ME 437 - Session #5ME 437 - Session #5

• Selection of term projects

• Assignment#4 : Review of manufacturing processes

• Presentations of PDS work

• Material selection

• Questions on material science review

Term ProjectsTerm Projects

• Let us select 3 projects • Each project would have 5 teams of size 2

working on them• Desired characteristic

– Consumer-type products, or common applications

– Simple: Not too many components

– Some plastic parts, some metal parts

– Some off-the-shelf components

ProjectsProjects

• Nut cracker

• Cat/Dog feeder

• Fish feeder

• Rock tumbler

• Envelope licker

• A bike rack for automobiles

• A device to test shear strength of wood dowels

• Device to test effect of vibration on light bulbs

• A battery-operated home lifter (or car jack).

• An easy-to-install tire traction device.

Material Selection CriteriaMaterial Selection Criteria

• Most designers select materials with equal consideration of:

– Properties

– Availability– Cost

AvailabilityAvailability

• Is the material available in the desired form?• Can it be obtained in a month? • Do we have to order 100,000 lbs?• Is the material available from multiple suppliers?• Does a particular material impose time delays in

part manufacturing?

COSTCOST

• The primary factor in material selection in industry.

• Comparative cost of engineering metals per pound.

• Cost per volume may be a better choice. The following table is $/in3 .

Cost FactorsCost Factors

• Cost of material in relation to the production cost.• The production volume?• The anticipated service life?• Manufacturability cost (Fabricability)

– Machinability

– Formability

– Weldability

– Castability

– Tolerances / Surface finish

Designer’s RepertoireDesigner’s Repertoire

• Carbon steels• Cast irons• Alloy steel• Tool steels• Stainless steels• Aluminum alloys• Nickel alloys

• Copper alloys• Titanium alloys• Magnesium alloys• Ceramics• Plastics• Composites

Materail SelectionMaterail Selection

• Plain carbon steels have very poor hardenability.• Cold finished steels (1006 to 1050) can come in various

tempers (hard to skin rolled). They can be strengthened up to 100% with cold working.

• Cold finished parts are stronger, have better surface finish, are more dimensionally accurate. But, they have less ductility, cost more, are dimensionally less stable in machining, and loose their strength if welded.

• High sulfur steels are ideal for machining (AISI 1112) as the chips break up easily. High sulfur content, however, causes weld cracking.

Material SelectionMaterial Selection

• Alloy steels– AISI 4340 or 4140 (Through hardening grades): Have

excellent strength and toughness. Available in most forms in hot rolled form. Designed for high hardenability. Best suited to high stress machine parts - gears, cams, shafting, piston rods, bolts, dies, keys, bending rolls. They can be finish machined but can not be easily welded without special precautions.

– AISI 8620 or 9310 (carburizing grades): These steels are designed for case hardening requirements. Deep cases can be created and hardened using carburizing. Used when high wear resistance is required. Suitable for gears, racks, cams, and sliding surfaces.

Alloy SteelsAlloy Steels

• High Strength Low Alloy Steels (HSLA)

– Used for Structural applications

– Stronger than plain carbon steels (40-80 ksi yield)

– Not heat treatable

– Corrosion resistant (not as much as stainless steel)

– Good weldability (low carbon content)

– Good formability

– Applications: (ASTM 242) structural bars, plates.

Alloy SteelsAlloy Steels

• Ultra-strength steels– High strength and reasonable toughness.– Used for heavily loaded machine parts.– 4340 and 4140 are most common

• Hard to weld but machinable

– 18% nickel maraging steel• Very high strength (200-300 ksi)• Good weldability• Very expensive

ExampleExample

• Suppose a designer has created a drawing for a shaft of a hypothetical device. The list of operational conditions have been determined as follows:– 1.25” diameters are to fit ball bearings

– The shaft is subjected to maximum shear stress of 10 ksi

– There is possibility of moderate shock load

– Small end must resist damage from frequent removal of a keyed gear.

– There are no inertial requirements.

– Surface roughness to be 32 micro inches max.

– Diameters must be concentric to 0.001 inch.

ExampleExample

• Based on the operational requirements, time constraint, and cost, the following selection factors have been established:– Hardness of at least 30 HRC

– Fatigue strength of 30 ksi

– Impact strength must be high

– Stiffness must be high

– The part must not rust in 50% RH room air.

– Must be dimensionally stable.

– Parts are needed in one week.

– Three units are required.

– Expected service life is 5 years

Example Example [[From BudinskiFrom Budinski]]

Solution ideasSolution ideas

• What kind of material– Metal?

– Plastic? Low stiffness

– Ceramic? Low impact strength

– Composites?

• Must be hardenable to 30 HRC– Hardenable steels 1050 or 4340/4140

• High fatigue strength– AISI 4340 or 4140

– Must be surface treated (nickel plating)

•