Man Pro Midterm 2

Why is the surface finish of rolled products better in cold rolling than in hot rolling?

Cold rolling produces a better surface finish because it avoids the oxidation and scaling that occur during hot rolling. It also provides better dimensional accuracy and closer tolerances.

What is the difference between blooms, slabs, and billets?

Blooms typically have square cross sections greater than 225 cm², while billets are smaller sections with a minimum size of 4 × 4 cm. Slabs have a cross sectional area greater than 100 cm² and a width at least twice their thickness.

What is forward slip? Why is it important?

Forward slip is the difference between the exit velocity of the metal and the roll surface velocity, expressed as s= (V_f-V_r)/V_r. It measures the relative slipping between the rolls and workpiece during rolling.

Define neutral point and draft in rolling.

The neutral point is the location where the metal velocity equals the roll surface velocity, resulting in no relative slipping. Draft is the reduction in thickness during rolling, calculated as d= t₀ - t_f.

Calculate the number of rolling passes to reduce a plate from 8mm to 5mm (friction = 0.15, roller diameter = 500 mm).

Using the maximum draft formula from the notes, d_max = friction² x R = (0.15)²(250) = 5.625 mm. Since the required reduction is 8 - 5 = 3mm, only 1 pass is required.

How does rolling differ from extrusion?

Rolling reduces metal thickness by passing it between rotating rollers that apply compressive forces. Extrusion forces metal through a die opening to produce a desired cross-sectional shape.

Explain the difference between extrusion and wire drawing.

Extrusion pushes metal through a die opening, while wire drawing pulls metal through a die to reduce its cross-sectional area. Both processes increase the material's length while reducing its cross section.

What is the purpose of dummy blocks in extrusion?

A dummy block is placed between the ram and billet to transmit the extrusion force while protecting the ram from direct contact with the hot metal.

What is impact extrusion? What are its applications?

Impact extrusion is a high-speed process where a punch strikes a metal slug, forcing it to flow into the desired shape. It is used to manufacture thin-walled products such as tubes and containers.

  In an extrusion process an Aluminium round billet of 150 mm diameter and 500 mm long is deformed into a channel shape product of 1500 mm long.  What is the cross section of the channel produced?

Using volume conservation, A₀L₀=A_fL_f, the final area is A_f = (pi x 150² × 500)/( 4 × 1500). Therefore, the channel's cross-sectional area is 5,890.49 mm².

Compare sizing and coining.

Sizing is a cold finishing process that improves dimensional accuracy by applying high force over a short distance. Coining uses high compressive forces to produce fine surface details and precise features.

Compare coining and embossing.

Coining applies high pressure to create intricate surface details through localized deformation. Embossing produces raised or recessed designs on sheet metal using matching dies.

What is the difference between cold, warm, and hot forging?

Cold forging occurs below the recrystallization temperature, warm forging uses intermediate temperatures to reduce forming forces, and hot forging occurs above the recrystallization temperature. Cold forging provides better surface finish, while hot forging allows easier deformation with lower forces.

What are the defects in flat rolling?

Common defects include wavy edges, zipper cracks, edge cracks, alligatoring, and surface defects such as scale, scratches, and pits. These result from uneven deformation, poor ductility, or impurities.

How are seamless tubes produced?

Seamless tubes are produced by piercing a round billet to create a hollow shell, followed by elongation and reduction of its diameter and wall thickness. Final sizing is performed through hot or cold rolling.

What factors affect rolling forces?

Rolling forces depend on the material's strength, thickness reduction, roll diameter, friction coefficient, and working temperature. Higher friction and greater deformation generally increase the required force.

What are the advantages and disadvantages of isothermal forging?

Isothermal forging maintains the workpiece at an elevated temperature using heated dies, improving metal flow and reducing thermal gradients and defects. Its main disadvantage is the high cost of equipment and operation.

What is upset forging? What are its applications?

Upset forging increases the cross-sectional area of a metal workpiece by compressing its length. It is commonly used to manufacture bolt heads, screws, rivets, and similar fasteners.

How can you differentiate whether a part is produced by forging or casting?

Forged parts typically have directional grain flow and improved mechanical properties due to plastic deformation. Cast parts are formed by solidification of molten metal and may contain porosity or casting-related defects.

What is the difference between punching and blanking?

Punching removes material from a sheet to create holes, with the removed material becoming scrap. Blanking cuts out a desired shape from the sheet, where the removed piece is the final product.

What is springback, and how can it be controlled in bending?

Springback is the elastic recovery of metal after the bending force is removed, causing the bend angle to change. It can be compensated for by overbending the material or using suitable tooling.

Differentiate compound dies and progressive dies.

Compound dies perform multiple cutting operations at a single station during one press stroke. Progressive dies use multiple stations where the workpiece moves sequentially until the finished part is produced.

What factors affect recrystallization temperature?

Recrystallization temperature depends on the nature of the metal or alloy and the amount of previous cold working. Pure metals generally have lower recrystallization temperatures than alloys, and increased cold working lowers the temperature.

What are the various applications of metal spinning?

Metal spinning is used to produce rotationally symmetrical hollow parts such as cones, cylinders, and other circular shapes. Applications include cookware, containers, and similar products.

What are the typical stages of a deep drawing process?

The stages include shearing a blank, clamping it onto the die, forcing it into the die cavity using a punch, and removing the formed part. The excess material is then trimmed.

What are the defects in deep drawing?

Common defects include wrinkling of the flange or cup walls, tearing near the cup base, earing, and surface scratches. These defects can result from excessive tensile stresses, sharp die corners, rough tooling, or poor lubrication.

Compare V-bending and edge bending.

V-bending uses a V-shaped punch and die to bend sheet metal into the desired angle. Edge bending bends the edge of a sheet using a pressure pad and punch.

What are trimming and shaving in sheet metal cutting?

Trimming removes excess material surrounding a previously formed part. Shaving removes a thin layer of metal from rough or uneven cut edges to improve dimensional accuracy and surface quality.

What methods reduce cutting forces in press cutting operations?

Cutting forces can be reduced by staggering punch lengths so multiple holes are not cut simultaneously or by providing shear angles on punches or dies. Both methods distribute the cutting load over a longer press stroke without reducing the total work.

Explain scrap-strip layout in blanking and its advantages.

Scrap-strip layout refers to arranging blanks along a metal strip to minimize unused material during cutting. Its advantages include improved material utilization, reduced scrap, and lower production costs.