Warping of workpiece

[CliveB]

Hello, I have been turning for a couple of years so am not a complete beginner, but am having problems with slight warping of most of my bowls during turning.

A recent example is a piece of apparently well seasoned chestnut 6" diameter x 2" thickness. After the first stage of forming the outside I get the dimensions shown in the schematic;-

Now, I know that 0.3mm might not sound a lot but with a thinnish rim it is noticeable, and sometimes I have had as much as 1mm in larger bowls. Funnily enough, the inner diameter (after later hollowing) comes out pretty close to round.

At first I thought it might be due to residual stresses in the wood, but the bowl has not been hollowed yet. I find it hard to believe that it would be due to drying of the wood when the process only took 15 minutes or so and the wood cut as if it was pretty dry. Perhaps it is due to differences in the machinability of the end grain versus the side grain?

Most of my bowls suffer from this and I wonder if anyone could suggest why I am encountering this and what I could do about it?

Many thanks,

Clive

 

[Linus]

I am no expert but the wood can be overheated whilst turning which will inevitably have an effect on the moisture content and may cause shrinkage. Also how dry is dry? If the dry blank is stored in a damp atmosphere and then taken to a drier location there will be some movement as the wood adjusts to different humidity levels. If it is turned before acclimatising the same will happen. Do you use a moisture meter on your timber?

 

[Paul Hannaby]

The bowl becoming oval when you have turned and sanded just the outside has nothing to do with shrinkage, warping or moisture loss. The cause is excessive sanding. More wood is removed from the side grain sections than from the end grain sections, just like your diagram. The cure is to keep the tools sharp and concentrate on improving your technique so you get cleaner cuts that need less sanding to remove torn grain etc.
Measuring the bowl before and after sanding will confirm if this is the cause.

 

[Linus]

I bow to your vastly superior knowledge on this Paul, but what is unclear is that according to Clive's diagram he is losing dimension on the end grain, or am I missing something?

 

[Phil Pascoe]

It would appear that the circle in the diagramme should be outboard of the elipse?

 

[JohnPW]

I read the diagram with the elipse as the out of round bowl, 0.55mm too wide in the grain direction.

 

[Linus]

Ignore my last. Just me being thick

 

[Phil Pascoe]

I read the diagram with the elipse as the out of round bowl, 0.55mm too wide in the grain direction.

Yes - but it must have shrunk across the grain, it couldn't have grown along it.

 

[CliveB]

Thanks for your replies, my apologies I should have made things clearer. The shrinkage is across the grain.

Paul, I did sand it which may not have helped much, but there was definitely ovality there before sanding although I didn't measure it carefully. I shall take another cut or several with a sharp tool to see what happens, hopefully this afternoon.

Linus - good point about my use of "dry" - completely subjective I admit. I'll look into getting a moisture meter. Is it possible that the turning tool could generate enough heat to dry the timber near to the cut?

Many thanks
Clive

 

[CliveB]

I sharpened up a gouge and took a few careful shallow cuts over the bowl. This gave me a round bowl as you'd expect with a decent finish. A little bit of sanding started to give me some slight ovality again. So thanks Paul for putting me straight on that one.

Clive

 

[Mark Hancock]

IMHO what you are experiencing is to be expected being a combination of factors. One being the wood acclimatizing to the environment it is in and the other being the release of the stresses and tensions in the wood fibers as you cut through them. The change of shape from round to elliptical is the classic movement you get when turning a green straight grain cross grain bowl blank either to a rough form for stabilizing or to finished wall thickness.
Never really thought of the effect of sanding but I generally don't get much torn grain

 

[Paul Hannaby]

One point in the original post was that this is happening before the inside of the bowl is turned so the usual warping due to stress or moisture loss probably wouldn't be significant at that point.

 

[Dalboy]

I agree that sanding can cause this problem, Once you have turned and sanded the outside how long is it before you notice the variation in the size is it immediately after turning the outside or do you turn the outside and return the following day to hollow the inside

 

[CliveB]

Yesterday, after my last post, having re-turned the outside to a satisfactory roundness, I then proceeded to turn the inside of the bowl. Having finished this, I see that the outside of the bowl is now 0.9mm wider on the diameter along the grain whilst the inner diameter is nicely round. The wood is not in the "green" state, it is seasoned but I have no idea of the actual moisture content. All I can think is that this is due to some form of stress relaxation in the wood - does this make sense to anybody?

Regards,

Clive

 

[Mark Hancock]

Yesterday, after my last post, having re-turned the outside to a satisfactory roundness, I then proceeded to turn the inside of the bowl. Having finished this, I see that the outside of the bowl is now 0.9mm wider on the diameter along the grain whilst the inner diameter is nicely round. The wood is not in the "green" state, it is seasoned but I have no idea of the actual moisture content. All I can think is that this is due to some form of stress relaxation in the wood - does this make sense to anybody?

Regards,

Clive

Yes. It's what I referred to previously
Another thing to bear in mind is that wood never stops moving. Simplest example a wooden front doors; loose fit in summer, tight fit in winter.

 

[CliveB]

Hi Mark, Maybe I'm chasing my tail trying to get the perfectly even rim. I did try to form the rim before I hollowed out most of the inside, but clearly that didn't work quite so well.
Clive