Cutting rectangular ductwork requires careful measurements and the right technique. Use a measuring tape and pencil to mark the cutting lines accurately. Tin snips are excellent for straight cuts on smaller ducts. For more extensive or thicker rectangular ducts, a reciprocating saw with a metal-cutting blade is effective. 

Ensure the duct is well-supported and secure while cutting to prevent wobbling and maintain precision. After cutting, use a metal file to smooth any sharp edges, ensuring safety during installation. 

Welding is the process of using extreme heat—and, sometimes, pressure—to attach two pieces of metal. In contrast, metal fabrication is a more involved process that can include welding, sheet metal work, bending, and other forms of metal manipulation. Metal fabrication is a large-scale project that may or may not involve welding, while welding is more often a step in larger fabrication projects.

The average cost for a welding project is about $300, with typical costs ranging from $125 to $490. However, prices can be as low as $125 for very small jobs or exceed $1,200 for large or complex projects. Welders typically charge an hourly rate between $65 and $150, depending on their experience, and many charge a minimum fee to cover overhead and travel expenses. The total cost is influenced by several factors, including labor, the cost of materials like metal rods and shielding gas, the type of metal being welded, the project's size and complexity, and your geographic location. For a precise quote, it's best to ask a local welder for an estimate based on your specific project.

Choosing the right metal fabricator for your project begins with research. Find a reputable fabrication company by searching for reputable contractors in your area, and then reviewing photos of their previous work, customer reviews, and photos of the company’s workshop.

Once you’ve narrowed down the list, contact the companies to ask about the types of metal fabrication projects they specialize and inquire about guarantees and warranties on workmanship. You should also ensure the company uses high-quality material, and ask about the schedule and turnaround time for your proposed project.