary the speed of a lathe spindles are (1) By belt drive on cone pulley with or without a back gear arrangement, (2) By all gear drive using sliding gears and (3) By variable speed motor. Headstock Slide 11: Headstock belt drive Back Gear arrangement Headstock Slide 12: Nose of the head stock, where various work holding devices may be fitted Headstock 3 Jaw Chuck : 3 Jaw Chuck All 3 jaws move together. Headstock 4 Jaw Chuck : 4 Jaw Chuck All 4 Jaws move independently
Very accurate Chuck Wrench : Chuck Wrench Never let me catch you doing this! Headstock : Headstock Spindle Speed Levers : Spindle Speed Levers Quick Change Gearbox : Quick Change Gearbox Used to select the rate of travel for turning, facing, and threading. Lead Screw and Feed Rod : Lead Screw and Feed Rod < Lead Screw
< Feed Rod Thread Chasing Dial : Thread Chasing Dial References the rotation of the lead screw. Slide 21: The tailstock is located on the inner ways at the right end of the bed.
It supports the other end of the work when it is being machined between centers, and
It holds a tool for performing operations such as drilling, reaming
The tailstock is non-rotating but on hardened ways, it can be moved, to the left or right, to adjust to the length of the work. It can also be offset for cutting small angle tapers. Tailstock Tailstock : Tailstock Supports long workpieces when machining. 60 degree rotating center point. Drill Chuck Turn the tailstock handwheel to advance the ram. Carriage : Carriage < Saddle
< Apron Slide 24: The carriage can be moved left or right either by hand wheel or power feed. This provides the motion along the Z-axis.
During this travel turning cuts are made.
Carriage consists of the following parts: (1) Saddle, (2) Cross-slide, (3) Compound-slide or compound rest, (4) Tool post, and (5) Apron. Carriage Slide 25: The saddle is an H-shaped casting that fits over the bed and slides along the bed ways.
It carries the cross-slide and tool post. Saddle Saddle : Saddle H-shaped casting mounted on top of lathe ways, provides means of mounting cross-slide and apron Slide 27: The cross slide is mounted on the carriage and can be moved in and out (X-axis) perpendicular to the carriage motion.
This is the part that moves when facing cuts are made with power feed, or at any time a cut must be made ‘square’ with the Z-axis.
This, or the compound, is also used to set the depth of cut when turning.
The cross slide can be moved by its hand wheel or by power feed. Cross Slide Cross Slide Cross Slide : Cross Slide < Used for facing. Slide 29: The compound rest is fitted on the top of the cross-slide, is used to support the cutting tool.
It can be swiveled to any angle for taper turning operations and is moved manually.
It can be moved in and out by its hand wheel for facing or for setting the depth of cut.
It can also be rotated 360 degrees and fed by its hand wheel at any angle.
he compound does not have any power feed but it always moves longitudinally with the cross slide and the carriage. Compound rest Compound Rest Compound Rest : Compound Rest Used for:
Short steep tapers
Accurate shoulder lengths Slide 31: The tool post is mounted on the compound rest.
This can be any of several varieties but in its simplest form is merely a slotted cylinder, which can be moved, left or right in the T-slot in the compound and clamped in place.
It can also be rotated so as to present the cutter to the work at whatever angle is best for the job. Tool Post Slide 32: (a) A tool post for single-point tools and (b) a quick change indexing square turret, which can hold up to four tools. Tool Post Tool Post : Tool Post Slide 34: Apron The apron attached to the front of the carriage, holds most of the control levers. These include the levers, which engage and reverse the feed lengthwise (Z-axis) or crosswise (X-axis) and the lever which engages the threading gears.
The apron is fastened to the saddle, houses the gears and mechanisms required to move the carriage and cross-slide automatically.
The apron hand wheel can be turned manually to move the carriage along the Lathe bed. This hand wheel is connected to a gear that meshes in a rack fastened to the Lathe bed.
The automatic feed lever engages a clutch that provides the automatic feed to the carriage Back Gear Mechanism : Back Gear Mechanism Tumbler Gear Mechanism : Tumbler Gear Mechanism Quick change gear box : Quick change gear box Quick change gear box : Quick change gear box Lathe Accessories : Lathe Accessories Lathe Accessories : Lathe Accessories Divided into two categories
Work-holding, -supporting, and –driving devices
Lathe centers, chucks, faceplates
Mandrels, steady and follower rests
Lathe dogs, drive plates
Straight and offset toolholders
Threading toolholders, boring bars
Turret-type toolposts Lathe Centers : Lathe Centers Work to be turned between centers must have center hole drilled in each end
Provides bearing surface
Support during cutting
Most common have solid Morse taper shank60º centers, steel with carbide tips
Care to adjust and lubricate occasionally Lathe Centers : Lathe Centers Revolving Tailstock Centers : Revolving Tailstock Centers Replaced solid dead centers for most machining operations
Used to support work held in chuck or when work is being machined between centers
Contains antifriction bearings which allow center to revolve with workpiece
No lubrication required between center and work
Types: revolving dead center, long point center, and changeable point center Revolving Tailstock Centers : Revolving Tailstock Centers Microset Adjustable Center : Microset Adjustable Center Fits into tailstock spindle
Provides means of aligning lathe centers or producing slight tapers on work machined between centers
Eccentric slide (dovetail) allows center to be adjusted limited amount to each side of center Self-Driving Live Center : Self-Driving Live Center Mounted in headstock spindle
Used when entire length of workpiece is being machined in one operation
Chuck or lathe dog could not be used to drive work
Grooves ground around circumference of lathe center point provide drive
Work usually soft material such as aluminum Chucks : Chucks Used extensively for holding work for lathe machining operations
Work large or unusual shape
Most commonly used lathe chucks
Collet chuck Three-jaw Universal Chuck : Three-jaw Universal Chuck Holds round and hexagonal work
Grasps work quickly and accurate within few thousandths/inch
Three jaws move simultaneously whenadjusted by chuck wrench
Caused by scroll plate into which all three jaws fit
Two sets of jaw: outside chucking and inside chucking Three-jaw Universal Chuck : Three-jaw Universal Chuck Three jaw self centering chuck : Three jaw self centering chuck Four-Jaw Independent Chuck : Four-Jaw Independent Chuck Used to hold round, square, hexagonal, and irregularly shaped workpieces
Has four jaws
Each can be adjusted independently by chuck wrench
Jaws can be reversed to hold work by inside diameter Four-Jaw Independent Chucks : Four-Jaw Independent Chucks Slide 53: With the four jaw chuck, each jaw can be adjusted independently by rotation of the radially mounted threaded screws.
