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My first few bowls (Read 1,012 times)
 
pratul sharma
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Nova Comet II
My first few bowls
Aug 24th, 2018 at 12:26pm
 
I got myself a Nova Comet II lathe about a month back and been turning bowls mainly out of green wood. I did a couple of segmented ones too using scraps pieces from other projects.

It is a lot of fun and I am addicted!!  Grin

Collecting a lot of green wood and trying to figure of the best way to turn them and dry them without a lot of warping. Been drying them in microwave but they still warp. Especially ones I am making out of Bradford Pear.

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John Grace
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Re: My first few bowls
Reply #1 - Aug 24th, 2018 at 1:21pm
 
Most woods will warp in the drying process and is nearly unavoidable.  You leave rough turned bowls about 90% complete and then 'true' them up again during the final turning.  You'll find there are many different ways of drying pieces.  One of the more common methods is to rough turn, wax them (to slow the drying process), date the bottom, and set them on a shelf to finish drying.  In place of wax, many will put their pieces in paper grocery bags with shavings, close, and date.  Depending on the wood, your climate, and where you store your pieces the final drying may take as little as a few months and up to 12.  As for myself, I like to rough turn in batches...setting the dated roughed pieces on shelves in my garage.  When I've run out of green wood I circle back to the beginning for final turning.  Once you get a rhythm going there's always something 'next' to turn.  Good luck...John

BTW...I really like that segmented bowl idea, what a great way to use up scrap pieces.
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pratul sharma
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Re: My first few bowls
Reply #2 - Aug 24th, 2018 at 4:19pm
 
Thanks John!
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Don Stephan
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Re: My first few bowls
Reply #3 - Aug 24th, 2018 at 5:07pm
 
Welcome to the community.  Many of your initial experiences can find suggestions in the video library here and at a local turning group.  Such a group is invaluable for sharing experiences, mentoring, sometimes classes, and much more.

As wood dries, it shrinks more along the growth rings than across them.  Hence any "flat cut" bowl will show warping as it dries, some species of wood more than others.  "Quarter cut" bowls will not, because the growth rings are parallel across the bowl, so uneven shrinkage does not show up much or at all.  There are some diagrams here, not sure of the link, that will help with deciding how to break a log into blanks.

If you want a perfectly round bowl, twice turning is the ticket.  Search the forum on twice turned for info.
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Glenn Roberts
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Re: My first few bowls
Reply #4 - Aug 24th, 2018 at 8:14pm
 
Multimedia File Viewing and Clickable Links are available for Registered Members only!!  You need to Login or Register
Tangential shrinkage is "circumferential shrinkage."
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« Last Edit: Aug 24th, 2018 at 8:38pm by Glenn Roberts »  

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Don Stephan
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Re: My first few bowls
Reply #5 - Aug 25th, 2018 at 9:57am
 
That's a very informative link I had not seen, Glenn.  Wonder if somehow that could be incorporated in the reference materials on this forum.

Relating the two different shrinkage rates to the growth rings, shrinkage along the growth rings is often called tangential, and shrinkage across the growth rings is often called radial.

Bruce Hoadley's book Understanding Wood, an excellent woodworking library addition, devotes quite a few pages to discussion and illustrations to the subject of wood and moisture.
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Glenn Roberts
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Re: My first few bowls
Reply #6 - Aug 25th, 2018 at 12:01pm
 
Don, Maybe Ed or Ron would know how to do that if possible.

Tangential is often twice the radial shrinkage. My guess is because both shrinkages(!) are in the same plane(?), with the radius being half the diameter. "Along the growth rings" is something I don't understand. If the pith is off center, would the tangential shrinkage vary - at a given point - because of the varying density?
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Ed Weber
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Re: My first few bowls
Reply #7 - Aug 25th, 2018 at 12:46pm
 
The only way to insure the grain orientation you prefer is to harvest your own blanks.
This is not always possible or economically viable. You waste a lot of what otherwise be viable wood when you harvest for a specific orientation. This is why historically quartersawn wood has a premium cost, there is a substantial amount of waset. see diagram

You have to pay for aesthetics, meaning if you want a certain look, you have to deal with everything that comes with achieving that look. Whether that's warping, cost or even availability, This brings us back to the start, you may have to harvest your own to get what you want.

