Hello all,
Probably a silly question but here goes:
Drill a hole into a piece of flat quarter sawn spruce about 1/2″thick by 3′ by 3′.
The hole is 1″ diameter.
The hole was drilled after kiln and or air drying while the moisture content was 13%.
Now the wood is 7%mc. Is the hole larger or smaller?
Thanks.
Replies
The 1" diameter in the same plane as the long grain will remain 1" as near as darn it.
The 1" diameter across the grain will shrink a bit due to moisture loss as the wood goes from 13% MC to 7% MC.
You'll end up with an ellipse with a long and short axis. Just how elliptical is debatable. It depends on the species and the movement characteristics in response to changes in MC. It would probably be hard to see any difference as the change in shape is likely to be very small over only 1" diameter.
Also debatable is how important such a small change from a circle to an ellipse might be. I can't think of too many circumstances where it would be critical. No doubt someone out there is able to suggest critical situations where the change is important, or perhaps unacceptable. Slainte.
Richard Jones Furniture
Thank you for the reply. The idea is a bit confusing to me. Analogy - the nail in the dry piece of wood is relatively loose. The nail gets tighter as the wood takes up moisture and the wood fibers swell. Sounds like just the opposite is true. Intuitively - to me - the idea that wood moves in three dimensions would suggest that the nail hole should get smaller with increasing moisture content. Thanks for the clarification.
Scutch,
It is conceivable that your analogy is also correct. If the nail were driven without benefit of a pilot hole, it will reside in a split, not a hole. The difference is the removal of the wood by the drill, vs separation of fibers by the wedging action of the nail.
Course, as Richard pointed out, this is all occurring on such a small scale, that it might be a challenge for anyone to measure the movement, unless by using one of their infinitely thin plane shavings as a feeler guage ;-)
Edit: To further confuscate the issue-- if the hole is filled with another piece of wood, as a tenon for a chair's stretcher, their different expansion rates as both swell and shrink (or if one begins the union drier than the other), could conceivably result in compression of the cells in the wood of either the tenon, or the sides of the hole, resulting in a loosening of the joint that was previously tight, or a tightening of a joint that was originally loose....
Ray
Edited 3/31/2008 8:23 am ET by joinerswork
It was not a good analogy.
Possibly a better one would be: a precision hole drilled into hard maple, maybe a smooth brass pin about .004" undersize inserted. Would the friction on the pin increase or decrease with an increase in mc?
Scutch,
We're getting close to debating how many angels can dance on the head of a pin here. As Richard stated, and as I understand it, the "sides" of the hole would get farther apart with an increase in moisture; that is, when the width of the wood above and below the hole swells, the hole will get wider.
Of course, if the temperature increased at the same time as that air got wetter, the brass would swell too :-)
I'll countersink the hole, and peen the ends of the brass pin, just to make sure it won't fall out.
Ray
Thank you all very much for your replies.
Actually in my experience, and I have seen it mentioned in several books and articles on building with green wood, nails driven into green wood are almost impossible to pull out later when the wood has dried and shrunk about the nail.John White
That's also related to the nails purposely 'rusting in' John. It's one reason why many pallets and crates are made while the wood still about 20- 23% MC. Slainte.Richard Jones Furniture
I wasn't going to complicate the purity of my argument by mentioning the rust factor but it certainly adds to the nail's holding power. The acids in oaks certainly react with the nails to create a hard crystalline rust that fattens the nail considerably.John W.
I am wondering if this may be a trick question ?
You say a piece of Spruce 3' X 3' ( three feet by three feet )
My thought is that a 3' X 3' piece may be glued up boards of various widths in that case the board with the hole in it say a 6" wide board will not be affected by the other boards .
So perhaps it is irrelevant how wide the stock is , rather how dry as you stated may be the more important detail .
As has been stated in Green chair construction the rungs are typically dry and the posts may still be Green . As the posts shrink the rungs fit becomes more snug .
The bent wood Cafe chairs made by Thonet of France and others have been able to take things a bit further by the exact control of the MC of each leg and the rim of the seat they are bored into . With the correct grain orientation the leg tenon turns to an elongated shape as does the mortise it fits into , the result a tenon that cannot spin and resists loosening .
dusty
Not a trick question. The spruce panels are sitka soundboards for pianos - all of the boards that make up the panel are about 4" wide. It is easy enough to predict and measure wood movement along all three axis in relation to grain for the entire panel. However, it has become a debated qestion about how a hole in the middle of the panel would move with changing mc. I have not been able to make accurate enough measurements using a dial caliper.
Scutch old boy , this should be the worst you have to worry over
I say the % of seasonal movement over the 3' is percentage wise the same
If that 3' sound board moves 1/2" ( sorry standard ) a 3 or 4" wide piece of the same stock will move perhaps one 1/9th of the total , of course that's on paper in a perfect world .
dusty
Th answer is very simple, the hole will move in the exact same way as the plug taken out of it. If the round disc taken out of the hole would expand across the grain with a change in moisture, then the hole will expand across the grain under the same conditions.
John White
"The 1" diameter across the grain will shrink a bit due to moisture loss as the wood goes from 13% MC to 7% MC."
In theory, the hole will get bigger through the cross grain axis and remain the same along the grain.
If wood shrinks as MC is reduced, the the wood will shrink towards its center and away from the hole center on each side along the cross grain axis and the hole will get bigger. Can one measure it, probably not! It will be mute point depending on the project.