Although accurate mounting of a workpiece can be time consuming, a four-jaw chuck is often necessary for non-cylindrical workpieces. Four jaw independent chuck Headstock Spindle Types : Headstock Spindle Types Threaded spindle nose
Screws on in a clockwise direction
Tapered spindle nose
Held by lock nut that tightens on chuck
Cam-lock spindle nose
Held by tightening cam-locks using T-wrench
Chuck aligned by taper on spindle nose Threaded Spindle Nose : Threaded Spindle Nose Tapered Spindle Nose : Tapered Spindle Nose Cam Lock Spindle Nose : Cam Lock Spindle Nose Collet Chucks : Collet Chucks Most accurate chuck
Used for high-precision work
Spring collets available to hold round, square, or hexagon-shaped workpieces
Each collet has range of only few thousandths of an inch over or under size stamped on collet Spring Collet Chucks : Spring Collet Chucks Spring-collet chuck
One form: Handwheel draws collet into tapered adapter
Another form: Uses chuck wrench to tighten collet on workpiece
Can hold larger work than draw-in type Slide 60: Collets are used when smooth bar stock, or workpieces that have been machined to a given diameter, must be held more accurately than normally can be achieved in a regular three or four jaw chuck.
Collets are relatively thin tubular steel bushings that are split into three longitudinal segments over about two thirds of their length.
The smooth internal surface of the split end is shaped to fit the piece of stock that is to be held.
The external surface at the split end is a taper that fits within an internal taper of a collet sleeve placed in the spindle hole.
When the collet is pulled inward into the spindle, by means of the draw bar that engages threads on the inner end of the collet, the action of the two mating tapers squeezes the collet segments together, causing them to grip the workpiece. A collet (a) and a collet mounting assembly (b) are shown here. Collet chuck: Collets : Collets Figure 23.7 (a) and (b) Schematic illustrations of a draw-in type collet. The workpiece is placed in the collet hole, and the conical surfaces of the collet are forced inwards by pulling it with a draw bar into the sleeve. (c) A push-out type collet. (d) Workholding of a workpiece on a face plate. Slide 62: Collet chuck: Slide 63: Collet chuck: Spring Collet Chucks : Spring Collet Chucks Spring Collet Chucks : Spring Collet Chucks Magnetic Chucks : Magnetic Chucks Used to hold iron or steel parts that are too thin or may be damaged if held in conventional chuck
Fitted to an adapter mounted on headstock spindle
Used only for light cuts and for special grinding applications Magnetic Chucks : Magnetic Chucks Faceplates : Faceplates Used to hold work too large or shaped so it cannot be held in chuck or between centers
Usually equipped with several slots to permit use of bolts to secure work
Angle plate used so axis of workpiece may be aligned with lathe centers
Counterbalance fastened to faceplate when work mounted off center
Prevent imbalance and resultant vibrations Faceplates : Faceplates Faceplates : Faceplates Slide 71: A face plate consists of a circular disc bored out and thread to fit the nose of the spindle.
This has radial, plain and T slots for holding work by bolts and clamps.
Face plates are used for holding workpieces which cannot be held conveniently held between centers or chucks. Face plate A face plate Lathe accessories Slide 72: This is a cast iron plate having two faces machined to make them absolutely at right angles to each other.
Holes and slots are provided on both faces so that it may be clamped on the face plate and can hold the workpiece on the other face by clamps and bolts.
Angle plates are used in conjunction with a face plate when the holding surface of the workpiece should be kept horizontal, as for example, in machining a flange of a pipe elbow.
When eccentric jobs are bolted on the face plate, a balance weight or counter weight must be added. Angle plates Lathe accessories Steadyrest : Steadyrest Used to support long work held in chuck or between lathe centers
Located on and aligned by ways of the lathe
Positioned at any point along lathe bed
Three jaws tipped with plastic, bronze or rollers may be adjusted to support any work diameter with steadyrest capacity Steadyrest : Steadyrest Slide 75: A steady rest consists of cast iron base, which may be made to slide on the lathe bed ways and clamped at any desired position where support is necessary.
This is so designed that the upper position is hinged at one end which facilitates setting and removal of the workpiece without disturbing the position of the steady rest.
There are three jaws on the steady rest, two on the lower base and one on the upper frame, the jaws may be adjusted radially by rotating individual screws to accommodate work of different diameters.
The main function of the steady rest is to provide support to a long slender work.
For a very long work more than one steady rest may be used.
However the carriage cannot be fed to the full length of the work when steady rest is used. Steady rest Lathe accessories Slide 76: The steady rest supports long, small diameter stock that otherwise could not be turned. The steady rest can also replace the tailstock to allow for cutting tool access at the outboard end of your workpiece.
To mount the steady rest:
Secure to bedway from below with the locking plate.
