IE 215: Exam 3

Discuss the reasons why most welding operations are inherently dangerous.

-Carried out at a very high temperature= harmful injuries -Gas welding process= fire hazard fuels used -Resistance and arc welding= high electrical supply (shocks) -Sparks, fumes, spatter, smoke

Why is a surfacing weld different from the other weld types?

-Done to place the filler materials onto the surface of the base metal in the form of weld beads -It is not made to join parts, but only to add filler metal to the surface

What are some of the simple measures used to assess the feasibility of a proposed cup-drawing operation?

-Drawing ratio (should be lower than 2) -Reduction (should be less than 0.5) -Thickness to diameter ratio (greater than 1%)

One way to classify forging operations is by the degree to which the work is constrained in the die. By this classification, name the three basic types.

-Open die forging: work is compressed between two flat dies, allowing metal to flow laterally with minimum constraint -Impression die forging: die contains cavity or impression that is imparted to workpart -Flashless forging: workpart is completely constrained in die

In conventional sheet metalworking operations, (a) what is the name of the tooling and (b) what is the name of the machine tool used in the operations?

-Punch and die is the tool equipment -Stamp presses are the machine tools

Why is friction generally undesirable in metal forming operations?

-Resistance offered due to friction can cause high residual stress in work resulting in undesirable effects like cracks -Resistance offered due to friction demands more to be applied to overcome it- increased power is required -Causes tool wear resulting in poor tool life and decreased dimensional accuracy

Name the four basic bulk deformation processes.

-Rolling= slab or plate is squeezed between opposing rolls -Forging= work is squeezed and shaped between opposing dies -Extrusion= work is squeezed through a die opening, thereby taking the shape of the opening -Wire and bar drawing= diameter of wire or bar is reduced by pulling it through a die opening

What is sticking friction in metalworking?

-Sticking occurs when the coefficient of friction increases beyond a certain condition -Sticking causes the work to stick to the tool instead of sliding over it -It occurs when the friction stress increases above the shear stress that keeps the work flowing over the tool

What does the centerburst defect in extrusion have in common with the roll piercing process?

-The centerburst is a defect of internal crack in extrusion process because of tensile stress along the centerline of the workpart -It is caused when the work piece is away from the center axis, and so the outside region of the workpart stretches the material along the center of work

Why is the term pressworking often used for sheet metal processes?

-The machine tool used to perform the sheet metal processes is a press, which has a die and a punch -The punch and a die is used for the bending operation -The punch applies hammering force to the sheet metal and forces into the die -Thus the sheet assumes the desired shape at the end of the operation

Name some products that are produced by extrusion.

-Tubes and pipes -Profile window casing -Telephone wires -Auto battery cables -Hardware components

What are the reasons why the bulk deformation processes are important commercially and technologically?

-When the metal is tested for hot working operation, the shape of the work parts change according to the desired shape of the product by massive deformation -When the metal is tested for cold working operation, the shape of the work parts become stronger through strain hardening -Little or no waste- some operations are near net shape or net shape processes

What are some possible defects in drawn sheet metal parts?

-Wrinkling -Tearing -Earing -Surface scratches

What are the advantages of snap fitting?

-components can be designed so that they can be self aligned -snap fits do not require any special tooling -parts can be assembled in very little time

Identify some of the general principles and guidelines that apply specifically to automated assembly.

-reliability of a system decreases with increase in the # of separate tasks to be accomplished at different workstations. Each module should have 12-13 parts produced at a single work station--> there should be modularity in product design -multiple components handled simultaneously should be reduced; instead the operations must be separated and allotted to different workstations -the # of directions in which the new parts are added to the subassembly should be limited -high quality and damage free components are to be added to every workstation of the automated system, or else jams may occur -the component must be designed in such a way that snap fits, integral fasteners, and retaining rings are utilized in place of threaded fasteners

Identify some of the general principles and guidelines for design for assembly

-the # of components used in the assembly should be minimized as far as possible during forming design for assembly -the component should be designed in such a way that snap fits, integral fasteners, and retaining rings are utilized in place of threaded fasteners -usage of different types of fasteners is to be minimized as it causes difficulty in automated assembly -the asymmetric features of the part are to be reduced while designing -parts with hooks, curls, slots, and holes are to be avoided as they cause problems of tangling

What is the heat-affected zone in a fusion weld?

