Machining Processes

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### Machine Tools - A machine tool shapes or machines metal/rigid materials by cutting, boring, grinding, shearing, or other deformation. - They employ a tool for cutting or shaping. - **Examples:** Broaching machine, Drill press, Gear shaper, Hobbing machine, Hone, Lathe, Screw machines, Milling machine, Shear (sheet metal), Shaper, Saws, Planer, Stewart platform mills, Grinding machines, Multitasking machines (MTMs - CNC with multiple axes for turning, milling, grinding, and material handling). ### Lathe Machine - Rotates a workpiece on its axis for operations like cutting, sanding, knurling, drilling, deformation, facing, turning. - Tools are applied to the workpiece to create objects with symmetry about an axis of rotation. - Used to remove metal from a workpiece to achieve a required shape and size. #### Block Diagram of Lathe Machine ### Parts of Lathe - **Bed:** Supports all major components. - **Carriage:** Slides along the ways; consists of cross-slide, tool post, apron. Used for longitudinal feed. - **Headstock:** Holds workpiece jaws, supplies power to jaws, and provides various drive speeds. - **Tailstock:** Supports the other end of the workpiece. - **Feed Rod and Lead Screw:** - Feed rod powered by gears from the headstock. - Lead screw transmits power from headstock to carriage for screw thread cutting. - Lead screw for threading, feed rod for turning. - Automatic feed requires engagement of feed rod or lead screw. - **Stand (or Legs):** Holds the lathe machine and elevates the bed to working height. - **Spindle:** Hollow horizontal axle with interior/exterior threads for mounting workpieces. - **Cross-slide:** Flat piece on the bed, perpendicular to the bed. Used for cross feed. - **Tool Post:** Sits on cross-slide, holds cutting tool. - **Tool Rest:** Horizontal area in line with spindle/tailstock, used to brace hand tools. ### 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 #### Cutting-tool-holding devices - Straight and offset toolholders - Threading toolholders, boring bars - Turret-type tool posts #### 1. Lathe Centers - Work turned between centers must have a center hole drilled in each end. - Provides support during cutting. - Most common: solid Morse taper shank 60° centers, steel with carbide tips. - Requires occasional adjustment and lubrication. ##### Type of Centers - **Live Centre (Revolving Centre):** Constructed so that the 60° center runs in its own bearings. Fits in the spindle (headstock) and rotates with it. - **Dead Centre:** Fits in the tailstock spindle, remains stationary while the work rotates. - **Difference:** Live center revolves with the work (headstock spindle), dead center remains stationary (tailstock spindle). #### 2. Chucks - Mounted on the headstock, manually or power actuated. - Holds rotating workpieces (bars, irregularly shaped objects). ##### Three-jaw Universal Chuck - Holds round and hexagonal work. - Grasp work quickly and accurately. - Three jaws move simultaneously via a chuck key. ##### Four-Jaw Independent Chuck - Holds round, square, hexagonal, and irregularly shaped workpieces. - Has four jaws, each adjustable independently via a chuck key. - Jaws can be reversed for inside diameter gripping. ##### Difference between Universal Chuck and Independent Chuck | Universal Chuck | Independent Chuck | |-----------------------------|-------------------------------| | Setting up workpiece is easy. | Setting up workpiece is difficult. | | Less gripping power. | More gripping power. | | Depth of cut is less. | Depth of cut is more. | | Heavier jobs cannot be turned. | Heavier jobs can be turned. | #### 3. Steady Rest / Fixed Steady - Used to support long work held in chuck or between lathe centers. - Located and aligned by ways of the lathe. - Positioned at any point along the lathe bed. - Three jaws (plastic, bronze, or rollers) adjust to support any workpiece diameter. #### 4. Travelling Steady/Follower Rest - Mounted on the saddle. - Travels with the carriage to prevent work from springing up and away from the cutting tool. - Cutting tool generally positioned just ahead of the follower rest. - Provides a smooth bearing surface for two jaws. ##### Differences between Steady