A single hex bolt, along with a nut and washer, is used to hold the steady rest in place. See Figure.
The bearing surfaces on the steady rest should receive periodic lubrication while in use to prevent premature wear. Steady rest Lathe accessories Slide 77: To adjust the Steady Rest:
1. Loosen the lock nuts.
2. Open the sliding fingers by turning the knurled screws until they fit around the workpiece. Secure the steady rest in position.
3. Tighten the knurled screws so that the fingers are snug, but not tight against the workpiece. Tighten the setscrews and the lock nuts.
4. Lubricate the brass points with machine oil. Steady rest Lathe accessories Follower Rest : Follower Rest Mounted on saddle
Travels with carriage to prevent work from springing up and away from cutting tool
Cutting tool generally positioned just ahead of follower rest
Provide smooth bearing surface for two jaws of follower rest Follower Rest : Follower Rest Slide 80: A follower rest consists of a “C” like casting having two adjustable jaws which support the work.
The rest is bolted to the back end of the carriage and moves with it.
Before setting the follower rest, the end of the workpiece is machined slightly wider than the jaws to provide the true bearing surface.
The tool is slightly in advance position than the jaws, and the tool is fed longitudinally be the carriage, the jaws always follow the tool giving continuous support to the workpiece.
The follower rest prevents the job from springing away when the cut is made and is used in finish turning operation. Follower rest: Lathe accessories Slide 81: The follow rest is normally used with small diameter stock to prevent the workpiece from “springing” under pressure from the turning tool. To install the follow rest:
1. The follow rest is secured to the saddle with two cap screws. See Figure .
2. The bearing surfaces on the follow rest are similar to those on the steady rest, and should be lubricated to prevent premature wear. Follower rest: Lathe accessories Mandrel : Mandrel Holds internally machined workpiece between centers so further machining operations are concentric with bore
Several types, but most common
Stub mandrel Mandrels to Hold Workpieces for Turning : Mandrels to Hold Workpieces for Turning Figure 23.8 Various types of mandrels to hold workpieces for turning. These mandrels usually are mounted between centers on a lathe. Note that in (a), both the cylindrical and the end faces of the workpiece can be machined, whereas in (b) and (c), only the cylindrical surfaces can be machined. Plain Mandrel : Plain Mandrel Expanding Mandrel : Expanding Mandrel Gang Mandrel : Gang Mandrel Stub Mandrel : Stub Mandrel Slide 88: For accurate turning operations or in cases where the long work surface is not truly cylindrical, the workpiece can be turned between centers.
Initially the workpiece has a conical center hole drilled at each end to provide location for the lathe centers.
Before supporting the workpiece between the centers (one in the headstock and one in the tailstock), a clamping device called a ‘dog’ is secured to the workpiece.
The dog is arranged so that the tip is inserted into a slot in the drive plate mounted on the main spindle, ensuring that the workpiece will rotate with the spindle. Work holding between Centers Slide 89: Lathe centers support the workpiece between the headstock and the tailstock.
The center used in the headstock spindle is called the ‘live’ center. It rotates with the headstock spindle.
The ‘dead’ center is located in the tailstock spindle. This center usually does not rotate and must be hardened and lubricated to withstand the wear of the revolving work.
The workpiece must have perfectly drilled and countersunk holes to receive the centers.
The center must have a 60-degree point. Work holding between Centers Slide 90: For accurate machining, cylindrical parts can be turned between centers. Hardened “dead” centers are mounted in the tailstock; they do not rotate with the workpiece and must be lubricated. Hardened “live” centers are mounted in the tailstock; they rotate with the workpiece and do not need lubricatio Work holding between Centers Slide 91: Carriers or lathe dogs and catch plates are used to hold workpiece when it is held between centers.
Carriers or lathe dogs are attached to the end of the workpiece by setscrews; catch plates are either screwed or bolted to the nose of head stock spindle.
A projecting pin from the carriers fits into the slots provided in the catch plate Carriers or lathe dogs and Catch plates or Drive plates A catch plate with live centre Lathe accessories Lathe Dogs : Lathe Dogs Drives work machined between centers
Has opening to receive work and setscrew to fasten the dog to work
Tail of dog fits into slot on driveplate and provides drive to workpiece
Made in variety of sizes and types to suit various workpieces Standard bent-tail lathe dog : Standard bent-tail lathe dog Most commonly used for round workpieces
Available with square-head setscrews of headless setscrews Standard bent-tail lathe dog : Standard bent-tail lathe dog Bent tail engages in slot on drive plate Straight-tail lathe dog : Straight-tail lathe dog Driven by stud in driveplate
Used in precision turning Safety clamp lathe dog : Safety clamp lathe dog Used to hold variety of work
Wide range of adjustment Heavy Duty Lathe Dog : Heavy Duty Lathe Dog Wider range than others
Used on all shapes Cutting-Tool-Holding Devices : Cutting-Tool-Holding Devices Available in three styles
Each has square hole to accommodate square toolbit held in place by setscrew
Angle of approximately 15º to 30º to base of toolholder Left-Hand Offset Toolholder : Left-Hand Offset Toolholder Offset to the right
Designed for machining work close to chuck or faceplate and cutting right to left
Designated by letter L Right-Hand Offset Toolholder : Right-Hand Offset Toolholder Offset to the left
Designed for machining work close to the tailstock and cutting left to right
Also for facing operations
Designated by letter R Straight Toolholder : Straight Toolholder General-purpose type
Used for taking cuts in either direction and for general machining operations
Designated by letter S Carbide Toolholder : Carbide Toolholder Has square hole parallel to base of toolholder to accommodate carbide-tipped toolbits
Holds toolbit with little or no back rake
Designated by letter C Cutting-Off (Parting) Tools : Cutting-Off (Parting) Tools Used when work must be grooved or parted off