-the third zone in the fusion weld -surrounds the fusion area in which melting does not take place -has a chemical composition equal to that of base metal, but this zone is heat treated because of welding temperatures

What are some of the reasons why assemblies must be sometimes disassembled?

-to maintain and repair the parts -to inspect the working condition and wear and tear -to make any slight adjustments -replace the parts that have been worn out

Define the term fusion weld.

-type of weld in which melting of the two surfaces takes place to obtain coalescence -obtained with help of filler materials -high quality and applicable to large amount of materials

Define and sketch a fillet weld.

-type of weld which has a triangular cross section -needs little or small weld preparation

Define and sketch a groove weld.

-type of weld which is made to fill in the space or gap between the adjoining edges of different types of weld, except lap

Name and sketch the 5 joint types

1. Butt joint= surfaces of the parts are welded at their edges which lie in the same plane 2. Corner joint= parts are joined at the corners which are at 90 degree right angles to each other 3. Lap joint= surfaces of the parts are overlapped 4. Tee joint= work parts are perpendicular to each other 5. Edge joint= joint is made at the edges of the parts which are parallel

Identify the 3 basic types of sheet metal-working operations.

1. Cutting- shearing action between 2 sharp edges 2. Drawing- cup shaped or box shaped parts 3. Bending- straining metals along a straight axis

What are the three ways in which a threaded fastener can fail during tightening?

1. When excess tensile strength is applied on the flank of the fastener 2. Stripping of one of the internal fastener threads 3. Stripping of threads of a bolt or nut

What is isothermal forging?

= a hot forging process in which work pieces are heated at an elevated temperature, slightly above the die temperature

What is forging?

= metal working process in which the metal is deformed plastically into a predetermined shape using two dies under high compressive stress -refines the grain structure and improves physical properties such as strength, ductility, toughness

How does mechanical assembly differ from the other methods of assembly discussed in previous chapters?

= process of attaching 2+ parts by mechanical means -the parts can be assembled with relative ease -the parts can be easily disassembled -there is no heat affected zone in the base part

What is isothermal forming?

= the process wherein both the work and the forming tool are heated to the hot working temperature of the work -Applied for materials used for high temperature applications like titanium alloys -These materials have high working temperature; thus heating the work alone during forming can cause the heat to be dissipated to the tool resulting in high residual stress in the work

What is the unit melting energy in welding, and what are the factors on which it depends?

= the quantity of heat energy needed to melt one cubic mm of material or one cubic inch Depends on: -Specific heat -Melting point temperature -Heat of fusion

What is extrusion?

=a compression forming process in which work metal is forced to flow through a die opening to produce a desired cross‑sectional shape

What are the advantages and disadvantages of welding compared to other types of assembly operations?

Advantages: -permanent joint is obtained -strength of the joint is normally higher than strength of the base metals -versatile in nature -most economical in terms of usage of materials Disadvantages: -process is conducted manually -skilled workers required so the cost of labor becomes high -welding is immanently dangerous -difficult to disassemble a welded joint

How does increasing temperature affect the parameters in the flow curve equation?

As temperature increases, there is a subsequent decrease in flow stress- leads to decrease in the values of K and n

What are the differences between bulk deformation processes and sheet metal processes?

Bulk deformation= a processing operation which causes the workpiece to deform plastically causing significant change in the dimension by application of force. -Low surface area-to-volume ratio -Involves a significant change in the shape of the workpiece -Rolling, forging, extrusion, bar drawing Sheet metal processes= a metal forming or cutting operation performed on a workpiece in the form of sheets or strips. -High surface area-to-volume ratio -Does not involve any significant change in the shape of the workpiece, just the geometry is altered -Bending, deep drawing, shearing

Indicate some of the advantages of cold working relative to warm and hot working.

