Cincinnati Woodworks, Inc.

Cincinnati Woodworks, Inc.

Chip’s Tips

Hints concerning various woodworking materials and techniques. These are short highlights of what I consider to be the essential ingredients of woodworking.

Expansion/Contraction

Sunday, June 11th, 2006

Many otherwise successful projects have been ruined by failing to consider this. Wood is very sensitive to humidity (but not very sensitive to temperature). This is why doors and drawers tend to stick in the summer. You should figure that, in most situations, wood will expand/contract .125” per foot of width through the seasons. This is just a rule of thumb and can vary in extrreme cases. Remember that wood expands accross its width, but almost not at all along its lenghth. Therefore, gluing two boards side by side, or end to end isn’t a problem, but gluing an end to a side can be. Generally, it there is 3” or less of contact, there will be no problem. 3” to 12” may cause problems . More than 12” will almost certainly cause problems. This is where veneer becomes useful. With a proper substrate, veneer will have no problems with expansion/contracction.

Bending

Sunday, June 11th, 2006

There are several ways to bend wood.

First of all, I don’t reccomend kerfing, the process of making multiple cuts in order to effectively make the wood thinner. The wood will bend only where you have made the cuts, rather than bend evenly. Where possible it is better to simply use a thinner piece of wood. There is no strength lost in doing this, since the kerfed wood has itself been weakened.

Bent laminations – This is self explanitory.  Multiple thin layers are laminated, glued, and bent to form the desired shape. Remember that there will be “spring back” so you may have to judge how much to over-bend the wood in order to get the desired result. You can often get the desired result with a combinaion of bent laminations and steam bending (see below)

Steam bending – This is a very versitle technique, but requires experience (and some caution). For wood to achieve maximum flexability it must be saturated with water vapor (steam) and be hot. Merely submerging the wood in water is a far inferior technique. Entire books have been written on this subjest, and to do this effectively you will need to do some reading and some practicing. 

Adhesives

Sunday, June 11th, 2006

Properties of adhesives that you must consider are hardness, gap filling properties, and moisture reistance.
Aliphatic resin glue – Commonly refered to simply as white or yellow glue. Yellow glue is a somewhat more user friendly version of white glue. Because of this, I don’t use white glue. Yellow glue is the primary glue of many woodworking shops. A properly prepared glue joint is stronger than the wood itself. Properly prepared means that there should be only about 0.001” gap between the pieces of wood. More than this will seriously weaken the joint strength (as the water evaporates, the glue shrinks and is stressed from having to span too large of void.) This problem can be alleviated somewhat by “priming” the joints with glue, allowing a few minutes dry time, and then reapplying more glue prior to clamping. Speaking of clamps. they should be used, so that the glued joint or seam can be pushed closed as the water evaporates.

In short, aliphatic resin glues are very effective when used propery, but the joint will be weak if there is excessive gap or if clamps are not used.  Remember also that glue smears, while drying almost invisibly, will repel the finish and show up as ugly spltches when finish is applied.

Polyurethane glue – This is useful for custom vacuum press venneering, as it has a long working time, and glue that has telegraphed thru the veneer has little effect on the finish. It is waterproof, and makes an extremely strong joint WHEN USED PROPERLY. If clamps or other pressure is not used, and if there are significant gaps, the glue will foam within the joint as it cures, and you will have minimum strength. Foaming outside the joint is of no concern.

Epoxy (fibrglass resin) – There are many formulations of epoxy and you want to be careful about the selection. Essentially, though, epoxy is a superior adhesive for many applications. It is waterproof, has excellant gap filling ability, and, depending on the formulation, can offer great stregth while still maintaing some flexibility.

Finish

Sunday, June 11th, 2006

First of all. finsihing should not be considered as an afterthought. It has as much to do with the final appearance as the wood itself. I have seen poor quality woods made to look good with good finishing techniques, and I have seen good wood and woodworking techniques obliterated by poor quality finish.

Without proper preparation, the actual finishing material can not do its job. To look its best, wood MUST be sanded prior to any finish application, even if the wood was carefully machined. Proper sanding involves more consideration than what we have time for here, but, besides grit, you must also consider the direction of the sanding, whether it is orbital or in line sanding, and whether the sander has padding or not.

STANDARD VARNISH- is a tried and true finish that’s been in use for about 100 years.  Be sure to allow adequate drying time.  The best finish work is either rubbed out or polished.  There just is no way to get a dust free perfect finish that doesn’t require rubbing.  For the woodies I use a varient of standard varnish called spar varnish.  Spar varnish is formulated to be somewhat soft so that it can expand and contract with temperature extremes, and has a U-V inhibitor to help protect the wood from sun damage.  Even so, clear finished wood that is constantly exposed to the weather will need consant maintainance, regardless of the finish.

POLYURETHANE-gives similar results to standard varnish.  It’s one advantage is that it is hard, making it desirable for use where there will be abrasion.  However that same hardness makes it brittle, and somewhat prone to cracking under impact or in high stress situations.  Also, applied improperly, there can be failure to adhere.

LAQUER-is best applied with spray equipment, though very small projects can be done with a brush.  Nitrocellulose laquer has been around for a long time and is realtively easy to use.  It is a fairly weak finish that is primarily used on furniture.  Modern technology has made numerous formulations of specialized laquers, each with it’s strengths and weaknesses.

 

 

 

Wood vs. Veneer

Monday, May 29th, 2006

Many people assume that veneer is inferior to wood. Actually each has its advantage and both are used in both the best and the worst products.

When veneer was first used in biblical times, it was hand cut and was an expensive process. Today veneer is mass produced, is relatively inexpensive and is sometimes used to reduce cost, but is also used to improve the product.

By bookmatching, or other matching techniques, veneered surfaces can give a much more uniform appearance than is achieved by lumber. Also, veneer construction is generally much more stable than lumber, with less expansion and contraction, less warpage, and usually no checking (splitting).

Veneer can not be formed into three dimensional shapes. For this and other reasons, wood products often contain combinations of both lumber and veneer.