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i-5359 Updated 04/07
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The PowerKnife works with Foredom's reciprocating handpiece to turn an irregular crack into a uniform channel that has consistent width and depth. This makes it much easier to do clean, professional-quality repairs of cracks in stringed instruments. With a uniform channel, it’s easy to make a splint of matching wood that fits well and visually disappears.
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Keep the blade sharp
The PowerKnife blade functions as a chisel, with an angled tip that needs to be good and sharp. To sharpen the blade, remove the wheels from their axle by taking off the retaining spring. Hold the angled tip on a sharpening stone (a diamond stone works well, StewMac #5257), and hone the end so the corners are square and sharp. Put the wheels back on and reinstall the retainer.
Chuck the chisel into the handpiece and tighten the collet with the wrench provided. Connect the handpiece to the Foredom power shaft. Hang the power unit in a comfortable location, and plug it into the variable-speed foot switch.
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Practice on scrap
Try the PowerKnife on scrap in order to get a feel for it. Use a scrapped instrument top, a piece of spruce patching wood, or other wood about 1/8" thick to simulate the instrument. Experiment with the reciprocating action before using the tool on the actual repair. Press the foot switch first, then apply the tip of the blade to the scrap piece. The “jackhammer” action engages only when you apply pressure. Push the chisel in the direction of the cut, and the hammer action will start.
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Controlling the cut
Tip the chisel up, and the slot widens. Keep the chisel low, and you get the narrowest slot. Keeping the angle constant, and running the wheels on the top will give a uniform channel along the cut.
Create a gradual taper at each end by making two PowerKnife cuts, starting at each end and meeting in the middle.
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Making the splint
After you’ve made a clean channel, you’ll need a wooden splint to fill it. This is easy to do on a table saw.
The PowerKnife blade is angled 4 degrees per side. We recommend making the repair splint on a table saw with the blade tipped at a 4 degree angle. Use repair material that matches the cracked wood as closely as possible, and cut one edge first. For repairing the spruce top in our photos, we’re using quartersawn spruce (tone bar wood). Make a cut, then move the fence toward the saw blade (in our example, approximately .200"). Flip the splint material end for end, and make another cut. The fall-off piece is the tapered splint.
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By fine tuning the angle of the saw blade, you can exactly match the angle of the PowerKnife channel. By changing the amount the fence is moved, you can make wider or narrower splints. The splint needs to be cut to length, then tapered on the ends to match the ends of the channel.
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When you’ve achieved a good fit, glue the splint in and trim away the excess. When sanded flush to the surrounding surface, and given the same color finish, a well-matched splint becomes nearly invisible.
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PowerKnife
Foredom tools, attachments
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