Having made his fortune as a Silicon Valley entrepreneur — including a nine-figure sum from his early involvement in the online-payment service PayPal — Mr. Musk was convinced that technology and vision could together conquer new frontiers, whether in space exploration (with his SpaceX venture) or everyday transportation. Replacing gasoline tanks with batteries was just a start. He was determined to reconceive their production as well, based on 21st-century advances in automation.

Established car companies master the process with assembly-line workers, and then find ways for machines to take over some of the work. Tesla did the opposite. It designed a highly automated production line populated by more than a thousand robots and other assembly machines.

Ron Harbour, a partner at the consulting firm Oliver Wyman, noted that in his annual survey rating auto plants worldwide, the most efficient ones use a lot of manual labor. “The most automated ones are at the bottom of the list,” he said.

In some instances, Tesla’s gamble on automation has paid off. A separate production line that makes the Model S and the Model X has a series of 14 stations, with 17 workers, in the area where battery packs and electric motors are married to the vehicles’ underbodies. For the Model 3, that function involves just five work stations and no workers at all, said Lars Moravy, the company’s director of chassis engineering.

For other tasks, the reliance on robots has proved a headache. For months, Tesla engineers struggled to get a robot to guide a bolt through a hole accurately to secure part of the rear brake. They found a maddeningly simple solution: Instead of using a bolt with a flat tip on its threaded end, engineers switched to a bolt with a tapered point, known as a “lead-in,” that can be guided through the hole even if the robot is a millimeter off dead center, Mr. Moravy said.

In a very tangible sense, Tesla views its production line as a laboratory for untested techniques. In recent weeks, company executives concluded they could produce Model 3 underbodies with fewer spot welds than they had been using. The car is still held together by about 5,000 welds, but engineers concluded that some 300 were unnecessary and reprogrammed robots to assemble the steel underbody without them.

“It’s unusual to be doing that at this point in time when the car is already launched,” said Mr. Harbour, a veteran manufacturing expert who has visited most of the world’s major auto plants. “Normally you’d make changes like that in the prototype stage.”

In another bid to push the limits of technology, Tesla at times pulls robots off the line and tests them operating at speeds greater than specified by the supplier, said Charles Mwangi, Tesla’s director of body engineering.

“We are actually breaking them to see what the maximum limit is,” Mr. Mwangi said. The idea is to find ways of accelerating production without spending capital on new machinery. In the future, rather than adding more machines to increase output, “we can just dial up our equipment,” he said.

The willingness to experiment with the production process even as cars are rolling off the line is perhaps the most significant way Tesla is defying the industry’s conventional wisdom. Automakers like Toyota, Honda and G.M. engineer manufacturing lines that can churn out cars or trucks at a rate of about one a minute, and essentially lock in the basic assembly process once they start production. While they make tweaks to improve quality or worker safety, they generally make major changes or introduce new techniques only every few years, when an old model is phased out and before production of a new one begins.

“The first step in auto quality is stability,” Mr. Harbour said. “Once you get a stable process that works, you can go back and make improvements.”

Tesla, by contrast, is tinkering with its production lines on the fly, and the tent is a stark illustration of that approach.

Beneath the peaked canvas, Tesla has hastily set up a third Model 3 production line. Like the other two, it handles final assembly, when trim and other finishing touches are put on the car. (Tesla did not include the tent on the tour of the plant.)

Adding a new assembly line, even temporarily, is a rare and risky move in the auto industry. A line set up hastily, in an untested environment, might not achieve the quality Tesla promises.

Two assembly lines inside the plant already exist to handle at least some of those tasks but they have proved troublesome and perform the work more slowly than Mr. Musk had hoped, in part because Tesla used robots for tasks that are better left to human workers.

Tesla engineering executives acknowledge that the company overestimated the rate at which it could produce cars, and designed a production system that proved to be too complicated — a problem that Mr. Musk lamented at the company’s June shareholder meeting.

“One of the biggest mistakes we made was trying to automate things that are super easy for a person to do, but super hard for a robot to do,” he said. “And when you see it, it looks super dumb. And you are like, wow! Why did we do that?”
Most automakers operate a single line to make two, three or sometimes four different vehicles, because using a second line would force them to invest in duplicate tooling and cut into profit margins.

And a third assembly line, outside the walls of a plant? “I’ve never heard anything like this, ever,” Mr. Harbour said.

Mr. Musk said the capital costs of the line in the tent were minimal because the company used equipment it already owned. (On Twitter he had called it “scrap we had in warehouses.”)

