Designing a 3D Printed Snake Robot: Body and Mechanics

3D printing cannot achieve the micrometer-level accuracy of machining; instead, it builds parts layer by layer with inherent dimensional variability. To compensate, designers must leave gaps between interacting faces—typically 0.2mm as a baseline. However, gap sizing depends on print orientation: parallel-to-bed features are most accurate and strongest, allowing smaller gaps, while perpendicular orientations introduce layer line variability requiring larger tolerances. Overhanging features need even more clearance due to potential drooping. For simple parts, gaps as small as 0.075mm work; for complex overhangs, 0.4-0.5mm may be necessary. Moving parts require additional clearance for wear. Understanding these relationships between design intent and manufacturing reality is essential for functional 3D printed mechanisms.

Reliable 3D printing requires following established rules of thumb for mating fitments, text, and angles. Design with 0.3mm clearance in each dimension to account for printer deviations - a 20mm female part should be 20.6mm to accommodate 0.3mm tolerance on each side. For text and labels, use debossed fonts (not embossed) with at least 0.5mm depth, avoiding serif fonts that don't print well. Recommended: DSO bold font at 6mm height, 6mm from base for repeatable orientation verification. Apply 2° draft angles as a starting point, recognizing that deep-drawn parts have inconsistent wall thickness requiring extra margin. These conventions enable consistent results across different printers and operators.

This segment addresses the critical topic of dimensional tolerances in 3D printing. The instructor explains that printed parts require dimensional adjustments due to printer accuracy limitations. For less precise printers, you may need to add 0.2mm to 0.4mm of tolerance to ensure parts can be assembled. For more precise printers, smaller tolerances (0.1mm) may be sufficient. The instructor demonstrates how to adjust tolerances by modifying the dimensions of threaded surfaces using the push/pull tool, and explains how to verify the results using the section analysis tool.

Different joint types require different tolerances for successful assembly. Axle joints (moving parts like wheels) need tolerance for smooth motion without wobble or friction. Snug slip fits (like eyeglass cases) need tight tolerances for secure hold while allowing removal. Compression fits (permanent assembly) require negative tolerance where pins are larger than holes. Additionally, 3D printers cannot print over air without support, so maximum bridging distance must be determined. These tolerances ensure functional assemblies that meet design requirements.

This video presents essential design rules for FDM 3D printing: walls must be at least 0.8mm thick (2 layers), embossed/engraved details require 0.6mm width and 2mm height, holes need minimum 2mm diameter, moving parts require 0.5mm clearance, minimum feature thickness is 2mm, pins need 3mm diameter, and hobbyist printers have ±0.5% tolerance requiring holes to be slightly larger than pegs.

A free rigid body in 3D space has six degrees of freedom: three translational (x, y, z) and three rotational. When constrained to a 2D plane, it loses three degrees of freedom, leaving three (x, y, and rotation). For multiple rigid bodies, total DOF equals 3 times the number of bodies. Kinematic joints constrain motion; specifically, a revolute joint (pin/hinge) eliminates two translational DOF while permitting one rotation between connected bodies.

Mechanics divides into statics (bodies at rest, F=0) and dynamics (moving bodies, F=ma). A rigid body undergoes no deformation (delta=0, elastic modulus=infinity). Motion types include translational (point to point), rotational (around axis), and complex motion (combination). Kinematics studies motion parameters without forces; kinetics studies forces causing motion. A mechanism is a collection of links connected through movable joints where parts move relative to each other without energy transfer. Links are rigid, flexible (deform but transmit force), or fluid (transmit pressure). Joints connect links and enable relative motion. Lower pairs have surface contact (revolute, prismatic, screw joints), higher pairs have point or line contact (cam, gear joints). Constraints reduce degrees of freedom: completely constrained (predictable motion), incompletely constrained (uncertain motion), successfully constrained (controlled motion). A body in free space has 6 DOF (3 translational, 3 rotational); a body on a 2D plane has 3 DOF. Gruebler's equation calculates mechanism mobility: M = 3L - 2J1 - J2. Joint order equals number of links minus one on that axis. A mechanism forms when a kinematic chain becomes completely constrained by fixing one link as ground.

Degrees of freedom refer to how much movement an object can have, specifically the number of independent parameters that define its configuration. A rigid body has six degrees of freedom: three translational movements (up-down, left-right, front-back) and three rotational movements (yaw, pitch, roll). These correspond to movement along the X, Y, and Z axes plus rotation about each axis. This concept applies to all rigid components in mechanical systems, including links, joints, and entire mechanisms.

