4_cutting

Traditional Manufacturing ProcessesCastingFormingSheet metal processingCuttingJoiningPowder- and Ceramics ProcessingPlastics processingSurface treatment

CuttingSawingShaping (or planing),Broaching, drilling,Grinding,TurningMilling Processes that involve removal of material from solid workpieceImportant concept: PROCESS PLANNING Fixturing and LocationOperations sequencingSetup planningOperations planning

SawingA process to cut components, stock, etc.Process character: Precision: [very low,, very high]; MRR: low

Sawing

Shaping A process to plane the surface of a workpiece (or to reduce part thickness

Process character: High MRR, medium Surface finish, dimension control

Broaching Precise process for mass-production of complex geometry parts(complicated hole-shapes)

Process character: High MRR, Very good surface, dimension control, Expensive

Drilling, Reaming, BoringProcesses to make holesProcess character: High MRR, Cheap, Medium-high surface, dimension control

Drilling basics- softer materials small point angle; hard, brittle material: larger point angle

- Length/Diameter ratio is large gun-drilling (L/D ratio ~ 300)

- Very small diameter holes (e.g. < 0.5 mm): cant drill (why?)

- F drilled hole > F drill: vibrations, misalignments,

- Tight dimension control: drill ream

- Spade drills: large, deep holes

- Coutersink/counterbore drills: multiple diameter hole screws/bolts heads

TappingProcesses to make threads in holesProcess character: low MRR, Cheap, good surface, dimension controlManual tap and die setAutomated tapping

Grinding, Abrasive MachiningProcesses to finish and smooth surfacesProcess character: very low MRR, very high surface, dimension control1. To improve the surface finish of a manufactured part (a) Injection molding die: milling manual grinding/electro-grinding. (b) Cylinders of engine: turning grinding honing lapping

2. To improve the dimensional tolerance of a manufactured part (a) ball-bearings: forging grinding [control: < 15 mm] (b) Knives: forged steel hardened grinding

3. To cut hard brittle materials (a) Semiconductor IC chips: slicing and dicing

4. To remove unwanted materials of a cutting process (a) Deburring parts made by drilling, milling

Abrasive tools and Machines

TurningProcesses to cut cylindrical stock into revolved shapesProcess character: high MRR, high surface, dimension control

Turning operations

Fixturing parts for turning

MillingVersatile process to cut arbitrary 3D shapesProcess character: high MRR, high surface, dimension control

Common vertical milling cuttersFlatBallnoseBullnose

Up and Down milling

Fixtures for Milling: Vise

Fixtures for Milling: Clamps

Process AnalysisFundamental understanding of the process improve, control, optimizeMethod: Observation modeling verificationEvery process must be analyzed; [we only look at orthogonal 1-pt cutting]

Geometry of the cutting tool

Modeling: Mechanism of cuttingOld model: crack propagationCurrent model: shear

Tool wear: observations and modelsHigh stresses, High friction, High temp (1000C) tool damage

Adhesion wear: fragments of the workpiece get welded to the tool surface at high temperatures; eventually, they break off, tearing small parts of the tool with them.

Abrasion: hard particles, microscopic variations on the bottom surface of the chips rub against the tool surface

Diffusion wear: at high temperatures, atoms from tool diffuse across to the chip; the rate of diffusion increases exponentially with temperature; this reduces the fracture strength of the crystals.

Tool wear, Tool failure, Tool life criteria Catastrophic failure (e.g. tool is broken completely) VB = 0.3 mm (uniform wear in Zone B), or VBmax = 0.6 mm (non-uniform flank wear) KT = 0.06 + 0.3f, (where f = feed in mm/revolution).

Built-up edge (BUE)Deposition, work hardening of a thin layer of the workpiece materialon the surface of the tool. BUE poor surface finish

Likelihood of BUE decreases with(i) decrease in depth of cut,(ii) increase in rake angle,(iii) use of proper cutting fluid during machining.

