Manual Assembly

Corrado Poli Mechanical and Industrial Engineering

UMass Amherst©Fall 2001

Two Phases of Assembly

• Handling

• Insertion

Can be done

• Manually

•Automatically

Manual Handling(Affected primarily by geometry)

Bins

Workstation

Fixture

OperatorOperator

• Reaches into the bin

• Grasps the part

• Transports and orients the part

• Pre-positions it

• Operator - places/fastens part onto a partially completed assembly/subassembly

BinsWorkstation

Fixture

Operator

(Affected by both the part geometry AND the part/parts to which it is placed/fastened/mated to.)

Manual Insertion

Automatic Assembly(free transfer/non-synchronous system)

Buffer Stock

Workstations

BowlFeeder

WorkheadAssembly Machine -

Workcarrier Partially completed assembly transferring to next station

Gravity Feed Track

• Parts emptied into a bowl feeder which contain orienting devices.

• Parts exit feeder go down track in preparation for insertion.

BufferSpace

Four Station Automatic AssemblySystem 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

4 screws1 screw

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

• Inserted

BinsWorkstation

Fixture

Operator

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

• Parts with no end-to-end symmetry

Dowel pin washer pin

nail bulbscrew disk

key

Examples of Rotational Symmetry

• Parts with rotational symmetry.

• Parts with no rotational symmetry.

washer pinbulb

screw

diskkey

Illustrations of Size and Thickness

size

Illustrations of Light and Heavy Mass

Light Heavy

Parts are Difficult to Grasp and Manipulate

•Nest or tangle(and degree of severity)

Slippery (ball bearings with oil)

Flexible - two hands to keep orientation prior to

insertion(belts, chains, gaskets)

Do not nest or tangle

Parts are Difficult to Grasp and Manipulate - continued

•Need Grasping Tools(tweezers, magnets, snap rings, )

Due to part size/thicknessDue to obscured view/access

• Two hands

Heavy Large

• FragileComputer chipExpansion 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*

* Reference: G. Boothroyd, “Assembly Automation and Product Design,” Marcel Dekker, NY, 1992

•Ease of alignment (provide feature to ease insertion)

No chamfers ChamfersDog point•Ease of insertion

(affected by clearance, jamming, wedging, pushing against a force)

Part jams on corners

Same clearance, no jamming

chamfers

Negative clearance (press fit) Push against a force

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:

• Use an approximate approach during configuration stage of design?

Precision needed if you need to do

process planning.

Assembly Advisor Allow 7-9 sec/part

• Precise quantitative method - such as the Boothroyd approach*.

* Reference: G. Boothroyd, “Assembly Automation and Product Design,” Marcel Dekker, NY, 1992

(Fails to focus attention on characteristics that make

assembly difficult)

(Compromise approach)

Assembly Advisor

Combining Parts

Question: Is the design on the right, which contains fewer parts, less costly?

If Cdcx<5:

yes!