Design guide Updated: Monday, March 14th 2022

Designing Machine Tending Equipment

1. Grippers and fingers
2. Part presenter
3. Robot base

Machine tending is by far the most common collaborative robot application. Despite widespread use, each application still requires customization to unlock the full potential value. How much autonomy is sought? Can machine-to-tray cycle be optimized? How should the tray be configured? How many parts can I tend?

This guide is intended to guide cobot users through each design consideration for their machine tending robot cell. The best practice is usually to start from the part you want to tend, and define your cell components accordingly:

Grippers and fingers
Part presenter
Robot base

1. Grippers and fingers

To choose the right gripper for your application, refer to our 2-finger gripper guide as a starting point.

When designing a robot cell for machine tending, there are additional design considerations pertaining to the end-of-arm tool:

Choose an electric gripper when compressed air is not available, when energy efficiency is required, or when controls flexibility is necessary.
Choose a pneumatic gripper when you need a high force to size ratio, a cost-effective solution, or a short actuation time.
Optimize the stroke needed to minimize the gripper size and optimize cycle time. The stroke should be determined by the clearance needed to get the fingers around the part (or within it, in the case of an internal grip) rather than the part’s size.
Add a tool changer if you intend on using the same robot for tending different parts that require different grippers.
Add a dual tool mount adapter to shorten the cycle if necessary. This will allow for a raw part to be added to the CNC while a machine part is removed, significantly improving cycle-time of the tending operations.

While collaborative grippers typically come with standard plastic fingers that allow flexibility for low-weight, easy-to-pick parts, it is possible to 3D print custom fingers for your specific needs. Nonetheless, most machine tending applications use custom-machined aluminum or steel fingers that fit the parts to be tended, increasing repeatability and cycle time.

Here are additional considerations for gripper fingers in machine tending applications:

Taper the fingers to increase the amount of material at the jaw.
Position the part to be tended as close as possible to the gripper to keep finger lengths as short as possible.
Use aluminum fingers instead of steel if you want to minimize the weight of the fingers and leave room for heavier payloads.
Aim to encompass the part and therefore use form-closure grasp to better secure the part.
Use rubber/urethane pads to increase the grip and avoid marking the part.
Select a 3-finger gripper if you are tending a cylindrical part.

2. Part presenter

Once you select a gripper and the appropriate finger design, the next step is to customize your part presenter. This is the structure that will hold the parts to be tended (pre- and post-machining). The two most common part presenter options are the configurable trays and the drawers.

2.1 Configurable trays

Configurable trays consist of a base plate to which are attached threaded nylon pins that act as datum locators. With nylon pins, the work surface can be configured to create repeatable locators specific to the part being tended. Part density on the base plate is easily adjusted by reconfiguring the pins. Trays are typically simple and inexpensive to deploy but offer limited autonomy in terms of part-holding capacity.

Depending on part dimensions, operators may typically load anywhere from 16 to 32 parts per base plate. To optimize your tray, consider your fingers’ design and size to determine the area they cover while the gripper is in the “open” position. The fingers are typically the main limitation in terms of number of parts to have on a tray since they can easily collide with other parts at the pick-up and drop-off stages.

Some users also prefer to have easily removable base plates, which enable off-line restocking of the trays and increase uptime. Also, cycle time can be improved by adjusting the angle of the tray facing the robot.

2.2 Drawer systems

Drawer systems typically involve 3 to 5 drawers, each holding up to 50 parts. Drawer systems have a high part-holding capacity — often ranging from 100 to 250 parts total — while requiring similar shop floor area than the trays. There are 3 main considerations when designing drawer systems:

Robot reach
Locking system
Open/close process

To position your robot correctly, consider positioning the robot base at a distance of 300mm to 450mm from the drawers to ensure all parts are accessible by the robot.

The drawer length must be equal to the rails’ maximum stroke to avoid overlap between different layers of drawers. For Vention telescopic slides, the maximum stroke is 666mm.

This type of part presenter can allow for 8+ hours of autonomy.

Typically, drawers have 3 locking positions: robot-side, closed, operator-side. While the robot tends the parts with a drawer open on the robot-side, an operator can open a drawer on the operator-side to remove machined parts and refill with raw parts. Drawer systems can be manual or automated, and locking mechanisms allow for high repeatability.

For the manual system, the robot arm simply pushes or pulls the drawers in and out as needed. An indexer system using plungers and catches is needed to ensure the positioning is accurate. To reduce the inertial load on the cobot during the drawer opening and closing process and ensure accurate positioning, the Machine Tending Indexer Catch along with an Indexer Spring Plunger can be used. When spacing these indexers, use either 666mm (rail stroke) or the maximum reach of the robot at that height, whichever is lower. By doing so, the cobot will be able to successfully complete its opening or closing process.

We recommend using one inductive proximity sensor per drawer. Ensure the chosen sensors have the right internal circuit design for your robot (e.g., Universal Robots controller allows for PNP sensors only). Calibrate the sensor position and the gusset below the drawer so that the sensor is activated both when the drawer is closed and when the drawer is fully open.

When designing and assembling a cobot drawer system, ensure that smoother sides of the indexer catches are both facing the inside of the drawer. The indexer plunger should be positioned in the “rear” position in relation to the cobot operation side. This will ensure that the cobot will have to overcome a much smaller amount of force to open and close, while also guaranteeing that the start and end points are clearly noticeable. An example of this setup is shown below.

For automated drawer systems, electric actuators open and close the drawers. These systems minimize the calibration time required compared to manual locking devices, but are limited to 1-way drawers only.

We recommend installing panels on top of the drawer as the installation process is simpler than panels that are inside or below the extrusions. We also recommend adding countersink holes and locating them on top of the extrusions that are perpendicular to the slides of the drawers (see image below) since pressure points should be avoided on sliding mechanisms.

When the robot is used to open and close drawers, avoid using the gripper fingers to do so. We recommend using the Machine Tending Robot Calibration Flange to interface with a round extrusion handle, or a custom hook with an assembly plate or handle. Ensure the wrist does not enter in collision with the structure or the parts.

2.3 Trays vs. Drawers

The table below compares the main characteristics of configurable trays and drawer systems for machine tending.

	Drawer systems	Configurable trays
Part Presenter
Parts Capacity	100 - 250 parts	Up to 50 parts
Configuration	Up to 5 drawers	Up to 2 trays side-by-side
Part Dimensions	Up to 150 mm x 150 mm x 150 mm	Up to 150 mm x 150 mm x 150 mm
Autonomy Gain (at 2 min/part cycle)	Up to 8 hours	30 min to 1 hour

Whether you choose a configurable tray or drawer system, your manufacturing process stands to gain improved productivity and autonomy.

3. Robot base

When it comes to the robot itself, we recommend keeping it fixed on the workstation. This allows for the maximum reach to the trays or drawers. The robot base should be as close as possible to your machine and to each part on the trays or drawers.

Robot bases can be fixed to the ground with floor anchors, or can be mobile using caster wheels. Many operators choose a mobile base as it allows easy redeployment to another machine. For such mobile bases, we recommend locating devices to position your station after each displacement. If you are using Universal Robots, Flexxbotics offers a plug and play software solution for fast redeployment.

Machine Tending Mobile Cart with 3 Drawers