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Welding

More than a microwave

By Gretchen Salois

An engineer creates a spot welder from a common kitchen appliance

October 2015 - A quick nuke in the microwave warms up a plate of food in seconds. When Matthew Borgatti found a microwave that had been discarded in a junk pile, he wasn’t interested in heating up some leftovers. He salvaged and dismantled the appliance because he needed a quick-fix spot welder and it was cheaper for him to make his own. 

In charge of a small research and development lab that specializes  in robotics, Borgatti creates prototypes and has worked on product development projects for Google as well as engineering for NASA through a Small Business Innovation Research (SBIR) grant.

“I need equipment for doing a lot of different flavors of prototyping and space is at a premium in my shop,” Borgatti says of his Brooklyn, New York-based lab. “If I can tailor something to my needs like this tool, I can modify it when a new problem comes up or scrap it to salvage components for another purpose.”

The majority of Borgatti’s prototypes are made from plastic and elastomers so when he needs a major piece of equipment for metal work, he finds it more cost effective to build it himself. “I don’t like to spend a lot of money on machined metal parts,” he adds, noting sheet metal and wire are what he likes to use as they are consistent, inexpensive, and readily available.

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Take care

 A standard microwave transformer was all Borgatti needed to get moving on his homemade spot welder. “Making the spot welder involved AC power, high amperage and high temperatures. While it’s relatively easy to construct, it’s also easy for someone to get injured during the process,” he explains. “The original microwave transformer turned up the voltage and simultaneously lowered the amperage. You can actually reverse that, turning down the volts and upping the amps, by changing up the number of turns of wire.

“You can get a pretty good approximation of the amperage the modified transformer can deliver with a simple equation. Just remember that, discounting losses, wattage in equals  wattage out,” he continues. “You divide the number of turns in the primary coil by the number of turns in the secondary coil and then multiply the result by the amperage coming into the primary.”

Borgatti opened the microwave, pulled apart the transformer and rewound with thick cable, creating a timing circuit. A thick, copper cable is necessary to handle the high amperage without heating up catastrophically. Even with copper, it is important to understand that it too will carry some resistance and will heat up so a user must use caution. “Using a spot welder continuously, especially with thin cables carrying the current out to the electrodes, will create a lot of heat and likely melt your cable’s insulation,” he explains on his website.

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Practical applications

Whereas the metal components Borgatti works with on a typical day are not very complicated, they serve a useful purpose. Borgatti is working on an exoskeleton for a medical device company meant to aid those with cerebral palsy or anyone with elbow motor problems. “I need an anthropomorphic measurement device to read how much torque the active orthosis is putting on the elbow,” he explains. “The measurement device has a 3-D printed shell derived from scans of an arm and is reinforced with CNC plasma cut sheet metal components.”

Using pieces of sheet metal fastened into the 3-D components, Borgatti needed to spot weld the metal to complete the piece. “Spot welding makes my life easier. I like to sandwich components almost like a master lock, fusing them together. As I develop the design if I need more rigidity I’ll plan to spot weld on another layer of metal. Or less if I need a bend,” he says.

The spot welder allows Borgatti to quickly tack pieces for brazing or weld sheet metal for strong enclosures. As far as future projects, Borgatti enjoys working with metal. “I’d like to put in a good word for sheet metal and wire fabrication. Sheet metal allows me to get quick results,” he says. “Having worked in a lot of machine shops I find myself tempted to get back into metalworking, using tools like the spot welder.” FFJ

All photos/images: Matthew Borgatti

Sources

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Flow International Corporation
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