Exoskeleton Arm Mount Assembly: Welding and Attachment for Expo Demo

The upper limb exhibits complex biomechanical movements governed by specific articulations and muscle activations: the glenohumeral joint allows three degrees of freedom (flexion/extension, abduction/adduction, and rotation) with the deltoid, pectoralis major, and rotator cuff muscles as primary agonists; the elbow joint permits only flexion and extension (145° each) driven by the biceps brachii and triceps brachii; the forearm undergoes pronation and supination (85-90°) controlled by the pronator teres, pronator quadratus, supinator, and biceps brachii; the wrist enables flexion, extension, abduction, and adduction (5-85° ranges) through the radiocarpal joint; and the hand achieves grasping capability through metacarpophalangeal and interphalangeal joints allowing flexion, extension, and circumduction.

The upper and lower limbs share homologous structures: pectoral girdle corresponds to pelvic girdle, shoulder joint to hip joint, arm (humerus) to thigh (femur), elbow to knee, forearm (radius/ulna) to leg (tibia/fibula), wrist to ankle, and carpus/metacarpus/digits to tarsus/metatarsus/digits. The upper limb's functional specializations enable skilled activities: smaller size, freely movable shoulder, carrying angle, supination/pronation for adaptability, two-row carpal arrangement, laterally placed thumb, separated digits, and nail protection. Forces travel through the limb via: hand → wrist joint → radius → interosseous membrane → ulna → elbow joint → humerus → shoulder joint → scapula → coracoclavicular ligament → clavicle → sternoclavicular joint → costoclavicular ligament → axial skeleton.

Joint systems range from single-axis hinges to complex three-axis articulations. Single-axis joints allow rotation around one axis, while two-axis systems enable terminal segment rotation around longitudinal axes. The shoulder exemplifies three-axis joints enabling flexion/extension, rotation, and abduction. The cervical spine demonstrates how oblique joint axes create coupled movements (inclination with rotation and extension), requiring compensation mechanisms at the occipital-cervical junction. The upper limb possesses seven degrees of freedom including elbow flexion/extension, forearm pronation/supination, and wrist movements. The pronation/supination mechanism, inherited from the ichthyostega's limb structure, is present in all vertebrates but not always utilized. The thumb's opposition mechanism involves oblique joint axes and counter-opposition from the fifth metacarpal, enabling complex functional movements.

The upper limb contains four major joints: the shoulder complex (glenohumeral and acromioclavicular joints) and the elbow complex (humeroulnar and humeroradial joints). The humerus has three key orientation angles: internal rotation of 10-18 degrees, glenohumeral angle of 94-98 degrees, and anterior inclination of 30 degrees. The elbow joint functions as a modified hinge joint allowing flexion (0-150 degrees) and extension, with the cubitus varus angle normally ranging from 13-16 degrees. Load transfer through the upper limb occurs primarily through the radius (80%) and ulna (20%), with the annular ligament stabilizing the radial head. The radiocarpal joint allows flexion (0-50 degrees) and extension (0-75 degrees), while the radioulnar joints enable pronation and supination through convex-concave and concave-convex relationships. Joint stability is maintained by both dynamic stabilizers (muscles like triceps and biceps) and static stabilizers (ligaments including the medial and lateral collateral ligaments).

The upper limb contains multiple synovial joints classified by their axes of movement: triaxial joints (like the sternoclavicular and glenohumeral joints) allow movement in three planes, biaxial joints (such as the radiocarpal joint) permit movement in two planes, and monoaxial joints (like the elbow and radioulnar joints) enable movement around a single axis; these joints are stabilized by various ligaments including the costoclavicular, coracoacromial, and transverse scapular ligaments, which provide structural integrity and limit excessive movement.

Four critical material properties govern plastic design: specific gravity (plastics weigh 3-4x less than steel), thermal expansion (filled plastics reduce expansion differences), strength (glass-filled plastics approach steel levels), and stiffness (both unfilled and filled plastics lag behind metals). Effective material selection requires identifying three key requirements (impact, thermal, fatigue) and considering worst-case environmental conditions (temperature extremes, humidity, UV exposure). Data sheets provide screening values only, as real-world properties degrade significantly under combined stressors.

