I have a 10 foot long board that I have run through the planer thicknesser. It's 2 inches thick and 14 inches wide. There is around 2mm twist at one end, and 3mm bow near the centre. Obviously I'd love it perfectly flat but I'm wondering can I get away with that small amount of variation? My plan is to screw it down to the the base at 8 points from underneath, then to biscuit it 4/5 times to the adjoining board before glue. If I sit on the board, or push it strongly with my hand, I can hold it down flat, so there's not a huge amount of pressure to fight.
How much twist is acceptable?
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custard
Established Member
In some circumstances that'd be just acceptable in my workshop, a lot would depend on the state of the board that you're jointing to. But if it's a table top that you're making, then 50mm is awfully thick given that 45mm would yield a perfectly flat board while still having an extremely chunky appearance, so on balance I'd machine the problem away.
Good luck!
Good luck!
AJB Temple
Finely figured
For me it would depend on the style I am making. For example I have a large oak table that is 70mm thick and is designed to be surface unfinished and quite rustic in a kitchen. It is about 2.5 metres long and that amount of twist and bow would not be noticeable and not be an issue even if it could be seen. However, if I were doing a fine finish I would want to operate to lower tolerances. Also I would be a bit concerned that twist and bow will get worse so I would acclimatise the board properly before doing anything else.
Also I have a 3 inch thick 16th century oak table that is about 14 feet long. It is three planks wide and despite having been used and abused for a few hundred years and has got knocks and gaps and repairs - it still looks great. Wood is quite an organic material!
Also I have a 3 inch thick 16th century oak table that is about 14 feet long. It is three planks wide and despite having been used and abused for a few hundred years and has got knocks and gaps and repairs - it still looks great. Wood is quite an organic material!
bugbear
Established Member
AJB Temple":2yv6jkb5 said:
Topic drift - how do you clean such a table? Quite important in a kitchen!
BugBear
Yes this is a concern, but the timber is kiln dried, has acclimatised in the room for 6 months and the underfloor heating keeps the temp at 19 degrees all year round, so I'm hoping movement will be minimal. Am I right?PAC1":27vdh6f0 said:
custard":84fdlfdi said:
There are 4 similar boards and I will have the same problem with all 4. My problem is I don't have a machine to solve this. I don't even have winding sticks at the moment. Elbow injury rules out hand planes. I have a cheap electric planer but it is a rough tool.
RobinBHM
Established Member
To me a 2mm twist and a 3mm bow on a board 3 metres long x 350mm wide is quite small. It would require a quite accurate reference surface to prove that the out of true measurements are the board.
It certainly isnt going to be easy to flatten on a planer thicknesser.
It certainly isnt going to be easy to flatten on a planer thicknesser.
AJB Temple
Finely figured
bug bear. It is actually in the breakfast area adjacent to the kitchen. We clean it with bleach actually. I had it made as I didn't have a suitable workshop at the time.
Helvetica":34yzzgen said:
In your favour is a constant temperature and it has acclimatised. Against it is 14 inches of real wood. The problem is that across that 14" the movement could easily be 1/2". No amount of screws, glue or biscuits will stop it from moving if it decides to go. You need to design something that will hold it in place but allow the width to expand and contract as it wishes.
Sgian Dubh
Established Member
That amount of movement in service is, I suspect, a bit over the top, even in the species American beech (Fagus grandifolia). I picked this species because it has one of the highest movement factors I'm aware of at 11.9% tangentially and 5.5% radially. Perhaps in circumstances where a 14" wide board of this stuff goes from green or above FSP (30% MC) to oven dry (0% MC) it might in theory shrink by about 1-1/2", but in reality this is unlikely.PAC1":30t7i5qy said:
Here we're surely talking about something that might in the extreme vary in MC from about 14% to 8% if used in a habitable building. Given that, my calculations indicate you're more likely to theoretically experience a change in dimensional width in this species across the seasons of about 1/3". In practice, the results of such calculations are generally overstated, and the actual dimensional change in service tends to be rather less than predicted. And we're not even sure the timber under discussion is American beech because I don't think Helvetica has specified the species - it could be one of the walnuts for all we know, a genus that is relatively dimensionally stable compared to American beech.
The main reasons for the discrepancy between calculations predicting wood movement and actual wood movement in service primarily relate to factors such as the effectiveness, or ineffectiveness of the barrier created by a polish, hysteresis effects in dried wood movement after seasoning, and the non-linear nature of wood shrinkage/expansion and cell distortion as it adsorbs or desorbs water vapour between FSP and oven dry. Slainte.
Richard, I admit it was a Friday night guesstimate rather than a calculation but the amount is not the issue it is the fact that the hold down system has to allow significant lateral movement. A further factor is that wood seems to react differently to underfloor heating (constant warmth) than radiators. This might reduce the movement yet further but I have not seen any research on the effect of UFH.
Sgian Dubh
Established Member
True, expansion and contraction is an issue, and slot screws, buttons, expansion plates, etc could all be used to take care of that.PAC1":1rt6nft5 said:
I can't comment on underfloor heating either, but unvarying conditions of relative humidity and temperature would of course eventually lead to wood reaching equilibrium moisture content (EMC), at which point it would cease moving. Such unvarying conditions never exist in the everyday world, except perhaps in some museum storage facilities or scientific research centres, and there are always seasonal variations in RH and temperature to account for in almost every habitable building. Typically, wood varies between about 8% MC and 13% MC in most habitable building with efficient climate controls systems in place, e.g., heating, cooling, insulation, etc. Slainte.PAC1":1rt6nft5 said:
I can't believe I didn't mention that the wood is mahogany. I don't know the genus but it's genuine. I understand mahogany to be extremely dimensionally stable, but where could I find some reliable data on this? Thanks all, food for thought.
Sgian Dubh
Established Member
Mahoganys are as you say some of the most dimensionally stable species. Shrinkage factors from FSP (~30% MC) to oven dry (0% MC) for your reference are:Helvetica":3s9qfy5e said:
American mahogany (Swietenia macrophylla): Tangential, 5.1%. Radial 3.7%.
African mahogany (Khaya ivorensis): Tangential, 4.5%. Radial 2.5%.
You might find further information of that nature at a site called the Shrinkulator. Slainte.
*Edit* I thought later to give you an idea of the sort of movement likely in a 14" (~350 mm) wide tangentially sawn American mahogany board. My calculations indicate, using a seasonal wood MC range of between 8% and 13%, a change in width of just under 3 mm. In practice, because all calculators overestimate dimensional change caused by moisture adsorption/desorption, it's likely this 14" (350 mm) wide board will only change in width by somewhere between 1.5 and 2 mm. The overestimation is caused by the fact that calculators generally assume a linear dimensional change as wood dries and gets wetter in what I call the Shrinkage Zone, i.e., FSP (~30% MC) down to oven dry (0% MC), whereas in reality shrinkage is non-linear.