Process modeling: empirical resultsExperimental chart showing relation of tool wear with f and V[source: Boothroyd]

Modeling: surface finishRelation of feed and surface finish

Analysis: Machining EconomicsHow can we optimize the machining of a part ?Identify the objective, formulate a model, solve for optimalityTypical objectives: maximum production rate, and/or minimum cost

Are these objectives compatible (satisfied simultaneously) ?Formulating model: observations hypothesis theory model

Analysis: Machining Economics..Formulating model: observations hypothesis theory modelObservation:A given machine, tool, workpiece combination has finite max MRRHypothesis:Total volume to cut is minimum Maximum production rateModel objective:Find minimum volume stock for a given part

-- Near-net shape stocks (use casting, forging, )-- Minimum enclosing volumes of 3D shapesModels: - minimum enclosing cylinder for a rotational part - minimum enclosing rectangular box for a milled part

Solving: -- requires some knowledge of computational geometry

Analysis: Machining Economics..Model objective:Find optimum operations plan and tools for a given partModel: Process Planning - Machining volume, tool selection, operations sequencing

Solving: - in general, difficult to optimizeExample:oror??

Analysis: process parameters optimizationModel objective:Find optimum feed, cutting speed to [maximize MRR]/[minimize cost]/Feed:Higher feed higher MRRFinish cutting:

surface finish feedGiven surface finish, we can find maximum allowed feed rate

Process parameters optimization: feedRough cutting:MRR cutting speed, VMRR feed, f cannot increase V and f arbitrarily V MRR; surface finish f(V); energy per unit volume MRR f(V) Tool temperature V, f; Friction wear V; Friction wear f For a given increase in MRR: V lower tool life than f Optimum feed: maximum allowed for tool [given machine power, tool strength]

Process parameters optimization: Speed provided upper limits, but not optimumNeed a relation between tool life and cutting speed (other parameters being constant)Model objective:Given optimum feed, what is the optimum cutting speedTaylors model (empirically based): V tn = constant

Process parameters optimization: Speed One batch of large number, Nb, of identical parts Replace tool by a new one whenever it is worn Total non-productive time = Nbtltl = time to (load the stock + position the tool + unload the part)Nb be the total number of parts in the batch.Total machining time = Nbtmtm = time to machine the partTotal tool change time = Nttctc = time to replace the worn tool with a new oneNt = total number tools used to machine the entire batch.Cost of each tool = Ct, Cost per unit time for machine and operator = M. Average cost per item:

Process parameters optimization: SpeedAverage cost per item: Let: total length of the tool path = L t = tool life Nt = (Nb tm)/t Nt / Nb = tm / tTaylors modelVtn = C t = C 1/n / V1/n = C/V1/n

Process parameters optimization: SpeedAverage cost per item:

Process parameters optimization: SpeedOptimum speed (to minimize costs)Optimum speed (to minimize time)Average time to produce part:

Process parameters optimization: SpeedOptimum speed (to minimize costs)Optimum speed (to minimize time)Average time to produce part:load/unload timemachining timetool change timeSubstitute, differentiate, solve for V*

Process PlanningThe process plan specifies:

operationstools, path plan and operation conditionssetupssequencespossible machine routings fixtures

Process Planning

Operation sequencing examples (Milling)step holeorhole stepbig-hole step small holeorsmall hole step big-holeor

Traditional Manufacturing ProcessesCastingFormingSheet metal processingCuttingJoiningPowder- and Ceramics ProcessingPlastics processingSurface treatment

Joining ProcessesTypes of Joints:1. Joints that allow relative motion (kinematic joints)2. Joints that disallow any relative motion (rigid joints)Uses of Joints:1. To restrict some degrees of freedom of motion2. If complex part shape is impossible/expensive to manufacture3. To allow assembled product be disassembled for maintenance.4. Transporting a disassembled product is sometimes easier/feasible

Joining ProcessesFusion welding:joining metals by melting solidification

Solid state welding:joining metals without melting

Brazing:joining metals with a lower mp metal

Soldering:joining metals with solder (very low mp)

Gluing:joining with glue

Mechanical joining:screws, rivets etc.

Arc weldingOxy-acetylene weldingarc: 30,000C manualroboticGas shielded arc weldingArgonMIGTIGAlTi, Mg,Thin sectionsFusion welding

Plasma arc weldingElectron beam weldingLaser beam weldingDeep, narrow weldsAerospace, medical, automobile body panelsFaster than TIW, slower than LaserNd:YAG and CO2 lasers, power ~ 100kW Fast, high quality, deep, narrow welds deep, narrow welds, expensiveFusion welding..

