Why are my blades tilted in the clamp?
- This topic has 33 replies, 10 voices, and was last updated 09/04/2014 at 11:43 am by
.
-
I did this quickly in CAD so bear with me, and I’ll try to explain as not to confuse anyone. I used CAD to create perfect lines and angles plus CAD has a measure utility where the angles of two lines can be measured. Everything I have written by hand was measured with CAD.
The first picture represents a FFG blade mounted in the vise, and shows a common scenario that happens to me almost every time in that one side of the blade is not at a perfect zero degrees (usually the LH side) and is canted towards the right. In all of these illustrations, towards the left is the negative direction and towards the right is the positive direction relative to the centerline of the blade. Our goal is to sharpen the centerline of the apex as not to create uneven bevels that are different sizes and not in line with the centerline of the blade. The blade in this example is ground at 10 degrees inclusive for a nice, round number. The dashed line running through the center of the middle triangle is the centerline of the blade, and the two right triangles on either side of the “blade” represent the vise jaws.
In this scenario, when we tighten the blade and place the angle cube on the LH side, we get a reading of 1.0 degrees. Placing the cube on the RH side, we get a reading of 9.0 degrees. Common sense tells you which direction these angles are relative to (9 degrees leaning left or right) but the angle cube has two tiny arrows that will tell you whether it’s leaning clockwise or counterclockwise relative to zero which in this case is equal to the vertical inner surface of the vise jaws. Knowing our positive and negative directions, the formula below shows positive 1.0 degrees plus a negative 9.0 degrees which equals negative 8.0 degrees or the blade in total is leaning 8.0 degrees toward the LH side…or does it? What we’re interested in is the centerline of the blade so dividing 8 in half gives us 4.0 degrees. As proved by CAD, the centerline of the blade is in fact exactly 4 degrees toward the negative direction relative to a vertical zero degrees. If we’re seeking to sharpen the centerline of the apex at a true 20 degrees per side, we would place the LH stone at 16 degrees and the RH stone at 24 degrees to have a true 40 degree inclusive cutting edge.
The second image shows what happens when the blade is tilted and we set our stones at 20 degrees to sharpen. Relative to the centerline of the apex, the LH stone set at 20 degrees is actually 24 degrees, and the RH stone set at 20 degrees is actually sharpening the apex at 16 degrees resulting in widely uneven bevels and an off-center apex.
The final image shows what happens when we use the math from the first image to make sure we are properly sharpening the centerline of the apex. Setting the LH stone at 16 degrees and the RH stone at 24 degrees puts our stones both a perfect 20 degrees relative to the centerline of the blade. I forgot to mention that the bolder lines on the outside represent the stones placed against the edge.
I hope this helps anyone that has trouble getting even edges on FFG blades or that has difficulty with the math. Now back to work. 😉
- You must be logged in to reply to this topic.