SAN FRANCISCO-Junior is showing off. A remote-controlled robot about the size of a microwave oven with a wicked-looking three-pronged arm in the front and a buzz saw mounted on its tail, the machine picks up a cinder block, pounds it into pieces and then turns around and attacks with the saw, turning the block into a pile of rubble. Then the 90-pound robot sprints across a condo parking lot like a rat and spins around gleefully in a circle, Not to be outdone, the robot's bigger cousin, Hercules, a 180-pound titanium box with a fierce-looking sharpened steel spike for a nose, does a back flip.

"Wow," says Jim Smentowski of Novato, a 29-year-old computer support technician, as he commands his robots from a $1,200 control panel normally used to fly model helicopters.

Hercules isn't always so well-behaved. The other day Smentowski was exercising his machine in a parking lot after work when the robot sped out of range of the remote controller and took off downhill at 30 mph, coming to a stop in a cloud of concrete dust against a curb. The mishap broke a couple of welds, and bent the steel frame, but, all told-the robot was little worse for the impact. "It was pretty cool looking," the impish and goateed Smentowski says.

With any luck Hercules will fare as well next weekend when, instead of battling curbs, it goes up against other lovingly built robots designed for a single purpose: to "kill" each other.

The annual machine melee started in 1994 as Robot Wars, but when a legal falling-out between the two promoters put this year's event on ice, Marin County computer programmer and robot-fighting fan Gary Cline decided to rent an arena and invite veterans of the earlier competitions to a free alternative demolition derby called Robotica. Although the name has changed, the event is basically the same: 56 robots in different weight classes (topping out at 180 pounds) will try to tear one another apart over three days of bouts lasting no more than five minutes. The contest starts Friday at the Cow Palace in San Francisco. Admission is free but limited to 450 spectators each day.

The thrill is hard to describe," says Cline. "It's better than fishing, a whole lot better than baseball. It's fighting and it's not fighting. It's just boys at play."

Boys, that is, playing with rotating metal-cutting saws, pistons that can punch holes, into aircraft aluminum at 300 pounds per square inch and toys with wheels that can tear up asphalt. Which is not to say there are no rules. "All explosives, corrosives, flames, pyrotechnics, and the use of gunpowder are prohibited," according to the regulations. untethered projectiles are prohibited. Tethered projectiles most not exceed 8 feet in range.

"In a boxing ring," Jamie Hyneman, (creator of the much-feared Blendo) once said, "if you knocked the other guy down and continued to beat him to death, that would be bad sportsmanship. But at Robot Wars, You're expected to do this." Other competitors downplay the violence and describe the happening as more of an engineering-meets-sporting event.

"I think it could take off as one of the major, hobbies people could do, right up there with stamp collecting," says Carlo Bertocchini, who designs plastic injection molds for industrial uses in Menlo Park and whose Biohazard robot has used its shovel-tipped arm to upend its way to victory in the heavyweight championship the last two years. "You just about have to see it to explain what you do. It's just a way to do something creative. There are a lot of tinkerers working in their garages who don't know where to apply their creativity and energy."

"Robot Wars is problem-solving and strategy," says Greg Munson, a member of Team La Machine, which has spent most of the last year, scheming to dethrone Bertocchini and Biohazard. "As violent as it seems, the people involved are really wimps," adds Edward Roski III, his teammate. "They'd never, get involved in a fistfight."

At least one, albeit legal, fight has broken out over Robot Wars; the battle between the two original promoters. The idea started with Marc Thorpe, a performance artist who once won an NEA grant to teach dolphins synchronous swimming and who went on to become a model maker on such films as George Lucas' "The Empire Strikes Back." He teamed up with Profile Records owner- Steve Plotnicki to produce what they called the sport of the future. "We foresaw the day where we could turn Biohazard, La Machine or the Master, as well as other robots, into household names, like the Ninja 'Turtles or other great action characters," Plotnicki wrote in one of the numerous Internet postings that document the long-simmering feud between the two.

Indeed, Robot Wars soon attracted a cult following, bringing worldwide attention to the San Francisco event, and inspired a series of televised robot competitions in England. But the two partners had a dispute over the business that almost derailed the event last year and briefly created a rival competition to Robotica this year. At one point robot makers were reduced to talking about meeting in a parking lot, but two months ago Cline dug into his own pockets to put together a tournament. He will only say it is costing him a "substantial" amount of money. He expects to recoup only part of it through T-shirt sales, so there won't be any cash prizes this year.

