Exam 2 Manufacturing
1. Why are END GRAINS generally undesirable in forged products? Give one examples of such product.
Exposed grain structure will corrode easier; the roughness of the grains at the surface also makes great crack starters in fatigue.
It is known that pouring molten metal at a high rate into a mold has certain disadvantages. Are there any disadvantages to pouring it very slowly? Explain
If a metal is poured too fast, turbulence may cause defects in the form of gas bubbles. If a metal is poured too slowly it may solidify while it is still in the gating system or before completely filling the mold cavities. This will result in an incomplete or partial casting. This situation can be overcome by using a mold with a lower thermal conductivity or a preheated mold, but these lead to reduced mold life and longer cycle times.
What are the mechanical properties of the main materials used in hot stamping?
Strength - Stiffness - Ductility - Plasticity- Toughness- Malleability - Brittleness
Give an advantage of rubber forming?
The advantages of rubber forming are the capability to form complex shapes, forming parts with laminated sheets of various materials and coatings, flexibility and ease of operation, the avoidance of damage to the surfaces of the sheet, low die wear, and low tooling cost.
What is a gutter and its purpose?
gutter is an extra portion which added to the workpiece when the forging operation are done then the extra material which is out from the workpiece will catch up by their gutters. The materials do not exceed 10% of workpiece in gutter
Describe the 4 major components of the Lathe and its accessories or parts:
(1) BED: The bed supports all major components of the lathe. Beds have a large mass and are built rigidly, usually from gray or nodular cast iron. The top portion of the bed has two ways with various cross-sections that are hardened and machined for wear resistance and dimensional accuracy during turning. In gap-hed lathes, a section of the bed in front of the headstock can be removed to accommodate larger diameter workpieces.(2) CARRIAGE: The carriage, or carriage assembly, slide along the ways and consists of an assembly of the cross-slide, tool post, and apron. The cutting tool is mounted on the tool post, usually with a compound rest that swivels for tool positioning and adjustment. The cross-slide moves radially in and out, controlling the radial position of the cutting tool in operations such as facing. The apron is equipped with mechanisms for both manual and mechanized movement of the carriage and the cross-slide by means of the lead screw.(3) HEADSTOCK: The headstock is fixed to the bed and is equipped with motors, pulleys, and V-belts that supply power to a spindle at various rotational speeds. The speeds can be set through manually controlled selectors or by electrical controls. Most headstocks are equipped with a set of gears, and some have various drives to provide a continuously variable range of speed to the spindle. Headstocks have a hollow spindle to which work-holding devices are mounted and long bars or tubing can be fed through them for various turning operations. The accuracy of the spindle is important for precision in turning, particularly in high-speed machining; preloaded tapered or ball bearings typically are used to rigidly support the spindle.(4) TAILSTOCK- The tailstock, which can slide along the ways and be clamped at any position, supports the other end of the workpiece. It is equipped with a center that may be fixed (dead center), or it may be free to rotate with the workpiece (live center). Drills and reamers can be mounted on the tailstock quill (a hollow cylindrical part with a tapered hole) to drill axial holes in the workpiece.
What are the 4 factors that influence the size and length of beads in sheet-metal forming operations?
. Press tonnage/ Blanking force2.Blank thickness3.Maximum radial Tensile stress4.Stamping finite radius
What are the 3 main defects found in sheet metal forming processes?
open die forging(2) impression die forging(3) closed die forging.
Describe term "fluidity of molten metal" and the 4 main factors that influence the molten's metal fluidity.
A term commonly used to describe the capability of the molten metal to fill mold cavities is fluidity. This term consists of two basic factors:(1) characteristic molten metal and(2) casting parameters.The following characteristics of molten metal influence fluidity.a) Viscosity. As viscosity and its sensitivity to temperature (viscosity index) increase, fluidity decreases.b) Surface tension. A high surface tension of the liquid metal reduces fluidity. Oxide films developed on the surface of the molten metal thus have a significant effect on fluidity. For example, the oxide film on the surface of pure molten aluminum triples the surface tension.c) Inclusions. As insoluble particles, inclusions can have a significant adverse effect on fluidity. This effect can be verified by observing the viscosity of a liquid such as oil with and without sand particles in it; the former has higher viscosity.d) Solidification pattern of the alloy. Fluidity is inversely proportional to the freezing range
PEARLITE:
A two-phase aggregate consisting of alternate lamellae of ferrite and cementite. The closer the pearlite spacing of the lamellae, the harder the steel will be
Give 2 advantages and 2 disadvantages of using a lubricant in forging
ADVANTAGES:(a) a reduction in the force and energy required(b) less localization of strain, resulting in improved forgeability(c) the lubricant acts as a thermal barrier, so that the part can remain hotter longer and thus have more ductility(d) the lubricant can protect the workpiece from the environment, especially in hot forging, and also act as a parting agent. DISADVANTAGES: (a) The lubricant adds cost to the operation (b) a thick film can result in orange-peel effect on the workpiece(c) lubricants may be entrapped in die cavities, thus part dimensions my not be acceptable(d) the lubricant must subsequently be removed from the part surface, an additional and difficult operation(e) disposal of the lubricant can present environmental shortcomings.
an alloy;
ALLOY: Composed of two or more elements, at least one of which is a metal. The alloy may be a solid solution, or it may form intermetallic compounds.
