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Fin Beveling Jig
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By Geof Givens

This article describes how I made a fin beveling jig for a standard vibrating floor/surface sander. To build and use this jig, you will need:

  • scrap lumber, including 28" of 2x4 and 34" of flat wood approx. .25-.75" thick *
  • power saw and drill
  • screwdriver and assorted screws
  • 2 metal shelf brackets
  • 3 clamps (or 2 if you're dexterous)
  • ruler and T-square
  • level
  • vibrating sander

* For the purposes of this article, we will assume it is .75" thick, as shown in the photos.

Figure 1 shows the completed assembly. Figure 2 shows it when the sander has been removed. The jig consists of three parts: a track, a sled, and an angle guide. It is critical that some portions of this assembly are square, plumb, level, or parallel. I have tried to emphasize these parts below. You cannot build this jig without a T-square and level. Work on a level surface to ease confirmation that parts are aligned correctly.

NOTE: All Figures can be enlarged by clicking on them

Figure 1Figure 2

Step 1: Construct the track (Figure 3)

Figure 3Cut 11" of 2x4 for the sled track base, X" of 2x4 for the upright, and 2 pieces of flat lumber each 11" long and 2.75" wide for the track rails. Assemble as shown in Figure 3, using screws into pre-drilled holes. It is important that the track and upright meet at a 90° right angle. If the wood is irregular, the interior angle is the crucial one. It is also crucial that the interior angle between the upright and the top edge of the sled track sides is 90°, and that the track base is parallel to the top edge of the track rails. The 90° angles are shown as A and B in pink in Figure 3; they look like acute angles because of the camera position.

You probably wonder how long X" is. It is convenient if the distance from the top of the angle base (see step 3) and the top of the track upright is a round number of inches. On my jig, this distance is 8" and is achieved with an upright 11.75" long.

Step 2: Construct the sled (Figure 4)

Figure 4The sled consists of a sled base, a leveling shim, a vertical clamping brace, and a plumbing shim (see Figure 4). First cut a 2x4 about 4.75" long for the sled base. This needs to be long enough to support the entire nose of your sander. The length of this sled base is important: your sander must fit flush with the vertical brace at the rear of the sled base and it must extend sufficiently off the front of the sled base so that the vibrating bed of the sander does not rub against the sled or the track rails (when the sled is placed in the track).

Next, cut a leveling shim. Your sander will rest as shown in Figure 1. The purpose of the shim is to ensure that the face of the sanding surface is perfectly vertical ("plumb"). Use your level to fashion exactly the right width shim. Mine is made from a scrap of .25" thick floor molding and a sandwiched piece of cardboard (!). It is important that the shim causes the sanding face of your sander to be perfectly plumb. It is also important that the leading edge of the shim and the sled itself (i.e. the pink, right edge in Figure 4) is square, so that when the sled sits in the track, the leading edge of the sled will be perfectly parallel to the facing edge of the track upright and perpendicular to the trackline.

Next attach the vertical clamping brace, made from a piece of 2x4. The particulars are not too important, but this piece should be at least as tall as the length of your sander, and should be attached approximately square to the base. Not owning a router, I sawed, drilled, and chiseled a notch in the brace to provide access to the sander's on/off switch.

Finally, place your sander in the sled as shown in Figure 1 and fashion a plumbing shim to barely fit in any gap along the top of the sander. It is important that this shim ensures that the sanding face of your sander remains perfectly plumb.

Practice clamping your sander into the sled and checking that the sanding surface is plumb. If your sander has a rounded top handle, it may have a tendency to rotate left or right relative to the trackline when clamped tight. If this happens, you can add small braces to the vertical brace and/or to the sled base to prevent this. It is important that the plane of the sanding surface is parallel to the plane of the track upright when the sander is clamped in the sled and the sled is placed in the track.

Step 3: Construct the angle (Figure 5)

Figure 5The entire angle assembly can be swapped out. Its leading edge may degrade with use, and you'll need a different one (or adjust an old one) for each different beveling angle you use.

The beveling angle is established using the right triangle outlined in pink in Figure 6. Notice that the bottom corner of this triangle is not quite at the leading edge of the angle base. Also, the triangle is formed by the interior surfaces of the track upright, the angle base, and the angle slope.

