Beaked vs Dovetail jaws for holding: tenon or recess?
I love the "beaked" jaws, actually small dovetails. They are easy to use and hold EXTREMELY well. I just examined about a dozen jaw sets on my various Nova chucks and some are dovetailed inside and out, some are "beaked" inside and dovetailed outside, some are straight inside and out, some are serrated inside - the point is examine the jaws carefully before deciding how to cut the recess or tenon.
Many people make a simple but problematical mistake when using internally "beaked" jaws with tenons. This method is recommended for tightening on a tenon (what some call a spigot)
Some occasionally make a similarly potentially disastrous mistake when expanding dove-tailed jaw into a recess. It is very difficult to make the dovetail exactly the same as the jaw but if erring, err on the side of less of an angle, as in the bottom sketch. The angle in the top sketch can put force on the weaker surface of the wood instead of the strong, well-supported wood at the bottom of the recess. I have witnessed bowls tossed from a lathes with this mistake. One guy did it more than once in a class.
Another thing I think most of us know: for the best holding strength make the diameter of the recess or tenon very close to the diameter of the "almost" closed jaws - this provides more surface area around the circumference than when the jaws are opened wider which can cause a weaker grip. One turner I know likes to leave a gap between the jaws when turning certain things held in a recess - he make the gap big enough to fit bowl calipers tips between the jaws to check the bottom thickness. I use other methods.
And be sure the contact ring at the bottom of the recess is flat, smooth, and clean!
Like Bill mentioned, I too rarely make a recess over 1/8" except for certain special design considerations. Even a 1/16" recess will work as long as it's made well, the recess is designed to carefully fit opened jaw diameter, and the gripping part of the jaws are sharp and not damaged or worn. (I usually cut a 2-1/16" diameter recess for 50mm Nova jaws.) I do make tenons deeper for obvious reasons.
This platter, inspired by a Frank Penta base technique, was held by a very small recess consisting of only
the short curved inner arcs of the three little curved triangular shapes just outside the shallow inner decorative dome. The platter was nearly 20", heavy sapele, and this unusual recess method held securely when I turned the upper side.
It should go without saying that when turning with a minimal recess:
No catches allowed!! Those prone to an occasional catch might want to use a different approach.
JKJ