Cold working= carrying out the metal forming process at room temperature Warm/Hot working= heating the workpiece and partially melting it during the process Advantages -Greater accuracy (does not involve heating and solidification) -Better surface finish (does not involve and scale formation over the workpiece) -High strength and hardness (workpiece is strain hardened) -Different properties in different directions (can result in deformed grain structure in the end product) -Cost efficient (does not involve heating)

What is the difference between a cutoff operation and a parting operation?

Cutoff operation= the shearing operation of separating blanks from the sheet metal by sequential cutting at its opposite sides Parting operation= cutting operation in which a punch that has two cutting edges is used to obtain the required part

What is the difference between deep drawing and bar drawing?

Deep drawing= a sheet metal operation performed on a sheet of metal -Output= cup-shaped parts Bar drawing= a bulk deformation process performed on a bar stock -Output= cylindrical parts of reduced diameter

In blanking of a circular sheet-metal part, is the clearance applied to the punch diameter or the die diameter

Die diameter greater than the punch diameter so the clearance is applied to the punch diameter

Differences between direct and indirect extrusion

Direct -Die at opposite end of container -Friction force between work surface and container walls -Container stationary, billet and die moving -Dummy block used to separate work parts Indirect -Die attached to ram -Friction force negligible -Die stationary, billet and container moving -Dummy block used to lock the container at one side and open it to the other side

What is direct and indirect extrusion?

Direct extrusion -Forward extrusion -A metal billet is loaded into a container, and a ram compresses the material, forcing it to flow through one or more openings in a die at the opposite end of the container -As the ram approaches the die, a small portion of the billet remains that cannot be forced through the die opening -This extra portion, called the butt, is separated from the product by cutting it just beyond the exit of the die. Indirect extrusion -Backward or reverse extrusion -The die is mounted to the ram rather than at the opposite end of the container. -As the ram penetrates into the work, the metal is forced to flow through the clearance in a direction opposite to the motion of the ram -Since the billet is not forced to move relative to the container, there is no friction at the container walls, and the ram force is therefore lower than in direct extrusion.

Why is it desirable to use energy sources for welding that have high heat densities?

During welding heat is concentrated only in a small region where the joint is to be made

What are the two discoveries of Sir Humphrey Davy that led to the development of modern welding technology?

Electric arc -discharge of current across a gap in an electrical current Acetylene gas -hydrocarbon compound

Extrusion is a fundamental shaping process. Describe it.

Extrusion= a bulk deformation process where the desired shape is given to the workpiece by allowing it to pass through the suitable orifice by application of force. -Results in the imparting of the shape of the orifice to the workpiece as it is forced to pass through it

What are the two basic types of forging equipment?

Forging hammer- upper die and lower die Forging press- KE is delivered through a force that acts over a distance as a gradual pressure

Define and distinguish the two terms heat transfer factor and melting factor in welding.

Heat transfer factor -ratio of amount of heat actually received at the surface of the workpiece to the total amount of heat produced at the source -depends largely on the type of welding process and also the capacity needed in converting electrical energy from source into usable heat received at the workpiece Melting factor -ratio of amount of heat needed for melting the workpiece to the amount received at the surface of the workpiece -depends on the type of welding process and is influenced by the configuration of welded joint, thermal properties of the workpart, and thickness of the part

Why is friction a factor in determining the ram force in direct extrusion but not a factor in indirect extrusion?

In direct extrusion the metal is forced to slide towards the die opening as the friction exists between the work surface and container walls

What is the difference between machine welding and automatic welding?

Machine Welding -human worker controls and directs arc movement and positions of workpiece continuously -Less accurate -only need human -productivity and operator efficiency at minimum Automatic Welding -has a welding cycle coordinator and/or controller which directs and controls the arc movement and positions of workpiece -More accurate -controller required -productivity and operator efficiency are increased

Distinguish between redrawing and reverse drawing.

Redrawing- if the shape is very complicated another drawing step may be required. Sheet metal is bent in opposite direction at inside and outside corners Reverse drawing- sheet metal is bent at corners in the same direction

What is the technical difference between a screw and a bolt?

