Why do the manufacturing companies always recommend using a chuck in compression rather than expansion? What is the difference and why is expansion considered more dangerous?—Annamarie Pennucci
Garrett Hack replies: An expansion chuck by its very name is tightened by trying to stretch the wood fibers. Think of it as a splitting force, one that wood is not strong at resisting. Tension failure means flying turning parts. But wood can resist lots of compression so you can really torque down your compression chuck for maximum hold.
Jon Siegel replies: When a lathe chuck is used in expansion (inside a recess) there is danger of the workpiece breaking apart from the outward force of the chuck aided by centrifugal force and obviously this is dangerous. Sometimes there are hidden cracks or other flaws that weaken the workpiece that you may not be aware of when you first mount it, but as the turning progresses and material is removed, it becomes weaker.
Jeff Roberts replies: In my experience it is definitely safer to compress onto a tenon. I like to prep a piece in advance by gluing a hardwood tenon of the appropriate size to what will be the bottom of a bowl. I have done many with the dovetailed recess in the bottom and have had plenty of success with that but I have had a couple blow out on the lathe from simply not having enough wood left on the outside of that recess to handle the outward pressure of the chuck. Also it gives you more of the thickness of your blank to work with if you add a tenon rather than cut a dovetail into the bottom.