While the diagram is designed to illustrate typical flat lumber yields, you can see how cutting a bowl blank from one area for a specific grain orientation will impact the remaining area of the log and what will be left that's usable.
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Mike Mills
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Re: My first few bowls
Reply #8 - Aug 25th, 2018 at 11:22pm
 
Glenn Roberts wrote on Aug 25th, 2018 at 12:01pm:
D

Tangential is often twice the radial shrinkage. My guess is because both shrinkages(!) are in the same plane(?), with the radius being half the diameter. "Along the growth rings" is something I don't understand. If the pith is off center, would the tangential shrinkage vary - at a given point - because of the varying density?


From Glenn - Tangential shrinkage is "circumferential shrinkage."  I prefer to use circumferential myself.

Take a 10" diameter log.  We will assume 5% and 10% for ease and that pi is really 3 and not 3.14.
Radial shrinkage would be 1/2" inch or .5% of the 10 diameter.
Circumference of the log is 30" and its shrinkage is 10%.  So circumferential shrinkage is a whopping 3".  The total width of the cracks around the exterior of the log should be about 2.5 to 3".
This log is shown for cutting. 
At B the growth rings are the same length so there will be no cupping since they will all shrink the same amount.
At A (second board) you can see the growth ring at the bottom may be 1.5" long where the ring at the top of that board may be 3 or 4 inches long.  As it dries the longer growth ring will shrink two to three times the distance of the short ring cupping the board up.

The pic is of a piece of holly about 6" diameter.  If you let your eye take a straight line from one side you can see it shrank about and inch on just one half of the log.




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« Last Edit: Aug 25th, 2018 at 11:24pm by Mike Mills »  

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Don Stephan
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Re: My first few bowls
Reply #9 - Aug 26th, 2018 at 5:00pm
 
Whether a green turned bowl warps as it dries is not a function of how it dries (air or microwave bursts) or how quickly it dries, but the tangential and radial shrinkage rates.

Can't find a chart on the Internet, and my copy of Hoadley's Understanding Wood is at work, but I remember that red oak has a fairly high tangential shrinkage rate.  So the rim of "flat cut" center facing once turned red oak bowls tends to remain high where the rim is closest to where the pith was in the log, and drop down where the rim is closest to where the bark was in the log.  Ash is not quite as pronounced, walnut and black cherry rims of "flat cut" once turned bowls tend to stay the flattest.

Make the wall of the bowl thinner and the warp may be greater - a thinner rim is more flexible - but even a thick wall will warp some.

In his bowl turning book Richard Raffan mentions heating a green once turned bowl in a microwave for 30 seconds or so, but that was in effect to steam the wood so that it could be shaped somewhat into a more dramatic warp.  Not knowing any better, I wanted to try this once on a black walnut bowl and set the microwave for several minutes.  I think in less than 2 minutes the bottom of the bowl was a glowing red coal and filling the utility room with smoke.  NEVER HEAT WOODWORKING PROJECTS IN THE KITCHEN MICROWAVE IF YOU ARE MARRIED, AND WANT TO REMAIN SO.

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Don Stephan
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Re: My first few bowls
Reply #10 - Aug 26th, 2018 at 5:04pm
 
Whether a green turned bowl warps as it dries is not a function of how it dries (air or microwave bursts) or how quickly it dries, but the tangential and radial shrinkage rates.

Can't find a chart on the Internet, and my copy of Hoadley's Understanding Wood is at work, but I remember that red oak has a fairly high tangential shrinkage rate.  So the rim of "flat cut" center facing once turned red oak bowls tends to remain high where the rim is closest to where the pith was in the log, and drop down where the rim is closest to where the bark was in the log.  Ash is not quite as pronounced, walnut and black cherry rims of "flat cut" once turned bowls tend to stay the flattest.

Make the wall of the bowl thinner and the warp may be greater - a thinner rim is more flexible - but even a thick wall will warp some.