Screw -externally threaded fastener which is tightened by applying torque fastened into a blind threaded hole -can never be used as a bolt -usually made of tapered threads -screws are held by the material -used in machine tool applications Bolt -externally threaded fastener -can be used as a screw -usually made of parallel threads -hold the material firmly -used to hold two objects together firmly

What is the difference between a shrink fit and expansion fit in assembly?

Shrink fit -the external part is heated to a sufficient high temperature, to enlarge it by thermal expansion -later, both parts are brought back to room temperature to form assembly Expansion fit -internal part is cooled to contract

What is the difference between industrial stitching and stapling?

Stitching -two parts are fastened together in a stitching machine by use of U-shaped stitches formed using metal wire Stapling -two parts are fastened together by punching U-shaped staples through the parts to be joined

Describe the effect of strain rate in metal forming.

Strain rate= the rate at which strain changes with respect to time during the forming process -As the strain rate increases, the material offers a greater resistance to deformation -Thus, a greater stress needs to be applied to continue the deformation process

Indicate the mathematical equation for the flow curve.

The flow curve depicts the behavior of the material in the plastic region -Yf=Ke^n (Yf= flow stress, K= strength coefficient, e= strain, n= strain-hardening exponent)

What is a trimming operation in the context of impression-die forging?

The purpose of trimming is to cut the flash between small gaps of die plates The purpose of flash friction is to oppose the continued flow of metal into the gap, which enables the bulk of work material to remain in the die cavity

Although the workpiece in a wire drawing operation is obviously subjected to tensile stress, how do compressive stresses also play a role in the process?

They play a role when the metal is pressed down as it passes through the die opening

Describe V bending and Edge bending.

V-bending= sheet metal is bent in the form of a v shape using v shaped punch and dies Edge-bending= sheet metal is held against the die and the punch forces the part down along the edge of the die

What is an autogenous weld?

When a fusion weld is made without the use of a filler metal

Why is flash desirable in impression-die forging?

When flash takes place between the gap of upper and lower die the friction opposes the flow of metal -This enables the work material to remain in the die cavity

What is wire drawing and bar drawing?

Wire drawing= a bulk deformation process to reduce the cross sectional area of a wire through a die opening by means of tensile force and compressive force at the exit side -Small diameter stocks Bar drawing= a metal working process to reduce the cross sectional area of a metal piece by means of tensile forces through dies -Large diameter stocks

For what is the bend allowance intended to compensate?

compensate the stretch loss in the metal that occurs when the bend radius is small compared to stock thickness

What is a stud?

externally threaded fastener that uses two nuts to assemble or hold parts together and does not have an unusual head like that of a bolt

What is the fundamental difference between a fusion weld and a solid state weld?

fusion weld= type of weld in which melting of the two surfaces takes place to obtain coalescence solid state weld= coalescence is achieved by heat and/or pressure (no melting)

What is the difference between a notching operation and a seminotching operation?

notching= cutting of a portion of metal from the side of sheet or strip seminotching= cutting from the interior of the sheet metal

What are integral fasteners?

permanent joints that involve deforming of component parts by use of forming operations so that they interlock and create a mechanical fastened joint -embossed protrusions, hemming

Define proof strength.

the maximum tensile stress that can be applied to the externally threaded fastener before it gets permanently deformed

Define drawing in the context of sheet-metal working

the metal to be drawn is held firmly between two blankholders and the punch is forced through the metal part deforming it in to the shape of the die

What is torque-turn tightening?

the method of application of preload or force to the fastener by applying the required amount of torque Initially the thread fastener is tightened to a certain low level torque and then the fastener is advanced by a specified amount of additional turn

What is meant by the term faying surface?

the workpart surfaces that are in contact and which are to be welded

What is a rivet?

unthreaded pin with a head on one end used to join two parts by passing them through the aligned holes in the pieces to be joined -the un-headed portion on the other end is then deformed by hammering, forming the second head

What is springback in sheet metal bending?

when the bending pressure is removed from the metal section, the elastic energy remaining in the part causes it to recover to its original shape. spring back= amount of elastic recovery