Critical success factors for joints

Whatever the type of joint under consideration, there are four crucial factors that determine success. These are, the nature of the stresses imposed on it and that it is designed to withstand, the grain direction of the joined parts, wood movement in response to moisture and the surface quality of the mating parts.

Although it is not usually important to be able to calculate loads precisely, it is essential to understand the types and relative sizes of loads that joints are able to support for furniture in use, in transit or in conservation treatments such as dismantling and cramping up. Most furniture is well designed for compression but trouble often arises from tension and racking (a racking load on a frame causes it to deform diagonally from a rectangle to a diamond shape).

The most difficult combination of grain to join is end to end. Although such a joint could easily support loads purely in compression, there is normally some bending as well. Many of the elaborate scarf joints designed by those who work with timber frames are designed to convert end-to-end grain combinations in joints to side-to-side combinations. End-grain to side-grain is a commonly encountered situation that works well for most loads except those of pure tension, when it tends to pull apart. In compression, this joint is usually limited by the strength across the grain of the side-grain member. In bending, either part may be the limiting factor. This type of joint is usually either made to be interlocking, as in the mortise and tenon, or reinforced by external members across the join. When adhesive-bonded, side-grain to side-grain joints can be as strong as the wood itself but when the grain of the two pieces is not parallel, complications arise in service as a result of moisture-induced dimensional change.

Dimensional change in response to changes in relative humidity can cause problems in joints when the response of the two members is different. This situation could arise because different timbers were used, or because of the difference in radial and tangential movement in two pieces that were cut differently but is most likely to cause serious problems when grain directions are at right-angles, such as a mortise and tenon jointing a chair leg to a seat rail (see Figure 2.28). This is particularly true if tangential and longitudinal cuts are opposed. Such joints are always apt to self-destruct no matter how firm they may be at first, and this is further discussed in Chapter 7.

Ideally, joints should have uniform dimensions, and regular, smooth, even surfaces that mate perfectly across 100% of their surface area so that applied loads are evenly distributed and unwanted movement in the joint is restricted. Deviation from this ideal may result in loads being unevenly distributed, stress concentrations arising, or movement that will lead to the destruction of glue lines, bearing surfaces and failure of the joint and the structure of which it is a part.

There are so many possible combinations of the above factors that may arise in different joints that only a few can be selected for further discussion.

Wood Working for Amateur Craftsman

Wood Working for Amateur Craftsman

THIS book is one of the series of Handbooks on industrial subjects being published by the Popular Mechanics Company. Like Popular Mechanics Magazine, and like the other books in this series, it is written so you can understand it. The purpose of Popular Mechanics Handbooks is to supply a growing demand for high-class, up-to-date and accurate text-books, suitable for home study as well as for class use, on all mechanical subjects. The textand illustrations, in each instance, have been prepared expressly for this series by well known experts, and revised by the editor of Popular Mechanics.

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