Take It Off: Robotic Material Removal Applications
| By: Bennett Brumson, Contributing Editor
In many robotic manufacturing processes, there is a need for some form of material removal operation, whether that is grinding, sanding, deburring or polishing.
'Robotic technology has relatively low risk,' says Lester Godwin, vice president of the Dallas, TX based Pushcorp, a maker of force control devices for robotic material removal applications.
Harley-Davidson's York, PA motorcycle production plant uses robots for preparing metal parts for finishing.
'Using a six axis cartesian robot, it takes us in the vicinity of three and a half minutes to prepare a fuel tank for painting. Gas tanks are robotically finished on a belt grinder and fenders on a flap wheel,' declared Mike Kunkle, a senior manufacturing engineer at Harley-Davidson.
'Grinding gas tanks for Harley-Davidson is a difficult job. The weld on the sheet metal seam is ground down flush and polished with a robot before painting. We had to weld on a Class A surface. If we ground the weld bead too hard or burned the metal, it would be discolored and rejected. It is a finicky application,' said Paul Miekstyn, senior product manager, at Acme Manufacturing. Acme of Auburn Hills, MI, is an integrator of robotic material removal workcells.
'We also use robots for metal preparation prior to the chrome platting process. The part is sanded to remove defects, in a similar process after spot welding,' Kunkle noted. Other parts that are robotically finished at Harley-Davidson include suspension arms, which are challenging due to many surfaces with blended contours. Manual finishing of these items was limited to about seven per day. With robotic buffing and polishing, seven suspension assemblies are completed per hour, a significant increase in throughput.
Aside from the ergonomic benefits to those workers who had to manipulate unwieldy parts by hand, there is less variation from part to part and greater repeatability.
Other contoured parts pose a challenge equal to that of motorcycle suspension arms.
'Contoured parts are the fun ones, those are tough,' said Jim Morris, president of Automated Cells and Equipment, Inc. (ACE), a systems integrator based in Painted Post, NY.. 'There is a need for force compliance, either passive or active. With contoured parts, our experience taught us to try several different media to come up with the right solution. We apply different grades and different amounts of cutting action. For instance, if we are going to grind on a bronze casting, we might send the part to several robotic stations to get at all the features. A wheel might do a portion of it and a sanding belt might do another.'
Another facet of robotic material removal applications, as mentioned by Jim Morris, is the need for force control compliance. Force control devices are either passive or active, depending on the amount of precision necessary in a particular material removal process.
'The part is pushed into the wheel. There is a passive force control built into the slide, which pushes back. The goal is to get a consistent amount of force around the edges you are trying to deburr or polish. Passive force control uses an air cylinder to do the pushing, usually with two to five lbs. of air pressure,' Morris said. Passive compliance is an open loop guiding system without an instrument to regulate the force applied by the robot arm. Active compliance is a closed loop control mechanism that adjusts the amount of force that the robot arm applies to the material removal tool.
Kunkle's experience at Harley-Davidson is similar to that of ACE's.
'Robots do not have the eyes and feel of a human operator. Because a robot cannot see defects that an individual does, it cannot react to apply more pressure or back off if a defect is there. This is partially addressed by using an active force device. This device can be applied through the media head or the end effector, and digitally tells the robot to adjust pressure as needed. Passive compliance is also applied to the end effector or the media device. For example, let's say passive force is set at five lbs. of pressure. With a passive force device, if I drop the robot head a quarter inch, it will still be reading at five lbs. With an active force device, if the robot is moved that quarter inch, it could be at three lbs. or seven lbs. of pressure,' he said.
Pushcorp makes both passive and active force control compliance devices. Their active force instruments maintain the applied force of the material removal tool. Should any variation in the force be sensed by the system, it will adjust accordingly. This device is capable of compensating for acceleration and gravity for a given force applied to the surface of the piece being worked. This facilitates the use of higher tool speeds, particularly when the surface has an irregular shape or profile.
'Pushcorp's patented active force controller makes the end of arm tooling as good or better than what can be done by a person. We can control contact between the material removal tool and the product. If can you can control media speed and force, robots can make consistently quality material removal operations,' claims Pushcorp's Godwin.
One of the pitfalls of robotic material removing applications is getting the proper finish.
'The customer requirements must be understood. For example, sometimes what they ask for is not what they truly want, especially in polishing. We learned that the hard way. They might specify a certain finish level, but when it comes down to it, they want something rougher, more of a modular finish. There is a tendency of over polishing a part,' indicated Jim Morris. Morris stressed that simply asking a customer what type of finish they want is not sufficient. Rather, history has told Morris and his colleagues at Automated Cells to examine sample parts from different runs in order to get a representative sample and not to look at back-to-back samples. For deburring applications, Morris also cautions against pulling the samples after installing brand new cutters on a milling machine and assuming these first few parts are typical.
'The hardest part is taking off the bigger burrs, not the smaller ones. Try to give us something from the bad end of the scale,' Morris advises customers. Morris stated that he learned this through trial and error. Automated Cells and Equipment has experience in working with aluminum, copper, ferrous metals, various aerospace alloys, titanium as well as plastics.
The media that is used for material removal varies for a particular activity.
'Robots use common coated abrasives, non-woven abrasives, flap wheels, router bits for deburring, and stone wheels. There are also hard grinding wheels, buffing wheels, random orbit sanders for wood and plastic parts. The sander leaves a uniform surface in preparation for painting,' observed Lester Godwin.
Non-woven abrasives are media such as scotch-brite™ made by the Abrasive Systems Division at Minnesota Mining and Manufacturing (3M) of St. Paul, MN. 3M's senior technical services engineer, John Barry, described their products:
'We make captive abrasives which are sheets bonded to cloth or paper, which are for surface finishing and conditioning. These abrasives come in sheet or disk forms, and are used for deburring and cleaning. We make different types of this non-woven media, including silicone carbide and aluminum oxide. Silicone oxide is a jagged, pointed media. Aluminum oxide is blockier and leaves a warmer finish, one with coarser scratches. Both silicone carbide and aluminum oxide are made either into layered wheels 20-25 cm in diameter or laminated sheets,' he said.
3M also produces a coated abrasive consisting of ceramic aluminum oxide. These ceramic abrasives are for applications that require higher pressures on the end effector, with the advantage of not dulling as quickly. Ceramic aluminum oxide is for heavy metal gate or flash removal, not for painted surfaces or softer metals. There are some exotic abrasives in 3M's product line, such as a diamond material, for shaping automotive glass and harder metals such as titanium.
Media endurance is an issue of great importance to users of robotic material removal systems. Harley-Davidson's Mike Kunkle addressed this concern.
'An important factor is media life. We cannot be going in to change the media every 20 minutes. By using the same belts that we use by hand, we can increase belt life two to four times. Flap wheel life has increased by as much as ten times. Even with increases in belt life, we had to change belts every 24 minutes or for every 48 parts processed. By playing around with belts, we can use them for up to 72 pieces. By increasing belt widths, we are able to get more parts done per belt, and by utilizing the entire surface of the belt,' he said.
John Barry's take on media life is favorable to use of robotics.
'Generally, media life is about doubled when using a robot over manual material removal because the speed of the media and pressures are more constant. There is more accurate control of the media by a robot versus manual control,' he asserts.