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I am wondering how a end grain bowl can be turned leaving the pith in the center. The question is not about how to mount it but when finished turning the pith remains centered in the bottom of the bowl. My limited knowledge and experience says it will crack significantly etc. I’m presuming green wood turning.
Can these cracks be filled with CA and thereby stop enlarging?
Drill out the pith area and after the bowl dries glue in a plug?
Are some woods better than others?
Any other pointers ?
Sometimes they do not crack, if they do you can put in a butterfly patch. I did this oak form and used CA glue while turning it green and waxed it as soon as i was done turning.Quote:
Originally Posted by Bernie Kopfer
The species is the most important IMO.
There are several species that are better suited to this type of turning. Mostly you see Norfolk Island Pine turned in this orientation.
Unless the pith is the focal point or an important aesthetic element of the piece, it's usually better to avoid.
I’m not one for putting a plug in the bottom. A wood turner would just think you went through the bottom. Walnut is bad about cracking. I don’t turn with the pith in the part, but if I was turning green wood I would use a recess instead of a tenon. With a tenon you have more mass that would minimize the chance of cracking. If you do get small cracks in the base are I think it is ok to fill those. Any cracks in the side would require a pewa patch if you want to save the bowl.
I(t is not uncommon to see martini glasses turned from a branch, with the pith near but not in the stem. No harm in experimenting if you are willing to risk significant cracking.
Not sure if I understand you observation about tenon vs recess.Quote:
Originally Posted by William C Rogers
The reason I brought this up is that the other day a newbie woodworker found a 20 in diameter piece of fruit cherry that he thought I would want to turn a bowl from. Unfortunately it was only 6” thick. He did not know that turning with the pith was a no-no. So I got to thinking that a lot of wood is wasted but could be salvaged if there was a reliable method to minimize cracking etc. Fruit wood would be my last choice even to practice on. Think I’ll try it on some western cedar this winter. Plugs might be ugly to Woodturners, but I’m not turning for them🤣
Some have described saturating the pith with ca glue successfully. Others have sliced off a layer to cut a thin plug which is slowly dried and used to fill a center hole in the dry vessel. A turned cove can hide the joint. No guarantees but those are two possible approaches. Some just accept the cracks as a feature.
If that cookie has sat around more than 2-3 days, it already has cracks started and the chance of success has dropped dramatically. I turned a walnut and red oak vessel with the pith coming out the sides. Both were dead green and I turned the wall around 1/8" thick. Both survived with just some tiny cracks The red oak dried looking like a football and the walnut less so.
With a tenon you have a lot more mass that will increase the chance for cracking because that area will dry slower than the rest of the wood. You don have that extra mass with a recess.Quote:
Originally Posted by Bernie Kopfer
The other thing you could try is once turning thin 1/4” or less. However it will warp. Some only turn once with the warp as a feature.
I would make a bunch of 6 inch bowls. You can make an end grain bowl with the pith in the center, but the probability of it cracking is very high.
robo hippy
recently a neighbor a neighbor asked me if I'd like to take a look at a bowl he'd bought at a silent auction on our very small rural island. It was a signed piece by a very well respected east coast family of turners. It was an end grain bowl from a species of pine that I didn't recognise.
It had a crack right through the pith centered bottom about a half inch wide and spreading nearly half the diameter of the 13" bowl. It was filled with what looked like sawdust mixed with whatever was used as a thick, film finish. All in all, I was quite surprised at the quality of the craft brought to the item. I was surprised to see it, but the neighbor got it at a decent price so kept my comments mostly to myself.
Others have covered the main issues. Species determines a lot of what you can get away with -- interesting that someone said walnut was bad about pith cracking, in my experience it is one of the most forgiving of the hardwoods in that regard (and fruitwoods some of the worst). I also wouldn't think that leaving wood thick is a good way to deal with pith cracking, as wood being thin makes it more able to deform during drying, and stress from differential shrinkage is what creates the pith cracks in the first place.
Something others have not mentioned is curvature. If the pith appears in a part of the turning that has a lot of curvature, then that allows some of the shrinkage stress to be taken up differentially. I would assert that part of the reason the vessel in Bruce Jones' post didn't experience severe pith cracking is that it is thin and the pith is in an area with high curvature. This helps because the ring an inch from the pith is not having to shrink over all the wood between it and the pith, a lot of that is now air. This is why a tree cookie cut on a slant is much less inclined to crack than one cut straight across.
In the case of a 20" wide, 6" deep bowl, it is going to be hard to be both thin and have curvature, since the pith is presumably right in the base of the bowl where it will be flat. And being fruitwood is not going to help. Most fruitwoods have both high overall shrinkage and high tangential to radial (T/R) shrinkage ratios (1.8-2.5ish), which increases problems with pith cracking. Black walnut has a lower T/R (1.4), and many pines have relatively low overall shrinkage (and are fairly flexible woods) which helps reduce pith cracking. For those unfamiliar with T/R ratios, more explanation here: https://www.wood-database.com/wood-a...nal-shrinkage/
and even more and better explanations in Hoadley's book "Understanding Wood" (IMHO the best "wood book" there is if you want to understand what makes wood do what it does).
Best,
Dave
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