Welding together car bodies on an automobile assembly line is a mind deadening job.

Wrestling with a 40-pound spot welding gun that dangles from the ceiling on umbilical springs, a worker guides the tool along a body seam, pulling a trigger, methodically zapping together sheets of steel with bolts of electricity.

The assembly line moves incessantly, hundreds of cars, tens of thousands of welds, shift after shift. It is a job more fit for a robot than for a man.

And that's how it's done in many automobile plants - by robots? Real robots. Not "Star Wars" fantasies but $50,000 mechanical men with mechanical names - Versatron, Unimate, Millicron, Prab.

There are 2,000 to 3,000 industrial robots working in American factories, estimates Ernie Sallot of the Robot Institute of America in Dear born, Mich., an organization of robot makers and masters.

Sallot said the U.S. robot population is growing at the rate of 125 to 150 robots per month. The five American robot makers will sell $50 million worth of them this year, he said.

By 1990 there will be a $3 billion a year robot market predicts Joseph Engelberger, first presidents of the Robot Institute and also president of Unimation, the Connecticut firm that is the largest producer of robots in America, turning out about 40 per month.

Engelberger, who is regarded as the father of the robot business, is also the man with the best answer to the question "What is a robot?"

"I don't know how to define a robot," Engelberger said, "but I know one when I see one."

Most "Star Wars" fans probably wouldn't know a real robot if they saw one. They aren't humanoid looking like CP30 or even lovable-looking garbage cans like R2D2.

"The word robot conjures up pictures of little mechanical men with flashing eyes and swinging arms," said Kim Cannon, sales manager for AMF Versatran, a robot maker at Herndon, Va. Real robots aren't anything like that.

A Versatran Series F - which the company says is the world's strongest robot - could easily pass for oversized, deformed drill press, dragging its hydraulic heart and electronic brain behing it. Maundane as it looks, the machine can learn 32 different jobs - and remember more with the aid of a cassette recorder. It is sensitive enough to pack eggs into cartons and strong enough to pickup a small car with its single hand.

Unimation's best selling Unimate looks like a Sherman tank turret with a mechanical hand stuck on the end of the gu barrel.

Neither robot can walk, talk or see, but lack of these faculties is no handicapin the jobs most robots do.

Welding together car bodies is the most common assignment, and the auto industry is the biggest robot employer.

There are also American robots that paint cars - weilding a spray gun in a hot, fume-filled room where the Occupational Safety and Health Administration would bar humans.

There are robots that unload glowing hot ingots with their bare hands, and robots that pack television tubes into boxes with suction cup fingers.

What makes them robots, but industry definition, is their ability to duplicate human motions. George Munson, marketing manager of Unimation, defines robots as "self contained computer-director machines with memories capable of performing human function."

Although most robots replace men in factories, robots are not marketed as competition for humans, said Cannon, who is Munson's counterpart at AMF Versatran.

Robots instead are an alternative to what is known as "hard automation" - machines built specifically to do a single job and nothing else.

A robot welder can be reprogrammed and given a new tool and put to work unloading a diecasting machine, noted Mort Sullivan, who runs the AMF robot factory near Dulles Airport.

But the robot salesmen do stress that their machines are often cheaper than people. On a two-shift job in an auto plant, a $40,000 or $50,000 robot welder can be cheaper to hire than two United Auto Workers Union (UAW) workers each collecting $20,000 a year in wages and fringe benefits.

Cannon said robots typically can pay for themselves in a year to a year-and-a-half. With that kind of economics they're a better investment than either people or automation.

Because robots are relatively simple machines, Sullivan said, they can deliver better than 95 per cent "up time" on a factory job.

"They're good workers," he smiled. On a repetitive job like welding car bodies, he said, "They don't get bored, they don't take coffee breaks, they don't go to the bathroom, and they don't get mad at the boss."

AMF - American Machine and Foundry, it used to be called - doesn't release sales op rrofit figures for its robot manufacturing operations, which recently were consolidated at the Herndon plant.

AMF Versatran is generally estimated to be the second or third largest in the business, competing for the robot market share with Cincinnati Millicron, which is the nation's number one maker of machine tools and is rapidly increasing its robot business.

The biggest robot maker, Unimation, built robots for years without making any money, but is credited with creating the commercial market with its highly successful teams of robot welders.

The United States is not the world leader in robots. The European and Japanese markets each post $100 million a year, in sales twice the size of the domestic market.

In Japan, government subsidies encourage the replacement of human workers with robots. Most well-known Japanese industrial companies produce their own robots - Kawasaki, Hitachi, and Mitsubish.

Italy's Fiat recently ordered $6 million worth of Unimates, the largest robot order ever. And some Olivetti typewriters are assembled by robots.

That sort of sophisticated work is not what most U.S. robot buyers are after, Cannon said.

But more sophisticated robots are in the works, according to Dr. John Evans, a National Bureau of Standards specialist in robots and robot control.

New robots, Evans predicts, will have computer systems that will enable them to learn faster. In the not too distant future, they will have vision.

Most robotsn ow learn by rote he explained. A skilled operator directs the machine through the desired task, manually moving the robot's arm and hand through the motion while feed back devices rocord every motion in the robots' computer brain. Flip a switch from "teach" to "run," and the robot repeats precisely the movement it has been shown.

One day soon, Evans predicts, robots will learn by being told what to do rather than by being shown what to do.