ME 330Manufacturing Processes

MECHANICAL ASSEMBLY:Joining Methods (cont.)

Two Major Types of Mechanical Assembly

1. allow for disassembly

2. create a permanent joint with no possibility of disassembly For example, adhesion and bonding

Use of the third part component Use of the deformation between two components to

be assembled.

Mechanical Assembly Approach:

Deformation to make two parts fit or lock each other together

Principle of the process

Structure and configuration

Process modeling

Defects

Design For Manufacturing (DFM)

Process variation

Law 1: Force-mechanical effect: elasticity and plasticity.

Law 2: Thermal-mechanical effect: varying temperature of an object can lead to expansion or contraction in geometry of the object.

Principle 1: Part A is in a forced state and Part B constrains Part A, which leads to the interference fit or assembly of A and B due to Law 1.

Principle 2: Part A is in an thermal or cooled state and Part B constrains Part A, which leads to the interference fit or assembly of A and B due to Law 2.

Principle 3: mutual interlock of A and B, which leads to a joint of A and B; the assemble process further follows Principles 1 and/or 2.

Principle of the process

Structure and configuration

Process modeling

Defects

Design For Manufacturing (DFM)

Process variation

Mechanical approach to introduce interference fit in assembling two parts

1. Press

2. Shrink and expansion

3. Snap

4. Retaining ring

5. Mold (permanent)

6. Integral

Press Fitting Press and then fit

Straight cylindrical pin of a certain diameter is pressed into a hole of a slightly smaller diameter.

Diameter of the hole (inner) is smaller than diameter of the pin.

After pressing on the perimeter of the pin, the diameter of the pin is smaller so that it can be inserted into the hole.

Spirol PinRoll Pin

Hole

Press Fitting

Functions:

1. Locating and locking components in place.

2. Facilitate the formation of pivot joint - to create shafts to allow one component to rotate about the other.

3. Facilitate the formation of safety device - to break in overload conditions to save the rest of the assembly from failure.

Facilitate the formation of a safety device - to break in overload conditions to save the rest of the assembly from failure.

(a) (b)

Assume the yellow is the pin. In (a), yellow is fixed with green and grey. Suppose the assembly of the yellow and green is based on the press fit, while the assembly of the yellow and grey is permanent joint. When there is a overloading along the shaft, the whole goes to the situation (b).

Shrink and Expansion

Shrink fitting - external part is enlarged by heating and

internal part is inserted, then shrinks to create

interference fit when back at room temperature

Expansion fitting - internal part is contracted by cooling

and inserted into external part, then expands to create

interference fit when back at room temperature.

Commonly used to fit gears, pulleys, and sleeves onto

shafts

Snap Joining two parts in which mating elements possess a

temporary interference during assembly, but once assembled they interlock.

During assembly, one or both parts elastically deform to accommodate the temporary interference.

Remark 3: Snap follows the interlock principle with Principle 1

Retaining Rings

Fastener that snaps into a circumferential groove on a shaft or bore to form a shoulder

Function: to locate or restrict movement of parts on a shaft or bore

Internal External

E-clip Push-on

Placement of a component into mold prior to plastic molding or metal casting, so that it becomes a permanent and integral part of the molding or casting: (a) threaded bushing, and (b) threaded stud

Molding Inserts

Pros of molding Inserts

1. Insert has better properties than molded or cast material.

2. Insert geometry is too complex or intricate to incorporate into mold cavity.

3. Applications:

Internally threaded bushings and nuts Externally threaded studs Bearings Electrical contacts

Integral

Both parts are deformed, so they interlock as a mechanically fastened joint. Methods include:

1. Lanced tabs

2. Seaming

Integral - lanced tabs

Lanced tabs to attach wires or shafts to sheet metal

Edges of two separate sheet metal parts or the opposite edges of the same part are bent over to form the fastening seam

Integral - Seaming

Principle of the process

Structure and configuration

Process modeling

Defects

Design For Manufacturing (DFM)

Process variation

Design for Assembly (DFA)

Keys to successful DFA:

1. Design product with as few parts as possible

2. Design parts so they are easy to assemble

Important to how a product will be assembled and the assembly costs.

DFA is done during the stage.

DFA Guidelines

Reduce number of threaded fasteners

1. Use quicker fastening methods such as snap fits,

retaining rings, and integral fasteners.

2. Use threaded fasteners where disassembly is required.

Reduce variety of fasteners

DFA Guidelines

Use modularity in product design: Design the subassembly around a base part to which other components are added.

Reduce the need for multiple components to be handled at once: Use separate operations for each component.

Limit the required directions of access: Adding all components from above is the ideal

Summary

Assemble without a need of the third party but two parts

themselves.

Two laws in physics and three principles of such kind of

assembly governs the process.

Laws Principles Structuring

Applications/Functions.

Design For Assembly (DFA).