The instructor discusses material properties including elasticity (return to original dimensions after load removal), plasticity (permanent deformation without breaking), ductility (significant deformation before failure with warning), and brittleness (fracture with little deformation without warning). Steel is the most commonly used structural material (over 90% of metal structures), followed by aluminum. Steel advantages include high ductility, simple weldability, easy cutting and bending, and widespread availability. Aluminum advantages include lower weight (one-third of steel's density) and corrosion resistance.

Steel braces exhibit different behaviors under tension versus compression. Under tension, they follow the theoretical elastic-plastic stress-strain relationship of metal materials. Under compression, they experience buckling before reaching yield stress, causing sudden load reduction. Plastic hinges form at the center due to geometric instability. After buckling, the brace enters a post-buckling state where load recovery is slow during shape reversal. While individual material points follow theoretical stress-strain rules, overall member behavior is governed by geometric factors like buckling, making structural behavior distinct from material properties.

The Push MetaGrip uses polyurethane material with an embedded metal piece for thumb CMC support, while the Comfort Cool uses neoprene for breathability. Both materials are latex-free and machine-washable. Neoprene provides better breathability for summer heat, while polyurethane offers more rigid structural support.

Thermosetting plastics (melamine, urea formaldehyde, phenolics, epoxies) cannot be reused once cured; thermoplastics (polypropylene, HDPE, ABS, PVC, polycarbonate, nylon) can be remolded. Plastic advantages include lightweight, low cost, recyclability, excellent finish, easy molding, and color variety for aesthetics. Limitations include lower strength, unsuitability for hot conditions, limited accuracy, disposal challenges, and inability to produce very complex parts beyond certain limits. Material selection matrix: for accuracy—steel is best; for cost—plastic is most economical; for aesthetic finish—aluminum provides best results. Cost ranges: mild steel ₹60-70/kg, cast iron ₹70-80/kg, aluminum ₹250/kg, copper ₹300-350/kg. These cost variations significantly impact material selection decisions.

When fabricating custom brackets, accurate measurement and marking are essential. The presenter demonstrates measuring from reference points (like a straight edge) to determine the required dimensions for each bracket. The measurements taken include 52, 95, 33, and 75 millimeters. These measurements are then transferred to the material to be fabricated, ensuring the brackets will fit correctly when installed.

Accurate bracket design requires precise measurement of existing components. Using a metric tape measure, critical dimensions were captured including overall width (460mm), vertical spacing between handle holes (54mm), horizontal spacing (380mm), and positions relative to edges. The designer measured the folded sheet metal lip and flange to ensure the new bracket wouldn't interfere with existing features. In Fusion 360, these measurements were used to create a sketch on the toolbox surface, projecting geometry points to establish the base flange location. A centered rectangle was drawn using diagonal construction lines to ensure proper alignment, with 5mm reveal maintained around the edges to accommodate the handle flange.

This segment demonstrates how to draw the base plate and brackets systematically. The instructor shows how to measure dimensions (120mm width, 15mm height, 15mm spacing), draw light construction lines first, and then solidify the final lines. The cutting plane concept is explained - when a cutting plane passes through a component, it appears solid without hidden detail lines. Brackets are drawn using symmetry, with measurements of 30mm length and 100mm height.

Fabricating custom metal brackets involves a systematic process: marking the halfway point on metal pieces to establish bend locations, measuring from center lines to determine bend positions (e.g., 3/4 inch), marking new lines at measured distances, and measuring 1 inch from center lines on each side to establish hole locations 4 inches apart. A punch is then used to create dimples at marked points, serving as centering spots for drill bits. For 6-inch plates, the center is marked at 3 inches. This precise measurement and marking ensures accurate hole placement for mounting hardware.

Creating custom metal brackets involves a systematic process: (1) marking required holes and cut lines, (2) drilling holes to specified sizes, (3) filing holes to create clean edges, (4) cutting excess material using a grinding wheel, and (5) finishing by rounding edges and cleaning surfaces. The process requires careful measurement, attention to detail, and proper tool selection to ensure the bracket functions correctly and safely.