Confused about wood movement

[Toller]

I am loosely following some plans for a chest of drawers. They say the drawer rails have to have oversized holes to allow for wood movement, since the grain in the rails is perpendicular to the grain on the panels they are screwed to.

That makes sense, but I have done it wrong many times without anything going wrong. I have put trim around the base of cabinets where the trim grain is horizontal and the cabinet is vertical, put shelf or drawer rails where the grain is perpendicular, etc. Rail and stile frames have to be perpendicular. Everything has survived okay. Admittedly I just started woodworking a couple years ago and nothing has stood the test of time; but I just checked out a 100 year old desk (mahogany?) and a 25 year old table (cherry), and they both have perpendicular grains fastened together..

Plywood is stable, so if you put plywood in with wood going both ways (vertical on the side and horizontal on the top) it has to be wrong one way or another. Doesn't it?

Have I just gotten lucky, or is wood sufficiently stable in a winter/humidified summer/airconditioned house that movement is no longer an important factor? Or do I have a profound misunderstanding of the process?

The issue at hand are the drawer rails for the cherry chest. Putting in oversized holes seems wrong, but I don't want to have it crack apart, now that I am warned.

[Swingman]

Oversize, or slotted holes, are easy to do, where they are called for, and will generally give you some insurance against inevitable movement. Cross grain face gluing, in particular, is asking for eventual trouble, IME.

Wood movement is MOST definitely a concern in furniture/cabinet making ... disregard it at your eventual peril. The effects can often take years in the same climate, or days/weeks in a new climate. Your projects have likely not been around long enough ... try moving them across country to a different climate and they may actually fall apart.

Time may be everything in this regard. Remember, "The oxen are slow, but the earth is patient."

A good book on the subject is Bruce Hoadley's "Understanding Wood."

[J T]

Fri, Mar 5, 2004, 11:09am (EST-1) [e-mail] (Swingman) says: Wood movement is MOST definitely a concern

Especially with sapient pear wood.

JOAT That the peope have a right to bear arms for the defense of themselves and the state. - Pennsylvania Constitution of 1776

Life just ain't life without good music. - JOAT

[George]

SOP is to run a regular-sized hole at the point you want to stay fixed - in drawers, the front, and float the back.

Traditionally, Norm bashers aside, nails were used to attach cross-grain moldings, because they bend. Screws won't, so give 'em room.

with frame and panel, as you noted, the dimensions don't really change much, but in solid wood carcasses, you can get into trouble unless you're fully climate-controlled.

[Alan Bierbaum]

Yes you need the slotted holes to allow for wood movement. Most North American hardwoods will move about 1/4" per foot of width, across the grain, between 6% and 12% moisture content. Your work can experience this range of moisture (depends on where you live and move to). A concrete example follows (quote from a past post by me, you can look up the series and pictures in google).

This is in response to several questions on wood movement. Almost everyone here knows that wood moves with relative humidity; some just don't know how much. Since I work mostly in maple (it is similar to oak, hickory, cherry and other American hardwoods), I used a maple table top end offcut as a gage. My shop is air conditioned in the summer and central heated in the winter (typical of local house conditions). I measure the stick about once a month and record the length, date and moisture content with a moisture meter. The meter always reads 8% (yes it does work and gives different readings on purchased wood); the date and length do change. Today the length is 25 7/8". The minimum recorded is 25 7/8 and the maximum recorded is 26 3/32".

snip

Alan Bierbaum

web site:

[Toller]

How much does it move with the grain? How about plywood?

[Bernard Randall]

Alan,

There are 3 different wood movements.

1) Longditudinal, this is about 0.1% from green to dry and so can be virtually ignored. 2) Tangental, to the center of the tree, this is where most of the change occurs and is about 2x the next. 3) Radial.

As most wood is slab cut the majority of the planks, except the center ones, are tangental thus percentage wise the width will vary much more than the thickness, the exact amount will depend on the particular wood. For example rock maple shrinkage, from green to 12% is 5.0% tangental and 2.5% for radial (from the 'Handbook of Hard Woods').

Ply-wood because of it's construction is for all intents and purposes stable.

Bernard R

[Alan Bierbaum]

Lengthwise - almost nothing - you can consider it nothing.

Flatsawn - across the grain (common lumber available) - about 1/4" per foot from 6% to 12% moisture content. About half of this amount through the board thickness on flatsawn boards. Reverse these numbers for quatersawn lumber.

Plywood can be considered as stable enough that you do not need to worry about movement (only warpage).

Invest in one of the many good books on wood properties (at least borrow one from the library and read it).

Alan Bierbaum

web site:

[Fred the Red Shirt]

I think it moves with absolute humidity.

[George]

Nope.

other sources give the tables in RH.

[Fred the Red Shirt]

If you are refering to figure 8, I suggest you review it. The plot is EMC vs Relative Humidity AND Temperature. Note the slope of the lines of constant EMC.

[George]

Are you by any chance related to Swingman?

You're sitting in front of a reference cardfile. I suggest you use it. You'll learn more with your brain open than your mouth.

your elementary science? Heat is energy, energized molecules are more active, therefore disperse through the wood at a more rapid rate. All of which, had you the inclination to read for information, rather than affirmation, would have been explained to you on page 8 of the document.

[Fred the Red Shirt]

As you will recall from your high school physics the partial pressure of a vapor over a liquid increases with temperature. The relationship is typically non-linear, that holds true for water. Thus if the EMC of wood tracked with Absolute Humidity the lines of constant EMC in Figure 8 (above) would be significantly curved.

It's a bit hard to tell on my monitor, I always have trouble with graphical representations of non-geometrical data, but they look pretty straight to me. That the lines of constant EMC have constant slope in Figure 8 shows that the EMC tracks (linearly) with Relative Humidity, not Absolute Humidity.

You still have neither confirmed nor denied that your argument is based on Figure 8. When you reference a 68 page document, it is helpful to be a bit more specific as to what part of it supports your argument.

No, I don't know who Swingman is.

[Swingman]

Neither does "George", but complete ignorance of a subject hasn't stopped him from posting his assumptions yet.

[Scott Duncan]

How much is wood movement effected by finish? If something is sealed, stained and covered with three coats of polyurethane on all surfaces, is the moisture level going to vary as much as unfinished wood? Do I have to worry as much about wood movement for this case?

Scott

[George]

The one with the number eight at the bottom.

Lots of paper collections were numbered like that. They were called "books."

All

[Alan Bierbaum]

covered with three

as unfinished

Finish slows down the moisture movement into or out of the wood; it does not stop it. You should still allow for the movement.

Alan Bierbaum

web site:

[George]

That's where the combined RH/OAT tables come in. They show how rapidly the untreated wood will gain moisture. And it _will_ gain or lose, regardless of finish, eventually, so smart money finishes inside and out to help equalize the rate.

FWW had a test (Jeff Jewitt?) of the rates through various finishes. Think shellac was the best.

stained and covered with three

as much as unfinished