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99 Cards in this Set
- Front
- Back
forming
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indicates changing the shape of an existing solid body
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shaping
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involves molding and casting of soft or molten materials, finished product is usually at or near the desired shape, forcing molten polymer into a mold and letting it solidify
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continuous products
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made by rolling, extrusion, drawing processes
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discrete products
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made by forging, forming, ...
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ingot
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metal cast into a block shape for convenience, to be further processed
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how are cast structures converted to wrought structures?
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by plastic-deformation
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sinter
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heat without melting
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forming and shaping processes
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rolling (flat, shape), forging, extrusion, drawing, sheet-metal forming, powder metallurgy, processing of plastics and composite materials, forming and shaping of ceramics
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flat rolling
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high speed foil production
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shape rolling
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high speed production of i-beams and rails
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forging
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production of discrete parts using dies
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extrusion
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production of long, constant shapes; cut into desired lengths
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drawing
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production of long rod and wire
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sheet-metal forming
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production of simple or complex shapes with thin walls
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powder metallurgy
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production of simple or complex shape by compacting and sintering metal powders
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processing of plastics and composite materials
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production of a wide variety of continuous or discrete products by extrusion, molding, casting, and fabricating processes
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forming and shaping of ceramics
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production of discrete products by various shaping, dring, firing processes
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rolling
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process of reducing the thickness or changing the cross section of a long workpiece by compressive forces applied through a set of rolls
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which forming and shaping process accounts for 90% of all metals produced by metalworking processes?
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rolling
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hot rolling
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first stage of rolling, rolling carried out at high temps, coarse-grained, brittle, and porous structure of ingot is broken down into a wrought structure with finer grain size and better properties
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cold rolling
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subsequently, rolling carried out at room temp
-good: improves strength and hardness and surface finish -bad: causes anisotropy |
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why does cold rolling require more energy than hot rolling?
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because of higher strength of the material at room temp
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anisotropic
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has different properties when measured in different directions
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plates
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high thickness (ship hulls, boilers, bridges, ...), more than 6mm
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sheets
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low thickness (automobile bodies), less than 6mm
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foil
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used for wrapping candy, thickness of .008mm
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flat-rolling process
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metal strip with thickness no enters roll gap and is reduced to thickness nf by rotating rolls (surface speed of rolls =Vr), velocity of strip increases from Vo (enters roll gap) to Vf (exits roll gap, highest)
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roll gap
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space between rolls
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neutral point / no-slip point
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-point along contact length where the velocity of the strip is the same as the roll
-left of neutral point, roll is faster -right of "", strip is faster |
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maximum draft (eq)
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different between initial and final thickness, ho-hf, function of roll radius (R) and coeff. of fric (u) s.t. ho-hf=(u^2)R
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roll force (eq)
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F=Lw(Yavg), F=roll force, L=roll-strip contact length, w=width of strip, Yavg=avg true stress of strip in roll gap
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how do you modify roll force eq to get ACTUAL roll force?
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add 20%
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torque
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F*a, a=L/2
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Power (kW)
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=(2piFLN)/60,000, N=revolutions/min of roll, F=newtons, L=meters
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Power (hp)
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=(2piFLN)/33,000, F=lbs, L=ft
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roll stand
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includes housing, chocks, bearings
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ways to reduce roll forces
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reduce friction at roll-workpiece interface, use smaller diameter rolls to reduce contact area, take smaller reductions per pass to reduce contact area, rolling at elevated temps to lower strength of the material, apply front or back tension to strip
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back tension
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tension at the entry zone, applied to sheet by applying a braking action to the reel that supplies the sheet into the roll gap
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front tension
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tension at the exit zone, applied to sheet by increasing rotational speed of the take-up reel
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steckel rolling
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process carried out by front tension only, with no power supplied to the rolls
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roll bending
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tendency of rolls to bend during rolling, caused by the difference between forces required to bend strip at center and edges
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crown
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tendency of rolled strip to be thicker at center than edges, result of roll bending
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camber
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way to counteract crown by grinding rolls in such a way that their diameter at center is slightly larger than at edges, so that strip has contant uniform thickness
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what is the difference between the radius of the max camber point and and the edges of the roll
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radius of max camber point is generally .25 mm greater than that of the edges
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thermal camber
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tendency of rolls to become barrel shaped because of the heat generated by plastic deformation during rolling
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what happens to the width of the strip when rolling plates and sheets have high width-to-thickness ratios?
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the width of the strip remains effectively constant
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spreading
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with small width-to-thickness ratios, width increases significantly as it passes through the rolls, like rolling dough with a rolling pin
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spreading increases with:
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a. decreasing width-to-thickness ratios
b. increasing friction c. decreasing ratio of the roll radius to the strip thickness |
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how do you prevent spreading?
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using additional rolls in contact with the edges of the rolled product in the roll gap (edger mills)
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chatter
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-self-excited vibration
-in rolling it leads to periodic variations in the thickness of the rolled sheet and in its surface finish, and consequently, leads to excessive scrap |
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tandem mills
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where chatter in rolling have been found to occur predominantly
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how much does chatter slow rolling?
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by 50%
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cause of chatter?
