Bowls from rough cut timber cupping

[Renoj]

When making blanks from rough cut timber is it best to leave them to settle before turning or use them straight away?

At the moment I use them as soon as they are cut and I’m getting quite a lot of cupping on the bowls.

I’m not finishing them straight away if that makes a difference.

Thank you in advance.

 

[CHJ]

If you are turning 'green' wood and wish to obtain a symmetrical finished item with minimum distortion then you need to turn them with a wall thickness about 10% of intended finished size. (so 10" bowl with 1" thick Walls and Base) and put it on one side for several weeks until it has dried out and finished distorting. (when it no longer looses weight) and hope it has not split, wrapping in newspaper or placing in a paper potato bag may help reduce splitting risk.

Then remount it on the lathe and finish to size.

There are a few notes here on storing green wood in the Help stickies
It's not unusual for stored logs and blanks to take several years to dry out sufficiently, the art is to get a feel for which method and how and where to store it that is likely to give you the least waste from splits etc.

 

[Renoj]

Should have said that it’s kiln dried. Oak, maple and beech.

 

[CHJ]

Should have said that it’s kiln dried. Oak, maple and beech.

Then if it's properly dried you should not be getting excessive movement.

It sounds as though there is a lot of stress relieving going on.

Are you cutting round blanks from slabbed stock or purchasing ready cut round blanks.

 

[Renoj]

Thanks for the reply.

Slabbed stock then cut on a bandsaw.

 

[CHJ]

Thanks for the reply.

Slabbed stock then cut on a bandsaw.

Then if you are getting significant distortion after turning I would put them on one side for a few weeks once in the round to see if they adjust to local humidity or stresses release (maybe go oval?)

in my experience the odd mm out of round once finished turning is not unusual in fact almost the norm but I would not expect excessive cupping.

 

[Renoj]

That’s super helpful thank you. Good to know a couple of mm is expected. Will try leaving as blanks and see how they get on.

 

[Dalboy]

Something worth noting is that even kiln-dried wood when stored outside or in a shed where there is no heating can still take on water only a small amount, then when moved into the house with central heating will dry and move. When I make flatwood items I tend to bring the wood in the house for a couple of weeks to allow any movement before working it.

 

[Renoj]

I was thinking temperature may have been a factor a couple of weeks ago so insulated the workshop and have an oil heater on it’s lowest setting all the time, guess it’s just a cost that has to be factored in.

 

[Jonzjob]

Oil? Paraffin?

That gives off a fair amount of water when burned.

 

[Renoj]

For anyone coming to the thread via search with the same issue.

I cut out a set of blanks and left them in the same room as the turned pieces and they didn’t move but the turned pieces 40mm x 300D cupped by about 15mm!

The humidity/temp is good and to answer previous post it’s a electric heater so no humidity there.

The possible and potentially all the reasons may be:

- Tension in the kiln drying process the wood measured at 6%
- I realised I was being pretty heavy handed with the sanding and speed which may have generated heat on one side so dried it out more than the other. It measures 0% on my cheapo meter.
- The most issues I had where the beech blanks which as a timber I believe has a lot of movement.
- All my beech is crown cut and by the looks of it fairly far out from the centre.

Solutions:
- I haven’t sealed any of the pieces yet which I have read may be a solution next time round to get them finished ASAP to slow moisture movement.
- As my pieces where all flat bottomed ( tray like) I carefully ran them over the planer which seems to have sorted it but time will tell.
- More gentle and slower speed when sanding on the lathe in the future.
- Choose my timber at the yard in future and try to get the planks with less of a radial pattern on the end grain.

 

[CHJ]

….
- As my pieces where all flat bottomed ( tray like) I carefully ran them over the planer which seems to have sorted it but time will tell.
- More gentle and slower speed when sanding on the lathe in the future.
....

Avoid Flat Bottom aim for a rim (slightly hollow base) gives you a better chance of it sitting level.
If you have a species or batch that always gives you problems lightly sand the base rim to give you three small equally spaced contact areas. (just a 1/4 to 1/2 mm is enough)

Doubt very much that your sanding influenced the final wood movement, (but sanding should not generate heat in the surface if the abrasive is cutting as it should.)