AUSTENTITE
Called gamma iron, it has a face-centered cubic structure. The fcc structure allows for higher solubility of carbon in the crystal lattice. This structure also possesses a high level of ductility, which increases the steel's formability.
Describe the difference between a compound, a progressive, and transfer die?
Compound dies perform several operations on the same sheet in one stroke at one station; however, they are usually limited to relatively simple shapes.Progressive dies use multiple strokes to perform multiple operations such as punching, blanking, and notching, at the same station.Transfer dies perform different operations at different stations. After the sheet metal undergoes one-step at one station, it moves on to another station for another step. These machines are arranged in either a straight line or a circular path.
What is a flash in die casting, Explain the reasons why the flash assists in die filling for hot forging?
Flash is thin web or fin which appears at parting of dies. Flash helps in filling the die by providing the flow direction. It helps in creating smooth flow lines.
MARTENSITE:
Forms by quenching austenite. It has a body-centered tetragonal (bct) structure, and carbon atoms in interstitial positions impart high strength to the structure. It is very brittle and hard.
List 2 possible consequences of rolling at (a) too high of a speed and two consequences of (b) too low of a speed.
HIGH SPEED ADVANTAGES• The lubricant film thickness entrained will be larger, which can reduce friction and lead to a condition where the rolls slip against the workpiece. This can lead to a damaged surface finish on the workpiece.• The thicker lubricant film associated with higher speeds can result in significant orange-peel effect, or surface roughening.• Because of the higher speed, chatter may occur, compromising the surface quality or process viability.• There is a limit to speed associated with the power source that drive the rolls. LOW SPEED DISADVANTAGES• Production rate will be low, and thus the cost will be higher.• Because a sufficiently thick lubricant film cannot be developed and maintained, there may be a danger of transferring material from the workpiece to the roll (pickup), thus compromising surface finish.• The strip may cool excessively before contacting the rolls. This is because a long billet that is rolled slowly will lose some of its heat to the environment and also by conduction through the roller conveyor.
1. Why are lubricants generally used in sheet-metal forming?
Metalworking lubricants are applied to tool-work interface in many forming operations to reduce harmful effects of frictionBenefits: reduced sticking, forces, power, tool wear, better surface finish, removes heat from the tooling
Explain the purpose of the following features in the mold design and what would happen if each of them would be missing
POURING CUP: designed to reduce the velocity of liquid metal which is entering in the sprue to minimize erosionSPRUE: designed to avoid air aspiration effect effect. Ideal shape of the sprue is parabolic. To minimize manufacturing deficiency, the shape of sprue is considered as tapered.WELL: forms a cushion for the metal pouring through the sprue so the sand is not washed into the casting OR changes the direction of flow of the molten metal to right angle and it passes it to the runner.RUNNER: takes the molten metal from sprue to the casting. To minimize turbulent losses, shape of the runner is considered as trapezoidal and has more surface area.RISERS: as the casting cools, it will shrink. Because the riser is larger, it will cool slower and feed liquid metal to the shrinking casting, thus presenting a number of defects.
Why would you recommend preheating the molds in permanent-mold casting? Also, why would you remove the casting soon after it has solidified? Explain.
Preheating the molds in permanent-mold casting is advisable to reduce the chilling etc. of the metal mold, which could lead to low metal uidity. Also, the molds are heated to reduce thermal damage (fatigue, shock) which may result from repeated contact with the molten metal. Considering casting removal, the casting should be allowed to cool in the mold until there is no danger of distortion or developing defects during shakeout. While this may be a very short period of time for small castings, large castings may require an hour or more.
Give an example of defects in (a) extrusion and (b) drawing
Recognizing that a defect is a situation that can cause a workpiece to be considered unsuitable for its intended operation, several defects can occur. Examples include poor surface finish or surface cracking (such as bamboo defect), tailpipe or fishtailing, and chevron cracking. In drawing, defects include poor surface finish and chevron cracking. Both extrusion and drawing also can have a loss in dimensional accuracy, particularly as attributed to die wear.
Why can blind risers be smaller than open-top risers?
