A six-axis robot combines six controlled rotary joints to place a tool at a position and orientation within its usable workspace. Its value is the ability to approach a part from different directions, reorient it and move through a planned sequence.
Choose the robot from the required tool poses, load and process result. This guide explains what to put in a specification and what to ask a supplier to demonstrate. It is intended for students and equipment specifiers; actual load approval and cell validation depend on the selected robot and application.
What six axes provide
In a typical articulated arm, the base, shoulder and elbow largely position the wrist, while three wrist joints orient the tool. All six joints interact in the full mechanism. Axis names, arrangement and permitted travel vary by model.
A pose includes both location and orientation. Imagine placing a nozzle above a part: its tip must reach the target and point in the required direction. A second pose might place the same tip beside the part with the nozzle horizontal. The gripper body, forearm and cables also need clearance in both poses and along the connecting path.
The tool center point, or TCP, is the tool reference used to describe that motion. It may lie at a gripper’s working center or at a nozzle tip, away from the robot’s mounting flange. Tool geometry therefore changes the position the flange must reach.
Six joints allow broad orientation control, but usable poses remain constrained by joint limits, obstacles and configurations where control becomes difficult. A singular configuration reduces the available independent motion or demands excessive joint motion for a requested tool movement. Ask the supplier to inspect the entire path, including entry, retreat and changeover positions.
Compare the geometry with the work
Begin with a sketch of the job. Mark pickup, process and delivery poses, the parts around them, and the required directions of approach. Then compare mechanisms. The OSHA robot-system overview describes articulated, SCARA, Cartesian and parallel structures.
| Robot type | Useful fit | Question to resolve |
|---|---|---|
| Six-axis articulated | Multiple approach angles and reorientation around a part. | Can every required pose be reached without collisions or joint problems? |
| SCARA | Planar assembly and transfer with vertical insertion. | Does the task need tool tilt beyond the available axes? |
| Cartesian | Motion aligned to a rectangular X/Y/Z work area. | Which additional tool rotations are required? |
| Delta / parallel | Rapid picking over a designed workspace. | Do payload, orientation and workspace fit the exact model? |
| Palletizing arm | Repeated stacking with a constrained tool orientation. | Are unusual carton angles or intermediate operations required? |
For example, a fictional tray-loading job keeps a part upright throughout the cycle. Start by evaluating a mechanism that supports that simple motion. If the process adds inspection of the underside followed by insertion at an angle, record the new orientations and reconsider the geometry. The extra axes earn their place when they support required work.
A catalog reach is a useful first filter. It does not establish that a particular tool orientation is available at the farthest point, or that the loaded robot can complete the required motion there. Include the base mounting position, tool length and all clearance objects in the application model.
Count everything carried by the wrist
The workpiece is only part of the load. Include the gripper, adapter, tool-mounted sensors and other carried equipment. Check the applicable load diagram and the manufacturer’s treatment of supported cables or hoses. Evaluate both empty and loaded tool states.
Universal Robots’ UR8 Long maximum-payload documentation shows that permitted payload depends on its center-of-gravity offset. Its UR15 payload configuration documentation distinguishes mass, center of gravity and inertia. These are model- and software-specific examples of the information a real selection needs.
The weighted-average calculation extends to three coordinates, each referenced to the same flange frame. It gives a center of gravity, not a complete inertia model. Two tools with the same mass and center can distribute their material differently and resist rotation differently. Obtain a suitable CAD mass-properties report or the manufacturer’s prescribed method for the actual assembly.
Record the result in the six-axis selection worksheet, including mass uncertainty and which accessories were counted. Carry an unresolved value forward as an open requirement, not a convenient zero.
Read precision claims in the context of the process
Repeatability concerns how closely the robot returns to a position. Accuracy concerns closeness to the intended reference. A repeated position can be consistently offset. NIST’s explanation of robot accuracy and repeatability shows why calibration, preparation and maintenance belong in the discussion.
Ask what the quoted value measures: a point or a path, translation or orientation, and under which load, speed and test conditions. Your process may care about a nozzle’s path while moving, a drill’s position under force or a part’s seating after release. A single repeatability number does not describe all three.
Define the complete tolerance budget. Errors can enter through TCP setup, fixture location, part variation, sensing, compliance and temperature. For a taught pickup at a stable fixture, repeatability may dominate the practical requirement. For offline programmed positions referenced to a CAD model, coordinate alignment and absolute accuracy also become central.
Likewise, a fastest-motion or standard test-cycle figure needs its path definition. Measure your cycle with gripping, machine handshakes, inspection and settling included. The manufacturing project guide connects that result to good output per shift.
Ask for a representative application demonstration
- Submit poses and paths. Provide part and tool geometry, approach directions, fixtures and expected variants.
- Submit load states. Include empty, full and intermediate configurations with mass properties and attached services.
- Define the output test. Specify the feature being measured, its reference, acceptable variation and the measuring instrument.
- Include operating conditions. State mounting, environment, process forces, expected duty and changeovers.
- Review the evidence. Ask for the actual configuration, successful and failed cycles, joint/path constraints and remaining assumptions.
Simulation can reveal access and motion problems before a physical trial. A physical trial then tests the effects of real parts, tools and the process. Retain both sets of assumptions so later changes to tool length or part weight trigger a fresh assessment.
Finally, include the whole cell in the safety work. ISO 10218-2:2025 covers industrial robot applications and cells. Determine the applicable requirements with qualified designers and the integrator; this selection worksheet is a technical brief, not a commissioning approval.