Robotics Case Studies
Motoman System Surpasses Cutting Accuracy and Cycle Time Requirements
by Sally Fairchild
Achieve 34-second cycle time, making a 7” bottom cut and a 5” top cut on 0.032” galvanized steel parts (body sides) weighing approximately 30 lbs. each.Produce straight, dross-free edge cut with accuracy within ± 0.020” on a contoured part.Provide flexibility to batch process either right-hand (RH) or left-hand (LH) parts for one model style with no manual changeover required.
Motoman SK45 robot with MRC controller.
8-KVA step-down transformer for robot.
54” robot riser base.
10-ft diameter 180° rotary turntable positioner with 800-lb. capacity and 8-9 second indexing time.
Motoman PanelMate operator station with 3-color status beacon.
Customer-supplied 070-Watt Nd:YAG laser.
58.5-KVA JVC step-down transformer for laser.
Precitec optical focusing system, including capacitance height sensor and controller.
Custom-designed spring-loaded device to protect fiber optic cables.
9’L x 14’W x 28”H steel platform, including steps and safety rails.
17’L x 14’W x 12’H Class 1 (completely light-tight) workcell enclosure, including LaserSafe glass viewing windows, two PLS zone scanners on platform in front of turntable, keyed laser disable switch, 3-color status beacon, and safety interlocked door.
Motoman provided two sets of identical custom pneumatic fixtures (one per side of the turntable). Each set of fixtures can handle either RH or LH parts without manual changeover.
Operator(s) load parts onto a universal locating pin and a main locating pin. Each set of fixtures has a total of eight clamps, but only a specific combination of six is used to locate either a RH or LH part.
Fixtures also include part-present and clamp-open/closed sensors.
LH or RH parts are batch-processed. An operator manually loads individual RH or LH side panels from racks onto the custom fixture on Side A of the turntable, where they fit onto a universal locating pin and a main locating pin. The operator clears the platform and activates dual opto switches on the Motoman PanelMate operator station.
A specific combination of six of the eight pneumatic fixtures automatically clamps the RH or LH part, and the turntable positioner indexes 180° to present the part to the robot.
The robot makes a 7” cut through the bottom portion of the side panel and then moves up to make a 5” cut through the top portion. Oxygen is used at the cutting tip to improve the quality and eliminate dross.
While the laser is cutting the part on Side A, the operator unloads the part cut during the previous cycle from identical fixtures on Side B, and transfers the part to the next operation in the existing automotive production line, where it is spot welded and then brazed. The operator also unloads the smaller cut portion of the body side, which is then scrapped.
The operator then loads Side B and the above cycle repeats to continue production.
The 700-Watt Nd:YAG laser is generated outside the robotic workcell enclosure and transmitted to the Precitec head at the end of the SK45 robot arm via a special fiberoptic cable. A water-chiller is used to cool the laser system.
Since the laser loses its coherency and diverges at approximately a 10° angle as it exits the fiberoptic cable, two lenses are used to bring it back into focus.
A collimating lens bends the diffuse laser back into a straight, coherent beam that a second optical lens then focuses into a single cutting point approximately 1 mm (0.039”) beyond the cutting nozzle tip.
A spring-loaded device protects the fiberoptic cable where it joins the Precitec head. Robot moves must be programmed so the movement of the arm does not bend the fiberoptic cable beyond its 10” minimum bend radius, which could cause fiberoptic cable burn-back.
For example, the fiberoptic cable cannot be allowed to bend back on itself with the loops closer than 5” apart on a 10” diameter.
Use of the Precitec optical focusing system was important to the success of this project. The Nd:YAG laser beam is focused to cut at a point about 1 mm (0.039”) from the end of the copper tip of the cutting nozzle. The cutting angle of the head is a function of the robot moves.
The capacitance height sensor moves in the Z-axis only and keeps the laser at the proper cutting distance as it travels over the part contours. The optical focusing system can compensate for height variations of ± 0.5” on the part while traveling at a maximum speed of 6.8” per second.
Cycle time 32-33.1 seconds, which meets 34-second requirement.
Cutting accuracy for the part tested is 0.026” -0.0295”, which is significantly more accurate than the 0.04” tolerance requirement for that cut.