In his bowl turning book Richard Raffan mentions heating a green once turned bowl in a microwave for 30 seconds or so, but that was in effect to steam the wood so that it could be shaped somewhat into a more dramatic warp.  Not knowing any better, I wanted to try this once on a black walnut bowl and set the microwave for several minutes.  I think in less than 2 minutes the bottom of the bowl was a glowing red coal and filling the utility room with smoke.  NEVER HEAT WOODWORKING PROJECTS IN THE KITCHEN MICROWAVE IF YOU ARE MARRIED, AND WANT TO REMAIN SO.

Once turned bowls are easier work, because green wood is easier on the tools than dry, but some warping is to be expected.  Get a copy of a chart showing tangential and radial shrinkage rates, and the ratio, and  make some once turned bowls from woods with low tangential shrinkage rates, and some from woods with high tangential shrinkage rates, to get first hand experience.
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pratul sharma
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Re: My first few bowls
Reply #11 - Aug 27th, 2018 at 9:12am
 
Thank you guys! This is very helpful!
So if I turn a bowl so that the bottom is towards the pith, the top is towards the bark and I avoid the pith altogether, the warping would be less severe. Is that correct?

Thanks!

Mike Mills wrote on Aug 25th, 2018 at 11:22pm:
Glenn Roberts wrote on Aug 25th, 2018 at 12:01pm:
D

Tangential is often twice the radial shrinkage. My guess is because both shrinkages(!) are in the same plane(?), with the radius being half the diameter. "Along the growth rings" is something I don't understand. If the pith is off center, would the tangential shrinkage vary - at a given point - because of the varying density?


From Glenn - Tangential shrinkage is "circumferential shrinkage."  I prefer to use circumferential myself.

Take a 10" diameter log.  We will assume 5% and 10% for ease and that pi is really 3 and not 3.14.
Radial shrinkage would be 1/2" inch or .5% of the 10 diameter.
Circumference of the log is 30" and its shrinkage is 10%.  So circumferential shrinkage is a whopping 3".  The total width of the cracks around the exterior of the log should be about 2.5 to 3".
This log is shown for cutting. 
At B the growth rings are the same length so there will be no cupping since they will all shrink the same amount.
At A (second board) you can see the growth ring at the bottom may be 1.5" long where the ring at the top of that board may be 3 or 4 inches long.  As it dries the longer growth ring will shrink two to three times the distance of the short ring cupping the board up.

The pic is of a piece of holly about 6" diameter.  If you let your eye take a straight line from one side you can see it shrank about and inch on just one half of the log.





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pratul sharma
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Re: My first few bowls
Reply #12 - Aug 27th, 2018 at 9:24am
 
another related question...
Found folks clearing to build a new subdivision in our neighborhood and picked up a decent amount of wood. Walnut, Bradford pear, cherry, poplar, maple, oak and pine. Right now, I have applied a thick layer of AnchorSeal to both ends and stacked them up on a platform I constructed using left over boards I used to build my deck. They are stacked under the deck where it gets very little direct sunlight. See attached picture.
Is this a good way to store these logs and dry them correctly? should I put a tarp on top of the logs or leave them as they are?
I will turn as much as I can while the logs are still green but this is a lot Smiley and I want to make sure the ones I don't get to in time are preserved and don't check.

Any suggestions?


Thank you in advance!
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John Grace
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Re: My first few bowls
Reply #13 - Aug 27th, 2018 at 7:25pm
 
My preference is to prep the blanks completely before storing them. Remove the pith by about 2 inches, seal the ends, and stack horizontally with spacers for breathing, and store out of the elements.  There's many 'right' ways to do it but the above has served me well.
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Don Stephan
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Re: My first few bowls
Reply #14 - Aug 27th, 2018 at 7:30pm
 
Pratul:

Whether the rim is towards the center or towards the bark, it is always a good practice to stay at least 1" away from the pith, and a bowl will warp.  And end coated sections of log will also lose moisture through the bark, although at a slower rate, so eventually shrinkage cracks will occur under the bark.

My suggestion is to make 3 or 4 "flat cut" bowls with the rim facing the center, 3 or 4 "flat cut" bowls with the rim facing the bark, and 3 or 4 "quarter cut" bowls
to gain first hand, direct experience with the different orientations and how they move as the wood dries out.

I tend to keep sections of log on their sides (bark down) rather than on end, in deep shade, and hope for the best.
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