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When are Six Axis Robots a Better Choice than Three Axis Gantry Robots?
03.20.20

When are Six Axis Robots a Better Choice than Three Axis Gantry Robots?

Six Axis are Better than Three, Usually
Six axis robots are the most widely integrated industrial robot at present! This is because the six degrees of freedom they offer deliver a near perfect, middle of the road combination of flexibility, strength, and reach for a majority of industrial operations [1].

Across the industrial world, the robot star is ascending. Robots continue their relentless march forward, handling lethal, tough, and repetitive tasks with effortless ease [2]. Superior controls have propelled robot user friendliness to unprecedentedly high levels [3]. Employing robots boost throughput, minimize cycle t imes, and remove bottlenecks [3].
Considering their fantastic applications, understanding the capabilities and limitations of different robot types is a worthwhile exercise.
What works today may not hold good tomorrow. Even if an economical three-axis gantry robot addresses a manufacturer’s requirements at present, it may not deal effectively with the rising operational complexities of the future. This can happen for many reasons.
For one, the simple manufacturing operation of the present day may evolve into a more intricate at a later date. Inspired by initial success, the manufacturer may diversify into more sophisticated projects wherein operations too get more convoluted by their very nature [4].
However, gantry robots have their own sets of advantages. They do a much better job when dealing with heavier loads across longer spans. Next, technicians can customize their features to a certain extent, the addition of a rotational axis to the arm being one such. The choice between six and three is, therefore, a more nuanced one.

Features of Multi-Axis Robots


Figure 1. Axes of a Six Axis Robot

Arms of six axis robots can rotate along six diverse axes, thereby permitting the robot to move in six directions as described below:

1. Axis 1 (Vertical): Robot pedestal can rotate around this vertical axis, thereby allowing the robot arms to move left-right in a full circle.
2. Axis 2 (Horizontal): Robot shoulder can rotate about a horizontal axis, thus enabling the robot arms to bend forward and backwards.
3. Axis 3 (Horizontal): Robot elbow can rotate about this horizontal axis, which is parallel to axis 2. Axis 3 facilitates the up and down movement of the robot arm.
4. Axis 4 (Vertical): Robot wrist can Yaw i.e. move left-right by virtue of rotation along this vertical axis. Axis 4 is parallel to Axis 1.
5. Axis 5 (Horirontal): Robot wrist rotates about this horizontal axis to create the Pitch movement i.e. up and down. Axis 5 is parallel to axes 2 and 3.
6. Axis 6 (Horizontal): Robot wrist can Roll i.e. rotates in 3600 around this horizontal axis which is perpendicular to axis 5.

A tool is attached to the wrist. It can be a drill, a weld gun, a material gripper or any other required instrument. Axes 1, 2, and 3 are the Position Axes whose purpose is to place the tool correctly in the work area. Axes 4, 5, and 6 are the Orientation Axes and their job is to align the tool correctly in relation to the workpiece [5].
Then, there is the SCARA i.e. Selective Compliance Articulated (or Assembly) Robot Arm robot with four axes of movement [6]. These can move along two horizontal and one vertical axis, along with rotational movement around the vertical axis. What the SCARA robots do not have is the pitch and yaw motions of the six axis robots [6].

Figure 2. SCARA Robot
Image Courtesy of Alexgace

Both, SCARA and six axis robots are articulate robots i.e. they have between four and ten axes. Generally, however, the number of axes on articulate robots is limited to between four and six [7].
Cartesian Robots move along three axes – two horizontal and one vertical. Designers can add a fourth, rotational axis to Cartesian robots at the tip of the arm for a specified operation [8].
Gantry Robots are Cartesian robots with two parallel horizontal axes instead of one. The horizontal member placed across these two parallel axes provides the second horizontal axis, which also houses the mechanism for the vertical motion.

Figure 3. Plotter / Printer is Similar to a Gantry Robot
Image Courtesy of Florian Schaffer

What this does is improve the load capacity. A Cartesian robot’s load capacity is limited because its second horizontal axis, which houses the tool, is inherently of a cantilever design, supported only at one end [8]. A gantry robot makes it a beam supported at both ends.
In figure 4, the plotter / printer is fairly similar to a gantry robot with the two parallel horizontal axes serving as the base axes. The cross horizontal axis can slide along the base axes. The printer tool slides along the cross horizontal axes and can also move up and down.

