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Back to turning - using collars on hollow forms

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4mikee

Joined
May 26, 2006
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N.W. Alabama
Can some of you good folks enlighten my limited knowledge on collars used on hollow forms. I'm aware that dry hollow form and dry collar wouldn't be a problem but how is it typically done. The hollow turning I've done is 99% green and finished soon after turning (days) which of course leads to warping. Also I just caved in and bought a Jamieson system, so I'm inclined to turn green hollow forms right now. What's the experience out there with turning green and when to finish. I tend to finish these things soon after turning as I metioned before.
1) Will the collar or base split if I have a green base and dry collar or is there more acceptable way of doing this.
2) Is the warpage usually acceptable as inevitable?
Thanks
 
The thinner green turning generally distorts in preference to the dry collar in my shop. After all, we're not talking great spans, where 10% might be more than a tenth of an inch.
 
In my experience, any time you're turning green, warpage is inevitable. How "acceptable" it is would, I suppose, depend on how much warpage occurs and how you feel about the appearance/function of the finished product.
If I were to collar a hollow form I'd probably turn it green, allow it to dry, finish turn it and then apply the collar (which I would rough turn and dry in the same fashion as the hollow form) as a part of the finish turning process. Two types of wood with different shrinkage rates and differing grain patterns assembled green sounds to me like a disaster looking for a place to happen. But then, when it comes to wood turning, I'm not much of a risk taker. IMHO, exotic wood is too expensive to take unnecessary risks with.
 
A dry collar, if it is a relatively strong wood, can actually help stop the green wood from splitting. Note: "can." That's not to say that it is guaranteed to.
 
I haven't added many collars to my green turnings but like MM said the collar is probably 2" or less and the wood movement will be probably 8% to 10% depending on which direction the grain runs. So you can see the movement will be rather small.
It will help keep the area from warping if you turn it thin enough and if the grain runs pretty circular around it. If you have some crotch flame or a knot running up near the lip it will probably shrink and move up or down in one area and you'll have a lump. This can often be carved and sanded down if the piece isn't too thin.
When I can I wait a few days until the piece has dried and then true up the hole for the collar. most of the time I'm in too much of a hurry and don't wait.
 
I haven't had a problem with collars or vessels cracking, but I have had a problem with joint creep. I attached a couple green collars to green hollow forms with titebond and polyurethane glue a couple years ago. The joint with the collars was flush to the piece. Within 6 months the joint was no longer flush. There was enough flexibility in the glue to allow the pieces to move without cracking. It was only a tiny bit of movement, but it was enough that it was very obvious to the eye and touch that one side of the joint was standing proud.

It was no big deal for me to repair, but it would have been a big deal if I had sold / given the piece to someone. I avoid this problem now by either using dry wood or having a collar design (e.g. collar sitting on a shoulder) where a little bit of movement won't result in a glaringly obvious flaw at the joint.

Ed
 
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