ME 330
Manufacturing Processes
MECHANICAL ASSEMBLY:
Joining Methods (cont.)
Two Major Types of Mechanical Assembly
1. allow for disassembly


Use of the third part component
Use of the deformation between two components to
be assembled.
2. create a permanent joint with no possibility of disassembly

For example, adhesion and bonding
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.
Hole
Spirol Pin
Roll Pin
 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.
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
E-clip
External
Push-on
Molding Inserts
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
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
Integral - Seaming
Edges of two separate sheet metal parts or the opposite edges
of the same part are bent over to form the fastening seam
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).