Hollow Form Calculator

Tool reach is calculated as interior depth plus neck length plus a flat 2-inch working clearance, because a hollowing tool bar needs room to maneuver above the opening, not just enough length to touch the bottom. Without that margin, you'd have the bar bottomed out with no leverage to angle the cutter against the far wall. If your setup needs more swing room than a standard handled tool, budget extra beyond this figure.

Max RPM is computed as 4000 divided by outer diameter, so a 4-inch vessel gets roughly 1000 RPM while an 8-inch vessel gets roughly 500. Surface speed at the rim rises with diameter at a fixed RPM, and hollowing already asks you to work blind through a small opening, so the cap scales down to keep the cutting speed at the vessel wall in a controllable range as the piece gets larger.

The depth-to-opening ratio — interior depth divided by opening diameter — drives three tiers: manageable at 2 or under, challenging from 2 to 4, and expert past 4. Once you're reaching more than four opening-diameters deep, you lose direct sightline and lighting into the cavity, and the tool is working almost entirely by feel and sound, which is why the calculator treats that threshold as a real jump rather than a gradual scale.

Both are built from a cylinder approximation of the interior cavity, since the engine doesn't model a curved or bulbous profile. A real hollow form's interior is usually more rounded than a straight cylinder, so it holds somewhat less volume than reported, which means the removed-weight figure is a conservative upper bound rather than the exact weight of wood you'll actually clear.