Long, thin cutting-off blade locked securely in toolholder by either cam lock or locking nut
Three types of parting toolholders
Straight Threading Toolholder : Threading Toolholder Designed to hold special form-relieved thread-cutting tool
Has accurately ground 60º angle
Maintained throughout life of tool
Only top of cutting surface sharpened when becomes dull Styles of Boring Toolholders : Styles of Boring Toolholders Held in standard toolpost
Light boring toolholder
Used for small holes and light cuts
Medium boring toolholder
Suitable for heavier cuts
May be held at 45º or 90º to axis of bar
Mounted on compound rest of lathe
Heavy-duty boring bar holder
Three bars of different diameters
May be held at 45º or 90º to axis of bar Compound Rest Tooling Systems : Compound Rest Tooling Systems Standard, or round, toolpost
Generally supplied with conventional engine lathe
Fits into T-slot of compound rest
Provides means of holding and adjusting type of toolholder or cutting tool required
Concave ring and the wedge or rocker provide for adjustment of cutting-tool height Conventional ToolPosts : Conventional ToolPosts Slide 108: After deciding on the machine tool and cutting tool, the following main cutting conditions have to be considered:
• Cutting speed
• Depth of cut
• Feed rate
Feed, speed, and depth of cut have a direct effect on productivity, tool life, and machine requirements. Therefore these elements must be carefully chosen for each operation. Whether the objective is rough cutting or finishing will have a great influence on the cutting conditions selected. Cutting Conditions Slide 109: In belt driven lathes the cutting speed may be changed using different pulley combinations Cutting Conditions (Changing cutting speed) Slide 110: In some lathes feed can be changed automatically using the levers in different positions as given in the chart Cutting Conditions (Changing feed) Slide 111: Cutting Conditions (Changing feed) Slide 112: When roughing, the goal is usually maximum stock removal in minimum time with minor consideration given to tool life and surface finish.
The first is to use a heavy feed because this makes the most efficient use of power and, with less tool contact, tends to create less chatter.
There are some exceptions where a deeper cut is more advantageous than a heavy feed, especially where longer tool life is needed.
Increasing the depth of cut will increase tool life over an increase in feed rate. But, as long as it is practical and chip formation is satisfactory, it is better to choose a heavy feed rate.
Even more important, tool life is greatly reduced at high cutting speeds unless coated carbide or other modern tool materials are used, and these also have practical speed limits.
Tool life is decreased most at high speeds, although some decrease in tool life occurs when feed or depth of cut is increased. This stands to reason, because more material will be removed in less time.
It becomes choice then, between longer tool life and increased stock removal. Since productivity generally outweighs tool costs, the most practical cutting conditions are usually those, which first, are most productive, and second, will achieve reasonable tool life. Roughing Cuts Slide 113: When taking finishing cuts, feed rate and depth of cut are of minor concern.
The feed rate cannot exceed that which is necessary to achieve the required surface finish and the depth of cut will be light.
However, the rule about speed will still apply. The speeds will generally be higher for finish cuts, but they must still be within the operating speed of the tool material.
Tool life is of greater concern for finish cuts. It is often better to strive for greater tool life at the expense of material removed per minute.
If tool wear can be minimized, especially on a long cut, greater accuracy can be achieved, and matching cuts which result from tool changes, can be avoided.
One way to minimize tool wear during finishing cuts is to use the maximum feed rate that will still produce the required surface finish. The less time the tool spends on the cut, the less tool wear can occur.
Another way to minimize tool wear during a long finishing cut is to reduce the speed slightly.
Coolant, spray mist, or air flow, will also extend tool life because it reduces the heat of the tool. Finishing Cuts: Lathe Cutting Operations : Lathe Cutting Operations Figure 23.1 Miscellaneous cutting operations that can be performed on a lathe. Note that all parts are circular – a property known as axisymmetry. The tools used, their shape, and the processing parameters are described throughout this chapter. Turning Operation : Turning Operation Figure 23.3 Schematic illustration of the basic turning operation, showing depth-of-cut, d; feed, f; and spindle rotational speed, N in rev/min. Cutting speed is the surface speed of the workpiece at the tool tip. Designations for a Right-Hand Cutting Tool : Designations for a Right-Hand Cutting Tool Figure 23.4 Designations for a right-hand cutting tool. Right-hand means the tool travels form right to left, as shown in Fig. 23.3. Right-hand Cutting Tool and Insert : Right-hand Cutting Tool and Insert Figure 21.20 (a) Schematic illustration of right-hand cutting tool. The various angles on these tools and their effects on machining are described in Section 23.3.1 Although these tools traditionally have been produced from solid tool-steel bars, they have been replaced largely with (b) inserts made of carbides and other materials of various shapes and sizes. General Recommendations for Tool Angles in Turning : General Recommendations for Tool Angles in Turning Three Important Elements : Three Important Elements Rotating SpeedIt expresses with the number of rotations (rpm) of the chuck of a lathe. When the rotating speed is high, processing speed becomes quick, and a processing surface is finely finished. However, since a little operation mistakes may lead to the serious accident, it is better to set low rotating speed at the first stage.
Cutting DepthThe cutting depth of the tool affects to the processing speed and the roughness of surface. When the cutting depth is big, the processing speed becomes quick, but the surface temperature becomes high, and it has rough surface. Moreover, a life of byte also becomes short. If you do not know a suitable cutting depth, it is better to set to small value.
