The first blade I made was using a file too! you should scribe a line down the center of the thickness of the blade. You can do this by grinding the end of a file into a V shape with the Vlining up somewhat where the center of the thickness of your material is. Then you just have to lay your blank beside the V portion of your file and drag the blank along the V, scratching a line into the edge portion. Flip the blank over and do this again and you'll end up with 2 parallel lines that indicate where center is. If you have a drill press you can take an old engine valve (ask a garage for one) and grind a burr along the disc portion using a sharpening sone, your file or sand paper, and chuck the valve into the press. Place your blank on a block of wood and raise it up to where the burr of your valve would be roughly in line with the center of the edge of your blank. turn on your drill press and drag the blank along the rotating valve and you'll scribe a line. Flip it over and repeat.
Whatever you do, scribe some center line on your edge. Since your blade is a double edge one, you'll have to scribe a center line on thetop and bottom of your blank.
If I were you, I'd file the main edge portion first. With the edge of your file, file in vertical lines every 1/2 to 3/4" appart going from the edge up. These shouldn't go all the way up your blank at all. The point is to file these lines so that at the edge they are deep enough to reach your scribed center line and then they should get less and less deep as you go up. the point of these lines is so that as you are filing away, yoi'll notice the gouges you filed disappearing going from the upper portion of your blade and finally disappearing at the edge. Once you have done this you have filed your edge in a nice flat V and right to your edge center line. Do the same on both sides at the same time. That is, mark your depth gouges on both sides and file on one side for a while, flip, file, flip, file, have a Pepsi, flip, file, etc. Constantly flipping allows you to keep an eye on the evenness off your filing. This is important for you especially since you are shooting for a double edged blade. if none of this makes sense, shoot me an e-mail and I'll try to scan you a pic and send it to you today. Constantly look at your work and check it over or your double edged blade could quickly become a single edged, flat ground blade as your edge creeps up and up and up!
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"Come What May..."
