Stress when turning

[Baldhead]

First off, go easy guys I'm not a turner and I don't have a lathe!

I was resawing some timber the other day and a couple of pieces ended up banana shaped, probably due to stress? Anyway it got me thinking, if you have a large blank and turn say a dish, is there a chance of the finished product distorting due to stresses built up in the wood, if yes how do you overcome it?

Stew

 

[Random Orbital Bob]

happens all the time Stew. In fact warping bowls is one of the primary pre-occupations of face grain wood turning ie how to avoid it.

The reason is simple, problems associated with moisture loss in wood are magnified by turning blanks because typically they're thicker than planked boards and therefore take longer (much longer) to dry.

The long term solution is to cut them carefully off the log (avoiding the pith) and then seal any end grain with either wax or pva or similar and let them air dry for many years. The shorter term solution is to rough turn them until you have much less material (notionally about 10% of the final desired wall thickness) and then package them up with wet shavings for a few months so they dry slowly enough to not crack. Then remount and finish turn them after the moisture loss is complete

This link from Chas's library of links has a couple of relevant links worth reading.

 

[Random Orbital Bob]

there is a specific form of turning green bowls where the very movement we're discussing is part of the design of the finished product. Thin walled bowls (so thin you can see light through them) are sometimes positively encouraged to warp to render interesting "organic" shapes as part of the art. Most of my early pieces did that without me even trying!

 

[CHJ]

Every bowl you turn is going to distort out of round as you turn and in most instances enough to prevent you going back to a previously finished surface to lightly skim it, in fact will continue to change it's ovoid dimensions during the year as humidity in the environment changes.

In most instances the odd millimetre or fraction thereof will not be visible or of any consequence but if it's an item with a lid then both components will move about and design of interface needs to take account of this as the lid may be placed back 90 deg. at odds with the body as far as grain orientation is concerned.

If making thin platters then the wood as Rob says needs to be as dry as possible and from stock that has dried out in a thickness as near the pieces finished dimensions, even then care is needed to take off material in equal proportions on each face to limit any curl, design of base configuration can be usefully employed to provide a three point contact if curl is anticipated and then any curl will be within the ability of the eye to discern and still sit soundly on the surface.

 

[Baldhead]

Bob, Chas thanks for the info, it makes me even more appreciative of turned bowls etc, as I said I haven't got a lathe as such, I do have a small drill powered lathe type thing which o bought years ago, I think I'll stick to more general type woodworking, I'll leave the lathe work to the experts.

Stew

 

[Random Orbital Bob]

I suspect you'd really enjoy turning Stew. It's ludicrously addictive once you get going

 

[Lons]

I suspect you'd really enjoy turning Stew. It's ludicrously addictive once you get going

If you want a taster just give me a bell Stew and call for a coffee.
I assume you got to the new Axi store at the weekend. I bit the bullet and bought a new lathe (axminster 1416vs) as I want to upsize from my little Jet lathe. Sue "allowed" it without a fuss so I guess it's going to cost me dearly somewhere down the line :lol:

cheers
Bob