# Mysterious cracks in bowls



## vhuffines (Jul 27, 2011)

Need advice. Fairly new to bowl turning compared to a lot of you all. I have made appx. 70 mostly segmented bowls in a little over a year and just recently found the walnut and maple (shown in my albums) to have cracks in the joints highlighted. Cracks about .0003 open and run from center 1/4" dowel to the joint with next segment ring. Made one like this for a friend and his cracked same place. Both were turned four months ago. Glued with regular Titebond and assembled and finished the same as all the other bowls. These are 11"on one and 13" on the other. Have repaired with CA and sawdust which seems to have worked. Thanks for your comments and suggestions.
Vernon


----------



## Dave Paine (May 30, 2012)

It will help others if you like the picture from the album, especially the folks looking at the forum on tablets or mobile devices which may not be easy to navigate around the forum.










The cracks look to be a result of wood movement. The bowl is going to want to move as moisture changes with the seasons.

The inner segments are almost end grain joints, so not very strong.

The outer moves more than the inner. Something is likely to give.

I have only made one segmented bowl. I used a solid piece for the base.

Nice bowl. I hope the fix holds up. :thumbsup:


----------



## WillemJM (Aug 18, 2011)

Wood movement and end grain glue tends to do that.

If you must use a segmented base, one alternative is to use a small thin segmented ring under the base as a foot. Any wide (distance from inner radius to outer radius) unsupported ring will put stress on your glue joints. In the picture below, my segments are tied with a foot ring on the bottom and the Cherry ring on the top. It means wood holds the inner circle together at the bottom and the outer circle together on the top of the platter.

Propose using solid wood for your base and if your base goes beyond 4" diameter, use a floating base. "Floating" means the base is sandwiched; free to expand and contract without putting any stress on the segmented rings, preventing the base from warping slightly, or cracking the segments.

Floating base example attached.


----------



## john lucas (Sep 18, 2007)

The cracks are definitely wood movement. Had that problem many years ago and it took a long time to find the solution. I found it in a very hard to read book by Jack Cox called Beyond basic turning. It all about segmented work but is like reading legaleeze or possibly an engineering text. 
Anyway what I found was you have to make your own plywood. then ideally you make the base float in rabbits or even a dado. 
Basically if you want to avoid cracks later on in segmented work you have to understand wood movement. Wood moves differently in the 3 planes. It moves a lot with the grain, less across the grain and even less from bark to middle. When you build a pie shaped segment which uses this movement to the worst advantage, what happens is the ring wants to try and get bigger and smaller because of the relatively large movement of across the grain vs side grain. This eventually opens up either the center of the ring or cracks on the outer edge. 
To make the plywood what I do is make up a ring and then slice off 2 layers of say 1/4". Then on the inside where you won't see it (unless you turn into it) you put several layers of say 1/8" wood with the grains oriented opposite each other. This will make the total stack into a sheep of plywood that won't move. Unfortunately the rest of the bowl will move slightly so you glue this into a rabbit and estimate how much slop to leave( which is usually pretty small). Ideally set this bottom pie plug into the next ring with dados so it's free to move but that's a real hassle because you have to glue up the next ring around it. I've only done that once. I prefer the rabbit because it's so much easier.


----------



## john lucas (Sep 18, 2007)

I glued up 4 clocks a few years ago and made the center portion pie shaped segments. I made the pie shaped portion very thin to see if this would crack. The outer rings were standard thick segmented rings. 2 of the 4 cracked. I haven't been able to figure out why since all were made of the same wood. Possibly glue failure but not sure about that since I've been gluing up wood for a long time and think I do pretty good with it. 
The best bet is to simply make up your homemade plywood out of solid piece and alternate the grain. This may not look as good but you can guarantee it won't come apart.


----------



## firehawkmph (Apr 26, 2008)

John.
We have a member in our turning club who specializes in very intricate segmented pieces that he sells in high end galleries. He did a demo couple of years ago and recommended doing the bottoms just as you described in your first post. If you make up the bottom disc and let it float in a dado type groove, you won't have the cracking issues.
Mike Hawkins


----------



## john lucas (Sep 18, 2007)

Mike a floating disc by itself won't stop the cracking in the pie shaped bottom. It will stop cracking between the base and the next ring. This is really important if you use solid wood for that reason. The problem in the pie shaped pieces is that ring itself. If you just glue up that ring and lay it out on a shelf it will often open up in the middle or leave cracks along the outer edge. 
When the wood shrinks it will shrink most from the middle out and not as much in the circular direction. This opens up a hole in the middle. When the wood expands it does the opposite. Since it can't move toward the middle the outer edge has to get larger and since the wood isn't expanding sideways as much as across the grain the outside circle splits.


----------



## vhuffines (Jul 27, 2011)

Thanks to you fellows. I may be old but I hope I can still learn. Gonna do bunch of reading and utubeing. I really hate it when a plan doesn't come together.
Vernon


----------



## john lucas (Sep 18, 2007)

Get Malcolm Tibbets books or videos. There are the best out there on segmented turning.


----------



## WillemJM (Aug 18, 2011)

vhuffines said:


> Thanks to you fellows. I may be old but I hope I can still learn. Gonna do bunch of reading and utubeing. I really hate it when a plan doesn't come together.
> Vernon


If you ever make it to NC, we have one of the best segmented turners I know of, give a free class about once a quarter at his shop. Does not advertise or promote his work, but the best I have seen.

http://www.ncwoodworker.net/forums/showthread.php?t=51905&highlight=charlie


----------

