Mechanical and Industrial Engineering Manual Assembly Corrado Poli Mechanical and Industrial Engineering UMass Amherst ©Fall 2001

Two Phases of Assembly Handling Insertion Can be done Manually Automatically Handling Insertion

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 Feeder Workhead Assembly Machine - Workstations Buffer Stock Workcarrier Partially completed assembly transferring to next station Buffer Space Parts emptied into a bowl feeder which contain orienting devices. Parts exit feeder go down track in preparation for insertion.

Schematic of assembly system Four Station Automatic Assembly System for Irrigation Mini-Drippers (Rotary Indexing Machine) Schematic of assembly system Mini-dripper has a base, body, regulator and cover.

Orienting system for cup shaped parts 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 Inserted Bins Workstation 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

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 bulb nail screw disk key

Examples of Rotational Symmetry Parts with rotational symmetry. Parts with no rotational symmetry. bulb pin screw washer Kollmorgen disk key

Illustrations of Size and Thickness

Illustrations of Light and Heavy Mass

Parts are Difficult to Grasp and Manipulate Flexible - two hands to keep orientation prior to insertion (belts, chains, gaskets) Do not nest or tangle Nest or tangle (and degree of severity) Slippery (ball bearings with oil)

Parts are Difficult to Grasp and Manipulate - continued Due to obscured view/access Need Grasping Tools (tweezers, magnets, snap rings, ) Due to part size/thickness Two hands Heavy Large Expansion board Computer chip Fragile

Parts are Difficult to Grasp and Manipulate - continued Sharp/hot/ undesirable to touch Grease on ball bearing Part with adhesive Sticky

Factors Affecting Manual Insertion* Dog point No chamfers Chamfers Ease of alignment (provide feature to ease insertion) 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 * 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) Yes No 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.

Precision needed if you need to do process planning. Assembly Analysis Three choices to analyze an assembly: Precise quantitative method - such as the Boothroyd approach*. * Reference: G. Boothroyd, “Assembly Automation and Product Design,” Marcel Dekker, NY, 1992 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)

Assembly Advisor

Combining Parts If Cdcx<5: yes! Question: Is the design on the right, which contains fewer parts, less costly?