do you remember this hollow form?

[cornucopia]

hello folks I turned a couple of hollow forms a few weeks ago out of the last piece of the burr oak i had- one form had the pith in the top and i was struggling to gauge the wall thickness in the bottom third of the piece (i have now made some ellsworth callipers which will help me next time) when i cut it of the lathe it was very bottom heavy and may be from half way it was possibly 1/2" thick-too thick
the piece on the right

theres the pith

well it did crack as it dried but not from the pith- the pith open up a bit but not to a bin it level.

so i have cut it open to see how right or wrong i was and why it has cracked in the way it did-

it was 1/8" at the top then quickly went to 1/4" then 3/8 near the base

the form cracked due to uneven wall thickness- the top dried out quickest and caused a stress to build up which is why it split from the hollowing hole.maybe the pith didn't help but the main problem was the wall thickness.

if one of these thickness 1/8", 1/4" or 3/8" had have been consistent and the piece dried slowly i doubt that it would have cracked.

so every now and then cut a piece open and learn how and why for next time.

 

[wizer]

a great shame George but a great lesson

 

[The Shark]

Hi George,
Thanks for posting that - very informative.
It's nice to know that I'm not alone in having "imperfections" :lol:

Malc

 

[PAC]

A very good lesson...

I'm fairly new to woodturning (since Sept. 2008). The bowls I roughed out in the winter, warped but didn't crack (with the exception of sycamore and yew, which dried incredibly well with no warping or splitting at all). These were all dried without any wax coating. The rough turning I'm doing at the moment, however, is splitting very quickly - probably due to the much hotter and drier conditions. Looks like I'll have to coat rough turnings with wax in the spring/summer and not bother in the winter.

 

[cornucopia]

or place them in plastic bags during the summer and turn the bag inside out every few days

 

[PAC]

or place them in plastic bags during the summer and turn the bag inside out every few days

Good tip - I'll try that. I've bought some brass powder to fill in some of those splits! One thing I found with the sycamore is that roughed bowls were stained with blue/green fungi, which I don't particularly like. The ones I microwave dried were much better as they were stain-free.

 

[big soft moose]

nice to see you back paul - how did the wood i gave you work out ? ( i havent done anything with the yew i took in exchange yet)

 

[cornucopia]

nice to see you back paul - how did the wood i gave you work out ? ( i havent done anything with the yew i took in exchange yet)

heh BSM

the pm function is there for a reason why not use it instead of my thread :roll:

 

[Jenx]

Wow, thats a pity its split, George ....
but what a great insight into the 'innards' of the form ...

Good lesson, as you say !
8)

 

[Lightweeder]

Seriously George, from 1/4 to 3/8 :!: :!: Unless you're stopping every few seconds and going in with the calipers, how are you to know :? :? :? :?:

 

[Richard Findley]

Hi George,

I reckon you're being a touch hard on your self there. I think it split more because of tension in the wood rather than your so called "uneven wall thinkness"!!

I will be the first to point out I'm no expert on hollow forms or working green timber but seriously mate, give yourself a break - that's bloody good turning =D> =D> =D> =D> =D> =D> =D>

Richard

 

[Bodrighy]

I am dead chuffed to get anything hollow down to 3/8th anywhere on small stuff. As one of the pros at the club put ti. "The trick is to make the hole too small to get your finger in so people can't check the inside" Seriously Your inside is damn good so I'm guessing you are a bit of a perfectionist.

Pete

 

[cornucopia]

lightweeder: thats what you have to do with h/f when your approaching the final wall thickness its a combination of feel sound and callipers.

Richard: thank you for your comments but trust me- if that wall had of been even it wouldint have split- dont get me wrong if this was beech or ash it would have been fine- this burr oak was full of tension for some reason. I made 18 h/f pieces from this batch of oak and i had two split- the other piece which split healed it self as it dried (which is also strange)

working with wet wood- it must dry evenly/slowly -it must be and even wall thickness.

 

[PAC]

the other piece which split healed it self as it dried (which is also strange)

I've noticed that too. I had an oak bowl that split and then the split disappeared when the bowl warped a little.

 

[mikec]

I also had a similar experience with a wet oak platter about 15" across.

The thinner section at the base of the platter opened up to about 1/4" as it dried quicker than the thicker rim section (which I deliberately left thicker to be able to turn down if it warped!).

After about 6 weeks the split closed up but the platter warped to such a degree that there was no way to turn it without losing the rim.

It is now a "rustic" piece :shock:

Mike C

 

[mrs. sliver]

I had noticed with a few I have done that split, that the bottom had a splits from the pith that looked like they would be trouble, but then, it was a crack that radiated from some of the smallest knots (as I see at the centre of the split on this one) that 'done it in'.
So I watch for the little buggers now, and glue them, inside and out (or turn them out if practical) should any start of a crack appear. :evil:

 

[big soft moose]

nice to see you back paul - how did the wood i gave you work out ? ( i havent done anything with the yew i took in exchange yet)

heh BSM

the pm function is there for a reason why not use it instead of my thread :roll:

Sorry george - consider the wrist slapped.

 

[mark sanger]

Hi George

Great thread. I have to say that I agree with both you and Richard F ( how;s that for sitting on the fence) :lol:

You are being hard on yourself. I love burr oak but damn does it move.

I have found that if it is heavily burred all the way through then it will hardly move but if it is partly burred then I break out in a cold sweat hoping it will turn out right. But isn't that just the best thing of working with it. We are not in control and it keeps us thinking.

But also this is as you say rightly why the wall thickness has to be spot on and I can also see from your cross section that this could have been an issue.

I admire you very much for pushing the boundaries. It was such a lovely piece.

 

[cornucopia]

thanks mark- watch out for those splinters eh :wink: