What is dimensional stability?

[John]

I have come across this term in descriptions of wood. Does it refer to changes with humidity, or something else?

[Bay Area Dave]

if you Google on "wood stability" you'll come up with pdf files and other web site information. as wood absorbs moisture it gets wider and contracts as it loses moisture. if one side of a board absorbs more than the other side it is gonna bow. (the dry side "cups") and there are other issues.

plain sawn boards are less stable than quarter sawn. and so on and so on.

dave

[Swingman]

A very much simplified explanation:

Wood is hydroscopic in nature and almost always contains water to some extent, more as a growing plant, less as a dead plank. Wood fibers exchange moisture with the surrounding air until an equilibrium is reached between the air and the wood. These fibers, being oriented lengthwise for the most part, expand across their width to a greater extent than they do length when they absorb water. When moisture content is high, once again generally caused by the relative humidity of the surroundings, and depending upon species and how it was cut from the log, wood will swell across its grain, sometimes as much as 1/4 ", or more, in a foot.

Conversely wood will shrink for the same reason when exposed to low relative humidity for any length of time.

When building furniture, or anything made of wood or wood products, you must take this usually seasonal change in humidity, and subsequent dimensional instability, into account.

Much of the art and science of joinery deals with the propensity of wood to be dimensionally unstable due to the relative humidity of its surroundings.

[Larry Capelis]

There was a great article in fine woodworking last month I think on the cellular reasons for dimensional instability . .please don't call me on this . .oh brother I gotta get over this head cold.

[Lee Michaels]

It is a science fiction term.

It has to do with being stable enough that you don't slip into other parts of time and space.

[George]

Where wood is always the subject.

Swingman's description is right on. The parts of the wood which expand most are those generally lighter colored, softer earlywood areas. Broader the earlywood areas on the visible surface, more the wood can expand. Latewood, the dense stuff, much less.

[Andy Dingley]

Changes with humidity. There's some change with temperature, but this is trivial and can be ignored.

Moisture related changes are really important for serious woodworking. If you're interested in timber drying and movement, read Bruce Hoadley's "Understanding Wood"

the US Forest Products handbook that George posted the link to. Lee Valley also sell a nice printed copy of this.

Wood is full of water. Freshly cut oak might be 80% moisture NB - as this is quoted as relative to the _dry_ weight, then this means that only about 45% of the log's weight is water, not 80%. Some softwoods are even wetter - maybe 120% when freshly cut.

This water is in two places within the timber; in the large spaces inside the vessels, and inside the cells themselves. Drying it down to about 35% water (or simply squeezing it !) removes the water from the large vessels first. The timber doesn't move during this process. Further drying (which needs time, although warmth and vacuum can speed it up) can remove (most of) the rest of this water from within the cells. During this process, the timber shrinks.

Looking at the moisture content for timber, the concept of EMC (equilibrium moisture content) is very important. Leave a piece of timber in the open air for a long time and it exchanges water with the air until each has its relative humidity / moisture content in equilibrium with the other. This is the EMC, and you can plot a graph of it for timber against air humidity. It's a well known graph, with a slightly S-shaped curve to it. Surprisingly (to me anyway) we discover two facts about it; it's almost the same graph for any timber species, and it's also independent of temperature. That's such a surprising result, but it's also useful because it means there's only one graph and we can make use of it. I can roughly draw it from memory (but not in ASCII), although the only points you really need to remember are that EMC for 35% RH (driest part of normal English Summer) is 7% EMC and that for 70% RH (damp English winter, when not actually raining) is 14% EMC.

So indoor timber "in service" will move its moisture through the year, every year, between 7% and 14%.

Semi-green timber stored in my workshop in Summer will get drier, down to about 7%

Kiln dried timber at 6% MC will _gain_ water when it arrives in my workshop. In Winter it might gain an appreciable amount and doubel its water content !

Now let's look at movement. All the movement is "sideways" to the central axis, and timber expands with increasing MC. There's negligible lengthways expansion with moisture. Tangential movement is greater than radial movement.

Movement behaviour is less consistent between species than the EMC curve, but we can make some approximations. All timber moves approx

10% maximum from "fully wet" to "fully dry" - although the moisture content to be regarded as "wet" varies between species.

Timber has approx twice as much tangential movement as it does radial, i.e. there's a maximum of 5% radial movement from our "fully wet" MC. This ratio also varies with species.

The tangential/radial ratio is what causes warping, cupping etc. If it was always 1:1, then timber would simply pump in and out in a linear manner. Unfortunately it isn't, but species with lower ratios will be less prone to it. Considering the geometry will allow you to work this out, but a rule of thumb is that "rings tend to straighten on drying". A quartersawn board is thus pretty stable, but a flatsawn board tends to cup with a concave outer face.

With age and number of moisture cycles, timber stops moving so much and settles down around the lower end. This takes 10-20 years, depending on species and extent of the cycles it goes through.

In the meantime, we have to allow for shrinkage. Read Hoadley et al for just how to do this, but the good news is that some very simple maths allows you not only to expect this shrinkage, but to _predict_ just how big it will be. Imagine making something, like a big greenwood timber frame, then knowing just _how_much_ to make it loose, knowing that it will shrink to fit perfectly over the next ten years. Well, I was impressed anyway.

Some timber species are stronger than others at resisting tensile or compressive stresses (forces). However they all fail at around 2% strain (change in length, as a percentage). Now if we compare this to our "10% total shrinkage" figure, it's a lot smaller. And _that_ is why you can't dry disks from the ends of logs without getting radial cracks, no matter how carefully you do it. Consider the log as a series of hoops, which try to shrink shorter, but can't get any smaller radius because there's another hoop inside. That's also why boring a big central hole allows you to dry these disks and avoid the splits.

[John, in MN]

[snipped a load of good info that even I could understand]

Thanks Andy for the info.

-- John, in Minnesota ........ who was not the OP, but finally comprehends the concept of moisture and wood movement.

[Phisherman]

I don't have a formal definition, but I think dimensional stability is high with plywood because it keeps its width-height ratio the same with changes in moisture content.

[Jeff Gorman]

It is worth knowing that over the long term, wood shrinks in width and thickness.

This is due to hysteresis - after a swell/shrink cycle the wood does not return to its exact previous state.

This is why through tenons, dovetail tails and pins will eventually appear slightly proud.

Jeff G

-- Jeff Gorman, West Yorkshire, UK Email address is username@ISP username is amgron ISP is clara.co.uk Website