Green Bowl Thickness - 10% Rule

[howlingbaboon]

Hi All,

This first Alder bowl has given me so much grief! When I green turned it I went with the 10% of diameter rule of thumb on wall thickness. During final turning I was forced to leave it slightly oval out of fear of turning it too thin and going through the side wall. I've learned that sanding and oval on a lathe just doesn't work well!  I sanded it as best I could but ended up with very thin wall in a couple of opposing spots, to the degree that it flexed when touched like a little wooden bubble. Still intact, I pressed on (unfortunately too literally!). I reverse mounted the bowl in my doughnut chuck to finish the base and, whilst immersed conversation, I tightened the wingnuts. To my horror I looked down and saw that my beloved bowl had crumpled in the 2 thin spots. After a flurry of swearing and frantic loosening I steamed the distorted areas over a kettle and tried to manipulate them back into shape by hand. This worked better than expected and the result is hard to spot by eye but obvious if you handle it.

This experience has left me with a few questions for you good folk:

If you follow it (?), which side of the 10% rule is it best to err on?

Does the degree of ovalling depend on the wall thickness and if so how? and how does this match up to chance of cracking during drying?

Do you ever get bowls that distort so much that they can never be round again? (as I did here)

Appreciate the advice muchly!

[Paul Hannaby]

As you found out, some types of wood move more than others so for some, you need to allow more than 10. I found out the hard way with some holly many years ago!

[howlingbaboon]

We should create a database! Or just a handy table. With everyone's combined knowledge and experience It could be quite comprehensive. I guess it would be difficult because of all the variation in wood and drying environments etc. Maybe there already is such a thing...?

[Les Symonds]

Essentially, the only rule is that are no rules. This wonderful material that we work with is too diverse in its density, moisture content and grain pattern to ever adhere to a rule that we may choose to write. However, there are guidelines!
The 10% concept is a good guideline, but only experience of working a wide variety of timbers will give you the knowledge that you need to estimate the degree of movement that is likely. Straight grained pieces of timber from the bole of a tall, straight tree such as ash, will probably move less than 10%, whilst a crotch of an apple tree might well move by 20%. The moisture content is another factor, as is the species that you are using and, perhaps most importantly, the way in which the bowl is orientated compared to the grain (side-grain bowl, end grain etc).
I for one would be disinclined to put my name to any entry in a list/table/database when there are so many variables to consider.
Meanwhile, my nest suggestion would be to err on the side of caution and do your best to control the drying process.
Les

Les

[crazylegs]

Well volunteered Mr Baboon!!
 A data base for the less experienced would give us a guide.

[seventhdevil]

the trouble with a database like that is that it won't be that accurate.

every piece of wood is different species to species yes but you can have a great amount of variation even within the same tree. i suppose it may aid a few scenario's but isn't that what asking a question can help with?


when teaching me to tenpin bowl my dad used to say to me you don't learn anything unless you do it wrong first...

[howlingbaboon]

Right, as I suspected, far too many variables for a list of any accuracy. I'm all for learning from mistakes as my story suggests, my own and from others however. We build on our own and each other's experience right? I suppose some mistakes are more costly (in time, money, energy etc) and sometimes time saved learning one lesson gives you more time to learn the next. I've seen wood databases which give some generalisations about each wood and it's character but I've not seen one specifically for green turning. The wood database seems very comprehensive (Just made the link Steve!). All we'd need is a section which describes how they typically dry and warp etc. A few brief notes on different species could be handy as long as one realises that there are a lot of variables at play, e.g. UK Grown Eucalyptus - Warps like crazy, Alder - Prone to mould, Cherry - Prone to checking (not saying any of these are accurate of even correct just an e.g.) No point reinventing the wheel when you can tweak it eh!:)

Les are you suggesting that to err on the side of caution is to turn them thicker rather than thinner then?

I've still not entirely got this (please be patient ) Can anyone help me out with this one?

Does the degree of ovalling depend on the wall thickness and if so how? and how does this match up to chance of cracking during drying?

The idea of finishing green and just embracing the warp is getting more appealing!

