A short-run prototype cell brings a futuristic welding project in under budget and on time.

Leslie Gordon, Associate Editor

The Taxi 2000 prototype’s welded skeleton.

The futuristic PRT gets finishing touches.

Working from geometry to prototype in less than 12 business days, Taxi 2000 Corp., Fridley, Minn., and Bauer Welding & Metal Fabricators Inc., St. Paul, used Bauer's new short-run prototype cell to engineer, fabricate components for, and final weld the framework at the heart of Taxi 2000's new Personal Rapid Transit (PRT) vehicle, a three-person computer-controlled and passenger-directed taxi. The cell made it possible for the project to come in under budget and on time as engineers from both companies worked together to "design on the go."

Providing the taxi's main structural support, the aluminum tubular frame consists of 1 3 1-in. aluminum, 0.065-in. wall thickness, square tubes MIG welded together. Each vehicle uses approximately 150 ft of this tubing. When assembled and welded, the frame, cabin, and chassis weigh about 1,500 lb.

Making the prototype
Instead of dimensioned part drawings, Bauer received datum points derived from the shape of the vehicle's outer shell. Bauer's engineers created a layout, and their CNC-bender operators then entered the datum points into the bender program and verified shape to the layouts. The bender used has a multiple-position head and roll forms and bends tight radii without removing the part. Although the machine bent most of the component shapes, Bauer had to do some blacksmithing because of springback in the material.

Many of the aluminum tubes have multiple radii and degrees of bend within a single component length, and several of the parts are bent in both planes. Bauer made certain bends on a bending machine, then on a roll former. While interpolated bends would have been easier with round tubing, project engineers believed that square tubing ensures better mount up of interior and exterior panels.

Bauer attached the bent tubing to other flat plate and straight tubing using TIG welding with filler. This formed the vehicle's finished skeleton which accommodates a domed hatch/ door that slides back far enough for wheel chair access. The framework required significant planning, sequencing, and correct weld points to ensure strength for safety.

The Taxi 2000's completed prototype consists of an indoor facility with one functioning vehicle containing the chassis and cabin components necessary to demonstrate vehicle operation on the 60-ft straight guideway.

Overcoming design challenges
The vehicle's designer, Dr. J. Edward Anderson of Taxi 2000, says the biggest design challenges were the vehicle chassis and the guideway. He explains, "There are a lot of ways to design a vehicle like this, and attach it to a skyway." For example, Anderson and his colleagues spent months deciding issues like how the door should work. "Our previous designs were always deemed unacceptable, but with the help of the Red Group, a Minneapolis design-engineering firm, we came up with this vehicle. We knew we needed to meet the human factors, and that led us to the design of this door where you walk straight in and sit down."

The Red Group's Allen Carlson says another important design/ manufacturing consideration focused on loading issues. "We wanted minimal deflection when the vehicle is heavily loaded (500 pounds) on one side, so there is no misalignment between the floor of the car and the loading platform."

Anderson explains the vertical aluminum framework is unusual, as most frameworks offered by other companies have been horizontal. He adds that guideways, which are expensive, need to be compact. This makes them light enough to be mounted to the sides of buildings, so there's no interference with foot, car, and bus traffic.