Theres been many times I have a piece turned around on my vacuum chuck working on turning off the tenon and sanding. More often than not Ill get tired or have some other distraction to take care of and I dont want to leave the pump running for extended periods unattended. Most times I can run my tail stock up tight to the work and shut the pump off without loosing position but if Ive sanded to finish and have wiped a penetrating finish on and need to leave it to dry I dont want to mark it up with metal contact or poke a hole in it otherwise. I found a golf ball placed over the center hole on the tail stock hardly makes any marks when used to hold the work in place.