Keep reading for our cheat sheet on different robot specifications, which will help you choose the right robot.
The robot’s reach, or work envelope, is the robot’s range of movement. If you want your robot to grasp parts, use a tool, and open a door, all these items must be located within the robot’s work envelope.
A machine-tending robot needs to take a blank part from somewhere (point A) and place it in the machine (point B). To calculate the minimum reach your machine-tending robot must have, measure the distance between these two points and divide it by two.
- If you need a large reach, your robot's payload will necessarily be higher; a larger robot is a stronger robot.
- A robot’s reach is also determined by its number of axes (degrees of freedom).
The robot’s payload is one of the most important specifications. Payload is the total weight the robot can carry. Since your robot will carry different types of parts, you will need to determine which parts will be the heaviest and select your robot consequently. To determine your required payload, sum the weights of your tool and the heaviest raw part you want to carry.
Calculate part weight by multiplying the part’s volume (W x L x H) by its density (g/mm^3). Or use CAD software to determine the weight of the part; that way you won’t need to do the math.
In order to have safe robot acceleration and speed settings, the robot should carry a weight no more than 90–95% of its maximum payload. In fact, carrying a load too close to the maximum payload can cause an error and stop your program.
Quick tip: Always add an extra 20% to your final payload calculation, to enable maximum accelerations and prevent errors while running the program.
Although CNC machines are super repeatable and you want your parts to be precise, you don’t need your robot to be as precise as your machine. Machine-tending robots are typically used to do first setups where the part is usually larger than the finished part, which allows for small variations in positioning.
The robot should be repeatable within 0.1 mm. To ensure repeatability, use mechanical stoppers or a force-torque sensor.
Quick tip: Make sure to have a compliant component in your program in order to allow the vise to close and not be restrained by the robot. In other words, leave at least one axis “free” when inserting a part in a vice or lathe, to prevent wear on the robot actuators.
Robot end effectors (grippers)
Your gripper is the robot’s “hand,” but it’s not nearly as versatile as your hand. Grippers work best when parts have at least two parallel surfaces. The stroke of the gripper will limit the range of parts you can handle.
Try to choose an adaptive gripper that can handle different shapes and, more important, different sizes of parts without modifying fingers or the robot’s programming.
Quick tip: Make sure your gripper can handle 85–95% of the parts. The rest can be loaded manually if need be.
Like robot payload, gripper payload is the amount of weight the gripper can handle. Make sure to respect this payload to ensure gripper longevity.
Quick tip: In addition to respecting payload, make sure not to max out the gripper’s allowable torque. If the bulk of the object's weight is always grasped by the gripper’s fingertips, the gripper will wear prematurely.
Grippers often have very limited dexterity. In fact, if your application requires the operator to handle several different parts at the same time and perform operations with both hands, proceed carefully. Robots can only do one thing at a time. This doesn’t mean you can’t use robots, but it does mean you will most likely have to redesign your process.
Here are some of the most important robot dexterity factors, along with questions to ask when defining your needs:
- Object size. How big are the parts? Are they identical, or a variety of sizes? How does this compare with the reach required of the robot?
- Object shape. What shape are the parts? Do they have many complex edges or a simple geometrical shape? Are they spherical or otherwise difficult to grasp?
- Gripping strategy. How could the parts be grasped (e.g. with an encompassing grip, internal grip, or suction)? Are there different ways to grasp the same parts? Are the parts delicate and so require a particular gripping strategy?
- Reach. How much does the robot have to “stretch” to reach all important locations in the workspace? Will the robot use its entire workspace, or just a small part of it? Does it need to approach locations from many different angles?
- Speed. What cycle time is required for each action?
People sometimes think factors 1-3 only relate to the robot's gripper, and factors 4-5 only relate to the manipulator, but they’re actually all interrelated! One factor in isolation does not necessarily make a robot “dexterous.” You can only get a full picture of dexterity when you consider all the factors together.
Your gripper selection will depend on the parts to be handled. If your parts vary in size, you will need a flexible gripper. Robotiq’s 2-Finger 85 Gripper can accommodate parts between 5 mm and 80 mm without any additional programming. If you will always be handling parts of the same size, you may want to go with a more rigid custom gripper that will perfectly fit your part.
Quick tip: A flexible gripper can grasp both raw and finished parts. A 2F85 Gripper will also allow you to adapt the force of the gripper in cases where the finished product is more fragile than the raw part.
In machine tending, the interface between the robot and the CNC machine is an important part of the integration. This is not always a given for different machines produced by different manufacturers. In order to optimize your cycle time, you should make sure the two machines can talk with each other so they know each other’s status. They will need to communicate statuses like CNC program done, door can be opened, vice closed, robot in motion, and part in place.
Quick tip: Make sure your robot and CNC or other machine can share the same interface. Some robot models may not speak the same language as your machine.
Picking the raw part and placing the finished part must be done in a structured way. We recommend starting with a simple device that will position the part in the same spot every time. This is relatively easy to program and highly repeatable, especially since most cobots have built-in wizards to simplify path programming.
Quick tip: Keep it simple by using matrices or stacks to position parts in the same place every time.