for david leonhardi, a union’s effort to increase its community service offerings has helped enhance his teaching.

Leonhardi is an instructor at the New England Regional Council of Carpenters’ training center in Millbury, where apprentice carpenters spend 16 weeks — one week per quarter for four years — learning the trade. In the past, his instruction was confined to the Central Massachusetts campus: He would teach carpentry using mock building projects, which students then tore down at the end of a course. While trainees learned the necessary skills, he says, they were often bothered by a sense of waste.

Increasingly, as part of NERCC’s Helping Hammers program, Leonhardi is teaching on real work sites, complete with wind, rain, and anxious customers. Through Helping Hammers, the union donates apprentice labor to nonprofits, allowing groups like the Pine Street Inn, the YMCA, and the Boy Scouts to save money on construction projects. Nonprofits welcome the free labor, and instructors like the positive effects on carpentry apprentices.

Leonhardi thinks apprentices are often more engaged — and more receptive to learning — on a site than in a classroom. “It’s hands-on stuff, stuff you can’t simulate in a lab,” he says. “A lot of our apprentices get into our program because maybe they didn’t like school. You get them out on a job site and you see a different person.”

He recalls one project, a senior center in South Boston, where his trainees enjoyed feeling like they were contributing to their own community. “A lot of the apprentices lived in Boston, so they really liked that,” he says.

According to Mark Erlich, NERCC’s executive secretary, the program started informally about a decade ago and has since assisted several dozen charities. But the more pro bono work they do, the more requests they get, he says, and he’d like to expand the program. So the name Helping Hammers is new, part of an effort to secure grant funding and increase the number of nonprofits served. Now, employer contributions — part of the union’s collective bargaining agreement — pay for all apprentice training and instructor salaries, including Helping Hammers.

“The apprentices like it because they build something that will stay as opposed to building in a classroom setting, where it will get torn down after they finish it,” says Erlich, a member of the MassINC board. “The owners like it because they get something built they couldn’t otherwise afford. It’s a win-win-win for everybody.”

The one drawback for nonprofits is speed. “If you take a group of apprentices, they aren’t going to be as productive as a seasoned union crew, but it’s free,” Erlich says. “It might take a little longer, but very few people get upset.”

As executive director of the Massachusetts Affordable Housing Alliance, Tom Callahan relied on Helping Hammers in 2008 to build MAHA’s new home ownership center in Dorchester. He said that the donated labor shaved 10 percent off the cost of a $2 million building, and that the project hit no significant delays. “We were very happy beneficiaries,” says Callahan.