Feed (Sending Speed )The sending speed of the tool also affects to the processing speed and the roughness of surface. When the sending speed is high, the processing speed becomes quick. When the sending speed is low, the surface is finished beautiful. There are 'manual sending' which turns and operates a handle, and 'automatic sending' which advances a byte automatically. A beginner must use the manual sending. Because serious accidents may be caused, such as touching the rotating chuck around the byte in automatic sending,. Summary of Turning Parameters and Formulas : Summary of Turning Parameters and Formulas General Recommendations for Turning Operations : General Recommendations for Turning Operations General Recommendations for Turning Operations : General Recommendations for Turning Operations General Recommendations for Turning Operations : General Recommendations for Turning Operations Range of Surface Roughnesses in Machining Processes : Range of Surface Roughnesses in Machining Processes Figure 23.13 The range of surface roughnesses obtained in various machining processes. Note the wide range within each group, especially in turning and boring. Setting of a Cutting Tool : Setting of a Cutting Tool In case a cutting tool is fixed to a table, thin metal plates are put between the tool and the table, and the height of the edge is adjusted to the center of material. In the case of using the general cutting tool, when the edge is higher than the center of material, the edge of a blade does not hit the material, and it cannot cut at all. Conversely, if the edge is low, it becomes impossible to cut the center of material. Moreover, the scale of a handle does not have correct value, then accurate processing becomes impossible.
Though the height of the cutting tool is adjusted in careful, we cannot unite with the center of material completely. Therefore, we have to set the tool to the direction, that the edge is easy to touch the material. The general cutting tool and the parting tool have to be set a few low position. The boring bar has to set a few high position. Slide 126: Turning Turning is a metal cutting process used for the generation of cylindrical surfaces.
Normally the workpiece is rotated on a spindle and the tool is fed into it radially, axially, or both ways simultaneously, to give the required surface.
The term ‘turning’, in the general sense, refers to the generation of any cylindrical surface with a single point tool.
Turning is the most commonly used operation in Lathe. By turning operation excess material from the work piece is removed to produce a cylindrical or cone shaped surface.
Two of the common types of turning are: Straight turning and taper turning. Lathe operations Slide 127: In this operation the work is held in the spindle and is rotated whole the tool is fed past the work piece in a direction parallel to the axis of rotation.
The surface generated is a cylindrical surface. Straight turning Slide 128: Minimize tool overhang
Support workpiece rigidly
Use machine tools with high stiffness and high damping capacity
When tools begin to vibrate and chatter, modify one or more of the process parameters, such as tool geometry, cutting speed, feed rate, depth of cut, or use of cutting fluid
Chip Collection Systems
Drop them on a conveyor belt
Dragging the chips from a setting tank
Using augers with feed screws
Vacuum methods Guidelines for Turning Operations Slide 129: Other related lathe operations Slide 130: Other related lathe operations Slide 131: Related turning operations: (a) chamfering, (b) parting, (c) threading, (d) boring, (e) drilling, (f) knurling. Other related lathe operations Slide 132: Facing: Facing is an operation for generating flat surface at the ends of a work piece. In this operation the feed given is in a direction perpendicular to the axis of rotation. First, clamp the part securely in a lathe chuck.
Then, install a facing tool
Bring the tool approximately into position, but slightly off of the part.
Always turn the spindle by hand before turning it on. This ensures that no parts interfere with the rotation of the spindle.
Move the tool outside the part and adjust the saddle to take the desired depth of cut.
Then, feed the tool across the face with the cross slide.
After facing, there is a very sharp edge on the part. Break the edge with a file. Other related lathe operations Facing : Facing Shouldering : Shouldering Parting : Parting Thread Cutting : Thread Cutting Drilling : Drilling Boring : Boring Slide 139: Chamfering: It is a operation of beveling the extreme end of a work piece. This done to remove unwanted metal projections at the ends and to protect end of the work piece from being damaged and to have a better look.
Knurling: Knurling is process of embossing a diamond shaped pattern on the surface of the work piece. The purpose of knurling is to provide an effective gripping surface on a work piece to prevent it from slipping when operated by hand. Knurling is done with a special tool called knurling tool. This tool consists of a set of hardened steel rollers in a holder with teeth cut on their surface in definite pattern. Other related lathe operations Slide 140: Grooving or Recessing Operations: Grooving or recessing operations is the operation of reducing the diameter of a workpiece over a very narrow surface. Grooving or recessing operations, sometimes also called necking operations, are often done on workpiece shoulders to ensure the correct fit for mating parts.
Drilling/reaming/ Boring: These are operations to accurately make holes on a workpiece. These operations uses the tailstock of the lathe. The tool is held on the tailstock and is fed toward the rotating work piece. Other related lathe operations Other related lathe operations : Other related lathe operations Parting: In. this operation a flat nose tool is used to cut the work piece, with feed in the direction perpendicular to the axis of rotation. A parting tool is deeper and narrower than a turning tool. It is designed for making narrow grooves and for cutting off parts. When a parting tool is installed, ensure that it hangs over the tool holder enough that the holder will clear the workpiece (but no more than that). Ensure that the parting tool is perpendicular to the axis of rotation and that the tip is the same height as the center of the part. A good way to do this is to hold the tool against the face of the part. Set the height of the tool, lay it flat against the face of the part, then lock the tool in place. When the cut is deep, the side of the part can rub against sides of the groove, so it's especially important to apply cutting fluid. In this clip, a part is cut off from a piece of stock. Slide 142: A taper may be defined as a uniform increase or decrease in diameter of a work piece measured along its length.
In a Lathe taper turning is an operation to produce a conical surface by gradual reduction in diameter from a cylindrical job.
Taper turning can be done by the following ways;
By a form tool.
By setting over the tailstock.
By swiveling the compound rest.
By taper turning attachment.