This comprehensive section covers the foundational concepts of mechanism analysis. A rigid body in a plane has 3 degrees of freedom (DOF): two translational movements along x and y axes, and one rotational movement around the z-axis. A rigid body in space has 6 DOF: three translational movements along x, y, and z axes, and three rotational movements around these axes. A link (khâu) is a generalized concept representing a rigid body that can move relative to other bodies, classified into rigid links, flexible links, and fluid links. A joint (khớp) is the connection point between two links that allows relative motion, classified by DOF constraints (Joint types 1-6) and contact type (lower pairs with surface contact, higher pairs with point/line contact). Specific joint types include spherical joint (surface contact, constrains 3 DOF), cylindrical joint (line contact, constrains 2 DOF), revolute joint (surface contact, constrains 5 DOF), and screw joint (surface contact, allows coupled rotational and translational motion).

A mechanical link is a relationship between two parts that constrains their relative movements, characterized by the number of degrees of freedom (DOF) it allows. A free body in 3D space has 6 DOF: 3 translational movements (along X, Y, Z axes) and 3 rotational movements (around X, Y, Z axes). The three basic mechanical links are: (1) Fixed link (رابط الارتكازي) - allows only 1 DOF (rotation around one axis), preventing all other movements; (2) Sliding link (رابط الانزلاقي) - allows only 1 DOF (translation along one axis), preventing all other movements; (3) Welded link (رابط الاندماجي) - allows 0 DOF, completely joining two parts as one unified body.

Fasteners (बंधक/फास्टनर) are mechanical devices used to join machine parts during assembly. They are classified into three categories: (1) Temporary Fasteners - used when parts need frequent disassembly, such as pins and press fit joints; (2) Semi-permanent Fasteners - used when occasional disassembly is needed, such as rivets and welding; (3) Permanent Fasteners - used when parts should not be separated, such as welding and soldering. Pins are classified into three main types: (1) Key Pin - requires slots in both hub and shaft; (2) Spline Pin - requires slots only in hub; (3) Spline Key - requires slots only in shaft. The spline key has a special head design with a chamfer angle of 30 degrees to facilitate easy removal. Plain pins are cylindrical fasteners with three main specifications: length, width (diameter), and thickness. They are typically made of 220F material with hardness of 1200. Round pins have a circular cross-section and are installed by cutting slots in the parts being joined. Cotter pins are classified into three types: (1) Flat Cotter - used for light-duty applications; (2) Tapered Cotter - used for medium-duty applications; (3) Tapered Cotter with Head - used for heavy-duty applications. The flat cotter is installed by cutting slots in the parts and bending the pointed end. The tapered cotter provides better fit and holding force. Tapered coppers are commonly used in rolling mills, installed at 120 degrees on wheel periphery.

Mechanical fasteners are critical design elements, with a Boeing 747 containing 6 million parts, half being fasteners. They divide into permanent (welding, gluing, riveting) and non-permanent (bolts, screws, studs). Threaded fasteners are most widely used due to reusability. Common types include hex head screws (most common), socket head screws (tight spaces), button heads (electronics), and flat head screws (countersunk). Materials range from low-carbon steel to titanium, inconel, and plastics. Zinc plating is economical for dry environments; chromate coating offers better corrosion resistance. Standards include ASTM, NAS, AAS, SAE for US fasteners and ISO/DIN for metric. Higher grades indicate higher tensile strength. Coarse threads (UNC) are standard; fine threads (UNF) offer 10% strength increase and vibration resistance. Thread engagement of 2x nominal diameter is sufficient.

Fasteners are mechanical devices with spiral threads that work in pairs (bolt and nut) to join components together, where the bolt's external threads mate with the nut's internal threads through profile matching rather than interference fit; metric screw threads follow a 60° triangular profile standard, with the nominal diameter referring to the outside diameter, and proper clearance (typically about 0.5mm) is essential for assembly; various locking mechanisms including locking nuts, nylock nuts, spring washers, castle nuts with split pins, keys for preventing rotation, and circlips for axial positioning are used to prevent loosening and ensure secure connections in mechanical assemblies.

Mechanical assemblies use various fasteners and retainers to secure components: shoulders (الكتف) are created by increasing shaft diameter, flat washers (الجاف) provide axial support, lock washers (حلقه مرينه) prevent loosening, set screws (صموله محزز) secure components, and cotter pins (سي بون) lock parts in place. These components are mounted on shafts and housings with specific conditions depending on whether the shaft or housing rotates. Understanding which components rotate is essential for proper retainer selection and placement.

Mechanical technicians must thoroughly understand machine components including fasteners (bolts, nuts), washers, bearings, shafts, bushings, couplings, and various joints. Critical distinctions exist between similar components—for instance, washers versus spring washers (mushroom washers), and right-hand versus left-hand threaded fasteners. Incorrect fastener selection based on thread pitch (e.g., M12x1.75 vs. M12x1.25) can cause component failure. Comprehensive knowledge of these elements enables proper machine assembly, disassembly, and repair.