Manual Assembly
Corrado Poli
Mechanical and Industrial Engineering
UMass Amherst
©Fall 2001
Two Phases of Assembly
• Handling
Can be done
• Manually
• Insertion
•Automatically
Manual Handling
(Affected primarily by geometry)
Bins
Workstation
Fixture
Operator
Operator
• Reaches into the bin
• Grasps the part
• Transports and orients the part
• Pre-positions it
Manual Insertion
(Affected by both the part geometry AND the
part/parts to which it is placed/fastened/mated
to.)
Bins
Workstation
Fixture
Operator
• Operator - places/fastens part onto a
partially completed
assembly/subassembly
Automatic Assembly
(free transfer/non-synchronous system)
Gravity Feed Track
Bowl
Workhead
Feeder
Assembly
Machine -
Workcarrier
Buffer
Stock
Buffer Partially completed assembly
Space transferring to next station
Workstations
• Parts emptied into a bowl feeder which contain orienting devices.
• Parts exit feeder go down track in preparation for insertion.
Four Station Automatic Assembly
System for Irrigation Mini-Drippers
(Rotary Indexing Machine)
Mini-dripper has a base,
body, regulator and cover.
Schematic of
assembly system
Automatic Handling
(From “Handbook of Feeding and Orienting Techniques for Small Parts,”
by Boothroyd, Poli and Murch)
Bowl Feeder
Orienting system for cup
shaped parts
Cost of An Assembly
• Number of parts
• Ease with which the parts can be
> Handled
> Inserted
Rule #1 - Eliminate Parts
• Outright elimination of parts.
(screws, nuts, washers, etc.- use snap fits)
• Combining 2 or more distinct parts into a
single molded, cast or stamped part.
9 parts
2 parts
Eliminate Fasteners
• If not possible, reduce the number or variety
• Incorporate the fastening function with
another feature
1 screw
4 screws
Snap
• Screws aren’t expensive - driving them is!
Can Parts Be Combined?
• Since there is no
relative motion between
the parts - and
• Since these parts can be
made of the same material
•Why not combine them?
Rule #2 -Design Parts
• So they can be easily handled and
Bins
Workstation
Fixture
Operator
• Inserted
Factors Affecting Manual Handling
Time (Cost)*
Part
• Symmetry
> End-to-end
> Rotational
• Size
• Thickness
• Mass
* Reference: G. Boothroyd, “Assembly Automation and Product Design,” Marcel Dekker, NY, 1992
And
If the part is Easy to Grasp
and Manipulate with One
Hand and No Tools
Examples of Symmetry
• Parts with end-to-end symmetry
Dowel pin
washer
pin
• Parts with no end-to-end symmetry
nail
bulb
screw
disk
key
Examples of Rotational Symmetry
• Parts with rotational symmetry.
washer
pin
screw
bulb
• Parts with no rotational symmetry.
disk
key
Illustrations of Size and Thickness
size
Illustrations of Light and Heavy
Mass
Light
Heavy
Parts are Difficult to Grasp and
Manipulate
•Nest
Do not
nestorortangle
tangle
(and degree of severity)
Flexible - two hands to
keep orientation prior to
insertion
(belts, chains, gaskets)
Slippery
(ball bearings with oil)
Parts are Difficult to Grasp and
Manipulate - continued
•Need Grasping Tools
(tweezers, magnets, snap rings, )
• Two hands
Due to part size/thickness
Due to obscured view/access
Heavy
Large
• Fragile
Computer chip
Expansion board
Parts are Difficult to Grasp and
Manipulate - continued
• Sharp/hot/
undesirable to touch
• Sticky
Grease on
ball bearing
Part with
adhesive
Factors Affecting Manual Insertion*
•Ease of alignment
(provide feature to ease insertion)
•Ease of insertion
No chamfers
Chamfers
Dog point
(affected by clearance, jamming, wedging, pushing against a force)
chamfers
Negative clearance (press fit)
Push against a force
Part jams on
corners
Same clearance, no
jamming
* Reference: G. Boothroyd, “Assembly Automation and Product Design,” Marcel Dekker, NY, 1992
Factors Affecting Manual Insertion continued
•Obstructed view and/or obstructed access
Task: Place battery in shaver.
View clear? Need to rely on tactile
sensing?
Access clear? Is there a feature
that restricts access to the operator?
•Self-locating, no need to hold
Additional Considerations
• Insert vertically from above (i.e. don’t fight
gravity)
No
Yes
•Provide features/obstructions to prevent incorrect
assembly of the part.
Additional Considerations
• Eliminate difficult to control operations - welding, brazing, gluing, etc
• Eliminate extra operations such as special
adjustments, torque adjustments etc.
Additional Considerations
• Design products into sub-assemblies that
can be assembled and tested separately.
• ‘De-couple’ manual and machine
operations.
• Group manual assembly operations to
optimize line balancing.
Assembly Analysis
Three choices to analyze an assembly:
• Precise quantitative method - such
as the Boothroyd approach*.
Precision needed if
you need to do
process planning.
• Use an approximate approach during
configuration stage of design?
Assembly Advisor
Allow 7-9 sec/part
(Compromise approach)
(Fails to focus attention on
characteristics that make
assembly difficult)
* Reference: G. Boothroyd, “Assembly Automation and Product Design,” Marcel Dekker, NY, 1992
Assembly Advisor
Combining Parts
If Cdcx<5:
yes!
Question: Is the design on the right, which
contains fewer parts, less costly?