Risers are used as reservoirs for a casting in regions where shrinkage is expected to occur, i.e, areas which are the last to solidify. Thus, risers must be made large enough to ensure that they are the last to solidify. If a riser solidi es before the cavity it is to feed, it is useless. As a result, an open riser in contact with air must be larger to ensure that it will not solidify first. A blind riser is less prone to this phenomenon, as it is in contact with the mold on all surfaces; thus, a blind riser may be made smaller.
Describe the mold and die materials used in the casting processes. Under each type of material, explain why these processes are suitable for that particular mold or die material.
SAND: Sand casting typically relies on silica-based materials, such as synthetic or naturally-bonded sand. Casting sand generally consists of finely ground, spherical grains that can be tightly packed together into a smooth molding surface. The casting is designed to reduce the potential for tearing, cracking, or other flaws by allowing a moderate degree of flexibility and shrinkage during the cooling phase of the process. METAL: Metal casting is a process in which hot liquid metal is poured into a mold that contains a hollow cutout or cavity of the desired finished shape. The liquid metal is then left to solidify, which is removed from the mold, revealing the end product, or the "Casting Form". These castings must go through rigorous quality checks.Die casting is a method of molding materials under high pressure and usually involves non-ferrous metals and alloys, such as zinc, tin, copper, and aluminum. The mold is coated with lubricant to help regulate the die's temperature and to assist with component ejection. Molten metal is then injected into the die under high pressure, which remains continuous until the workpiece solidifies. This pressurized insertion is rapid, preventing any segment of the material from hardening before being cast. After the process is completed, the component is taken out of the die and any scrap material is removed. GRAPHITE: Graphite mold casting technique is a modern affordable, quick and perfect casting methodology to produce graphite molds. Aluminum alloy and special zinc-aluminum alloys like ZA-12 can be best casted using this casting process. Graphite mold casting is a cheaper alternative to other casting methods which helps mass production of quality casting like camera housings. Graphite mold casting process is a simple process where a plastic aggregate consisting of graphite grains, pitch, carbonaceous cement, starch and water are use for making the mold. The mixture is squeezed around pattern and allowed to set. When the mold dried, it is fired to dry to form a strong mold. The mold is then assembled by fixing the sprue, cores and gates in its.Graphite mold which is easily machinable can be created and modified quickly and economically .PLASTER: Plaster casting is similar to the sand casting process, using a mixture of gypsum, strengthening compound, and water in place of the sand. The plaster pattern is typically coated with an anti-adhesive compound to prevent it from becoming stuck against the mold, and the plaster is capable of filling in any gaps around the mold. Once the plaster material has been used to cast a part, it usually cracks or forms defects, requiring it to be replaced with fresh material.
What is electrically assisted forging?
The mechanical properties of metal change temporarily or permanently under application of electrical currents under deformation. This phenomenon is often referred to as electroplasticity. The electroplasticity of metals and their alloys has been investigated under various load conditions. Their electroplasticity can be utilized in electrically assisted forging operation. Due to it's technical advantages such as enhanced formability and reduced spring back, electrically assisted forging is energy efficient with reduced process time.
Give 2 example of limitations associated with the stretching process of materials
The stretching process is a batch operation and it cannot be continuous as in rolling.The reduction in thickness is limited by necking of the sheet, depending on its strain-hardening exponent, n.As the sheet is stretched, the surface finish becomes dull due to the orange-peel effect, and thickness and width control becomes difficult.Stretching the sheet requires some means of clamping at its ends which, in turn, will leave marks on the sheet, or even cause tearing.There would be major difficulties involved in applying high temperature during stretching of less ductile materials.
Describe the importance of controlling roll speed, roll gap and temperature in a tandem-rolling operation.
We note that mass continuity has to be maintained during rolling. Thus, if the roll speed is not synchronized with the strip thickness in a particular stand, excessive tensions or slack may develop between the stands; some rolls may slip. Also, if the temperature is not controlled properly, strip thickness will change, thus affecting reduction per pass and, consequently, the roll forces involved. This, in turn, will also affect the actual roll gap and roll deflections. Complex control systems have been developed for monitoring and controlling such operations at high rolling speeds.
CEMENTITE
known as iron carbide (Fe3C). Cementite is a hard and brittle phase.
1. What is a pattern in sand casting, why are allowances or tolerances provided for in making patterns? What do they depend on?
pattern is replica of project with slight modification and used to make the cavity. allowances/tolerances are provide for the reasons below: Machining allowance: product produced from casting have poor surface finish and dimensional accuracy.shrinkage allowance: liquid metal upon solidification shrinks.draft allowance: for easy removal of pattern from cavity.distortion allowance: for thin casting to prevent bending in final product opposite bend is provided in opposite direction.rapping allowance: when pattern is removed from the cavity size of cavity increases slightly, to compensate this negative allowance is provided on pattern.
1. What is nesting?
refers to the process of laying out cutting patterns to minimize the raw material waste
