Removing Turning Blank Pith

      Techniques to prevent cracking from the pith. January 6, 2005

Question
Why does the pith cause cracking? What exactly is going on there? Does the pith cause some sort of pressure that's being relieved?

Also, if I cut out an 8 x 8 x 15 inch vase turning blank from a chunk of wood that contained the pith down the center, would I be successful in drilling out the pith with a Forstner bit, and then gluing a dowel into its place, and then turning?
I'm curious as to whether or not the removal of the pith via drilling and replacement with glued dowel (or similar) would be successful.

Forum Responses
(Sawing and Drying Forum)
From contributor I:
The wood doesn't shrink evenly. It may shrink by 4% in the radial dimension, but 6% in the tangential dimension. If both shrinkage rates were the same, then you could dry a round without it distorting. But back in the real world, the radius of the round has shrunk 4% and the circumference has decreased 6%. The missing 2% is the crack. If the pith isn't included in the blank, the effect is much reduced and the piece usually just pulls itself slightly out of square.

Your idea of drilling out the pith may work. I'd suggest you drill out something like a 3 inch hole removing the pith. Then dry your blank. Once dry, the 3" hole would probably be 2 3/4 (just guessing here). Then you should be able to insert a dry and stable plug and hopefully it will all hold together.



From Professor Gene Wengert, forum technical advisor:
Great answer. Indeed, there is no room for shrinkage, so the hole idea is a good one and it does work if the hole is large enough. Just do not put the dowel into the hole until the piece is fully dry (and you may have to re-drill the hole to get it to be round and full size for your dowel).

This effect occurs in all large timbers, but it is the pith where the shrinkage forces are the greatest. It is also greater with a round shape than with a square or rectangle - more volume in a circle per area, etc.



From the original questioner:
Wait for it to dry? 8" of wood dry? Never gonna happen. These blanks even take a while after they are turned down to an inch or so of thickness.

I was hoping to drill out the pith and glue in a replacement using Gorilla glue, because of its reaction to moisture. If drilling maple, I was hoping to dowel it with a contrasting color wood, like walnut, and make the final turning decorative.



From Gene Wengert, forum technical advisor:
Just to make sure you understand, the hole provides room for shrinkage, so the dowel is not a good idea until the piece is fully dry.


From contributor B:
The only guys I've ever seen preserve a round like that from cracking do it by soaking in PEG glycol solution. Even then, it's hit or miss.


From contributor I:
There should be no reason you couldn't drill out your blank and rough turn it while green, with the hole in the centre. Then let it dry - you need to do this anyway. Once it's dry, insert the dowel and do your final turning and finishing.


From the original questioner:
But Dr. Wengert, if the hole provides room for shrinkage, won't the wood simply get nice and tight around the dowel? Or will the dowel act as the new pith, and the bowl blank still end up cracking?

Contributor I, I want to sell these, and was hoping the blank would be whole and complete before I sell them, but what you suggest may be the only way.



From Gene Wengert, forum technical advisor:
The dowel will act as the new pith, and the bowl blank still end up cracking.


From the original questioner:
Okay. Thanks for the education. Should drilling out 3" be sufficient? Or will that depend on the diameter of the round?


From Gene Wengert, forum technical advisor:
I cannot suggest a size that will work for sure... 3" sounds like a good starting point, but the bigger the piece, the bigger the hole.

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