Tip #28
Table Saw Special Operations (continued)
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Pg. 1-2, Pg 3-4, Pg 5-7

Bending Wood

Whether in a home workshop or a commercial establishment, most woodworkers often find it necessary to bend wood to a particular shape. It may be needed on a furniture project-the apron on a drum table or round stool-or on other projects like a garden arbor with an arched top or a doorway with a semi-circular upper structure. Commercial establishments, of course, can bend wood to almost any shape by steaming it or using chemicals to make it flexible. In the home workshop, you don't usually have the special bending equipment, but you can still do an impressive amount of wood bending just by working on the table saw and using either the kerfing technique or a thinning out process. You must use straight grain lumber for any bending.

Kerfing

Figure 4-1. A good amount of wood bending can be done mby using the kerfing technique. Kerfs can extend the full length of the workpiece or be confined to an area.

The most popular method of bending wood without steaming is by kerfing (Figure 4-1). What this method accomplishes is a reduction in stock thickness, while allowing room (between the cuts) so the wood can bend back on itself. The depth of the kerfs and their spacing are the important factors and are variable. Deep kerfs, closely spaced, allow the sharpest bends (Figure 4-2).

Figure 4-2. The kerf spacing can be veried, depending on the sharpness of the bend. (A) The kerfs should be more closely spaced in the sharpest bend area. (B) Greater specing is sufficient wher bend begins to straighten.

To bend wood with minimum loss of strength, proper kerf depth and spacing should be determined using a simple test.

Make a sample kert in a test piece of stock that you wish to bend (Figure 4-3). In 3/4" stock try a kerf that is 5/8" to 11/16" deep. Position the test piece, kerf side up, on a flat surface and hold it in place with a clamp on the right side of the kerf. The distance from the kerf to the edge of the surface on which you have placed the work should equal the radius of the bend you need. Lift the stock at its free end until the kerf closes and then measure the amount of lift at the edge of the table. This tells you the distance required between kerfs.

Figure 4-3.Make this test to determine, at least as a start, the kerf depth and the spacing you need to make a particular bend. Click on image for larger view.

Figure 4-4. Use a miter guage extension to mke the kerf spacing guide. The distance from the pin (8d nail) to the slot automatically spaces the kerfs.

Since work like this calls for a considerable number of kerfs correctly spaced, you should work with a kerf spacing guide like the one shown in Figure 4-4. After you have secured the guide to the miter gauge, cut a saw slot through it and then drill a hole for a nail to serve as the guide pin, spacing it away from the slot a distance equal to the required kerf spacing. Make the first kerf with the workpiece butted against the guide pin. The distance between the remaining cuts is automatically gauged by placing the last kert over the guide pin (Figure 4-5). When the kerfs must be cut in a central area of the stock, make the first cut without using the guide.

Figure 4-5. By placing the last kerf over the guide pin, the work is accurately positioned for the next cut.

To calculate the number of cuts, first find the circumference of the circle that will form the corner of the project. Divide this number by the total number of corners on the project. This will be the length of one corner. Divide the length of the corner by the total width of the saw kert plus the spacing between the kerfs. This will give you the number of cuts you'll need to make. The formula for this is:

Circumference ÷ Number of Corners = Corner Length
Corner Length ÷ (Kerf Width + Kerf Spacing) = Number of Cuts
Example: Calculating the number of cuts for a 12" dia. circle, used on a four corner project, with a saw kert of 1/8", and kerf spacing of 3/4".

Circumference = 3.14 x 12" = 37.68"
37.68" ÷ 4 = 9.42"
9.42" ÷ (1/8" + 3/4") = 9.42" ÷ 7/8"(.875) = 10.77 or 11 cuts

When kerfing is complete, bend the wood slowly until the curve you need is achieved. Wetting the wood with hot water (even if you must soak it awhile) will help the bending process. Also, use a tie strip, tack-nailed in place, to hold the part's shape until it is permanently attached on an assembly.

You can form irregular curves if you do the kerfing on both sides of the stock and/or vary the kerf spacing. When the kerfing is exposed, veneers may be glued in place to conceal the cuts. If you're working on an outdoor project, coat the kerts with waterproof glue before making the bend. Wood dough or putty can be used to fill the crevices. When the work has been correctly sanded and finished, it will require a close examination to reveal the method used to make the bend.

Thinning Out

Figure 4-6. Wood can easily be bent when you reduce its thickness in the bend area. This is the "thinning out" technique.

When thinning out, the stock's thickness is reduced the full length of the bend area (Figure 4-6). In effect, you are producing a length of veneer which is an integral part of the wood. The thinning out can be done with the dado or molder head. It can also be done by resawing the stock on the bandsaw. This method permits very sharp bends; but, since the veneer area won't have much rigidity or strength, corner blocks should be used to provide structural strength (Figure 4-7). Thinned out sections that have the wood grain running lengthwise will be stronger than those where the grain runs crosswise.

Figure 4-7. Thinned out areas, even kerfed areas, cna be reinforced by using corner blocks. Click on image for larger view.

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