Comparison of Six Axis, SCARA, & Three Axis Gantry Robots

Selecting from among robot types for a particular task requires an analysis of the application’s LOSTPED considerations [9]. These are:

Load: Gantry robots are best suited for high load and long stroke operations. The working arm of six axis and SCARA robots is a cantilever, which cannot support very heavy loads. Plus, cantilevers deflect at the free end when supporting heavy loads, something that lowers their accuracy.
Orientation: Handling complex part movements along four or more axes is best left to six axis and SCARA robots. Intricate movements include rotating the part or locating the tool at diverse positions and angles.
Speed: Six axis and SCARA robots are faster as they follow an interpolated path between points. Cartesian and gantry robots, on the contrary, move along the linear axes, one at a time.
Travel: Gantry robots are most compatible for moving loads across longer spans while six axis robots are better off guiding the tool along complicated trajectories.
Precision: SCARA and six axis robots have predefined precision levels. Operators can customize the accuracy of gantry and Cartesian robots. Deflection of the cantilever arm tends to lower the precision of six axis and SCARA robots.
Environment: Compact six axis and SCARA robots require little floor space. Six axis robots are best suited for applications where mechanical interference is a reality such as extending into boxes to extract parts. They are also less affected by dirt as it is possible to completely seal rotary joints.
Duty Cycle: Is the time needed for one complete operation cycle. This duration will vary based on the intricacy of the involved operation.

Table 1. Comparison of Six Axis, SCARA, and Gantry Robots

Six axis robots score over gantry robots when [9]:
• Motion and speed parameters are clearly defined.
• Manufacturer requires the new robots to start delivering urgently.
• Operations are complicated.
• Intricate movement sequences need to be executed.
• Task necessitates flexible arm extensions, for these robots are designed light.

Arguably, six axis robots (articulated robots in general) are the most versatile because [10]:
• Their size ranges from huge to tiny.
• When installed on walls or ceilings, they produce minimal impact on shop floor footprint.
• They work well in dusty-dirty environments as sealing rotary joints (all six axis joints are rotary) is simpler than doing that with linear slide ways.
• Adding a linear rail for their movement facilitates motion along a seventh axis.

Six Degrees of Freedom: The Six Senses
With greater freedom comes greater functionality.
Each axis of a robot lends it one degree of freedom i.e. enables its movement in one direction. With six axes, the robot commands greater freedom of movement and, consequently superior functionality. What is more, it is tougher – making light work of dusty ambiences that can clog a sliding gantry robot.

Since 1989, Cybernetik Technologies has been seamlessly and completely integrating reputed six axis robots including Kuka, ABB, and Yaskawa into industrial spaces. Beginning with robot selection based on payload, size, and speed, we continue with prompt installation and after-sale support services.
Contact us at +91 20 6790 9600 or sales.automation@cybernetik.com and experience the sheer delight of end-to-end automation services.

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References
1. https://www.robots.com/articles/benefits-of-six-axis-robots
2. https://www.newequipment.com/industry-trends/article/22059066/whats-the-difference-between-industrial-robots
3. https://www.robotdigg.com/news/131/The-Difference-between-Cartesian,-Delta,-SCARA-and-Six-axis-Robots
4. https://blog.universal-robots.com/how-to-get-all-the-advantages-of-a-6-axis-robot-for-the-price-of-a-3-axis
5. https://www.youtube.com/watch?v=EjvdpkX9fD8
6. https://www.fanuc.eu/de/en/robots/robot-filter-page/scara-series/selection-support
7. https://www.robots.com/articles/articulated-robots-more-axes-equal-more-possibilities
8. https://www.linearmotiontips.com/when-do-you-need-a-gantry-robot/
9. https://www.machinedesign.com/mechanical-motion-systems/article/21831692/the-difference-between-cartesian-sixaxis-and-scara-robots
10. https://www.kellertechnology.com/blog/how-to-use-4-types-of-industrial-robots-to-your-advantage/

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