Mortise size

[Hal Mann]

What's the best diameter make a mortise in the bottom of a bowl? As wide as the chuck jaws will handle, smallest, or somewhere in between?

And for depth, do you want the bowl bottom up against the chuck rim? Or should the jaws bottom out inside the mortise?

hal

[Chris A Lawrence]

Mortise should be slightly bigger then the jaws fully closed. That gets you the most surface area of the jaws touching the wood. The ends of the jaws should be touching the bottom of the mortise.

[John K Jordan]

What's the best diameter make a mortise in the bottom of a bowl? As wide as the chuck jaws will handle, smallest, or somewhere in between?

And for depth, do you want the bowl bottom up against the chuck rim? Or should the jaws bottom out inside the mortise?

hal

As Chris said, the best holding is with the jaws nearly closed since at that point the outside of the jaws are nearly circular. For 2" jaws I might make a recess (mortise) about 2-1/16". (Frank Penta recommends 2-1/8 so he can fit calipers in the gaps between the jaws to measure the bottom thickness.)

The face of the jaws should rest tightly against a cleanly cut flat or almost flat. It's also important that the dovetail angle is such that the jaws rest firmly in the recess without the side of the recess touching the side of the jaws. Hard to explain but here's a diagram someone did. The bottom diagram is the correct way. Make sure the "corner" is clean with no sawdust or fibers.

440455

The recess should never be so deep that the wood is touching the parts of the jaws that are held by the screws. I often make recesses 1/8" or so deep in good wood, sometimes less, sometimes deeper in softer wood.

JKJ

[Reed Gray]

Check out my video on 'Mounting things on the lathe'. I use my big Vicmark chuck fro bowls up to about 16 inch diameter, though most of the time, I turn 14 inch and smaller. The chuck jaws measure just under 2 5/8 inch diameter. I make my recess just a hair bigger than that. For bowls under about 6 inches, I will use my smaller Vicmark, and the jaws on it are about 1 3/4 inch diameter. So, for the big jaws, I make the recess maybe 2 3/4 max. That gives a very secure mount. If I made the recess 4 inch diameter, that could work, but would not be nearly as secure. Since the jaws are dove tailed, I want that angle on the recess to match pretty close as well.

robo hippy

[Hal Mann]

Thanks all. Very helpful information. Now I understand why I've got to change the way I was doing it.

hal

[Robert D Evans]

If you are planning on roughing the bowl, letting it dry and then turning it again, (twice turning) I've found that a tenon is easier for me to true up. It's kind of tricky to get the mortice trued up after it warps. There are ways to do it but not as easily as with a tenon.

[Hal Mann]

Great point Robert. The current work is on dried wood, but will certainly do some work on green wood soon. So very glad you mentioned this.

hal

[Pat Scott]

I agree that if you are making a mortise you should size it 1/8" or so bigger than when the jaws are closed. But I don't think anyone has mentioned the size of the mortise should be proportional to the size of the bowl. I would never use the standard 2" or 2-1/2" jaws that come with most chucks and create a mortise for a 16" bowl! I don't care how perfect you make the mortise, that's way too small for a bowl blank that size. Buy some bigger jaws and/or bigger chuck and jaws if needed. The same applies to tenons. I see far too many people make small tenons that fit their jaws and think it's OK to use for a large bowl. Everyone says the foot or base on a bowl should be 1/3 of the overall size, that same thinking applies to the size of tenon or mortise.