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CNC Machining Glossary
Boss (Raised Feature) in CNC Machining
A boss is a raised section of a part used to mount, locate, or support another component. Think of a short post or platform standing above the surrounding surface. It may be solid or contain a drilled or threaded hole.
When machined from solid stock, the boss is formed by cutting away material around it, not by adding material.
What Matters in Design and Machining
Design the boss and the surrounding space together. A tall, slender boss can flex during cutting. Placing it close to a wall also restricts cutter size and access. Leave enough clearance for the tool to reach the sides and base without excessive overhang.
Allow clearance for the base fillet. This rounded transition can reduce stress where the boss meets the surrounding surface. However, a mating plate may slide over the boss but catch on the fillet, preventing it from sitting flat. Check both the diameter fit and the clearance at the base.
Details Designers and Machinists Can Overlook
Check wall thickness after adding holes or threads. A boss with a large outside diameter can become a thin sleeve once its center is removed. Evaluate the remaining material, not just the outside dimensions.
Check fit along the height. Cutting forces can deflect the tool or boss, leaving variations in size. For a close-fitting round boss, measure near both the top and base rather than relying on one diameter measurement.
Match tolerances to the boss’s job. A locating boss needs appropriate size, position, and alignment. A supporting boss needs the correct top height and seating surface. Specify precision where it affects the assembly, rather than applying tight tolerances to every surface.