[Les Symonds]

Les are you suggesting that to err on the side of caution is to turn them thicker rather than thinner then?

Does the degree of ovalling depend on the wall thickness and if so how? and how does this match up to chance of cracking during drying?

Yep....err on the side of caution by leaving a thicker wall and allowing longer for it to dry

As for the degree to which a piece becomes oval, it isn't determined by wall thickness. Take a typical, side grain bowl blank and look at the grain. When this dries it hardly shrinks along the grain at all, but shrinkage across the grain can be considerable, so what started off as a circle becomes an oval.

Les

[howlingbaboon]

ah OK, cheers for clearing that up Les. That's where I've gone wrong, I thought, for some reason that a thinner wall would oval less. I hope that all these dried bowls I've got stacked up are not all too thin! Eek! I'm happy for them to take a bit longer drying, and will green turn a little chunkier in the future.

Another glance at the Wood Database and it seems like what we are seeking is already there in the shrinkage data. It's just going to take me a while to gain enough experience to interpret the numbers I guess.

[seventhdevil]

it is there in the shrinkage data and you have to read between the lines somewhat to realise how the shrinkage ratios can affect the timber you are drying.

eric accumulated all the data on the wood database so if you contact him he may be able to help you further if you can't find the answers you are after through the normal Q&A's.


in theory the timber with the lowest shrinkage ration should be the most stable but it won't always be the case simply because these figures are all obtained from averages and different pieces of  timber can be specifically different from others in the same species.

most of the time you can ignore the tangential shrinkage as it usually doesn't affect turners with the lengths we use but the radial shrinkage can be very high and therefore can really ruin your day when you find a very oval bowl in your pile.

the worst one i've come across to date is whitebeam and i had an 11" shallow bowl that lost at least 1 1/2" in width meaning i nearly lost the bits i'd rough turned but i was able to rescue them, still it was a wake up call reminding me that even though i probably know the most about timber and their properties out of the members on this forum i still have plenty more to learn about the behaviour of some species.


as i said, we learn by our experiences and mistakes and sometimes you just have to make them...

[Lazurus]

I go about green wood in two different ways depending on the job in hand, either finish it completely with thin walls and let nature have its way, or leave at 1" thick and turn when dry (storage in a bag with shavings) whilst I have lost one or two general the 1" leaves enough to turn out the "oval".
As many have said it is experience and trial and error, natural edge with thin walls left to dry can produce fantastic warping on some species, if I want to encourage the shaping a few seconds in the microwave and the wood is very pliable and can be "formed" by hand. Never a mistake..... Just a design opportunity!!

[howlingbaboon]

yeah, I've gone by leaving around an inch on everything so far as it's easier to eyeball and I'd figured it was close enough. That included the bowl in question. I've measured all of my other dry rough outs including the other Alder bowls and they should all true up fine without being dangerously thin.

I'm wondering if I've somehow perpetuated or even accentuated the ovalness of this bowl. Is it possible that I could've somehow been cutting oval if my tool was going in and out with each rotation ( or at least some of them ) or is that just crazy talk. I suppose this could be like when turning air on a NE bowl, and falling into the "holes". Does that make any sense?

I had wondered at the time if the wood was flexing and deflecting away from the tool as I had a very tough time final turning it. As I say, I had to leave it oval in the end.

[BrianH]

when the tool goes in and out with the ovality (is that really a word???) the natural tendency is to press harder towards the spinning wood. WRONG.... press the tool down onto the rest instead.
Brian

[Derek]

Also something worth thinking about is a 9" bowl may not shrink much in the direction of the grain but say shrink by 1/2" across the width but a larger bowl of the same wood will appear to have shrunk a lot more so for larger bowl blanks leave them a little thicker that smaller ones.

The minimum that I rough turn is 1" and going up from that for larger pieces or different woods that I know have a tendency to shrink more, learnt from experience. So far I have been ery lucky not to have lost any through being oval due shrinkage but have due to splitting

[bodrighywood]

Also don't assume that once it reaches a certain moisture content that is it, that it won't move again. Different woods can take on and lose moisture quicker than others and so if the atmospheric humidity changes they can move again.

Pete