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.
- 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.
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.
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.