Rest and Follower Rest - Both hold long workpieces steady during turning. - Steady rest: Mounted to lathe bed, does not move with lathe. - Follower rest: Fastened to carriage, moves with cutting tool. #### 5. Faceplates - Used for holding workpieces that cannot be conveniently held in a chuck. #### 6. Lathe Dog or Carrier - Rotates work placed on a mandrel or held between centers by clamping to the end of the work. - Engaged with a pin attached to the drive plate or face plate. #### 7. Mandrels - Cylindrical bar with center hole at each end. - Used to hold hollow workpieces for machining external surfaces. - Work revolves with mandrels mounted between lathe centers. ### Working Principle of Lathe - Holds work between two supports (centers) or with a chuck/face plate. - Chuck or face plate is mounted on the machine spindle. - Cutting tool is held and supported on a tool post. - Workpiece rotates about the spindle axis. - Tool is fed against the revolving work. - Tool movement is parallel or inclined to the work axis. - Parallel movement forms a cylindrical surface. - Inclined movement produces a taper surface (taper turning). ### Lathe Operations - **Turning:** Produce straight, conical, curved, or grooved workpieces. - **Facing:** Produce a flat surface at the end of the part or make face grooves. - **Boring:** Enlarge an existing hole or cylindrical cavity; produce circular internal grooves. - **Drilling:** Produce a hole by fixing a drill in the tailstock. - **Threading:** Produce external or internal threads. - **Knurling:** Produce a regularly shaped roughness on cylindrical surfaces. - **Chamfering:** Essential operation after thread cutting. ### Classification of Lathe Most lathes used today fall into these classifications: #### i. Speed Lathe - Named for its high headstock spindle speed (1200 to 3600 rpm). - Simplest form: headstock, tailstock, tool-post. - No gearbox, lead screw, or carriage. - Tools are hand operated. - **Cone-pulley** is the only source for spindle speed variation. - Applied in wood turning, metal spinning, and polishing. #### ii. Engine Lathe or Centre Lathe - Most important and widely used machine tool in the lathe family. - Early models driven by separate engines or central engines with overhead belts/shafts. - **Stepped cone-pulley** or **geared head** vary spindle speed. - Cutting tools controlled by hand or power, fed crosswise and longitudinally via carriage, feed rod, and lead screw. - Can fit a wide range of attachments. #### iii. Turret Lathe - Performs many operations simultaneously. - Several tools set on a **revolving turret** for efficient operation with minimal time wastage. - Indexable square tool post on cross-slide for turning and parting-off tools. - Turret accommodates six tools (drilling, countersinking, reaming, tapping, etc.), brought into position by indexing. - Widely used for repetitive batch production. #### iv. Capstan Lathe - Similar to turret lathe. - Hexagonal turret carried on a slide mounted in a saddle bolted to the bed. (Turret lathe's turret is on a slide directly on the bed). - Best suited for fast production of small parts due to light weight and short stroke of capstan slide. #### v. Tool Room Lathe - Modern engine lathe with all necessary accessories for accurate tool room work. - Geared head driven machine with considerable range in spindle speeds and feed. - Best suited for small tools, dies, gauges production. #### vi. Bench Lathe - Small lathe mounted on a workbench for small precision and light jobs. #### vii. Gap Bed Lathe - Has a gap in the bed near the headstock to handle workpieces with flanges or other protruding parts. #### viii. Hollow Spindle Lathes - Spindles have large through bores to facilitate turning ends of long tubular workpieces. #### ix. Vertical Turret Lathes - Vertical orientation, used for turning large components easily mounted on the machine table. #### Difference between Centre Lathe and Capstan Lathe | Centre Lathe | Capstan Lathe/Turret Lathe | |---------------------------|--------------------------------| | Manually operated | Semi-automatic | | Fewer speeds | More speeds | | One operation at a time | More than one operation | | Has tailstock | Turret head instead of tailstock | | Only one tool fitted | Six