How much extra blank do I need for the chuck and tailstock, and how do I hold a 3D rotary carving with tabs?

Three lengths to keep straight

• The blank: the real piece of wood, including what sits inside the chuck.
• The job Length: the part of the blank the bit is allowed to reach. It must stop short of the chuck jaws and the tailstock.
• The design: shorter again, so there is room at each end for tabs that keep the finished piece attached.

The official guide puts it this way: part of the blank is inside the chuck, the tool must keep a safe distance from chuck and tailstock, and the design is shorter than the job to leave space for tabs.

Planning steps

1. Add holding stock. One machine maker asks for at least about 3/4 inch of extra stock for its chuck jaws. Another published guide leaves about an inch at each end of a long log. Check your own chuck.
2. Decide on the tailstock. One maker's rule of thumb: it is generally needed once the piece is about 3 times as long as its diameter.
3. Put the XY datum at the chuck end of the job and zero the machine a safe distance from the jaws.
4. Add tabs to a 3D model. The Clipart tab has a 3D Tabs folder. Place a tab at each end so it touches both the model and the job edge, and stretch it across the wrapped direction to the job boundaries so it forms a full disc of support.
5. For a hollow or deeply undercut model, add a tab that runs the full length as a spine. It stops the bit from trying to carve below the center line.
6. For 2D work, part the piece off with a Profile Toolpath that stops short of the center (see the entry on coves, rings and parting cuts).

Pro tips

• If a faceplate is screwed to the blank, keep the job clear of the screws.
• Put the thicker end of the model at the motor end so the drive torque goes through the strong section.
• Forgot the allowance? Change the blank size later in Job Setup and re-center the design.