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Help! My bowls are cracking

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Forum topic by shopmania posted 586 days ago 1267 views 0 times favorited 5 replies Add to Favorites Watch
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shopmania

692 posts in 1807 days


586 days ago

Topic tags/keywords: question sanding finishing lathe

So I;ve been turning, but just pens and bottle stoppers, and making square sticks round. Got some nice wood (logs) from a friend, and decided to start trying bowls. First one in cherry turned out great. The blank was from the end of a log, about 5 inch diameter. The second one turned well, but cracked some on the bottom. The third one was holly, about 7” finished, but was too set to sand well in the middle. I let it dry for a 4 days, and it finished pretty well, but cracked just a little. Made an 8” shallow bowl from cherry where branch split off the trunk. Very cool grain patterns, but the crack grows by the hour.
/Users/timothyoneill/Desktop/pictures/IMG_0781.jpg

I know I’m having issues because of wet wood and uneven drying, but how do I fix it? Are full log/branch sections the best way to make blanks? How long after I cut them should they sit in the shop? Are there other good ways to get blanks from logs? What moisture content is good in the blank before you turn? Do you finish as soon as you turn/sand, or wait a while?

I’m very happy with how beautiful the bowls can be, but frustrated by this problem. Plan to try turning some goblets soon. Any different advice for those?

Thanks for the help guys/gals.

Tim

-- Tim, Myrtle Beach, DrTim@ONeillChiro.com- Just one more tool, that's all I need! :)


5 replies so far

View David Craig's profile

David Craig

2135 posts in 1734 days


#1 posted 586 days ago

Hey Tim,

I generally use the whole log when turning as well and cracks are usually part of the package. Many turners rip the log in half and cut a circular blank from there. When you deal with smaller logs, though, it is hard to get a good size blank without using the whole. Problem is that you have that pith in the middle

There are ways to slow down the cracking and make it more livable. If you notice any cracking as you are turning, squeeze CA glue into the cracks and rub sawdust in it. This will help stabilize the turning and prevent the cracking from spreading. Sometimes you can get lucky and turn past it with the wood stabilized. If you want more even drying, seal the ends of the logs when you get them with either anchor seal or latex paint. You can also round the logs on the lathe, turn it to shape, and then store them in plastic bags for a few weeks and then pull them out for final dimensioning and shaping.

Hope this helps,

David

-- There is little that is simple when it comes to making a simple box.

View Fettler's profile

Fettler

117 posts in 622 days


#2 posted 586 days ago

I just got mostly through “Understanding Wood” (book) which goes into this topic indepth. I would approach this problem using the scientific method. How much moisture does your log start with? What is the moisture / temperature in your shop? What time of year was the log harvested (fall/winter wood is harder)? What does the grain pattern look like? How much movement can be expected in that species of wood?

I’ve never tried to turn a bowl and can’t say that I truly grasp all the concepts in the afforementioned book. Might be (since its winter) that cold dry air is causing the bowl to loose moisture too quickly? Potenial solution: Try putting the bowl in a piece of Tupperware and vent every day or so (have used this trick for drying herbs), checking moisture content along the way.

Anyway, sounds like an interesting problem, best of luck to you.

-- --Rob, Seattle, WA

View Wildwood's profile

Wildwood

987 posts in 760 days


#3 posted 586 days ago

You did not say whether turning end grain bowls or side grain bowls. Normally diameter determines choice but no hard & fast rules. Moisture Content (MC) of cut trees varies a lot depending upon just cut or not sure when tree taken down. Still good idea to end seal as process a log.

Two ways to turn wet bowls, rough turn set aside to dry or turn thin, sand, and finish. Rough turning is just general shape to uniform thickness time to dry can vary by species of wood to few months or longer. You want to wait until wood has reached equilibrium moisture content (EMC) of plus or minus 1 or 2% of in use. Indoor use could be 6 to 10% depending upon where you live. Remember wood will lose MC as you turn too. Some species more prone to crack a little or lot during drying period.

Turning thin normally uniform wall thickness of ¼” to 3/8.” Turning thin does not stop warping of bowl (going oval) or cracking. When they do not crack lots of interest.

Wood from base of tree normally contains straighter grain than crotch, limbs or roots. Splitting logs regardless of where comes from a tree depends upon diameter and intended use; talking about bowls, hollow forms, or spindles. Lot of fuss made about pith or juvenile wood but will see bowls and hollow forms containing the pith. My general rule for splitting wood regardless of use is 12” or more in diameter split. I end seal green wood ASAP, but do not end seal after rough turning a bowl blanks.

For years use weight as a guide to whether finish turn a bowl blank. Simply picked-up the blank after few months or more to check. Still do even though now own a moisture meter. Some folks recommend buying a scale to weigh blanks, when weight remains the same (stopped losing moisture) ready to turn.

-- Bill

View RussellAP's profile

RussellAP

2944 posts in 912 days


#4 posted 586 days ago

It is easier to turn a wet piece than a dry one, but turning a dry piece isn’t all that hard and it has many advantages. I’ve seen some woodworkers take a piece of wet wood and just dowel the heck out of it with both wood and aluminum dowels. I tried it myself but I didn’t glue the dowel in, brain fart. It cracked overnight with a cheap clock blank in it and crushed that blank to rubble. I still have the piece, it’s elm ad it’s swollen up like it has cancer. The summer grain has receded and the winter growth stands out like tiger stripes.

If you turn wet wood, patience is a must. Turn some and let it sit a couple days. Then some more. and leave it thick on a dry warm shelf for about 6 months before you finish it.

-- A positive attitude will take you much further than positive thinking ever will.

View Jim Jakosh's profile

Jim Jakosh

11284 posts in 1731 days


#5 posted 584 days ago

Tim. Green wood is the best to turn and you can do it right away. But it has a lot of drying to do. If you don’t mine an oval bowl, you can turn it complete in one set up, but it is hard to sand when wet.
the best thing to do is to rough turn the bowl to a uniform wall thickness- about 3/4 to 1” and then let it set for about 6 months to let it dry out. It will warp as it dries and may crack in the solid part of the base because it has no where to move to like the walls do. You can put anchor seal on the end grain to slow down the drying and you can pack the rough bowl in wood chips to also slow the drying.

...........Jim

-- Jim Jakosh.....Practical Wood Products...........Learn something new every day!! Variety is the Spice of Life!!

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