By compound feed. Taper turning Slide 143: Where, D = Large diameter of taper in mm.
d = small diameter of taper in mm.
l = length of taper part in mm
2α = full taper angle
α = angle of taper angle or half taper angle. The amount of taper in a workpiece is specified by ratio of the difference in diameters of the taper to its length. This is termed as conicity and designated by letter K. Taper Geometry Slide 144: Taper turning by a form tool Taper turning by a form tool uses a tool which is a broad nose tool having straight cutting edge.
The tool is set on the work piece at half taper angle, and is fed straight into the work to generate a tapered angle.
This method is limited to turn limited length taper only.
This is due to the reason that the metal is removed by entire cutting edge, and any increase in length of the taper will necessitate the use of a wider cutting edge.
This will require excessive cutting pressure, which may distort the work due to vibration and spoil the work due to vibration and spoil the work surface. Taper turning methods Slide 145: Taper turning by setting over the tailstock The principle of turning taper by this method is to shift the axis of rotation of the workpiece, at an angle to the lathe axis, and feeding the tool parallel to the lathe axis. The angle at which axis of rotation of the workpiece is shifted is equal to half angle of taper. The amount of setover is limited. This method is suitable for turning small taper on long jobs. The main disadvantage of this method is that the live and dead centres are not equally stressed and the wear is not uniform. Moreover, the lathe dog being set at an angle, the angular velocity is not constant. Taper turning methods Slide 146: Taper turning by swiveling the compound rest This method employs the principle of taper turning by rotating the workpiece on the lathe axis and feeding the tool at an angle to the axis of rotation of the workpiece. The tool is mounted on the compound rest, is attached to a circular base, graduated in degrees, which may be swiveled and clamped at any desired angle.
Once the compound rest is set at the desired half taper angle, rotation of the compound slide will cause the tool to be fed at an angle and generate the corresponding taper.
This method is limited to turn a short but steep taper owing to limited movement of the cross slide.
The movement of the tool in this method is controlled by hand, thus this gives low production rate and poor surface capacity. Taper turning methods Slide 147: Taper turning by taper turning attachment The principle of taper turning by taper turning attachment is to guide the tool in a straight path set at an angle to the axis of rotation of the workpiece, while the work is being held by a chuck or between centres aligned to the lathe axis.
A taper turning attachment consists of a frame or bracket which is attached to the rear end of the lathe bed and supports a guide bar pivoted at the centre.
The bar having graduations in degrees may be swiveled on either side of the zero graduation and is set at any desired angle with the lathe axis.
When taper turning attachment is used, the cross slide is first made free from the lead screw by removing the binder screw. Taper turning methods Slide 148: Taper turning by taper turning attachment The rear end of the cross slide is tightened with the guide block by means of bolt. When longitudinal feed is engaged, the tool mounted on the cross slide will follow the angular path, as the guide block will slide on the guide bar set at an angle to the lathe axis.
Taper turning by this method does not disturb the alignment of the live and dead centre.
By this process both steep and small taper can be made over any length of the workpiece. Taper turning methods Slide 149: Taper turning attachment Taper turning methods Cutting Screw Threads : Cutting Screw Threads Fig : (a) Cutting screw threads on a lathe with a single-point cutting tool. (b) Cutting screw threads with a single-point tool in several passes, normally utilized for large threads. The small arrows in the figures show the direction of feed, and the broken lines show the position of the cutting tool as time progresses. (c) A typical carbide insert and toolholder for cutting screw threads. (d) Cutting internal screw threads with a carbide insert. Thread cutting operation : Thread cutting operation Thread cutting operation : In thread cutting operation the first step is to remove the excess material from the workpiece to make its diameter equal to the major diameter of the thread to be cut.
The shape or form of the thread depends on the shape of the cutting tool to be used. The tool point must be ground so that it has the same angle as the thread to be cut. In a metric thread the included angle of the cutting edge should be ground exactly 600.Typical angles are 60° for Vee threads, and 29° for ACME threads. A thread gauge can be used to measure thread angles. (also called Centre Gauge or Fish Tail Gauge).
The top of the nose of the tool should be set at the same height as the centre of the workpiece.
The correct gear ratio is required between the machine spindle and the lead screw. This can be determined in the following manner: Thread cutting operation Thread cutting operation : Thread cutting calculations:
To calculate the gears required for cutting a thread of certain pitch can calculated from the following formula:
The gear of the spindle shaft is the driver and the gear on the leadscrew is the driven gear. Note: Often engine lathes are equipped with a set of gears ranging from 20 to 120 teeth in steps of 5 teeth, and one gear with 127 teeth.
To cut metric thread on English leadscrews: The cutting of metric thread on a lathe with an English leadscrew may be carried out by introducing a translating gear of 127 teeth.
If the leadscrew has n threads per inch to cut p mm pitch then, The factor 127/5 from the fact that 25.4 mm is equal to 1 inch. So one translating gear, with 127 teeth is necessary. Thread cutting operation Thread cutting operation : 5. Change gears of correct sizes are then fitted between the spindle and the leadscrew. When the Change gears are not fitted and when the Change gears are fitted (in this case a compound drive is used) Thread cutting operation Thread cutting operation : To change gears in a all geared lathe:
1. Loosen the nut below the middle gear and rotate the bracket so the middle gear moves away from gear F.
2. Loosen the cap screw at the center of the middle gear and slide it away from gear G.
3. Gear F can be removed by loosening the cap screw in its middle. Gear G has a setscrew in its rim. Loosen this screw and pull the gear off of the shaft.
4. Replace these two gears with the gears which will produce the desired pitch and secure with screws provided. Thread cutting operation Thread cutting operation : The speed of the spindle should be at a lower value and the half nut is engaged.