Remember, the more a hole is cut the larger it gets. Take away wood matter due to shrinkage and the hole gets bigger.
Ron T says:
"If wood shrinks as MC is reduced, the the wood will shrink towards its center and away from the hole center on each side along the cross grain axis and the hole will get bigger. Can one measure it, probably not! It will be mute point depending on the project. "
This is what my intuition tells me. I have tried to plot this using the onine wood movement calculator http://justwoodworking.com/software/wood_move.php but it is not particularly clear regarding the idea of shrinking toward its center as you have mentioned.
My reasoning for asking this question is: I have always told clients that piano actions become more sluggish in damp weather because wood parts swell and bind at the bearing points. The bearings are holes drilled in hard maple and the parts are joined with a brass center pin - the hole in the pivoting part is lined with felt bushing cloth and the felt acts as a bearing, stabelizing the povoting part and allowing free rotation. Recently this idea has been challenged and the suggestion is that as wood mc increases the hole should expand and it is the felt that swells and causes increased friction and sluggishness. I have been at a loss to give evidence to support my side of the arguement.
I did not want this conversation to diverg into piano actions but thought that I would let you know why I ask. Also, wood movement on this scale is probably in the 2 or 3 thousandths of an inch range - unmeasurable - but certainly relevant in regards to changing friction at a bearing point.
Many thanks.
I don't have a piano handy to check this...
How much free play is there along the axis of the pivot? The wood swells and shrinks in that dimension, too.
-Steve
Hi Steve,
Free play If I understand you correct is measured as grams of friction at the end of the movable flange - it is about 20mm long and ideal friction would be 2 to 3 grams at the end of this lever arm. Friction is adjusted by reaming the felt bushing that the brass pin is inserted into.
Part of the problem is that the part manufacturers do not necessarily pay attention to uniformity in regard to how the holes are drilled with respect to grain orientation. So it could easily be paralell to the grain or tangential, radial or some other variation??
It is easy enough to measure increased friction at the moving flange as rh/mc increases but I am at a loss to explain weather or not it is a result of swelling wood or wool or possibly both. The farther I dig into this(on the piano industry side) the more I find that much is assumed and not really understood.
The wood aspect of this I would really like to understand.
Ron T, "If wood shrinks as MC is reduced, the the wood will shrink towards its center and away from the hole center on each side along the cross grain axis and the hole will get bigger. Can one measure it, probably not! It will be mute point depending on the project."
Scutch, "This is what my intuition tells me."
I don't agree with Ron's diagnosis. The hole is neither separate nor independent of the plank of wood it's bored in. The plank shrinks in the width as it loses moisture, therefore the hole will shrink across the width with the rest of the plank.
If you had two planks butted edge to edge where one half circle on the edge of one plank met and matched another half circle on the edge of the next plank to form a 1" circle that would be different. And it would match Ron's diagnosis. In that case if both planks lost moisture each would shrink away from the centre of the original circle formed. True, you also end up with a gap between each plank, but that's not the point I'm making. Slainte. Richard Jones Furniture
Not being able to look at the design aspects of the application, it is difficult to draw a conclusion.
Based on the additional information provide regarding the pivot point, wood shrinkage may not be the issue at all. Consider the relationship of the felt bearing to the wood. That felt has to be far more forgiving in terms of compressibility relative to wood shrinkage that the shrinkage itself is a non-consideration.
That then leaves the interface of the brass to the felt. Water is a lubricant, therefore, lubricity is changes with a change in RH. It may be wise to look at friction characteristics of the two interfacing materials. How much moisture can the felt absorbe, if any. And how is friction affected between these two materials with change in RH.
RonT
Thanks again for your reply. I tried to upload a pic but do not know if it will work. The felt is very hygroscopic and has an impact on flange friction but it is also much more compressable than the wood. The extent of this interaction is unknown.
I can measure friction resistance of the flange but how wood impacts this is also unknown. It is probably very little as the parts are small but even .001 or .002" will have an effect.
In looking at your photo, my first hunch is, the wood shrinkage/expansion is not your issue. The degree of same is a function of the size of the material and your members are very small.
Hygroscopic by definition is "readily taking up and retaining moisture" . That given then, moisture in a container, in this case the felt medium, will take up space. Hence, will expand the area in the felt zone and will tighten up around the brass pin and effect how easily it moves.
I am not saying this is it, I'm just thinking out loud with you.
I believe that your assessment is correct.
However, my mind will still be searching for the answer as to how the hole changes diameter as the mc changes. Will it expand or contract with increasing mc. I would assume that it is predictable in a similar way that wood movement in general is but appears counterintuitive to consider the hole enlarging as the wood swells.
I have always believed that the hole would contract as the wood swells but this idea is now in doubt.
"However, my mind will still be searching for the answer as to how the hole changes diameter as the mc changes."
Is the question then; why is the piano sluggish or does the pin hole expand/contract with a MC change?
For sure, MC is creating a dimensional change in the wood. Is it enough to create sluggishness, I would think not. I would be looking for other causes!
I think I've got this thing figured out. Take the area of the circle (radius squared times pI), divide this by flux of the board, multiply times the thickness (accounting for humidity), add the warpage factor, and the net result will give you the shrinkage factor, accurate, give or take an inch.
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