As discussed in Step 1, it is convenient if the distance from the top surface of the angle base to the top of the track upright is a round number, u. In any case, the angle formed by the angle assembly is tan-1(b/u) where b is the length of the base of the triangle in Figure 6.

We specify u by the length we choose for the track upright. We also specify the angle, Ð, the beveling angle desired. I recommend Ð=10° for a finer edge and Ð=15° for a more durable edge. Choose a smaller Ð if your fin is thick or you want an especially gentle taper. Important: when doing the math, some calculators want Ð to be expressed in radians. Radians = Degrees * 2 * 3.1416 / 360.

For a 15° bevel, we solve to find b=u*tan(Ð). For my jig, b= 8*tan(0.2618 radians) = 2.14". The angle base must have length equal to b plus ½ of the thickness of your slope piece (e.g. 2.14+0.75/2 = 2.52"). Cut a rectangular angle base having this length and having width at least as wide as the width spanned by the track rails. This rectangular angle base must have each corner exactly 90°.

Mark a line across the width of the angle base at a distance of b from one edge. This will be where the edge of the angle slope piece touches the angle base.

Cut an angle slope piece having the same width as the angle base, and having sufficient length to surpass the top of the base upright by about 0.5" when it is aligned against your pencil line. (See Figure 6.) Screw the angle base and slope pieces together so they touch along this line. Use the shelf brackets for this. Note that the leading edge of the slope will protrude past the edge of the slope base. This is necessary, and this protruding material should be considered somewhat sacrificial when sanding fins. This leading edge must be exactly perpendicular to the trackline and parallel to the facing edge of the track upright (and to the sanding face when the sled is put in place.)

The angle assembly shown in these pictures allows the angle to shift somewhat under pressure because each bracket has only one point of contact with each piece of the angle assembly. This is undesirable unless you favor flexibility over stability. I later sunk a nail in the interior of the triangle to prevent slippage.

Figure 6Step 4: Final assembly (Figure 6)

Clamp the angle assembly to the track upright. Ensure that the leading edge of the angle slope piece is perpendicular to the trackline. Clamp the sander to the sled. The vibrating bed of the sander should not contact the sled (or the track).

If you want, clamp the sled (with the sander clamped in it) to the track. You may not clamp if you prefer to use one hand to press the sled/sander against the angle as you work. I prefer to clamp so I have both hands available for working with the fin.

When the whole jig is assembled and the sander is clamped in, the sanding surface must be flush against the leading edge of the angle slope piece and parallel with it. There should be no significant gap or uneven seam there. With the type of clamps I have, I put the whole assembly in a vise so that the sled clamp doesn't interfere with how the base rests on a table.

Step 5: Usage (Figure 7)

Figure 7Use suitable safety gear, including mask and eye protection as necessary. If you're worried about sanding your hands, wear gloves. There is actually adequate room to work unless you have big hands, a small fin, and/or a very fine angle to bevel. I use medium grit sandpaper.

This sanding jig works like a chef's knife sharpener. The fin will be held flat against the angle slope piece. You will slide it along the notch-shaped path formed by the sanding surface and the angle slope. See Figure 7. You do not need to get the fin to pass below the seam where the sander and slope meet. Only slight downward pressure is needed. It is important to keep the fin flush against the guide, to keep the speed of the fin though the path fairly constant, and to keep the fin edge parallel with the bottom edge of the angle guide. With slight downward and forward pressure, pass the entire fin along the sander at the pace of about 1 inch per second. Repeat for about 4 passes for 0.062" G10 fins, or until the desired bevel is reached. Flip the fin over and bevel the other side of the edge.

Figure 8When I'm finished, I like to take some sandpaper and manually dull the knife edge of the bevel, because I think it will be less prone to chip (e.g., with G10 fins). I also lightly sand the imaginary line where the beveled portion of the fin meets the flat surface of the fin, thereby rounding the transition from flat to bevel.

Figure 8 shows the end result on some 0.062" G10 fins. It took me a couple hours to design and build this jig, but it takes less than a minute to bevel a fin now.

"" (X.X.)


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