I am building an oak box (1/2 inch stock) that will be about 5 1/2 inches high, 9 or 10 inches long and 5 inches wide or deep (from front to back). The lid will be a piece of 1/4 inch oak stained a different color. It will be sort of inlaid, that is it will rest in a rabbet cut around the sides and top of the box. The inlaid piece of 1/4 inch oak will be about 4 1/2 inches wide by 9 inches long. My question is “do I need to account for seasonal movement of this 1/4 inch inlay or can I just glue it tightly in the rabbet? The 4 1/2 inch dimension is across grain where I would expect the movement to occur. (I hope that I have explained this so that it is understandable). Thanks from this beginning woodworker, for any help.
Frank
Replies
Yes, you will have to account for wood movement in this situation. It won't be much across a 4-1/2" span, but it could be enough to cause the top to split. I'd recommend that you install a 1/8" rabbit on the inside of the box that is 1/8" down from the top. Then rabbit the top edges of the top panel to create a 1/8" tongue on all four edges to fit in the grooves in the box. This will result in a flat top with the 1/4" panel allowed to float. DON'T glue the panel in.
Oh there we go again Jon! In and out the same entrance via the revolving door, and nary a sight of each other, ha, ha. I like your tongue and groove solution better, but Frank is new to woodworking, so didn't want to suggest it. Slainte.RJFurniture
Thanks for youq quick responses. You have been very helpful. I don't know which suggestion I will take but I'm sure it will be one of them.
Frank
Jon, If I use your suggestion (tongue and groove) it seems there will need to be a slight gap between the edges of the panel and the sides of the box to allow for the movement. (at least if I want to keep the top flat.) How much of a gap do you think will be necessary?
Frank
Frank, about 0.75 mm either side, or a slightly fat 1/32" or very shy 1/16" should be enough. Slainte.RJFurniture
Frank, I think you can go with Sgian's calculations. They sound about right...I'd give you a more definitive answer, but I'd have to find my algebra book and spend the weekend catching up with Sgian's intuitive sense for math.
Seriously, the gap will be very small and easily hidden by giving the meeting edges a slight bevel. In other words, hide the gap in a v-groove. The fact of the matter is, only the front and back gaps are functionally necessary. Because the grain of the top panel and the front and back of the box will be parallel...and wood hardly shrinks at all in terms of its length, anyway...the only movement you have to be concerned about is where the top panel joins the ends. With changes in humidity, the top panel will want to expand and contract in width, while the ends will remain static in terms of their length...so, it will be the front and back gaps that open and close as the wood's moisture content fluctuates.
While the objective is to allow the top panel to "float", you don't have to live with a box top that rattles. If you put just a drop of glue in the center of the grooves in the ends of the box, it will lock the panel in place so it can't slip around. This will also ensure that the expansion and contraction will be shared equally by the front and back gaps and thus be less noticeable.
...And one final note on this subject. When the width of the panel is narrow enough to make the issue of expansion and contraction a borderline risk (as it is in your case where the panel is only 4-1/2" wide), you can sometimes reduce the risk by putting the panel in a more arid environment for a few days before you fit it to the box. This will ensure that the initial stress relationship is one of compression, meaning the panel will want to expand into the frame that is confining it. A compression relationship does put additional stress on the joinery holding the frame together, but this typically isn't a problem. The bigger risk is that the panel is more likely to fail (split) if it is put into a tension relationship across its grain...So, said simply, compression stress is always preferable to tension stress in borderline situations.
Funnily enough Jon, I was just called in on a consultancy basis to advise on a set of oak tables, maybe only 18 months old that were exhibiting splits in the grain length. When they were described to me I had a shrewd idea what was going on before I even went to look at them.
They turned out to be constructed much as I'd expected. Long tops about 33" wide completely trapped in an enclosing framework glued and biscuited solidly with no allowance for shrinkage and expansion. Here in the UK wood in service in houses and so on can be expected to read in a range of approximately 7% to 11% MC, with perhaps a percentage or two greater in either direction in sustained arid or humid conditions.
Anyway, I calculated that the top could be expected to vary in width by approximately 8- 10 mm over the seasons-- that's about 3/8" for those of a metrically challenged mind. No wonder the oak had split both on the join lines and along grain and the mitred and butted corner joints of the encasing frames were starting to show signs of failure too. There is enough table perimeter length for about 60 diners, so you can perhaps imagine how many table tops there are. Slainte.RJFurniture
An long time ago friend of mine made high dollar pistol cases and he stablized all the wood that went into his cases with PEG. It involves soaking the pieces in the stuff then letting them dry.
Can someone explain what PEG is? I've never used it.
PlaneWood by Mike_in_Katy (maker of fine sawdust!)PlaneWood
Planewood, it's a chemical compound called Poly Ethylene Glycol (PEG). Basically, it impregnates the wood with plastic, which locks the fibers in place so that the wood can't expand and contract with humidity.
I've not seen any data on the use of PEG to stabilize wood. First, PEG is not plastic like what we typically think of plastic.....it isn't like the plastic you think of as in plastic bags or plastic toys or plastic in your keyboard....ie, it is not a thermoplastic.
PEG is a polymer ...hence the poly ethylene glycol. Does it penetrate into the wood? Perhaps. It is a bit puzzling to think it stabilizes wood....and I did think it is used primarily on GREEN lumber....and keeps green lumber from drying and so keeps it stable....but that is just IIRC.
