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Help mounting segmented blanks

Dean Center

Dean Center

Joined
May 4, 2010
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Our high school shop teacher has added a segmented bowl project to the curriculum of the Advanced Wood class. I've been helping with an after-school wood club there for a number of years and he asked me to help the regular classroom students with their turning. The glue ups are done and they start turning them this coming week, but I cannot for the life of me figure out how to mount the blanks to start things out. I've never turned a segmented bowl.

I'm used to solid wood mounted with a screw chuck, face plate or between centers. Since there's nothing in the center of the blank above the inside bottom, my usual methods of mounting won't work.

The shop teacher proposes the students glue a base with a premade tenon onto their glued up rings. This would work, but the cumulative effect of 'a little off' from one ring to the next and especially the glue block will make the complete blank unbalanced and possibly without adequate wood in some places. Is this the right way to mount them?

If that's the direction we go, how do the students get access to the entire outside of the blank? (two of the lathes are Powermatic 3520s and I KNOW they won't be able to get a normal base-to-rim push cut at the base of the bowls, even with a real bluntly ground gouge) I suppose we could turn the inside and the outside of the rim, then reverse mount on a jam chuck and turn the outside after the inside. Does this sound like a good solution?

Thanks for your help.
 
Dean, I don't know squat about segmented turning, but it seems to me that you don't have to do a push cut from the headstock end if it is a segmented turning. I took a quick look at Malcolm Tibbetts book and it looks like he uses an MDF glue block on a faceplate. You also might be able to use a stable wood such as mesquite.
 
One method that I have seen Ron Browning use in a demo is to mount a disc of Formica ( sink cutout) on a faceplate.
Circles drawn on the Formica are used to center and hot-melt glue the rim ring to the Formica.
Each ring is glued on top of a flattened ring. Last ring is the base. The outside including the tenon is turned almost to the rim using the tailstock as much as possible.
Then the rim is freed from the Formica using alcohol and a flat chisel with care taken not to cut the
The rim finished turned and the inside turned.

Next time you might include the mounting with the bowl glueup.

If the rim ring has a true surface you can adopt this method by glueing the rim of the glue up to a disc of Formica, MDF, plywood....
The centering needs to be near perfect. With MDF you could cut a shallow recess for the rim to get it centered.
 
Most of my turning is segmented pieces, either closed or open.

I start with the base glued to a waste block screwed to a faceplate. It's simple to get the base centered with the tailstock brought up and used as a press to hold it in place while the glue sets. Then, I turn the base to final size and face it off flat if it isn't already. I uses a cone that I have turned to center each of the rings as I add them. The cone is held in the tailstock on a faceplate, using the adapter that takes the threads on a Oneway live center to the same as my face plates. I turn inside after each two rings. That way, it is easy to keep the subsequent rings centered. If the piece is open segment, I turn the outside after every two rings, too.

Before I made the cone, I would take the piece off the lathe after every ring, eyeball the next ring on center, hot melt glue popsicle stick pieces to the ring to keep the ring centered when I pressed it on. It worked reasonably well, but my eyeballing was not great. Using the cone has made things much quicker and more accurate.
 
As Grant mentioned above a cone is a quick and easy way to center rings on a lathe mounted segmented piece, there are several manufacturers that make morse taper adapters with popular sizes of thread adapters on the rotating end that you can mount a face plate on with a wood turned cone.
 
Thanks for the responses. I'll be much better prepared for next time. This time, we'll muddle through and laugh at our booboos.
 
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