"We don't care," says Smentowski. "We just want to get out there and tear up some other robots and show off what we're doing. It's the fame and the glory you win."

Smentowski has been an inveterate inventor, since he was a kid. After a friend suggested he check out Robot Wars in 1996, he got the bug to build his own. "I can do that," he thought. He spend $2,500 and nine months of weekends creating Hercules; half a minute into the robot's first fight last year, Hercules was history.

Hercules faced Blendo, two blunt blades mounted on a whirling dome spinning at 70 mph that resembles nothing so much as the business end of a blender. Blendo was thrown out of Robot Wars in 1995 after it spit shrapnel over a barricade into the audience. Two years later, it tore a 12-inch stainless-steel plate of armor off Hercules and hurled it at the bulletproof glass protecting the crowd, carving a 2-inch-long gash. The second hit from Blendo flipped the 170-pound Hercules five feet through the air, bending its case, pinching wires and shorting it out, Blendo, once again, was ejected.

For most of the last year, Smentowski has been plotting revenge in his small garage. The new and improved Hercules-a titanium shell, a pair of special-order 24-volt motors that put out six horse-power, knobby go-cart tires, speed controllers from a golf cart ("They have all sorts of safety features that I had to disable") and power wheelchair batteries with 20 minutes of juice-cost $4,500.

There are wheels on the top and bottom so the robot can run even if turned upside down; then there's a 2-foot-long sharpened steel spike. "I can impale just about anything," Smentowski says. "Until now, there haven't been a lot of robots that could do internal damage."

Alas, a rematch will not take place. Team Blendo had a scheduling conflict, as well as some safety concerns. "Team Blendo has just looked over the arena specs," Jonathan Searles wrote to Robotica, "and we feel that the arena is not up to Blendo holding/audience protection specs. Our robot is lethal against these types of materials and could eat its way into the crowd. . . ."

Also looking forward to another shot at Blendo had been Team La Machine in northern Marin County. Unlike Smentowski's modest garage workshop, Team La Machine operates out of a spacious, professional-looking space filled with serious industrial tools, including a $17,000 milling machine. It was here that Scott LaValley, 22, spent much of the time in his high school years, immaculately restoring a 1931 Ford Model A. "Other kids were out partying and I was in the garage at night banging metal together," says LaValley, who works as a computer systems administrator for Marin County.

After his father showed him a news story about the upcoming inaugural Robot Wars, LaValley put together a rhomboid-shaped box that shot a piston, which he called Doo Little. LaValley, a rail-thin, ponytailed tinkerer, stayed up all night before the big match, only to have a speed controller burn out the morning of the event. He found a replacement at the last minute but it would work only while going forward. The fight ended almost as soon as it began when Doo Little drove into a wall and couldn't back up.

Last year LaValley joined up with Munson and Roski, cousins who run a digital design business in San Francisco. Team La Machine, which landed more than $25,000 in corporate sponsorship from companies like Newtek, an animation software firm, fielded two robots: La Machine, a wedge-shaped robot with a battering piston, and Doo All, basically a metal-cutting saw mounted between two tank-like treads.

This year the team's mainstay will be Ginsu, a box with four 20-inch, tungsten-tipped steel blades that is designed to carve its opponents into shrapnel. They have four complete sets of blades to replace any that go dull in battle. "Slice 'em, dice 'em, make them into robot fries," says Munson, who has cut his fingers numerous times just lifting his robot. "Die Biohazard," he wrote with his own blood on the side of Ginsu after one recent mishap.

"This is the most dangerous robot I've ever seen," says Roski, "It's a go-cart with saw blades." So far, saws haven't proved terribly effective in robot fights, although they do generate a lot of sparks, which usually pleases the crowd.

To demonstrate, LaValley drags out Doo All and its chew toy, a rusted steel trunk door from his old Model A, onto his parent's driveway. Munson dons safety goggles, bends down and yanks the starter cord on the saw, the same kind used by fire departments to cut people out of car wrecks. The engine roars to life. LaValley steers the robot, which climbs atop the hunk of steel, and lowers the blade. Sparks fly off wildly.

"Scott," his father says insistently from the safety of the porch, "could you move that can of gasoline." LaValley lifts his goggles, notices the gas a few feet away, as Roski scampers to carry it out of the danger zone. "It wasn't as close as last time," LaValley says.

For further information about Robotica, see the competition's Web site: http://www.clineworks.com/robotica.html.