Direct answer: Choose by the motion. A step that picks from a fixed height and places at a fixed height, with rotation about the vertical axis only, is planar work and suits a SCARA. A step that changes the attitude of the part needs an articulated arm. Both configurations frequently appear on one line, each taking the steps that match it, and feed stability plus fixture repeatability set the ceiling for either.
Who this is for: Written for engineers specifying automation for small moulded or machined parts fed from bowl feeders, trays or rails into fixtures.
Scope: This EVST guide covers how to split assembly work between planar and articulated configurations, and why feeding and fixturing cap what either can deliver. It does not cover vision selection, gripper design or line balancing.
Start from the motion, not the axis count
The common framing is four axes against six. It rarely helps, because it compares hardware before the work has been described.
In the reference footage a SCARA lifts small moulded parts from a straight vibratory feeder rail and sets them into a locating fixture. The cycle picks at one height, travels in plane and places at another height, with rotation about the vertical axis. That is planar work by definition.
A six-axis robot works further along the same bench on another part of the process. Two configurations on one line is a normal arrangement, and it is what a step-by-step split usually produces. According to ISO 10218-2:2025, it is the application that is assessed rather than the robot alone, which is why a SCARA robot and a six-axis arm can sit on one line without one being the safer choice by category.
What planar work gets from a SCARA robot
A SCARA concentrates its stiffness in the horizontal plane and gives a vertical axis for insertion. For repeated pick and place at fixed heights, that is a direct match between structure and task.
The trade is attitude: the part arrives and leaves in the same orientation apart from rotation about the vertical. Any step that has to tilt a part, approach at an angle, or present a face for inspection is outside that envelope.
According to ISO 9283:1998, pose repeatability is stated under defined test conditions, so it is worth reading the number for the pose a SCARA robot actually uses in your cell rather than the headline figure.
What articulated arms are for
An articulated arm buys attitude. Steps that rotate a part into a fixture, follow a contour, or approach from a direction the fixture blocks need the extra freedom.
That freedom costs envelope and usually cycle time on simple moves. Using it for a step that never leaves the plane is paying for capability that stays idle.
The practical split is not by robot preference but by whether attitude changes anywhere in the step.
Feeding and fixturing set the ceiling
In the footage the parts come off a feeder rail. Whether they arrive in a consistent orientation, and whether the rail keeps them there, decides whether the arm can pick without searching.
The fixture side is the mirror image: if a placed part has a unique seated position, the next operation does not have to find it again.
Repeatability figures are quoted under laboratory conditions. On a real line the delivered result is bounded by feed consistency and fixture design long before it is bounded by the arm. According to ISO 12100:2010, determining the limits of the machinery comes before protective measures, and in assembly those limits usually sit in the feed and the fixture rather than in the SCARA robot itself.
What the footage proves and what it does not
It proves that a planar-configuration robot runs a repeated pick and place from a feeder rail into a locating fixture, and that a six-axis robot works on the same line.
It does not prove cycle time, yield, placement accuracy, or how the two arms divide the work between them. Those come from process documents and measurement, not from watching.
A selection argument can be built from observed motion. A performance claim cannot.
A short decision table
Pick and place at fixed heights, rotation about the vertical only, short travel: planar configuration.
Attitude change, angled approach, contour following, or access blocked in the plane: articulated configuration.
Mixed line with both kinds of step: split by step and accept two configurations, rather than forcing one arm to cover both.
| Step motion | Configuration | Why |
|---|---|---|
| Pick and place at fixed heights | SCARA robot | Planar work matches the structure |
| Rotation about the vertical only | SCARA robot | No attitude change required |
| Part tilted or approached at an angle | Six-axis | Attitude change needs the extra freedom |
| Access blocked in the plane | Six-axis | Approach direction is the constraint |
What to prepare before asking for a quotation
Part drawings and the supply method: bulk, tray, or bowl feeder with a rail. Assembly sequence broken into steps, marking every step where attitude changes.
Cycle requirement, number of variants and changeover expectations. Station space and where operators load and unload.
With those in hand, configuration and fixture concept can be discussed together instead of separately.
Frequently asked questions
Is a six-axis robot always more capable?
It has more freedom, which is only an advantage where a step uses it. For planar pick and place the extra axes usually add envelope and cost without adding capability.
Can one arm cover the whole line?
Sometimes, but it often means the arm is oversized for most steps and marginal for one. Splitting by step is frequently cheaper and steadier.
Does repeatability decide the choice?
Rarely on its own. Feed consistency and fixture design usually bound the result before the arm’s repeatability does.
What if part variants differ in attitude?
Then attitude change exists somewhere in the family, and the step that handles it needs an articulated arm even if the others do not.
Project inputs for an application review
If you are specifying small part assembly, a step-by-step description of the motion gets a more useful answer than an axis count. EVST reviews the motion of each step before proposing a configuration.
- part drawings and how parts are supplied
- assembly sequence with attitude changes marked
- cycle requirement, variants and changeover expectations
- station space and operator access
Send those and EVST will map configurations against your actual steps. Related reading: robot machine tending, collaborative robots.