In thread cutting there are two methods of feeding the tool into the workpiece. In the first method the tool is feed perpendicularly into the workpiece. In the second method the tool is feed at half the angle of thread by swiveling the compound rest. The second method has distinct advantages over the first as it permits to have a top rake, cuts with single cutting edge, allows chips to flow easily, and reduces the strain on the tool. So the later method is used for roughing cuts and the first method is used for finishing cuts.
After the tool has produced a helical groove upto the end of the work, the tool is withdrawn by the use of cross slide.
Thread catching: The complete depth of cut of the thread cannot be attained in a single pass. Several cuts have to be taken till the required depth of cut is obtained. For this, the tool has to be withdrawn from the thread groove after completing each cut and then brought back to the starting position. Therefore we should have a suitable method so that the tool follows the previously cut thread groove, otherwise the threads will be spoiled. The process of engaging the thread with the same groove is called thread catching or thread chasing. The following methods can be used for thread catching: Thread cutting operation Thread cutting operation : When the length of the threaded part is short, after each cut, the carriage is brought back to its starting position by reversing the direction of rotation of lead screw. Therefore in this case the half nut is not disengaged from the leadscrew so the relative position is maintained.
When threading long jobs, the above mentioned method is not suitable, as it requires lot of time. So after each cut the machine is stopped, the carriage is disengaged from the leadscrew, by disengaging the half nut. It is then brought back to the starting position by rotating the hand wheel in suitable direction. If the leadscrew pitch is an exact multiple of the pitch to be cut than the half nut can engaged anywhere and the tool will follow the previously cut groove. But if not, a reference dial present on the right hand side of the apron called thread chasing dial has to be used. A fixed zero mark is provided on the saddle surface adjacent to the periphery of the dial. When the first cut is to be taken, the half nut is engaged when zero mark and in subsequent cuts the half nut should be engaged when the zero mark coincides with the same mark on the dial. Thread cutting operation Grinding Attachment : Grinding Attachment Milling Attachment : Milling Attachment Machining time calculation for turning operation : This is the time required for one pass. A job is completed in several passes. For facing operation the diameter used for calculating is the average of the blank diameter and the lowest diameter (zero in case of complete facing). Machining time calculation for turning operation Turret Lathe : Turret Lathe Figure 23.9 Schematic illustration of the components of a turret lathe. Note the two turrets: square and hexagonal (main). CAPSTAN AND TURRET LATHE : CAPSTAN AND TURRET LATHE The standard engine lathe is versatile, but it is not a high production machine. When production requirements are high, more automated turning machines must be used. The turret lathe represents the first step from the engine lathe toward high production turning machines. The turret lathe is similar to the engine lathe except that tool-holding turrets replace the tailstock and the tool post-compound assembly. The ‘skill of the worker’ is built into these machines, making it possible for inexperienced operators to reproduce identical parts. In contrast, engine lathe requires a skilled operator and requires more time to produce parts that are dimensionally the same. The principal characteristic of turret lathes is that the tools for consecutive operations are set up for use in the proper sequence. Although skill is required to set and adjust the tools properly, once they are correct, less skill is required to operate the turret lathe. Slide 163: The difference between the engine and turret lathes is that the turret lathe is adapted to quantity production work, whereas the engine lathe is used primarily for miscellaneous jobbing, toolroom, or single-operation work. The features of a turret lathe that make it a quantity production machine are:
Tools may be set up in the turret in the proper sequence for the operation.
Each station is provided with a feed stop or feed trip so that each cut of a tool is the same as its previous cut.
Multiple cuts can be taken from the same station at the same time, such as two or more turning and/or boring cuts.
Combined cuts can be made; tools on the cross slide can be used at the same time that tools on the turret are cutting.
Rigidity in holding work and tools is built into the machine to permit multiple and combined cuts.
Turret lathes can also have attachments for taper turning, thread chasing and duplicating, and can be made. Advantages of Turret Lathes Slide 164: : Differences between a Ram type or Capstan and Saddle type or a Turret lathe The turret of a capstan lathe is mounted on a short slide or ram which slides on the saddle. The saddle is clamped on bedways after adjusting the length of the workpiece. Thus in a capstan lathe, the travel of the turret is dependent upon the length of the travel of the ram. This limits the maximum length of the work to be machined in one setting.
The turret of a turret lathe is mounted on a saddle which slides directly on the bed. This feature enables the turret to be moved on the entire length of the bed and can machine longer work. Slide 165: : In the case of turret lathe, the turret is mounted on the saddle which slides directly on the lathe bedways. This type of construction provides utmost rigidity to the tool support as the entire cutting load is taken up by the lathe bed directly. In the case of a capstan lathe as the ram feeds into the work, the overhanging of the ram from the stationary saddle presents a non-rigid construction which is subjected to bending, deflection or vibration under heavy cutting load.. Differences between a Ram type or Capstan and Saddle type or a Turret lathe Slide 166: : On a capstan lathe the hexagonal turret can be moved back and forth much more rapidly without having to move the entire saddle unit. Thus capstan lathes are particularly handy for small articles which require light and fast cuts. While operating the machine by hand, there is less fatigue to the operator, due to lightness of the ram, whereas in the case of turret lathe hand feeding is a laborious process due to the movement of the entire saddle unit. Differences between a Ram type or Capstan and Saddle type or a Turret lathe Slide 167: : Some turret lathes are equipped with crosswise movement of the hexagonal turret. The crosswise movement may be effected by hand or power. This feature enables turning of large diameters, facing, contour turning and many other operation on the lathe.
Heavier turret lathes are equipped with power chucks like air operated chucks for holding large workpieces quickly.