PEG is hygroscopic....if you leave some out exposed to the air, it will absorb moisture out from the air....Tim
Could be, Tim. I'm not a chemist, so I don't know exactly how PEG reacts with the wood. It does stabilize it, preventing dimensional change (distortion).
If it does so because it is hygroscopic and therefore causes the wood to maintain a higher than normal Equalibrium Moisture Content, then it must also sterilize it. Otherwise, it would promote decay...and I haven't heard any reports that that's the case.
Personally, I don't use the stuff. I've seen pieces that have been PEGed and it gives the wood all the charm of formica. Makes it look fake.
You ought to allow for a bit of movement Frank. Oak is classified as having medium movement in service, i.e., between 3% and 4.5% when it dries from 20% MC to 12% MC (World Woods in Colour, William A Lincoln.) If you start with a piece of oak 100 mm wide (about 4") at 20% MC and it dries to 12% MC it can be expected to shrink by between 3 mm to 4.5 mm, about 1/8" to 5/16"
Let's say your wood is seasoned and it will range somewhere between 8% and 12% MC in service with 10% MC as your starting point, you can expect a little movement, say about 1% to 1.5%. A 4-1/2" initial width equals about 115 mm, so it might get wider by 0.75% and get narrower by the same 0.75%. Expect a movement range about 1 mm (3/128") either side of your initial starting width-- that's 2 mm overall or about 3/64".
It'll move, but how you allow for this small movement is up to you. Maybe you shouldn't make it too good a fit in the rebate at either edge of the width and use PVA glue, a renowned creeper, to glue it in? Slainte.
RJFurniture
Edited 5/8/2004 12:57 am ET by Sgian Dubh
1mm (3/128) 2mm (3/64) Sgian I hope this is a typo error or maybe one too may brews. 1mm is .040". 3/64" is .047" Somehow ya managed to cut things about in half. Did you mean 5/128 (.039) and 5/64 (.078)? Maybe it's something to do with the different time zones. You are in a metric time zone now aren't you? ;>)
RichThe Professional Termite
Yeah, I'm in a metric time zone, trialnut, and was definitely short of a brewski or five at the time of day I posted. I shouldn't bother trying to translate metric into fractions of inches for the metrically challenged in my head. I was never any darned good at fractions anyway, ha, ha.
Using my reliable Construction Master calculator, I see that 1 mm is actually close to 3/64"and 2 mm is close to 5/64" and 1.5 mm is close to 1/16" which is not really a surprise as 3 mm is pretty close to 1/8". That's enough conversions for me for one day.
So you are correct, I about halved the values,....................................... and then you threw me completely for a loop by introducing that darned decimal inch thing.
Jings ma boab! Isn't the conversion from fractions to millimetres complex enough for you guys that work in inches and other antideluvian(sp?) units of measure without tossing that one in too? Slainte.RJFurniture
>> antideluvian(sp?)
Close, very close. Antedeluvian. Before the Flood.
I knew I was close Dunc, but was too lazy to check with a dictionary in my hastily typed reply-- anti, ante. Close, but far enough apart.
Cubits anyone, or royal cubits? Slainte.RJFurniture
'Antedeluvian'?
Close ... it's 'antediluvian'
;-)
Main Entry: an·te·di·lu·vi·an View ImagePronunciation: "an-ti-d&-'lü-vE-&n, -(")dI-Function: adjectiveEtymology: ante- + Latin diluvium flood -- more at DELUGE1 : of or relating to the period before the flood described in the Bible2 : made, evolved, or developed a long time ago <an antediluvian automobile>- antediluvian noun
Clay
Hey now. I didn't invite corrections to my spelling. That was Richard.
I hang my head, et cetera, et cetera.
Edited 5/11/2004 12:32 am ET by Uncle Dunc
Several years ago I glued up a four foot section of plain sawn red oak cutoffs from a table top I made and measured the change in dimension over a couple of years. While the MC of the wood (in my shop) varied from 5% to 13% (probably more than you would see in a living space) the four foot dimension changed 1 5/16 inches or approximately 5/16 inch per foot. Hope this Helps. Art
Hey Art -
According to my tables, Red Oak has a Tangential Coefficient of Expansion (Ct) of 0.00369. So, changing from 5% to 13% humidity a 48" glue-up should expand and contract as much as 1.41". So, your right on!
48(.00369(13-8)) = 1.41
Cr is 0.158 (Radial Coefficient of Expansion)
Is it feasible to soak an entire table top in PEG?
PlaneWood by Mike_in_Katy (maker of fine sawdust!)PlaneWood
Quite some time ago, in methods of work I think, someone described how to make a moisture guage using a section of edge glued cutoffs. I have used my four foot section to make such a guage and now use it to determine where I am in the cycle when making raised panel doors. Beats guessing. Art
How exactly do you use that information? Do you size your panels differently depending on the MC?
Planewood - you can also use penetrating (thinned) epoxy to stabilize wood and reduce movement. Expensive, though. "Everything should be made as simple as possible, but no simpler." A. Einstein
http://www.albionworks.net
Yes, but unless it's a really wide panel or I am at the extremes of the MC cycle, the adjustment is usually minimal. Art
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