Robot arm design: CAD, joints and materials

Most student robot arms do not fail in CAD. They fail at the joint. A printed hinge with a servo screwed to it will flex, wear and develop backlash within a week.

This guide covers the design sequence for a small robot arm: reach and payload, degrees of freedom, link and joint design, materials, bearings, cable routing, and the CAD checks that catch problems before you print.

Start from the task

Write down four numbers before opening CAD: the reach you need, the payload including the gripper, the cycle time, and the repeatability the task requires. Every later decision follows from them, and a design that ignores them becomes a sculpture.

Degrees of freedom, one at a time

• Base rotation: sweeps the workspace; the heaviest joint because it carries everything.
• Shoulder: raises the arm; the highest torque demand with a horizontal arm.
• Elbow: sets reach; lighter than the shoulder because it carries less.
• Wrist roll: orients the tool around the approach axis.
• Wrist pitch: points the tool up or down.
• Wrist yaw or a sixth axis: full orientation, rarely needed for a first build.

Four axes handle most pick-and-place tasks. Five add orientation flexibility. Six give full pose control but multiply cost, calibration work and the chance of a self-collision.

Link length and the reach trap

Reach grows with link length, and so does the moment on every joint behind it. Doubling the forearm does not double the payload capacity; it roughly halves it at full extension because the same mass now sits twice as far from the shoulder.

static torque at a joint = sum(mass_i * g * horizontal_distance_i)

Example: forearm 0.3 m, 0.8 kg, payload 0.5 kg at 0.45 m
T = 9.81 * (0.8 * 0.15 + 0.5 * 0.45) = 9.81 * (0.12 + 0.225)
T = 3.38 Nm before acceleration and safety factor

With a safety factor of 2 and 5 m/s^2 acceleration:
T_required is roughly 6.8 Nm at the shoulder

Design for the worst pose: arm horizontal, payload at full extension. Any other pose is easier, and the worst pose is the one that breaks the servo or the printed bracket.

Joint design

A joint needs a bearing path, a drive path and a structure that keeps them aligned. In hobby arms, all three are often the same printed boss around a servo horn, which is why backlash appears quickly.

• Carry load through a bearing, not through the servo shaft.
• Use two bearings separated along the axis for a moment-resistant joint.
• Drive through a coupling or a gear that tolerates small misalignment.
• Keep the motor mass close to the previous joint to reduce inertia.
• Provide an adjustable stop for each axis.
• Leave access to the fastener that holds the joint together.

Materials

Printed links are fine for light payloads if the load path is designed properly. A printed part in pure tension or compression behaves well; the same part in bending creeps and cracks. Add ribs, increase the second moment of area, and orient the layers along the load.

Bearings, shafts and backlash

• Use ball bearings for rotation and thrust washers for axial load.
• Press bearings into a printed housing with a slight interference fit, or use a metal insert.
• Avoid a printed hole as a bearing surface; it wears oval.
• Preload the joint lightly to remove play without adding friction.
• Keep the bearing span as wide as the joint allows.
• Check backlash by holding the link and rocking the drive.

Fasteners and printed threads

Self-tapping screws into printed plastic work once. After two or three assemblies the thread strips, the joint loosens and the arm loses repeatability. Heat-set inserts cost a little and solve the problem permanently.

• Use heat-set brass inserts at every serviceable joint.
• Leave a clearance hole for the screw shank and a boss for the insert.
• Use a washer under the head to spread load on plastic.
• Do not over-torque; printed bosses split along layer lines.
• Keep a spare set of inserts and screws.

Cable routing

Cables fail at the joint that rotates most. Route them through the axis of rotation where possible, leave a service loop, and strain-relieve at both ends. A cable tie pulling a wire tight around a hinge will break the conductor inside the insulation long before it looks damaged.

Gravity, springs and counterbalance

The shoulder motor works hardest holding the arm up, not moving it. If the arm is heavy, a spring or a counterweight reduces the holding torque and lets a smaller servo do the job. It also makes the arm safer when power is lost: a balanced arm falls slowly instead of dropping.

• A gas spring or extension spring across the shoulder is the simplest counterbalance.
• Place the spring so its moment arm is largest at the horizontal pose.
• A counterweight works but adds inertia and total mass.
• Check that the mechanism cannot over-centre and pull the arm up on its own.
• Design a mechanical stop so a power loss does not drop the arm onto the fixture.

Print settings that change strength

Layer orientation decides whether a printed link survives. A part printed flat on its largest face has layers running across the bending load and splits; the same part printed on edge puts the layers along the load. Increase wall count before infill, and add fillets where a boss meets a plate.

• Orient load-bearing parts so layers run along the main load path.
• Use four or more perimeters on structural parts.
• Keep infill moderate; walls carry most of the load.
• Add fillets at every sudden section change.
• Print a test joint before printing the whole arm.

CAD workflow

• Draw the kinematic skeleton first, with joint axes and link lengths as named parameters.
• Build every part from that skeleton so a length change propagates.
• Model the servo, bearings and fasteners at real dimensions, not as blocks.
• Assemble with real clearances; a 0.2 mm gap in CAD is 0 mm in printed reality.
• Run a motion study through the full range and look for collisions.
• Export each part with its own print orientation in mind.

Testing the first article

• Clamp the base, hang a known mass at full extension and measure deflection.
• Command the same point ten times and measure the spread.
• Rock each joint by hand and feel for backlash.
• Run a full-range sweep slowly and check for cable snagging.
• Run it hot; printed parts soften and a marginal joint becomes loose.

Mistakes that ruin a good design

Related: 3D printed robot parts, robot arm kinematics, pick and place arm with Arduino, gearbox types, motor selection.

Link lengths from CAD feed straight into the inverse kinematics for a two-link arm.

Leave mounting space for a vacuum cup array when you model the wrist flange.

Every CAD decision is questioned in an interview, as this mechanical design question list demonstrates.

Size the joint motor from the load case, then verify it against the published torque speed curve at your top speed.

Sources and further reading

• ISO 9283 — robot performance criteria and test methods.
• SKF — bearing selection and mounting guidance.
• Prusa Research — material properties and print orientation.
• ISO 10218 — robot system safety for arm design limits.