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Marking Out the "Pith Slab" (Read 113 times)
 
Don Stephan
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Marking Out the "Pith Slab"
Jul 25th, 2020 at 6:36pm
 
This spring/summer I've been gifted a number of fresh-cut logs 12" to 20" in diameter. The larger ones all were used for the bowl type most of my customers want - a salad mixing bowl 14" to 16" in diameter - and all the logs were cut to make "side grain" pairs of bowls facing the center or pith of the tree.

For those new to turning bowls, the "pith" is the tree's growth center, the middle of the annual rings, and may not be the geometric center (if the tree was leaning). The pith is almost always off the geometric center of branches as well, but branch wood has internal stress due to the pull of gravity, is referred to as "reaction wood" and not something I turn.

The pith itself is almost always hollow, a sixteenth or strong sixteenth in diameter. According to Bruce Hoadley's excellent book Understanding Wood, the wood immediately around the pith is a particular type of cell, extending the length of the tree and branch tips. I believe this wood immediately around the pith is called "juvenile wood" and the common recommendation for side grain bowls, facing the pith or facing the bark, is to exclude the pith itself and the wood within about 1" of the pith from side grain bowl blanks.

The process I use to make side grain bowl blanks from a section of log, which no doubt I learned from other bowl turners when I was starting out, was to cut a section of log 1" to 2" longer than the diameter of the section, and lay it on its side so the grain of the wood is horizontal.

First I would roll or lift the section of log onto a cutting stand (which needs to be redesigned).  Looking at each end for any cracks at the pith, and other defects, I would roll the section one way or another until the pith-containing slab I wanted to remove was vertical.

Using a two foot level, I would draw a vertical line through the pith at each end, measure out 1" to each side of those vertical lines through the pith and draw two more vertical lines on each end of the section of log,l marking out a 2" wide slab on each end centered on the pith.

With a chain saw, I would then cut along the inside edge of those outer lines, removing a 2" width of wood containing the pith.  (The resulting slab is of course only about 1 1/4" wide due to the kerfs made by the chain saw.)

After doing this a hundred times or more there seemed to be an opportunity to make the process a bit more efficient and less a literal pain in the (bent over) back.

In the proverbial scrap bin (things we never throw away even though we call them scrap) I found a piece of 1/8" thick plexiglas about 24" x 12". On the tablesaw I cut a 24' long piece 2" wide.

Carefully, keeping a 24" ruler from slipping, I scratched a continuous center line the length of this piece of plexiglas.

Now, after marking a center line on each end of a section of log using my level, I center this plexiglas strip on a line through the pith and without any measuring quickly make a line along each edge of the plexiglas, marking out a 2" wide pith-containing slab to be removed.

Anticipating an occasional need to move one of the cut lines a bit further from the line through the pith, I made intermittent scratches 1/4", 1/2", and 3/4" from the plexiglas strip's center line, but have not yet had the need to do so.

No doubt there will be calls for a photograph of the plexiglas strip, but clear plexiglas is not likely to stand out against any wood background. It is just a 24" by 2" piece, with a 24" long center line scratched the full length.
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« Last Edit: Jul 25th, 2020 at 7:16pm by Don Stephan »  
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Bill Neff
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Re: Marking Out the "Pith Slab"
Reply #1 - Jul 27th, 2020 at 7:55am
 
Great idea!  What I do on really large logs, is I mark the center of the pith with a line running the width of the log.  Then I lay an old yard stick with one edge on the center line and draw a line on the log along the outside edge of the yard stick.  Then I flip it over and do the same  on the other side of the pith.  This gives me a slab of  about 1.5 - 3 inches thick which I can get 2-4 small quarter sawn bowls out of.
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Some people are like slinkies... totally worthless but it puts a smile on your face when you push them down the stairs.
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robo_hippy
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Re: Marking Out the "Pith Slab"
Reply #2 - Jul 27th, 2020 at 10:34am
 
Sounds similar to what I do. Not sure if you have seen my 'Chainsaw Chopsaw' video or not... Plumb line through the pith, then strips of 1/4 inch plywood, 1/2 inch increments, from 1 to 8 inches. That gets just about every thing.

I don't think I have ever had a log section that didn't have a crack off of it. Some times a tiny hole in the center. Saw some paolonia/royal empress tree sections once that had a 1 inch diameter hole in the center...

Can't think of a tree where the pith was ever centered, though some are close. I did get a chunk of Osage once which was a hill side tree. The pith was on the uphill side, right on the bark line, and all the rest of the tree was on the down hill side, maybe 10 inch diameter.... You just never know.

robo hippy
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