How rapidly should a blank be turned

[thelastgizmo]

Not in RPM. But time elapsed from acquisition of the blank to the finished article. I have been turning a 15 inch by 2 1/2 inch oak blank that I had bought from an online retailer. I turned the base after a few days and finished it with hard wax oil a few days later. I have been leaving it in a cool room indoors for the finish to dry. I have noticed now that several checks have developed at the end grain. I contacted the retailer who stated that the bowl should have been turned immediately upon receipt of the blank, finished and the inside turned and finished immediately after that. This seems unreasonable to me and not what I remember from my experiences of turning when I did it as a hobby several years ago (recently retired so have time again for wood!). The blank was kiln dried and "ready for the lathe" according to the retailer. I'm just wondering whether their statement is correct and that I have to complete a project within a day and not use finishes that take several coats that each take four hours to dry.

 

[CHJ]

....The blank was kiln dried and "ready for the lathe" according to the retailer....

Kiln Dried? to what moisture level?
Was the blank waxed around the periphery? if so then the producer was aware that there was an end grain split risk, I doubt you will ever see a turning blank for sale that has not been edge waxed or sealed in some way, it's virtually impossible to cut through a reasonably thick slab of wood and not experience end grain splitting if you leave it exposed.


On the turning timescale, not completing the outer and inner profiling in one session is always a risk.

Internal wood stresses and moisture equalisation are bound to cause movement, even turning a complete bowl in one session needs caution to complete the rim area and move on through the form at final thickness if you are to minimise problems from the piece going oval.

Going back to attempt to rework a thin rim after you have removed the bulk of a piece is asking for trouble unless approached with experience and appreciation of the fact that it is most unlikely to be a true circle any longer.


If it's unavoidable that you leave a partially turned item on the lathe for any reason it's almost essential that you wrap it in cling film or similar to prevent surface moisture loss between sessions, the closer it is to finished dimensions the more important this is.

I personally never start a single piece wood turning without the full intention of completing it in that session. My expectation being that it's more than likely to be a failure if not completed in one go.

Any slow curing finish such as a polymerising oil, or any sealant for that matter, should be applied all over, not just to outer or inner, to equalise any moisture loss over the whole surface.
Delays or timing of subsequent coats is not a factor other than observing suppliers recommendations on re-coat times to get correct bonding and curing characteristics.

 

[thelastgizmo]

Thanks for the informative reply Chas. The moisture content on delivery was about 9 percent (measured on the periphery). It's actually gone up a bit now and is at 10 percent. On my meter anyway. Blank was not waxed although website says it should have been.

I've only turned the bottom of the bowl (see my previous post regarding chucking). I should say I've only removed about 10-15 % of the wood.

With regards to turning the whole project in one go, other commitments meant I did not have a day or two continuous time in one lump. If I had realised that that was what was required then I would have planned accordingly. Next time I will take precautions as you outlined.

A very informative reply which I will take note of in the future. A bit of an expensive lesson though!!

 

[eddieperkins]

Generally two options I think. Either turn and finish in one session or rough turn keeping the bowl an even thickness then finish later.

The second option is usually used with green wood then the blank is dried for 6 month plus then finished.

Unfortunately wood will move as the moisture content changes regardless.

Are you sure the checks were not as a result of heavy handed sanding, this is not uncommon.

In any event all is not lost, you can fill the cracks with CA glue and saw dust, resin ( can be coloured) or metal power and CA glue or change the design.

Keep going with it, it may evolve into something really special.

 

[thelastgizmo]

Eddie, the checks have developed on the unturned side of the blank (see images) so not due to overzealous sanding. My original design has a "skirt" around the edge so the top of the rim will be sunken so I can lose some of the checks. I am hoping that I can work round them and disguise any remaining ones.

 

[eddieperkins]

I would flood them with thin CA glue before continuing, they should then be stable and virtually invisible once finished

Good luck

 

[thelastgizmo]

Thanks Eddie. I'll do that. I see you're not too far from me (20 miles). Do you attend any local turning club?

 

[RobinBHM]

Oak is quite weak in shear and is prone to surface checking and those deep hairline cracks.

where do blank suppliers obtain their material? - I wonder because boards of timber often have splits and checking, we often have to lose a foot off the end of a board, I just wondered if turning blanks are board ends rather than centre of boards. Im not suggesting this is the case, just interested to know the source of the material.

 

[thelastgizmo]

I'm reading that sanding sealer should be applied before the CA glue as it could stain. I'm finishing with hard wax oil. Will I have problems with this over small areas of sanding sealer?

 

[CHJ]

I'm reading that sanding sealer should be applied before the CA glue as it could stain.

Staining only a problem on a finished surface or one very nearly finished where bleed stain is not likely to be removed.

TIP:- If you have figured wood such a Yew or a Burr, when flooding split or weak areas quickly wipe the glue residue across the piece to spread the 'stain' this will give you a colour blend indistinguishable from the base wood figuring, just take care with heat on cloth used to spread it and keep fingers clear.

I'm finishing with hard wax oil. Will I have problems with this over small areas of sanding sealer?

Yes, finishing oils need to penetrate the wood pores, applying over sealed wood is likely to result in a blotchy surface colouring because of uneven or none existent penetration.

Be aware that surface CA will have the same sealing effect on subsequent oil coatings.