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interaction between structural dynamics of the mill stand and the dynamics of the rolling operation
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ways to reduce chatter
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increasing dist between the stands of the rolling mill, increasing the strip width, decreasing the reduction per pass (draft), increasing roll radius, increasing strip-roll friction, incorporating dampers in the roll supports
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cast structure
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dendritic with coarse and nonuniform grains, brittle and porous
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dendritic
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having a branching structure similar to a tree
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flat rolling process
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1. break down material by hot rolling, converts cast structure to wrought structure-->resulting wrought structure is bloom, slab, or billet
2. cold rolling |
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wrought structure
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finer grains and better ductility by breaking up brittle grain boundaries and closing up internal defects (porosity)
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bloom
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square cross section, becomes i-beams and railroad rails
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slab
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rectangular cross section, becomes plates and sheets
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billet
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square with smaller cross section than bloom, rods and bars
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conditioned
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prepared for subsequent operation; surface of bloom, slab, and billet is usually conditioned before rolling
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scarfing
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conditioning by torch
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pickling
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acid etching, by blasting with water or grinding
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pack rolling
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flat-rolling operation in which 2 or more layers of metal are rolled together to increase productivity
ex: foil is pack rolled in 2 layers, only the top and bottom outer layers have been in contact with rolls, foil-to-foil side is matte, foil-to-roll side is shiny |
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yield-point elongation
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phenomenon that causes surface irregularities called stretcher strains or luder's bands, happens to rolled mild steel when subsequently stretched during sheet-forming operations
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how do you correct yield-point elongation?
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sheet metal undergoes a final light pass of 1% reduction known as temper rolling or skin pass
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leveling rolls
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to improve flatness, rolled strip goes through series of "", the workpiece is flexed in opposite directions as it passes through the sets of rollers
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(T/F) Each roller is usually powered individually by a separate motor.
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T
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surface defects
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scale, rust, scratches, gouges, pits, cracks
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wavy edges
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result of roll bending, strip is thinner along edges than at its enter causing the edges to elongate more than the center, the edges buckle because they are constrained by the central region from expanding freely in the longitudinal (rolling) direction
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alligatoring
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complex phenomenon and typically is caused by nonuniform bulk deformation of the billet during rolling or by the presence of defects in the original cast material
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residual stresses in rolled metals
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small diameter rolls: compressive "" on surfaces and tensile "" in bulk
large diameter rolls: opposite |
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gage number
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number identifying the thickness of a sheet, smaller=thicker
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two-high rolling mills
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used for hot rolling in initial breakdown passes on cast ingots or in continuous casting with roll diameters from .6m to 1.4m
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three-high mill
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direction of material movement is reversed after each pass, using elevator mechanism and various manipulators
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four-high mills and cluster mills (sendzimir or Z mill)
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based on the principle that small-diameter rolls lower roll forces (why? because of small roll-strip contact area) and power requirements and reduce spreading, also smaller rolls are cheaper to replace
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tandem rolling
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strip is rolled continuously through a number of stands to thinner gages with each pass, speed and thickness of strip is critical
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stand
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consists of a set of rolls with its own housing and controls
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train
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group of stands
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heat checking
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cracking from thermal cycling, results from using rolls made for cold rolling for hot rolling
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spalling
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cracking or flaking of surface layers
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why does the bottom surface of an aluminum beverage can have longitudinal scratches on it?
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because the surface of the can is a replica of the surface finish of the roll, which is produced by grinding, makes it easy to see the rolling direction of the original aluminum sheet
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are lubricants used in hot rolling?
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hot rolling of FERROUS alloys is carried out without lubricants although graphite may be used, NONFERROUS alloys are hot rolled with various compounded oils, emulsions, and fatty acids
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are lubricants used in cold rolling?
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cold rolling is carried out with water-soluble oils or low-viscosity lubricants, such as mineral oil, emulsions, paraffin, fatty oils
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shape rolling
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forms straight and long structural shapes (i-beams, rails), the stock goes through a set of specially designed rolls
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cold shape rolling
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can be done with the starting materials in the shape of wire with various cross sections, roll-pass design requires considerable experience
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roll-pass design
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the design of a series of rolls, requires considerable experience to avoid external and internal defects, hold dimensional tolerances, and reduce roll wear
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roll forging (also cross rolling)
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the cross section of a round bar is shaped by passing it through a pair of rolls with profiled grooves, used to produce tapered shafts and leaf springs, table knives
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skew rolling
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similar to roll forging, used for making ball bearings, round wire is fed into the roll gap and spherical blanks are formed by the action of rotating rolls
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ring rolling
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a thick ring is expanded into a large-diameter thinner one by placing it between 2 rolls and driving 1 while the other is idle, thickness is reduced by bringing the rolls closer together as they rotate
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thread rolling
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cold-forming process by which straight or tapered threads are formed on round rods or wire, threads are formed with each stroke of a pair of flat reciprocating dies, makes screws and bolts
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thread rolling advantages
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threads with good strength and without loss of material, smooth surface finish, induces compressive residual stresses on surfaces (thus improving fatigue life)
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internal thread rolling
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carried out with a fluteless forming tap, similar to external thread rolling, produces internal threads with good strength
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lubrication in thread rolling
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important to obtaining good surface finish and surface integrity
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rotary tube piercing
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also mannesmann process, hot-working operation for making long, thick-walled seamless pipe and tubing
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tube rolling
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used to reduce diameter and thickness of pipes and tubing
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integrated mills
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large facilities that involve complete integration of activities, from production of hot metal in a blast furnace to casting and rolling of finished products
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minimills
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scrap metal is:
a. melted in electric-arc furnaces b. cast continuously c. rolled directly into specific lines of products each minimill makes 1 type of product (rod, bar), scrap metal is obtained locally from old machinery (cars) |