different tools fitted | | Longer tool changing time | Shorter tool changing time | #### Difference between Capstan Lathe & Turret Lathe | Capstan Lathe | Turret Lathe | |-----------------------------------------------|-----------------------------------------------------| | Light duty machine. | Heavy duty machine. | | Short workpiece can be machined. | Long workpiece can be machined. | | Saddle does not move during machining. | Saddle moves during machining with the turret. | | Turret head mounted on ram, ram on saddle. | Turret head directly mounted on the saddle. | #### Copying Lathe - Reproduces shapes faithfully and consistently from a template. - Types: Mechanical, Hydraulic, Electric. #### Hydraulic Type (Copying Lathe) - Servo-motor system that magnifies small input force/signal to provide large output force/signal for machine operation. ### Size and Specification of Lathe Lathe size is specified by: 1. Height of centers measured over the lathe bed. 2. Swing or maximum diameter over the bed ways. 3. Swing or diameter over carriage (largest diameter over the saddle). 4. Maximum job length (mm) between centers (headstock and tailstock). 5. Bed length (meters), including headstock length. 6. Diameter of the hole through the lathe spindle for bar material. Additional specifications for ordering a lathe: i. Length, width, and depth of bed. ii. Depth and width of the gap (for gap lathes). iii. Swing over gap. iv. Number and range of spindle speeds. v. Number of feeds. vi. Lead screw diameter. vii. Number and range of metric threads that can be cut. viii. Tailstock spindle travel. ix. Tailstock spindle set over. x. Back gear ratio. xi. Power rating of electric motor. ### Milling Machine - Machining process using rotary cutters to remove material. - Cutter advances (feeds) at an angle to its axis. - Covers diverse operations from small parts to heavy-duty gang milling. - Widely used for machining parts to precise sizes and shapes. - Milling cutter: Rotary cutting tool, often with multiple cutting points. - Cutter usually moves perpendicular to its axis, cutting on its circumference. - Cutting edges (flutes/teeth) repeatedly cut into and exit material, shaving off chips via **shear deformation**. #### Specifications of Milling Machine 1. **Size of the work table:** Length x width (e.g., 1500 x 30mm). 2. **Longitudinal movement (X-direction):** Total table movement (e.g., 800mm). 3. **Transverse movement (Y-direction):** Total saddle movement with table (e.g., 200mm). 4. **Vertical movement (Z-direction):** Total table, saddle & knee movement (e.g., 380mm). 5. **Range of the speed:** Speed variation in gearbox (RPM) (e.g., 45 to 200 rpm). 6. **Power capacity of the motor:** (HP) (e.g., 2 HP). #### Principal Parts - **Base:** Provides rest for all parts, made of grey iron casting. - **Column:** Rigid vertical box housing spindle driving mechanism; knee fixed to guide ways. - **Knee:** Adjustable height on column, houses table's **feed mechanism** and controls. - **Saddle:** On top of knee, provides **guide ways** for table movement. - **Table:** Rests on saddle, has T-slots for clamping workpieces. **Feed motions** controlled by lead screw. - **Overhanging Arm:** Mounted on column, **serves a bearing support for the arbor**. Adjustable for cutter bearing support. - **Arbor:** Holds **rotating milling cutters** rigidly, mounted on spindle. Can be supported as a cantilever (stub arbor). - **Front Brace:** Adjusts relative position of knee and overhanging arm; extra support for rigidity. - **Spindle:** Projects from column face, tapered hole for arbor. Accuracy, strength, rigidity are critical. Transfers motive power to arbor. ### Types of Milling Machines #### I. Knee and Column Type a. Horizontal b. Vertical c. Universal d. Turret type #### II. Production (Bed) type a. Simplex b. Duplex c. Triplex #### III. Plano Millers #### IV. Special Type a. Rotary table b. Drum type c. Copy milling d. Key way milling machines e. Spline shaft milling machines #### Universal Milling Machine - Performs a **large variety of operations**. - Distinguishing feature: **table** mounted on a circular swiveling base with degree graduations. - Table can be **swiveled to any angle**. - **Helical milling operation** possible as table can feed cutter at an angle. - Large number of auxiliaries (dividing head, vertical milling attachments, rotary table) make it versatile. ### Milling Operations 1. **Angular Milling:** For angular cuts like V-notches & grooves (double angle cutter). 