How deep of a recess is needed for expanding jaws?

When turning something using a recess and expanding the jaws of a chuck how deep of recess do you need? I assume it somewhat depends on the size so how deep on these sizes:
8" bowl?
12" bowl/platter?
16" bowl/platter?

Thanks,
Greg

Greg, it depends on the dia.. of your jaws. For a good grip you want the hole just big enough to insert your jaws. This allows for the best grip. Personally I slightly undercut the hole creating the reverse dovetail shape--some guys do not.

The same depth for all of them. More important is the size of your jaws. I go no more than 1/8 inch deep. My big Vicmark chuck has 2 5/8 inch jaws, and is fine for 8 inches up. For turning and coring a 16 inch bowl, it is getting a little iffy. It can be done, but you don't want to really hog off material. Like Robert said, you want the size of the hole to match as closely as possible the size of your chuck jaws, and the angle of the jaws as well. Dove tail holds better than straight.

I posted this a few weeks ago:

https://www.youtube.com/watch?v=5KHkkws9lWA

robo hippy

I agree with Reed - only about 1/8" - more important is how much wood is outside the ring to resist the force of expansion.

I use my Talon chuck with the #2 jaws when making 15" platters. I drill a 2" hole about 1/8" or so. Seems to hold fine.
Joe

For the purpose of flexibility in design, I like to leave a little more wood on the bottom, so I use a 1/4" deep recess.....that allows me to put a foot on it or just keep the recess when I finish off the bottom. I have found at times that I like to change the bottom and having that extra 1/8" allows me to adjust the curve, and still have enough to go right up to the jaws in the cut.

Like the others, I also highly recommend that you try to put a dovetail cut down into your recess....your hold will be more secure than with a straight sided recess.

I'm more in the deeper camp, 2 inch is maybe 1/4 inch, while 5 inch is more 3/8.
If I'm coring, it's deeper for certain.

I did watch Mike Mahoney turn a walnut platter that was maybe 15 inch diameter. He used a 1/16 inch deep recess. He had a new chuck with sharp edges. Mine now are a bit rounded over, and I should probably refresh them some time, but I am not in a hurry. You can get a secure grip with minimal depth, but you have to be VERY PRECISE when you cut it. I have to leave mine 1/8 inch deep, because some times the bowls warp so much that it is impossible to remount them for sanding. With tenon or recess, going deeper does not add strength, and some times can actually lead to a weaker grip if the diameter and angles are not dead on. What is on the outside or inside (recess or tenon) is most important.

robo hippy

Thanks everyone for the information. I do make sure the recess isn't any larger than it needs to be to fit over almost closed jaws, but wasn't sure on the depth that was needed.

Reed, thanks for pointing me to your video...good information.

Greg

One more thing--we lump all wood together--not so. For green or even weak woods go to 3/8" at least. They will pop out. On dry ebony I recently did a 6" box with maybe 1/16" groove maybe. Impressed me. Also consider the width of wood just outside the recess--jaws can break out the side too.

Robert, you are right, it depends on how hard the wood is, like soft maple compared to Burl wood, or how green the wood is, also is it out of balance.

I go 1/4 to 3/8" as some of the others do, and as Reed states in his video.
That may be deep for some platters and I may go a little less.
My chucks are Nova which use a 15* dovetail so check your own chuck for the appropriate degree as they may vary.
I cut the recess as close as possible to the fully closed position where the jaws barley slip in. Nova states 2mm are removed when the jaw billets are cut apart so each jaws only has to move 1mm to come to complete circle position.
The 1mm is expandable within the recess if you have a dovetail.