A beginner’s guide to automation

Cell requirements

Here are a few questions to ask when considering an automation cell. You may want it to unload and load the machine tool, but do you want something designed to load only one type of part or something flexible enough to handle other parts as well? A system handling a family of parts could keep the change-over to a minimum, or the cell could be designed to handle almost anything within a certain size range – but flexibility tends to cost more money. How often and what kind of change-over are you expecting? Automation can be configured to allow some components to be automatically changed, but this is more expensive than manually changing components over. How much infeed capacity would you like the cell to accommodate? A lot of capacity is great, but too much capacity can take up a lot of space and be a waste. How much room do you have for the cell? Do you plan on running the cell with the robot all the time or will the machine be used manually without the robot for periods of time?

The robot

The main component in the automation cell tends to be the robot, the device that provides most of the required motions. Automation robots are divided into Cartesian- and articulating-style. Cartesian (X/Y) style robots are basically grippers on two rails and are often seen on turning centers. One rail brings the robot over the machine and one rail brings the robot down into the machine to the spindle. Occasionally, a third axis is used to allow the robot better access to the spindle when overhead obstacles, such as the door configuration, limit direct access to the spindle. Quite often, Cartesian robots are developed and sold as a semi-standard option by the machine tool company and are purchased as part of a package along with the machine tool. These Cartesian robots tend to be less costly than the articulating arms, but they are not as flexible. They can only move back/forth and up/down along the rails. If all you are doing is loading/unloading, then this may be a good solution.

Articulating robots are the 6-axis arms traditionally thought of as industrial robots. Very flexible, they can reach into some hard-to-reach places and perform many other tasks beyond basic loading/unloading. These robots can be mounted on the floor, on a wall, or upside down, depending on the requirements of the cell.

It is important to make sure you purchase a robot that is designed for the intended application. A robot designed for use in a clean laboratory would not be suitable for the coolant-dripping and chip-covered environments prevalent in most machine tools. To determine if your robot is suitable for the environment, robot parts will have an ingress protection (IP) rating. These ratings are a two-digit number with the first digit being the solid particle protection and the second digit being the liquid protection. For example, a rating of IP63 is dust tight and will handle dripping water, where a rating of IP66 is also dust-tight but will allow water/liquids spraying directed at the robot.

Industrial robots are sized by reach and payload. Maximum robot payload is normally stated from a specific point on the center of the faceplate on the robot. Everything that gets attached to the end of the robot needs to be included as part of the payload. This includes not only the components to be loaded and unloaded, but also the grippers or other part-picking devices that are mounted to the end of the robot.

The distance that the robot grippers are mounted from the face of the robot can affect the payload. Most robotics companies have payload calculators which allow you to evaluate the details of the EOAT for compliance to the maximum robot payload. While there is no substitute to using a formal payload calculator program, a good rule of thumb when using basic EOAT is to assume half of the robot payload for the parts and half for the grippers/mounting plates.

The reach of the robot is another important consideration, it will need to reach both the machine and the infeed/outfeed system.

End-of-arm tooling

EOAT is the generic term for the tooling secured to the robot arm that performs the tasks the robot needs to do. In machine tool load/unload, pneumatic grippers typically grab the parts. Suction cups and/or magnetics are sometimes used, but the vast majority of machine tool load/unload EOAT’s are pneumatic grippers. Similar to the robots, make sure you are using good industrial-grade grippers designed for the appropriate environment.

How many grippers are needed on the EOAT? When loading a single workholding position such as one chuck on a lathe or one vise on a mill, the typical answer is two – one to grab the finished machine part and remove it from the workholding and one that has the incoming raw blank and loads it into the now-open workholding. Using two grippers to unload and load with one trip into the machine reduces the load/unload time but increases the weight on the end of the robot. For example, if you are loading two workholding positions, the first operation in one vise and second operation in the second vise, the number of grippers will increase even further. It is important to factor this in when calculating EOAT weight and robot payload.

The next question is the type of gripper. One look at a gripper catalog and the choices will seem overwhelming, however, once you focus on the grippers designed for the machine tool loading/unloading task, the choices are usually much more manageable. When grabbing square parts, rectangle parts, or parts with opposing flat surfaces, a two-jaw parallel close gripper is the gripper of choice. When grabbing round or hex parts, three-jaw centric close grippers are preferred. Four-jaw grippers lend themselves to picking parts out of a tray. Three-jaw grippers tend to have a jaw out of place when picking from a tray or a place where parts are presented in rows and columns. A four-jaw gripper will place a jaw in each corner, eliminating the interference. Grippers come in many different sizes and are rated by gripping force and stroke. Try to select the smallest gripper that will do the job to keep the robot EOAT weight down.

Most gripper companies offer finger blanks that can be machined and/or modified for gripping the parts, similar to what is done with chuck or vise jaws. Unfortunately, these fingers cannot be machined in place like a vise or chuck jaw, so a simple fixture can be used to hold the fingers in proper place for machining. When machining a set of fingers, machine them to allow the best possible opening for easier picking of incoming blanks. A chamfer or radius will also help facilitate better picking. Incoming parts are not always as consistent. The extra opening and chamfers simply makes it easier to accommodate the unplanned differences.

Methods Machine Tools Inc.

www.methodsmachine.com

Part 2, will appear in the March 2016 issue, and focuses on infeed and outfeed systems, probably the most important, difficult decision to make when designing an automated cell.