Solid state weldingDiffusion welds between very clean, smooth pieces of metal, at 0.3~0.5Tm Cold welding (roll bonding)coins, bimetal strips

Solid state welding..Ultrasonic welding Medical, Packaging, IC chips, Toys

Materials: metal, plastic

- clean, fast, cheap

Resistance weldingWelding metal strips: clamp together, heat by currentSpot welding Seam welding

BrazingTorch brazingFurnace brazingTm of Filler material < Tm of the metals being joinedCommon Filler materials: copper-alloys, e.g. bronze

Common applications: pipe joint seals, ship-constructionSolderingTin + Lead alloy, very low Tm (~ 200C)

Main application: electronic circuits

Gluing

Mechanical fasteners(a) Screws (b) Bolts, nuts and washers (c) Rivets (a) pneumatic carton stapler (b) Clips (c) A circlip in the gear drive of a kitchen mixer Plastic wire clipsWire conductor: crimpingPlastic snap-fasteners

Traditional Manufacturing ProcessesCastingFormingSheet metal processingCuttingJoiningPowder- and Ceramics ProcessingPlastics processingSurface treatment

Surface treatment, Coating, PaintingImproving the hardnessImproving the wear resistanceControlling friction, Reduction of adhesion, improving the lubrication, etc.Improving corrosion resistanceImproving aestheticsPost-production processesOnly affect the surface, not the bulk of the material

Mechanical hardening Shot peening precision auto gears[source: www.vacu-blast.co.uk][source: www.uwinint.co.kr]Shot peening Laser peening

Case hardening

ProcessDopantProcedureNotesApplicationsCarburizingCLow-carbon steel part in oven at 870-950C with excess CO20.5 ~ 1.5mm case gets to 65 HRC; poor dimension controlGears, cams, shafts, bearingsCarboNitridingC and NLow-carbon steel part in oven at 800-900C with excess CO2 and NH30.07~0.5mm case, up to 62 HRC, lower distortionNuts, bolts, gearsCyanidingC and NLow-carbon steel part in bath of cyanide salts with 30% NaCN0.025~0.25mm case, up to 65 HRCnuts, bolts, gears, screwsNitridingNLow-carbon steel part in oven at 500-600C with excess NH30.1~0.6mm case, up to 1100 HVtools, gears, shaftsBoronizingBPart heated in oven with Boron containing gasVery hard, wear resistant case, 0.025~0.075mmTool and die steels

Vapor deposition Deposition of thin film (1~10 mm) of metalSputtering: important process in IC Chip manufacture

Thermal sprayingHigh velocity oxy-fuel sprayingThermal metal powder sprayPlasma spray Tungsten Carbide / Cobalt Chromium Coatingon roll for Paper Manufacturing Industry [source: www.fst.nl/process.htm]

ElectroplatingDeposit metal on cathode, sacrifice from anodeAnodizing chrome-plated auto partscopper-platingMetal part on anode: oxide+coloring-dye deposited using electrolytic process

PaintingType of paints:

Enamel: oil-based; smooth, glossy surface Lacquers: resin based; dry as solvent evaporates out; e.g. wood varnish Water-based paints: e.g. wall paints, home-interior paintsPainting methods

Dip coating: part is dipped into a container of paint, and pulled out. Spray coating: most common industrial painting method Electrostatic spraying: charged paint particles sprayed to part using voltage Silk-screening: very important method in IC electronics mfg

PaintingElectrostatic Spray Painting Spray Painting in BMW plant Silk screening

These notes covered processes: cutting, joining and surface treatment

We studied one method of modeling a process, in order to optimize it

We introduced the importance and difficulties of process planning.SummaryFurther reading: Chapters 24, 21, 30-32: Kalpajian & Schmid

Friction wear: increase in V increases relative speed of tool and chip; however, increase in Feed increases area of contact of chip and tool rate of tool wearis not affected, just the area of wear is larger.Avg cost = Non-productive cost + Machining time cost + Tool change time cost + Tool costWe need to replace all time components in terms of VelocityThis gives us cost per unit as a function of Velocity.Note: the solution is different.