2. **Form Milling:** Produces various types of forms (form cutter). 3. **Plain Milling/Slab:** To get flat surface on workpiece (plain cutter). 4. **Face Milling:** Produces flat surface (face milling cutter). 5. **End Milling:** Produces flat surface (end milling cutter). 6. **Slot Milling:** Produces slots like T-slot, plain slots. 7. **Gang Milling:** Produces many surfaces of a workpiece. #### Two Major Classes of Milling Process - **Face Milling:** - Cutting action primarily at the **end corners** of the milling cutter. - Used to cut flat surfaces (faces) or flat-bottomed cavities. - Cutter axis is **perpendicular** to the surface being machined. - **Peripheral Milling:** - Cutting action primarily along the **circumference** of the cutter. - Milled surface cross-section receives the shape of the cutter. - Cutter axis is **parallel** to the surface being machined and cutting edges are on the outside periphery. ### Indexing - Method of dividing the periphery of a job into an equal number of divisions. - Operation of rotating the job through a required angle between successive cuts. - Accomplished with a **dividing head**, an accessory to the milling machine. - Divides job periphery into equal divisions. - To rotate a job through a required angle, one needs: - A device to rotate the job (index-crank). - A source to ensure job rotation through the desired angle (index plate). - Index plate has concentric circles with equally spaced holes. - Crank rotation transmitted to job via gear. - **Ratio of crank to shaft (job mounted) is 40:1** (40 index crank revolutions = 1 spindle revolution). #### Methods of Indexing 1. Direct Indexing 2. Simple Indexing 3. Compound Indexing 4. Differential Indexing 5. Angular Indexing Method #### Index Plate Types ##### Brown and Sharpe Type - 3 plates, 6 circles each. - Plate I: 15, 16, 17, 18, 19, 20 holes - Plate 2: 21, 23, 27, 29, 31, 33 holes - Plate 3: 37, 39, 41, 43, 47, 49 holes ##### Cincinnati Type - One plate, drilled on both sides. - First side: 24, 25, 28, 30, 34, 37, 38, 39, 41, 42, 43 holes - Second side: 46, 47, 49, 51, 53, 54, 57, 58, 59, 62, 66 holes #### Simple Indexing or Plain Indexing - Index plate selected, fitted on worm shaft, locked via pin. - Work indexed by withdrawing index crank pin, turning crank a calculated number of revolutions and holes, then relocating pin. - Formula: $N = 40 / Z$, where $Z$ is the number of divisions. - Example: 8 divisions -> $N = 40/8 = 5$ turns. - Example: 10 divisions -> $N = 40/10 = 4$ turns. - For 16 divisions: $40/16 = 2.5$ turns = 2 complete rotations + 8 holes on the 16-hole circle (Plate I). ### Milling Machine Comparison #### Difference Between Horizontal and Vertical Milling Machine | Horizontal Milling Machine | Vertical Milling Machine | |--------------------------------------|------------------------------------| | Spindle horizontal to worktable. | Spindle vertical to worktable. | | Cutter cannot move up/down. | Cutter can move up/down. | | Spindle cannot be tilted. | Spindle can be tilted. | | Operations: plain, form, gang milling. | Operations: slot, T-slot, angular, flat milling. | | Cutter mounted on arbor. | Cutter directly mounted on spindle. | #### Difference Between Up Milling & Down Milling | Up Milling (Conventional Milling) | Down Milling (Climb Milling) | |--------------------------------------|------------------------------------| | Workpiece fed opposite cutter direction. | Workpiece fed same cutter direction. | | Cutting force directed upwards. | Cutting force directed downwards. | | Strong clamping required. | Strong clamping not required. | | Chips progressively thicker. | Chips progressively thinner. | | Poor surface finish. | Good surface finish. | | Used for hard material. | Used for soft material. | ### Shaper - Machine tool producing flat surfaces (horizontal, vertical, inclined) depending on cutting tool orientation. #### Features - Single point cutting tool used. - Tool clamped in tool post on ram. - Ram reciprocates; tool cuts on forward stroke, no