Explain bevels to me...

[ratdog]

I have a sharpmaker. Should I start with the 30 deg and then switch to the 40 deg? I've always just done 10-20 equal strokes on each side with the medium then the fine. Am I doing this wrong? I don't understand the concept of back and miro bevels.

 

[yablanowitz]

If you use the thirty degree setting to establish the main edge bevel on your knife, it will be quite sharp. Each time you touch it up, you have to remove material from the entire face of the bevel to do anything to the edge itself.

If you set the main bevel with the thirty degree setting, then use the forty degree setting for touch ups, you only have to remove material from a small portion of the bevel next to the edge. This is commonly referred to as a microbevel or secondary bevel. The knife will not be quite as sharp, but the edge will be a bit more durable, plus it will take much less time to sharpen and will remove less material, making the blade last longer.

Eventually, the edge bevel will end up being forty degrees and you will have to reset the main bevel to thirty degrees, which can take a lot of time on a Sharpmaker.

 

[GigOne]

A double bevel adds a second, more acute, angle behind the edge bevel. This secondary bevel is generally called a back bevel or relief angle. Its purpose is to thin the metal behind the edge. The thinner the edge, the greater the cutting ability. However, an edge that is too thin is susceptible to damage. By adding a smaller, more obtuse primary bevel to the very edge, it gives the edge the strength to avoid damage from impaction, chipping or rolling. In addition, as a knife is sharpened over time. . . .the metal behind the edge becomes progressively thicker. As a result, the knife doesn’t cut as well and becomes harder and harder to sharpen. The answer is to grind the shoulders off the edge at an acute angle (i.e., add a back bevel, then reestablish the primary bevel).

For a better understanding. . . . .look at blade #5 in the pic below.

A micro bevel is a very tiny bevel at the very edge of a knife. The principle reason for a micro bevel is to provide toughness right at the edge, without sacrificing the geometry of the blade.

 

[Cougar Allen]

Bizarro

 

[ratdog]

Can someone explain how to rig the sharpmaker to do a 25 degree angle?

 

[yablanowitz]

The only way I know of is by placing a shim under the base. I think one of the brass guard rods placed under the center of the base will give you about 25 included if you rock the base from side to side, running the knife on the more upright stone. One of the triangle stones will give you about 20 degrees included.

I think it is easier to sharpen freehand than messing with these makeshifts, but that's just me.

 

[Bucktool_02]

Can someone explain how to rig the sharpmaker to do a 25 degree angle?

25 degrees, as in 12.5 degrees per side?

You have to use the hole on the end of the Sharpmaker, the one for sharpening scissors. That hole is supposed to be 12.5 degrees.

I put something heavy on top of the Sharpmaker to hold it down. The rod points away from me. With 2 hands, I hold the knife as horizontal and flat as possible and sharpen it on the flat of that single rod.

 

[StretchNM]

Approximately 1/4" under the base, moved back and forth until just right (and checked with a protractor) will give you 25 degrees. The reason I know this is because I ground a couple of knives at 12.5 degrees (25deg) and I needed to adjust the 30 degree holes to 25. Once you get the angle right, mark the location of the shim and don;t forget to swap it end-for-end when doing the other side of the edge.