Technique for a bowl with a very small base and wide top

[Dan Gaylin]

Hi all,

I would like to make a bowl that is 11” in diameter at the top, 2” deep and 1.5” in diameter at the bottom (approximate dimensions). That base diameter will be inadequate for holding the blank in a chuck to hollow it out after I have done the outside shaping. How can I solve this? I have thought about using a 3” deep blank and creating a 1” deep by 1.5” diameter tenon that could then be glued into a bigger piece of wood that has an appropriately large diameter for the blank and will fit in a chuck with normal jaws on it. But I’m still worried that sheering force could cause that tenon to break and send the bowl flying

thanks,

-Dan

[John Keeton]

First, perhaps you have a particular application in mind, but for accepted design proportions the base should be somewhere between 33-38% of the diameter of the bowl. That would mean a base between 3 5/8 - 4”. Additionally, an 11” bowl with a base of 1.5” would be really tipsy.

That said, I reverse my bowls against a jam piece with tailstock pressure and finish the base and peel away the tenon to a small 1/4” post. I remove the post with a sharp chisel and sand the bottom by hand. That should work for you. Just use a larger tenon to shape the outside and hollow. Then flip it and shape the base by blending it into the outside contour.

[Dan Gaylin]

Hi John thanks for your reply. The inspiration for this project was seeing several objects like this one:
447957

Admittedly not traditional shape but I have seen some wooden bowls like this and they are beautiful. Your technique is smart. It will be hard for me I think because I will have to get the inside contour correct first and then match the outside contour to it. I have not done that before.

[John Keeton]

Dan, you may want to consider doing the outside contour first. That would be the customary approach. Then hollow the inside an inch or two at a time. The rim will likely move as you hollow the form and you will not be able to return to it except for sanding.

[Reed Gray]

For a piece like that, I would start with a blank that was 3 to 4 inches tall. This way, you can start with an properly sized tenon. I would shape most o the outside first, but leave the base thick. I would turn out all of the inside, then finish turn the outside, and turn at slow speed for the final bit. Then part it off. With a base that small, even a tiny catch would be enough to shear off the blank. I would suppose that you could also finish turn the inside, and most of the outside, then reverse it and turn off the base. I would use a jamb chuck, with the bowl being inside the jamb chuck. This would allow you easy access to the bottom of the bowl, and very little stress. You would have to be careful about not going too deep when finishing the bottom part of the bowl.

robo hippy

[Dan Gaylin]

John, yes I understood. I meant that after doing the outside shaping but leaving the bottom thicker I would then have to get the correct final contour inside and then match it on the outside. Robo’s thoughts confirm this. One shot at the inside and then trying to match it “blind” on the outside with the jam chuck in there. This is definitely a challenge. I may experiment with some smaller pieces of wood first. Thank you both this is helpful. Of course I may now be convinced that this is best left to people with more skill — I am a weekend turner and have only been doing it for a couple of years.

[roger wiegand]

Mount the blank between centers, turn the outside shape as much as you can leaving a ~3-4" tenon, mount it on the tenon and hollow and finish the inside completely. reverse it using a friction drive with the tailstock in place, turn away the tenon and finish shaping the outside, shape as much of the bottom as possible, mount on a vacuum chuck or cole jaws to finish turning, sanding and finishing the base. Without the vac or cole jaw chuck turn the tenon down as small as you are comfortable with and take the nib off with a chisel. You'll probably have to accept a less than well done inside bottom edge if you take that approach.

[John K Jordan]

... One shot at the inside and then trying to match it “blind” on the outside with the jam chuck in there. This is definitely a challenge.


I almost always shape the outside first because of that. If you do have to do the inside first then finish by jamming to shape a bit of the base to match one thing I did once was make a cardboard template of the inside profile. Then I traced that onto another piece of card stock and drew my desired wall thickness and cut that strip away. Used that as a template for turning the outside profile.

Alternatively, you could also do some careful measurement of the inside diameter at a series of distances from the top then use calipers to check/set the outside profile to keep the wall thickness as desired.

BTW, a potter friend likes to make large pieces (several feet tall) with small bases like that since he likes the look. They do tend to be unstable without taking care. I remember a tall and skinny one at his mother's house he glued to a coffee table to keep it from getting knocked over!

If at all interested in his shapes, some are on his website under the gallery, pots:
http://paulmenchhofer.com/gallery.html
BTW, most of these are Raku fired - he makes the largest Raku-fired pottery in the world! (or used to, I don't know who is doing what these days)

[roger wiegand]

I think the key is to establish enough of the outside curve so that it is obvious where it is heading. On the example shown you'd only have to extrapolate maybe an inch and a half to get up to a fairly stout tenon. Just let the momentum of the curve carry you through to the obvious ending point both inside and out. Nothing very "blind" about doing that. It's the method I use on most all of the hollow forms I've attempted. I use a big tenon when hollowing something 10-12" deep, the base is almost always significantly smaller than the tenon was. Often the bottom of the vessel ends up in the tenon wood when you flip it over.

[Dan Gaylin]

Thanks Roger abs John for the additional thoughts. All very helpful. I’m probably going to give it a go next weekend. But maybe widen the base just a bit. Thanks everyone.

[Dan Gaylin]

Oh and John, I checked out Paul Menchhofer's work -- very interesting and very skilled. Also I think the carboard contour template is a great idea. Roger I do have a good Cole jaw and donut chuck so I think I've got my work cut out for me. Thanks for the encouragement

[John K Jordan]

Oh and John, I checked out Paul Menchhofer's work -- very interesting and very skilled. Also I think the carboard contour template is a great idea. Roger I do have a good Cole jaw and donut chuck so I think I've got my work cut out for me. Thanks for the encouragement

Dan, stop in sometime (post-virus) when you get down this way and you can go see Paul's house and ceramics studio - he often give people an introductory lesson in throwing clay. His house looks like a museum inside.

BTW. I wouldn't trust something that tall on Cole jaws (or vacuum chuck or jam chuck) unless supported by the tailstock the entire time.

JKJ

[roger wiegand]

BTW. I wouldn't trust something that tall on Cole jaws (or vacuum chuck or jam chuck) unless supported by the tailstock the entire time.
JKJ

LOL, I aspire to the day when having a piece come off the chuck at the final step is the worst thing that will happen to the piece at my hands!