Compression or Expansion Tenon for Chucking Bowls

[MichaelMouse]

Good Catch

I'm a mortise grip sort, and this experiment, along with sloth, is one reason why. Quickly sanded to 320 and set aside so I could get upstairs and get dinner fixed, as SWMBO was due in an hour. Felt pretty good about the concept and result at the time. Next development will be one gap side, if I can pull it off.

This morning I went down to hand sand with some 400, and horrors , I could feel a bit of a lump about 3 inches off the bottom. Curve wasn't fair. Though there was nothing visible, I knew if I decided to use a semi or full gloss finish, it would mock both the eye and hand. Better firewood than foolish, so I chucked it up. Pleased to have left myself the option.

Two thin passes with the gouge and a blend with soft velcro disks, and the curve was fair. Not that I have a vacuum chuck, but I think that would have been my only other holding option had I done a more exotic bottom, and a chancy one at that. It was a piece of firewood, and had come a little bit out of round sitting under the heat duct to dry. It's back there now, to spend a couple more days to stabilize.

Sometimes virtue comes out of necessity. Or is it fools and drunks?

 

[S. Clark]

I was just telling myself, as I was fashioning tenons on a couple rough-outs, that I aw'ta mortice some bottoms. A couple years ago I was doing that to everything and told myself I aw'ta use tenons on some of my bottoms.

- Scott

 

[underdog]

"There's a hole in the bucket,
dear Liza, dear Liza.
There's hole in the bucket, dear Liza!"



I'm not sure why more of us don't use this method. Once the piece gets to a certain width, the foot needs to be larger than a tenon can manage. Having a mortise style foot, makes it possible to re-turn it also.

I also don't like the look of a lot of bottoms if you don't get it quite centered on your jamb chuck, vacuum chuck or what-have-a-you...

 

[pencheff]

Counter-chuckwise?

Once the piece gets to a certain width, the foot needs to be larger than a tenon can manage. Having a mortise style foot, makes it possible to re-turn it also.

Glad this topic came up. You old pros can answer some newbie questions about mortises, tenons and chucks.

I have two different PSI chucks for my little midi lathe. They're both the lever type and the lever action used to tighten the two is opposite. I've picked up from the "rusty chuck" thread that Vicmarks tighten in the oppo direction of Oneways /Novas and others.

A typical drill chuck tightens counterclockwise (or however you wanna look at it) so that "forward" rotation keeps the chuck tight on the bit.

So does the direction in which a 4 jaw chuck tightens make it self tightening in one direction? That is to say is the Oneway/Nova/et. al. type best at holding a tenon and the Vicmark best at mortises? Vise versa? Neither. Am I, once again, thinking about it too hard?

Enquring newbies need to know.

 

[Steve Worcester]

I'm not sure why more of us don't use this method. Once the piece gets to a certain width, the foot needs to be larger than a tenon can manage. Having a mortise style foot, makes it possible to re-turn it also.
..

I don't use an expansion because it is more likely to tear out more open-grained woods under greater pressure and would require you to make adjustments to the base/foot design.

When I turn larger objects, I have jaws that will accommodate about a 5" compression tenon and the outside/bearing surface can get up to about 6" (with jaw widths,etc). When the bowl is dry, I just return the tenon round and the bearing surface and I am good to do whatever I want in the way of re-turning.

 

[MichaelMouse]

A typical drill chuck tightens counterclockwise (or however you wanna look at it) so that "forward" rotation keeps the chuck tight on the bit.

So does the direction in which a 4 jaw chuck tightens make it self tightening in one direction? That is to say is the Oneway/Nova/et. al. type best at holding a tenon and the Vicmark best at mortises? Vise versa? Neither. Am I, once again, thinking about it too hard?

Thinking's good. With a fixed inner and a rotating outer, the rotation of the lathe against the inertia of the ring will expand the old Novas and similar. Means it's tightening if you're using a recess. My first chuck (a nightmare!)was made the same way for apparently the same reason. Doubt this makes a lot of difference with the keyed chucks. Does with my one remaining Tommy bar/pin spanner Nova.

Steve mentions the two most commonly given objections to a mortise. Makes sense that the outside dimension will always be larger than the inside dimension by the thickness of the jaws. I think this is somewhat offset by the practice of using the log with the round side toward the bottom. If you make a 2" flat spot, the wood still curves out in two directions, which means you can run your mortise in, follow the natural curvature, and your piece is held on a 2" base. You have to pare down the sides of the same 2" area to get your tenon, which gives you a pair of shoulders to reference. But you still can't get a 2" base, because you have to get rid of those shoulders. Makes an inch or so base if you trim, or wider, if you turn the tenon away. If you have other size jaws, it's certainly less of an issue. You choose the size which gives you the base you're after. Further, if you use a mortise, you can visualize and turn the bowl shape entire, without mentally subtracting the tenon.

Second is the business of overtightening the chuck and splitting out. This is a real factor with ring-porous woods like oak, but there is a ready cheat, if you want to use it. Tape the outside of the endgrain around the mortise and run CA from the inside. That will firm it up nicely, but you'll have to contend with the saturated area in your finishing considerations. Tape will limit the spread on the outside surface. Of course, the real answer is not to overtighten the chuck. Consider that if you make your mortise the proper diameter, have less force per unit of contact area, and you don't need to do more than snug. The dovetail will press the face of the jaws nicely up against the bottom of the mortise, and the sides will hold things against the force required to remove shavings. This is what makes the dovetail a no-compromise, no-runout hold, as opposed to a serrated hold, where you often have to settle for less-than-perfect when you reverse or try to re grip. You've put the force on a smaller area because of the serrations, which can crush the wood. Where it's crushed, it's no longer circular, and that can be a problem.

I keep my bowls snug between centers until the inside is hogged out and I'm ready to pare. I hog with pressure downward toward the bottom, rather than outward , which puts the load on the shoulders of a tenon or the inside of the mortise, thus against the face of the jaws. Then, of course, when I release the tailstock to pare, the piece is also balanced as it is going to get, and nearly as light. Paring creates very little outward pressure, since the bevel is steady and the cut is made in a more or less downward direction, across the fibers.

I suppose you pay your money and take your chances, but if you have a mortise, you can do some things like rework the inside (or outside, with clearance) of a bowl pretty easily. I've even been known to re-chuck a piece to cut off the finish if I wasn't satisfied with it.

 

[pfduffy]

chucking bowls

Some great ideas and a terriffic discussion. It would seem the "automatic response" is to build a tendon but I too find that the mortise is most useful in many applications. One has to be careful, as ususal, to know where the inside of the mortise ends so the bowl does not blow up. That know, by measurement, one can proceed with confidence to complete the inside. It also can be advantageous to turn the bottom clear and clean of the mortise by one of several mounting methods. Inspite of all the hoopla over vacumn chucking I still like my old cork covered 18in disk with the tailstock holding it in place until I get it right, and the little piece at the center is turned away. The use of a dummy plug to do the interface with the live center works very well. OTOH, being able to remount on the mortise is a great advantage. Each piece has its own special challenges and these are just some ramblings that may be useful to others. Turn On! Phil