cutting on return stroke. - Job held rigidly in a vice. #### Principle of Working - Job fixed rigidly on machine table. - Single point cutting tool on reciprocating ram. - Reciprocating motion via quick return mechanism. - Tool cuts during forward stroke; return stroke is idle (no cutting). - Forward and return strokes form one operating cycle. - Quick return mechanism (crank and slotted-link design) reduces idle time. #### Quick Return Mechanism - Mechanism for faster return stroke (than cutting stroke). - Converts rotary motion to reciprocating motion. - Reduces idling time. Shaper machine is a prime example. ##### Working of Quick Return Mechanism - Crank AB (adjustable length R) rotates at uniform angular speed. - Crank pin B (die block) slides in slotted lever OBC. - Lever OBC pivoted at O, other end C connected to ram. - Crank AB rotation from AB1 to AB2 moves ram forward (left to right). - Crank AB rotation from AB2 to AB1 returns ram to original position. - Forward stroke time proportional to angle $\alpha$, return stroke time proportional to angle $\beta$. ##### Why Quick Return Mechanism? - Shaper cuts only on forward stroke; backward stroke is idle (waste). - To minimize waste, return stroke must be as quick as possible. #### Specification of Shaper Machine 1. Maximum length of stroke of Ram 2. Power input 3. Floor space required 4. Weight of the machine 5. Cutting to Return Stroke ratio range 6. Feed range 7. Adjustable range (depth) #### Difference Between Shaper and Planer Machine | Shaper Machine | Planer Machine | |--------------------------------------|--------------------------------------| | Cutting tool reciprocates. | Cutting tool stationary. | | Workpiece stationary. | Workpiece reciprocates. | | Suitable for small workpieces. | Suitable for larger workpieces. | | Tools are smaller. | Tools are larger. | | Only lighter cuts can be taken. | Only heavier cuts can be taken. | | Less initial cost. | High initial cost. | | Occupies smaller work area. | Occupies larger work area. | ### Other Machining Operations #### Boring Operation - Enlarging an already drilled or cast hole using a single-point cutting tool or boring head. - Example: Boring a gun barrel or engine cylinder. #### Broaching Operation - Machining process using a toothed tool (broach) to remove material. - Types: linear and rotary. - **Linear broaching:** Broach runs linearly against workpiece. - Used for precision machining, especially for odd shapes (circular/non-circular holes, splines, keyways, flat surfaces). - Workpieces: small to medium-sized castings, forgings, screw machine parts, stampings. - Broaches are expensive but favored for high-quantity production. - Broaches are saw-like, with teeth height increasing along length. - Three sections: roughing, semi-finishing, finishing. - Feed is built into the tool; machined surface profile is inverse of broach profile. #### Sawing Operation - Uses a multipoint cutting tool (saw). - Many teeth move through workpiece; each successive tooth deepens the cut. - Feed given to saw or workpiece. #### Honing Machine - Abrasive machining process creating a precision surface. - Abrasive stone scrubs workpiece along a controlled path. - Primarily improves geometric form, also surface texture. - Applications: finishing cylinders for engines, air bearing spindles, gears. - Types of hones: one or more abrasive stones under pressure. - Hone usually turned in bore while moving in/out. - Special cutting fluids used for smooth cutting and material removal. - Machines can be portable, manual, or fully automatic. #### Hobbing Machine - Machining process for gear cutting, splines, and sprockets. - Special type of milling machine. - Teeth or splines progressively cut into workpiece by a series of cuts from a hob (cutting tool). ### Drilling, Boring, and Reaming #### Difference among Drilling, Boring, and Reaming ##### Drilling - For making cylindrical bore. - Drill bit is used. ##### Boring - For enlarging a drilled hole. - Boring bar is used. ##### Reaming - For finishing holes or slightly removing material. - Reamer is the cutting tool. #### Difference among Counter Boring, Counter Sinking & Counter Drilling ##### Counter Boring - Enlarge one end of an existing hole, concentric with original