Drives: "looks great from my chair and those guns look too nice to shoot."
I would counter, "They shoot too nice to just sit pretty!"
Drives, I want to clarify something if I am reading your post correctly.
Concerning wood movement, the outward appearance of the joint needs to be perfect and tight. A loose appearing joint or one with gaps is amateurish. Come see some of my early pieces, those that are still together that is. It is the inside where the movement is needed to prevent warping and cracking. That statement is way oversimplified, see below.
A loose joint is cosmetically distracting and likely weak. We strive for the observer to not really see the joint line. For instance, if the shoulder of a mortise and tenon joint is loose, or there is space there where the cheek meets the crosspiece, the mortise is weak and could fail if stressed as in a chair.
Let me try to describe wood movement as I learned it. I went to a class taught by Kelly Mehler (TS guru). He used a handful of drinking straws to illustrate the orientation of wood fibers. Some of the parallel fibers are actually vessels for carrying water and nutrients in the living plant (remember xylem and phloem?).
The drinking straws illustrate how the fibers run parallel as in a solid wood board. Add or remove water content and the fibers swell or contract. That is what happens in the Southeast when the humidity changes from summer to winter. The fibers swell or contract along the parallel lines.
So a board that is six feet long but 3 inches wide, will swell along the direction 90* to the fiber direction---the 3 inch width becomes 3 1/16". My 40 inch maple and walnut top grew 3/16" on each side! But there is almost no growth in the length.
MDF is powdered wood and resin . Plywood and OSB has fibers at random or varying orientation, therefore there is less directional movement.
So this principle needs to be considered in designing wood furniture made with solid wood, less so with all ply or MDF.
Example:
Jim had a great tutorial on sliding dovetails. When you use a dado or sliding dovetail made of solid wood, you must understand and consider the stresses placed on the woods that will be joined at 90*. This is especially important when gluing up wide panels. If the wide panels grow or shrink, no problem UNLESS you have fixed or glued the panel along its entire width to the other cabinet or table support pieces. A sliding dovetail 90* to a wide panel will cause the panel to crack or explode if glued all the way across the width. The strength of a sl dovetail is mechanical and one may only add glue at one end (not both ends) or in the middle of the panel.
There are all sorts of strategies to overcome this ranging from the old board and batten (the wide boards are only fixed loosely by the cross piece, batten) to the loose tenon (NOT a loose tenon joint but a rounded tenon in a slightly oversized mortise, oversized only enough to accomodate a good glue surface), to bread board table ends (where the table end keeps the wide board flat and together but is held with glue only in the center of the wide panel or held on by loose oversized screw holes or pins.
This also influences the basics of joinery from a strength standpoint. A **** joint is terribly weak. Think of the drinking straws. Try to join two handfulls of straws end to end. That is a **** joint. Now join them side to side. Probably a lot stronger. That is why **** joints fail, and why miter joints, a modified but joint, fails. The glue is joining air rather than substance.
So how do you make a **** or miter better?
Well you could "size" the glue joint. This is a wierd term meaning that you put glue on the fibers' ends and let it partially dry, clogging the pores, then add another layer of glue to hold the joint. This is called "sizing".
Or better, put a solid or hardboard spline in the joint. This gives more side surface area. Or create a mortise and tenon where the cheeks are **** joints (weak) and the shoulders are long joints with great glue surface area. That is why, when you see crush tests (?correct term?) of different types of joints, the M&T is superior in strength, followed closely by the loose tenons and the biscuit or double biscuit joint.
I am a bad typist and have a short attention span so this is oversimplified. I bet there is a lot on the Google Search to explain this better than I but this is my attempt at explaining the concept. I know others will chime in and correct my generalizations.
I hope this helps.
