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Keys to dowel holding strength.

Dowel joints can be a cost-effective assembly alternative

Dowel joints are one of the most common adhesive-based furniture assembly joints. Doweling is a simple, inexpensive and reliable means of making butt and miter joints. Dowel joints have three advantages:

* They allow precise alignment of parts without special jigs or fixtures.

* Machining is simple - only a drill is required.

* The dowels substantially reinforce the joints.

In many applications, joint strength is largely determined by the holding power (withdrawal resistance) of the dowels. A number of factors govern the holding power of dowels used with particleboard and medium density fiberboard. This article discusses those factors and their relationship to dowel holding power.

Stress on joints

Dowel joints involve two types of stress: shear and withdrawal. Shear is the stress encountered when the two joined pieces slide against each other. The stress is perpendicular to the longitudinal axis of the dowel pin. Withdrawal stress is the force in pounds needed to overcome resistance when pulling a dowel out of the hole drilled for it. This stress is along the longitudinal axis of the dowel pin.

Factors affecting strength

The characteristics of three joint components determine the strength of a doweled glue joint: the dowel, the panel composition and the adhesive. In addition, two processes - glue application and drilling the hole - affect joint strength.

Dowels

A major factor in resistance to shear is the selection of wood species. Where high shear stresses are anticipated, dowels cut from woods with high shear strengths such as yellow birch, beech or sugar maple should be used.

Dowels should be cleanly machined with no loose or torn fibers on the surface. A rough surface or loose fibers will diminish the ability of the adhesive to bond to the dowel. Spiral grooved or multi-grooved dowels are generally preferred over plain dowels since they allow air to escape from the dowel holes.

The moisture content of dowels at the time of use should average 7 percent +2 percent. Dowels with too high or too low moisture content may have a significant loss in withdrawal strength. Excessive moisture content will result in an oversized dowel. The oversized dowel may cause panel-end splitting due to insufficient dowel/bore tolerance, while eventual dowel shrinkage may lead to dowel or glueline fractures. A lower moisture content will result in a slightly undersized dowel and diminish the ability of the adhesive to bond to the wood.

Plastic dowels can be used in place of wood dowels. While they may deform more than wood dowels under stress, in many uses they are as strong as wood dowels, and they have the advantage of not being sensitive to humidity. In addition to standard straight dowels, right-angle plastic dowels are available for reinforcing miter joints.

When doweling to an edge, the diameter of the dowel should be no greater than half the thickness of the stock. For example, for 3/4-inch stock, a 3/8-inch-diameter dowel should be used; for 5/8-inch stock, a 5/16-inch dowel would be appropriate. For 32mm construction, 8mm (or 5/16-inch) diameter dowels should be used to attach cabinet sides to the bottom and to the top rails. Dowel diameters should be held to within +0.005 inch. This tolerance reflects the machining of the dowel as well as the moisture content.

The longer the dowel, the greater the holding strength. This is particularly useful where a dowel fastens a panel edge to a laminated panel face. Increased depth into the edge can give that panel as much withdrawal resistance as the face of a laminated panel.

Panel composition

Internal bond strength is the strength of the bond between the wood particles in the panel. High internal bond strength corresponds to greater dowel withdrawal resistance from the face. This is not a critical factor for edge doweling because the depth of the hole (and length of the dowel) can be increased to compensate for low internal bond strength.

In general the higher the density, the greater the holding power. Panel products with a decreasing gradient from the surface to the center - higher density on the surface than in the center - exhibit higher dowel holding strength on the face surface than panel product of the same average uniform density. This can be a critical factor if the joint will be subject to heavy stress.

Laminated or veneered surfaces have considerably higher dowel holding strength than plain surfaces. This is true for both bonded edges and laminated or veneered faces.

The fit of the dowel in the hole is critical to withdrawal strength. Holes drilled in the edges should be 0.005 inch oversize to prevent the sides from splitting as the dowel is inserted. Holes drilled in the face of the panel should, at most, be the same diameter as the dowel. Studies have shown, however, that when dowels are inserted in holes 0.004 inch undersize in the panel face, they retain their withdrawal strength under cyclical loading. Loose-fitting dowel joints tend to lose much of their strength under the same conditions,

Dowel holes should be cleanly machined with no loose or torn fibers. All loose particles should be blown from the holes. Typical drilling speeds should be around 2,800 rpm. Heating and polishing of the hole walls indicate a faster feed speed should be used, since polished walls will not bond well to the dowels.

Adhesive

Polyvinyl acetate (PVA) adhesives are commonly used for the assembly of dowel joints in particleboard and MDF. For high strength or limited hole depth situations, a high solids content is preferred.

The strongest bond between panel and dowel is obtained by double gluing. That is, applying an even coat of adhesive to both the dowel and hole wall with a stick or brush. A double glued joint is roughly 20 percent stronger than a joint in which the adhesive was applied to the hole walls only. The joints are strongest when enough adhesive is used to produce "squeeze out" when the dowel is forced into the hole. However, this should be weighed against potential cleanup required.

The "Anker and Leimperle dowel joint system," or glue bullet system, consists of a specially machined spiral-grooved wood dowel and a capsule containing adhesive. The glue capsule (slug) is inserted into the hole. The dowel is pressed in behind it. The dowel, pointed on each end, punctures the glue capsule. The glue is forced up and around the dowel. The manner in which the dowel has been machined ensures proper distribution of the adhesive. The glue is pushed through a hole drilled from the end of the dowel to a groove cut around the circumference of the dowel. From that groove, the adhesive is forced up the outside of the dowel. Hole depth must be exact in order to puncture the capsule and assure the adequate lateral adhesive penetration.

BASIC DOWELING RULES

* Control moisture content of the dowels to 7 percent +2 percent. * Use quality dowels with minimum machine tolerance +0.005 inch. * Apply an even coat of adhesive to both the hole wall and the dowel. * Dowel hole diameter should be no larger than the nominal diameter of the dowel on the panel face and no more than 0.005 inch oversize on the panel edge. * Spiral or grooved dowels have better holding power than plain dowels. * Dowel diameter should be no greater than half the thickness of the panel. * Dowel holes should be clean, free from glazing burn marks.
COPYRIGHT 1992 Vance Publishing Corp.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:fastening systems for particleboard and medium density fiberboard
Publication:Wood & Wood Products
Date:Apr 1, 1992
Words:1243
Previous Article:Pressing thin MDF panels.
Next Article:Industrial particleboard shipments set record in 1991.
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