hole. ##### Counter Sinking - Enlarge top portion of an existing hole to a conical shape. ##### Counter Drilling - Drill in the opposite direction. ### Cutting Tools - Any tool used to remove material from a workpiece to create required size and shape. #### Types of Cutting Tools 1. Single Point Cutting Tool 2. Double Point Cutting Tool 3. Multi Point Cutting Tool #### Characteristics of Cutting Tools - Hot hardness - Wear resistance - Toughness - Chemical affinity - Favorable cost - Availability - Low coefficient of friction - Shock resistance #### Difference between Single Point and Multi Point Cutting Tool | Single Point Cutting Tool | Multi Point Cutting Tool | |-------------------------------------------|-------------------------------------------| | One cutting edge. | Two or more cutting edges. | | Simple shape. | Complex shape. | | Generally used in lathe machine. | Generally used in milling and drilling machines. | #### Cutting Tool Material - Diamond - Tool steel - High speed steel - High carbon steel - Ceramics ### Working and Auxiliary Motion - **Auxiliary motion:** Motion not directly part of surface formation but essential for working motion to fulfill its functions (e.g., clamping/unclamping workpiece). - Machining process involves a relationship between cutting speed and feed rate. - **Cutting speed (V):** m/min - **Feed rate (S):** - mm/rev (rotary motion: lathe, boring, drilling) - mm/tooth (M.T.C: milling machine) - mm/stroke (reciprocating motion: shaping, planning machine) - mm/min (milling machine) - **For Rotary working motion:** $V = \pi DN/1000$ - D = diameter (mm) - N = revolutions per minute (rpm) - **For Reciprocating motion:** $V = L/(1000 T_c)$ - L = length (mm) - $T_c$ = time of cutting stroke (min) - **Relation between cutting stroke and cutting speed:** $V = n \cdot L \cdot (K+1)/(1000K)$ - $K = T_c/T_i$ - $T_i$ = time of idle stroke (min) - $n$ = number of strokes per minute = $1/(T_c+T_i) = K/(T_c(1+K))$ - **Feed per revolution and feed per stroke to feed per minute:** $S_m = S \cdot n$ - $S_m$ = feed per minute - $S$ = feed per rev or stroke - $n$ = number of rev or stroke per minute - **Feed per tooth in multiple cutter:** $S = S_z \cdot Z$ - $S$ = feed per rev or stroke - $Z$ = number of teeth on cutter - $S_z$ = feed per tooth of cutter - **Machining time (Tm):** $T_m = L/S_m$ - $L$ = length of machined surface - $S_m$ = feed per minute ### Mathematical Problems - **Sample Problem 1:** Determine the spindle speed for a high steel drill (12 mm diameter) cutting medium carbon steel at 28 m/min. - **Sample Problem 2:** Determine the machining time to drill a 20 mm diameter hole in a 25 mm thick workpiece with a drill at a cutting speed of 30 m/min and a feed of 0.2 mm/rev. - **Sample Problem 3:** A 30 mm H.S.S. drill is used to drill a hole in a cast iron block (100 mm thick). Determine the time required to drill the hole with a feed of 0.30 mm/rev. Assume an over travel of drill as 4 mm. The cutting speed is 20 m/min. - **Sample Problem 4:** A 600mm×30mm flat surface of a plate is to be finish machined on a shaper. The plate is fixed with the 600mm side along the tool travel direction. If the tool over-travel at each end of the plate is 20mm, average cutting speed is 8m/min, feed rate is 0.3mm/stroke and the ratio of return time to cutting time of the tool is 1:2, Determine the time required for machining. - **Sample Problem 5:** Bars of 250mm length and 25mm diameter are to be turned on a lathe with a feed of 0.2mm/rev. Each regrinding of the tool costs is Rs. 20. The time required for each tool change is 1 min. Tool life equation is given as $VT^{0.2}=24$ (where cutting speed V is in m/min) and tool life T is in min). Determine the optimum tool cost per piece for maximum production rate. - **Sample Problem 6:** A workpiece of 2000mm length and 300mm width was machined by a planning operation with the feed set at 0.3mm/stroke. If the machine tool executes 10 double strokes/min, Determine the planning time for a single pass. - **Sample Problem 7:** A 31.8 MM HSS drill is used to drill a hole in cast iron block 100 mm thick at cutting speed of 20 m/min and feed 0.3 mm/rev. If the over travel of drill is 4 mm and approach 9 mm, determine the time required to drill the hole.