Roughing out... Hollow forms?

[underdog]

I was roughing out a bunch of bowls yesterday, and decided to sacrifice one to playing with practicing hollow forms (playing) since I'd never done a HF before.

Since it's green and I got it pretty thin in places, it's starting to crack and separate as it dries. Not such a problem as it was just an experiment anyway, but it leads me to a question..

Most of us rough out bowls and I've seen several demonstrations on that subject, but I've never heard anyone mention how hollow forms are done in regard to cutting green....

 

[dkulze]

three ways.

First, turn it really thin and consistant to finished shape and thickness then let it move as it dries.

Second, turn it medium thick and paint the outside with anchorseal then let dry just like a bowl blank (don't anchorseal the inside as it is already gonna dry slow from less exposure and it won't hurt if it dries first".

Third, turn the HF end grain and without the pith. This will usually be enough to prevent cracking.

Have fun,
Dietrich

 

[wdspinner]

Jim,

Another way to do it is to turn it to about 1/2" thick for wall thickness. Soak in DNA (Denatured Alcohol) for about 24 hours. Wrap and set aside for about 1 month. Unwrap and set aside for about 1 week. Remount and finish the form. Best of luck on your hollowing adventure.

 

[Anthony Yakonick]

Turn it green start to finish to a 1/4" or so, it will be dry when you're done sanding it but will tend to move some. NO pith!!

 

[MichaelMouse]

Couple of things working against you when hollowing green wood. First, most people hollow along the grain, which puts the quick-drying stuff up and down as they set it aside. In my reply to Frank I mentioned that inequality in dry rate is going to create a bad situation likely to promote radial checking. You've got a built-in inequality with a hollowed form, however, so you want to slow the end loss from the outside to match what you can't affect - the inside. Those of us who use the mark one mod zero lung to blow through a tube and eject shavings hurt our case. Those who use dry air help it.

Second is the hollowing devices we use, which are generally high-friction, and therefore high heat, which can boil the end grain and get things checking if we're not careful. Initially a help, because it gets moisture moving out, it can develop into local disaster as it dries the convex surface. Same with sanding, as you know. I hollow outside, inside, outside to compensate. Rough the out, hollow the in, leaving end grain fat to tolerate heating, then trim the outside to desired thickness with a low-friction tool.

Since I changed to air ejection I'm no longer quite as concerned about the interior/exterior drying differential, but I still will not sand finer than 120, which means most of what I put aside to dry is from the cutting tool. I let'em warp, because I can't stop it, and they're not, in my market, worth re-turning.

 

[Bill Grumbine]

There are a couple of ways to do it, as you might suspect. Anthony is right about green to finish. As long as you get it to 1/4" or less, it should dry just fine. There are always exceptions of course, but in the vast majority of cases, 1/4" seems to be the magic number. Bowl or spindle orientation seems to matter not. I turn a fair number of bowl oriented hollow forms with the same results Anthony gets with his spindle orientation.

If you want to see some truly spectacular work, look for Wally Dickerman's hollow forms. Wally uses a lot of burls, but he does regular wood too, and he often roughs them out and lets them dry before finishing them off to 1/8".

In all cases, a consistent wall thickness is important. If the wall thickness is not consistent, you are going to have trouble with cracking.

Bill

 

[john lucas]

I rough out hollow vessels to 1" or less depending on the shape and the wood. I seal all endgrain surfaces and any that I suspect might even look like endgrain. Then I put the piece on my shop floor in the far corner. it's cooler and has less air movement there. If it's a piece I really don't want to lose I'll put it in a box with the lid lightly closed.
If I leave a thick tenon on the bottom I seal the whole tenon with endgrain sealer. Most of the ones I've lost have either checked at the tenon on the bottom or on the upper surface near the lip so those areas are the ones I really try to protect.
I will agree with Bill and the others. If you turn them to 1/4 to 3/8" thick and put them up to dry where there's not a lot of air movement you shouldn't have any problems.

 

[dkulze]

One of our club members, John Moore, is the same way as Wally. He fell in love with a laser pointer capture rig and does the most beautiful 1/8 thick hollowforms from green wood. They ovalize a bit when they dry but they are so thin, they don't even crack when he includes the pith. He's truly great at finishing too. Brings out all the depth.

Dietrich

 

[Andy Coates]

Hollowforms

I'm simply too impatient to do anything other than green-to-finish...although I should say that the finishing, more and more, is done off the lathe over a period of days.

My routine is just about the same as anybody else who turns green HFs to finish. And I just realised that MMs outside-inside-outside is something I have been doing for quite some time without actually analyzing why. I hollow to about the same thickness 1/4", or 5mm if you want it in a foreign language Thinner is just as easy once your arm and eye are in, but I find distortion more of a problem with anything under 5mm. You do get some problems if you have branchlet knots, faults, Etc. where differential drying will cause them to shrink, or worse yet the timber around them shrinks and they fall out, but CA the moment you notice it, cures it.

I tend to leave the base un-sanded and un-sealed for a week (indoors) after turning...I don't know if I'm kidding myself but it seems that after I started doing this the occaisional problems I had with bases cracking stopped. Or maybe I just get my bases the correct thickness now. Who knows!

Over the last two weeks I've turned over twenty green to finish HFs and not had a problem with any. All are stored indoors, roughly 20degC, and the bases are sanded and finished after a week or two.

After I first started turning HFs I burnt an awful lot of them along the way, but the only wastage I have now is the odd one that I'm not happy with for reasons of shape, form, finish. They go on the fire. The others seem to sell. So happy turning!

Andy

 

[Ed Koenig]

I would like to give my thoughts on your question. I rough turn my green HF's to about 1" and more for larger forms. I like to boil my wood as it speeds up the drying time and usually helps to stabilize the wood. I usually boil for at least one hour at a full boil. After boiling, I place the form in a brown paper shopping bag and begin weighing the form daily after about a month. When the form weighs the same for three or four days in a row, it is dry enough to finish. I do the same for green bowls. You must be patient and if your are you will be rewarded.