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Cutting Bowl Blanks: How far from the pith should you be? (Read 1,607 times)
 
Chris Gunsolley
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Re: Cutting Bowl Blanks: How far from the pith should you be?
Reply #15 - May 23rd, 2016 at 10:52am
 
robo_hippy wrote on May 23rd, 2016 at 10:15am:
Well, how far away is like drying, every piece is different, so it is more art than science as the exact same method will work some times and not others. Only real rule I try to apply here is to make sure you turn away any cracks, and then a little bit extra because some times they extend past what you can see. Cracks always get bigger.

robo hippy


Let's assume you're cutting your blank for a bowl that will be 10" in diameter. Do you leave an extra inch, for example, on each side, cutting it to 12", in order to allow a little extra room for the potential of cracking/checking even after you've sealed it?
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Re: Cutting Bowl Blanks: How far from the pith should you be?
Reply #16 - May 23rd, 2016 at 11:46am
 
Chris Gunsolley wrote on May 23rd, 2016 at 10:52am:
robo_hippy wrote on May 23rd, 2016 at 10:15am:
Well, how far away is like drying, every piece is different, so it is more art than science as the exact same method will work some times and not others. Only real rule I try to apply here is to make sure you turn away any cracks, and then a little bit extra because some times they extend past what you can see. Cracks always get bigger.

robo hippy


Let's assume you're cutting your blank for a bowl that will be 10" in diameter. Do you leave an extra inch, for example, on each side, cutting it to 12", in order to allow a little extra room for the potential of cracking/checking even after you've sealed it?



That is always a good idea...
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Ron Sardo
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Re: Cutting Bowl Blanks: How far from the pith should you be?
Reply #17 - May 23rd, 2016 at 12:45pm
 
Chris Gunsolley wrote on May 23rd, 2016 at 10:52am:
allow a little extra room for the potential of cracking/checking even after you've sealed it


and warping
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David Hill
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Re: Cutting Bowl Blanks: How far from the pith should you be?
Reply #18 - May 23rd, 2016 at 2:13pm
 
Chris-  nice discussion now.
What I was saying ---Mesquite moves the least.  All the others that I'd named (trying to think of those that might grow where you are) move a lot more than Mesquite.  And yes, it's the "comfort rule" that comes with experience.
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Re: Cutting Bowl Blanks: How far from the pith should you be?
Reply #19 - May 24th, 2016 at 8:28am
 
Also, remember that the slab of pith removed isn't waste wood.  Each side of that slab (away from the pith) is a great piece for pens or other small-ish spindle projects.

Even when I thought I had excluded enough pith and my bowl blanks were pretty dry when I turned them, I find that now, years later, very few of my bowls will sit totally flat (ie, without any rocking) when top down.  Some don't just rock, they are visibly oval!
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robo_hippy
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Re: Cutting Bowl Blanks: How far from the pith should you be?
Reply #20 - May 24th, 2016 at 9:24am
 
Chris,
I turn green to final thickness, then let them dry and warp, then sand and finish. The more they warp, the better they seem to sell for me. For twice turned bowls, rule of thumb is 10% thickness, so 10 inch bowl is left about 1 inch thick. Some woods like mesquite move less, some like madrone move way too much for twice turning so you have to boil it to stabilize it, or turn to final thickness and let it warp.

robo hippy
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Re: Cutting Bowl Blanks: How far from the pith should you be?
Reply #21 - May 24th, 2016 at 12:32pm
 
Ok, I'll add my $0.02 worth. I split right down the center. Do not seal as general rule and let it crack. yes, I loose some, but I have fun filling the cracks with metal flake & epoxy. My wife & family likes them.

Glenn J.
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Ron Sardo
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Re: Cutting Bowl Blanks: How far from the pith should you be?
Reply #22 - May 24th, 2016 at 1:01pm
 
Chris Gunsolley wrote on May 23rd, 2016 at 10:48am:
Well, I suppose one lesson here is don't expect to be able to accurately predict what will happen with each blank. All of this really has me thinking about wood stability. The link at Multimedia File Viewing and Clickable Links are available for Registered Members only!!  You need to Login or Register made me realize that the rate of change as the wood dries, which depends on its density, can actually be measured, and the woods thereby ranked in terms of stability. Here are some of their results, which are generally consistent with some of the woods David listed as being particularly stable:

Most stable woods (of those measured):

1) Walnut
2) Merbau
3) Ash
4) Cherry
5) Birch
6) Oak
7) Maple
8) Beech



I think you missed this from Multimedia File Viewing and Clickable Links are available for Registered Members only!!  You need to Login or Register

Quote:
Table 1. Average change rates of unglued timber parameters, measured when wood moisture changes from 8 to 9 percent and vice versa.
Type of Wood      Average Change of Wood Parameters %
Oak                           0.25
Ash                          0.22
Beech                        0.32
Maple                        0.30
Birch                           0.24
Merbau                        0.21
Cherry                        0.23
Walnut                        0.20


Here in Pennsylvania air dried wood rarely gets down to 12%. In a Arizona desert I wouldn't be surprised if wood can get down to 6%.

Also one need to consider how the timber is cut (quartered, rift, or plain sawn) with quarter sawn wood the most stable and plain sawn the least. (The link you provided are for floor boards which is a different animal than wood bowls. Timber, as its used here, is wood that has been cut into boards)

On a typical wood bowl all three grains (quartered, rift, and plain sawn) along with end grain can be present, so different parts of the bowl will act differently depending on its environment.

I find wood from fruit trees the least stable, followed by nut trees. The most stable are wood from trees like Maple, Oak, Mesquite and so on.

This said, my favorite local wood to turn is Cherry followed by Walnut, I just had to learn how to deal with it. I love Curly Maple but I'm allergic to it so I hardly use it.
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Ed Weber
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Re: Cutting Bowl Blanks: How far from the pith should you be?
Reply #23 - May 24th, 2016 at 2:06pm
 
Flooring manufactures need to be concerned because a .25% over the width of a 10' floor is about 5/16". This is why we leave room around the perimeter for expansion.
This is also cut 3/4' thick for flooring
A .25% potential movement on a 10" blank isn't much to worry about at 3/4' but bowl blanks are typically much thicker. This may not effect the movement in one direction but may in another, which is usually where the warping comes into play.
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Mike Mills
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Re: Cutting Bowl Blanks: How far from the pith should you be?
Reply #24 - May 26th, 2016 at 10:46am
 
On the sizes you mentioned, 6" - 12", I cut both directly through the pith leaving 2" on each end for waste.
I do have an old electric hand plane that I may run down the pith area if it looks like I did not remove the pith entirely.
It also leaves a true flat surface for a small faceplate or the top of the jaws for a wormwood screw.
Differences may be with sycamore and oak which I normally cut about 1" from each side of a 12" in order to have a quartersawn section.  On larger logs I remove about 1/2" from each side.
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John Cepko
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Re: Cutting Bowl Blanks: How far from the pith should you be?
Reply #25 - May 27th, 2016 at 8:16pm
 
Like usual, it depends.
On the smallest logs, I try to split right down the pith with one shot.
Logs a little thicker, I may cut an "X" through the pith with 2 cuts.
On the widest logs, a slab up to a couple inches thick is cut.
.
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