In the case of a capstan lathe, the cross slide is mounted on a carriage which rests on the bedways between head stock and the ram. The carriages rests on both the front and rear ways on the top of the bed. Some turret lathe are equips with side hung type carriage. The carriage of this type does not require support from the rear bedways but slides on the top and bottom guideways provided at the front of the lathe. This construction enables larger diameter of work to be swung above the lathe bedways. There is no rear tool post on this type of machine as the carriage does not extend upto rear bedways. Differences between a Ram type or Capstan and Saddle type or a Turret lathe Slide 168: The turret 1 is mounted on the spindle 5, which rests on bearing on the turret saddle.
The index plate 2, the bevel gear 3 and the indexing ratchet 4 are keyed to the spindle 5. Turret indexing mechanism Slide 169: The plunger 14 fitted within the housing and mounted on the saddle locks the index plate by spring pressure 15 and prevents any rotary movement of the turret as the tool feeds into the work.
A pin 13 fitted on the plunger 14 projects out of the housing.
An actuating cam 10 and indexing pawl 7 are attached to the lathe bed 9 at the desired position. Turret indexing mechanism Slide 170: Both the cam and the pawl are spring loaded.
As the turret reaches the backward position , the actuating cam 10 lifts the plunger 14 out of the groove in the index plate due to the riding of the pin 13 on the beveled surface of the cam 10 and thus unlocks the index plate 2.
The spring loaded pawl 7 which by this time engages with a groove on the ratchet plate 4 causes the turret to rotate as the turret head moves backward. Turret indexing mechanism Slide 171: When the index plate or the turret rotates through one sixth of revolution, the pin 13 and plunger 14 drops out of cam 10 and the plunger locks the index plate at the next groove.
The turret is thus index by one sixth of revolution and again locked into the new position automatically.
The turret holding the next tool is now fed forward and the pawl is released from the ratchet plate by the spring pressure. Turret indexing mechanism Slide 172: When the index plate or the turret rotates through one sixth of revolution, the pin 13 and plunger 14 drops out of cam 10 and the plunger locks the index plate at the next groove.
The turret is thus index by one sixth of revolution and again locked into the new position automatically.
The turret holding the next tool is now fed forward and the pawl is released from the ratchet plate by the spring pressure. Turret indexing mechanism Slide 173: The ratio of the teeth between the pinion and gear are so chosen that when the tool mounted on the face of the turret is indexed to bring it to the cutting position, the particular stop rod for controlling the longitudinal travel of the tool is aligned with stop 12. Turret indexing mechanism Slide 174: The setting of the stop rods 8 for limiting the feed of each operation may be adjusted by unscrewing the lock nuts and rotating the stop rods on the plate.
Thus six stop rods may be adjusted for controlling the longitudinal travel of the tools mounted on the six faces of the turret. Turret indexing mechanism Slide 175: Different types of tool holders used in turret lathes Slide 176: Different types of tool holders used in turret lathes Slide 177: Different types of tool holders used in turret lathes Slide 178: Different types of tool holders used in turret lathes Slide 179: Different types of tool holders used in turret lathes Slide 180: In order to perform any work on turret lathes, proper planning for systematic operations to be carried out in advance before setting the work on lathe. The following procedures should be adopted to plan and execute a work.
For effective planning and control, for each turret lathe upto-date capacity chart is an essential requirement. This chart is supplied by the manufacturers contains every working details of the machine such as the maximum and minimum diameter of the work that can be mounted, maximum length of stroke of the turret and saddle, maximum length of the cross slide movement, tools available etc.
For tooling layout, a drawing of the finished part is required.
Proper selection of tools and tool holder is to be made.
Then the finished drawing is to be superimposed on the capacity chart and the tools to be used are drawn out in proper sequence. The length of travel of each tool is now calculated from the chart and position of stop decided.
Proper spindle speed, feed and depth of cut is then decided.
The work and the tools are then set on the machine according to the planned chart.
A typical example of such chart is given below. Tool Layout Slide 181: Tool Layout Slide 182: The planning for Production of a hexagonal bolt is given below:
The capacity chart is made available.
The drawing of the finished hexagonal bolt is taken into consideration. The tools and equipments such as bar stop, roller steady turning tool holder, roller steady bar ending tool holder, self opening die head, chamfering tool, parting tool are collected.
The drawing of the work and tools are superimposed on the capacity chart to decide the length of travel of the tool and the position of stops. Production of a hexagonal bolt Slide 183: Proper speeds and feeds for each operation are next calculated. Production of a hexagonal bolt Slide 184: Setting and machining operation are performed in the following order:
Setting of bar stops: The bar stop is placed on the first turret face. The bar stop is set at a distance of 70 mm from the collet face. An extra length of 10 mm than the bolt length is allowed, 4mm for parting off and 6 mm clearance of the collet face so that the parting off tool may penetrate deep into work without interference. Setting of the roller steady box turning tool: The roller steady box turning tool is set on the next turret face for turning a diameter of 16 mm. The stop for turning tool is set 20 mm from the collet face. Production of a hexagonal bolt Slide 185: Setting of bar ending tool: The bar ending tool is set on the next turret face and is brought into operation after turning the bar. The stop is adjusted in position accordingly. Setting of self opening die head: The self opening die head is mounted on the next face of the turret and dies are fitted into it to cut a thread of 16 mm diameter. The stop is adjusted in position keeping in view the pulling out length of the die for self releasing. Production of a hexagonal bolt Slide 186: Setting of chamfering tool: The chamfering tool is mounted on the four station turret on the cross slide and the extreme longitudinal position of the saddle is adjusted by a stop. The cross feed movement of the cross slide is also adjusted by a stop. Setting of parting off tool: The parting off tool is set on the rear tool post on the cross slide and longitudinal position of the parting tool is adjusted by the stop set at a distance of 6 mm from the turret face. Production of a hexagonal bolt