Run the Envelope-Before-Payload Test
Arm families from FANUC, ABB Robotics, and Universal Robots publish payload and reach as headline figures. Both numbers are true and neither is the specification you actually need.
The Bot Scout envelope-before-payload test asks whether the arm can reach every point of the task with the real end effector attached, at the orientation the task requires. Rated payload is measured at the wrist; a gripper, camera, and cable pack subtract from it, and reach is quoted to the wrist rather than to the tool tip.
The mismatch shows up late and expensively. A cell passes on paper, then the arm cannot angle into a machine door, clear a pallet corner, or reach the far side of a fixture without colliding with its own second axis.
| Specification | What it means | What it hides | How to verify |
|---|---|---|---|
| Rated payload | Mass at the wrist | Gripper, camera, and cables subtract from it | Weigh the complete tool assembly |
| Reach | Distance to the wrist | Tool length extends beyond it | Model the tool tip, not the wrist |
| Axis count | Degrees of freedom | Orientation limits at the envelope edge | Simulate the awkward pose, not the easy one |
| Repeatability | Return-to-point precision | Not the same as accuracy to a coordinate | Ask which one the vendor is quoting |
| Cycle time | Motion speed | Excludes gripper open and close time | Time the complete cycle including the tool |
Choosing Between Arm Types
A six-axis arm buys orientation freedom and pays for it in speed and footprint. A SCARA is faster in a plane and cannot tilt. A collaborative arm trades outright speed for easier deployment near people, though safety is still assessed per application as covered in the cobot guide.
Match the type to the operation first, using the manufacturing robots guide, then specify the individual arm. Reversing that order produces a shortlist built around a vendor relationship rather than the task.
The commonly skipped requirement is the cable and utility routing. Air, power, signal, and vacuum lines all take space in the envelope and are the usual cause of a collision that simulation did not predict.
For the motor and gearbox side of an arm specification, the robot motors guide covers torque sizing, gearbox type, and encoder choice.
- Weigh the complete tool assembly before comparing rated payloads.
- Model reach to the tool tip in the worst-case orientation.
- Ask whether quoted figures are repeatability or accuracy.
- Route air, power, signal, and vacuum lines in the model.
- Time a full cycle including gripper actuation, not just motion.
Robot Arm Types: Articulated, SCARA, Delta, and Cartesian
An industrial robot arm is classified by its joint geometry, and the geometry decides which jobs it can do before any brand comparison starts. Four types cover almost every machine on a factory floor.
An articulated robot arm is the shape most people picture: a chain of rotary joints, usually six, giving full orientation freedom within its envelope. It is the general-purpose choice and the reason "robot arm" and "articulated arm robot" are used interchangeably.
SCARA arms are rigid vertically and compliant horizontally, which suits fast pick-and-place and assembly onto a flat plane. Delta arms hang above the work and move a light tool very fast, which is why they dominate food and packaging lines. Cartesian arms move along three linear axes and are the simplest to program and the cheapest per unit of reach.
Robot manipulators is the umbrella term for all of them, and it is the wording used in research and in control software. If a datasheet or paper says manipulator, it means the arm plus whatever is mounted at its end.
Humanoid arms are a separate design problem from industrial arms. An industrial arm optimises stiffness and repeatability from a fixed base, while a humanoid arm has to be light enough for a moving body to carry and compliant enough to be safe near people, so it trades precision for weight and safety.
Robots with arms are not automatically manipulators in this sense either. A mobile robot carrying a small arm is a different machine from a fixed cell, because its base position adds error that the arm has to absorb.
| Arm type | Geometry | Strength | Typical job |
|---|---|---|---|
| Articulated | Chain of rotary joints, usually six | Full orientation freedom in its envelope | Welding, machine tending, general handling |
| SCARA | Two parallel rotary joints plus a vertical axis | Fast and rigid in the vertical direction | Assembly and pick-and-place onto a plane |
| Delta | Parallel linkages driven from above | Very high speed with a light payload | Food, pharmaceutical, and packaging lines |
| Cartesian / gantry | Three linear axes | Cheapest reach, simplest to program | Large-envelope pick-and-place, palletising |
Buying and Supporting an Arm
An arm is rarely bought alone. Most industrial purchases run through an integrator who supplies the tooling, controls, guarding, and commissioning, which is why the arm price is a poor basis for comparing proposals — see the robot arm price guide.
Safety obligations attach to the application, not the arm. ISO 10218-1:2025 sets the requirements and OSHA notes that many incidents occur during programming, setup, and maintenance rather than production.
Ask where spare parts are held, what the controller software costs to license, and who is qualified to program the arm in your region. Those three answers determine downtime more than any specification on the datasheet. Hobbyists and students learning arm motion before scoping an industrial cell should start with the DIY robot arm kits guide, and one common learning project that transfers cleanly to industrial cells is the DIY line-following robot walkthrough.
Bottom Line
Specify a robot arm by working envelope with the real tool attached, then by payload, axes, and cycle time. The arm is one line in a cell, and support determines its uptime.
Model reach to the tool tip in the worst-case pose before shortlisting on payload.
FAQs
How do I choose a robot arm payload?
Weigh the complete end effector assembly including gripper, camera, and cables, then add margin. Rated payload is measured at the wrist, so the tool subtracts from what is available for the part.
What is the difference between repeatability and accuracy?
Repeatability is how closely an arm returns to the same taught point. Accuracy is how closely it reaches a specified coordinate. Datasheets usually quote the first, and applications often need the second.
How many axes does a robot arm need?
Six axes give the orientation freedom needed at machine doors, weld joints, and pallet corners. Planar pick-and-place often works with fewer, which buys speed and a smaller footprint.
Can I buy a robot arm without an integrator?
You can buy the arm, but most industrial applications need tooling, fixtures, controls, guarding, and commissioning. Those are usually supplied by an integrator and cost more than the arm.
Where can I find industrial robot arm cost breakdowns, schematics, and programming courses?
See The Bot Scout's dedicated industrial robot cost guide for arm-only versus installed-cell pricing by payload class, and the industrial robot programming guide for course and training options. Manufacturer datasheets, not general schematics, are the reliable source for a specific arm's dimensioned drawings.