“It does everything that the other assembly lines do but with fewer people, lower labor costs and much higher uptime,” he said. “Our unit cost for vehicles is lower on that line than on the other lines, and we’re seeing higher initial quality.”

Whether that remains the case as Tesla speeds up production will be revealed in a few months if the company reports a profit, as Mr. Musk has promised.

For many years the Fremont plant was a joint venture of Toyota and General Motors known as New United Motor Manufacturing Inc., or Nummi. After G.M.’s bankruptcy, the factory closed in 2010 and the site was acquired by Tesla.

Today, the four-million-square-foot plant alongside a busy freeway sees a steady stream of deliveries and tractor-trailers departing with new vehicles. Each afternoon, line workers in matching black pants with a white Tesla logo on one leg stream out of the plant and descend upon packed parking lots in the sprawling, bayside suburb.

Workers feel the pressure to speed output. In interviews away from the plant, several said they had been putting in 10- and 12-hour days, sometimes six days a week. They report that turnover among line workers is high, and that sometimes supervisors join the line during extended shifts.

Jose Moran, a five-year Tesla veteran who for the last 10 months has worked as a Model 3 quality-team lead, said the already taxing production demands of previous models had intensified. “It’s a constant ‘How many cars have we built so far?’ — a constant pressure to build, especially with the Model 3,” he said. “It gets desperate sometimes, especially right now.”

One challenge that workers see is the rapid influx of new workers. The company aims to hire about 400 employees a week to help accelerate Model 3 production. After Tesla’s most recent earnings announcement in early May, Mr. Musk said he hoped eventually to have three shifts a day, essentially running the assembly line around the clock.

“Everyone I talk to has only been here for two weeks, a month, and those people don’t last long,” said Jonathan Galescu, a Model X body repair technician who has worked at the plant for four years.

Mr. Harbour, the manufacturing expert, says automakers typically give new workers several weeks of training before putting them into production work. Bringing in large numbers of new workers can hurt quality because they may not perform their work properly or fail to notice when problems crop up.

New workers at the Fremont plant get three days of training before going to work on a production line. This includes a day of computerized virtual training on doing their jobs safely, and a day of instruction on the area to which they will be assigned.

Worker safety at the Fremont plant has come under scrutiny after a nonprofit news organization, the Center for Investigative Reporting, cataloged a series of injuries suffered by Tesla factory workers. California’s job safety watchdog is investigating a recent incident that left a worker hospitalized with a broken jaw.

Michael Catura, a 33-year-old battery-pack line worker who has been with Tesla for four years, said he had suffered hand, shoulder and elbow injuries because the company had sometimes dispensed with rotating workers to different jobs around the factory floor.

“We need to make sure people are thoroughly trained,” he said, and not just getting “cookie-cutter training.”

Mr. Moran, Mr. Galescu and Mr. Catura are involved in efforts by the United Automobile Workers — a group reviled by Mr. Musk — to organize the plant.

Asked to comment on the intensity and the safety of Tesla’s workplace, a company spokeswoman said, “We care deeply about the well-being of our employees.” Tesla’s efforts reduced the injury rate by 25 percent last year, she said, and “with each passing month, we improve it further.”

In early June, Mr. Musk said Tesla was making 3,500 Model 3 sedans a week, and vowed to reach 5,000 a week by the end of June. In the interview on Thursday, he voiced confidence that it was nearing that elusive goal, the pace he has said is needed for the company to turn a profit.

Tesla has already spent heavily on the Model 3 assembly process, and modifications mean machinery purchased for hundreds of millions of dollars is likely to be discarded. Mr. Musk essentially acknowledged this point after the earnings announcement when he said he did not expect the gross margin on the Model 3 — the share of revenue retained after the cost of goods sold — to reach the 25 percent target until early next year, six to nine months later than previously forecast.

Max Warburton, an analyst at Sanford C. Bernstein, estimates that Tesla spent about $2 billion to set up the Model 3 production line. “This is vastly more than we’ve seen any other car company spend on new capacity,” he said, adding, “$2 billion is a mind-blowing amount to spend on a second assembly line at an existing plant.”

For now, Tesla generates most of its revenue from the Model S and the Model X, which are priced at about $70,000 and up. Combined, their global sales add up to about 100,000 vehicles a year — too few to offset the billions Tesla has been spending to build its gigantic battery factory in Nevada, develop new cars and a semi truck, and equip its car plant.

That means the company’s future hinges on the assembly lines Tesla has set up to produce the Model 3 — and whether the company can make them hum.

Lauren Hepler contributed reporting.