Edge Geometry and Sharpening

[tippit]

Hey Lin,
I read your comments on the ABS forum about Edge Geometry and Sharpening.  I've been playing with taking my entire last grind on my Rotary Platen so not only is the edge rolled but the entire finished blade is slightly rolled.  So far I've been real happy with the blade finish and rolled edge...Jeff

[Bladepeek]

Jeff, I have a flat platten on my Burr King that I need to resurface. Could I get the same effect by making the surface slightly concave?

[Lin Rhea]

That's great Jeff. I also use the rotary platen for sharpening and setting the geometry along the edge. The transition from blade flat to absolute cutting edge blends to where the natural 12 degree per side angle is tangent. Hunters are thinner than large knives depending on the purpose of the knife.

Ron, if you don't have a rotary platen, just use the slack belt near one of the wheels with the belt set kind of tight. Some will slide a cushioned pad behind the belt in front of the flat platen to soften the grind. Usually leather. This pad has a "hook" of sheet metal riveted to the leather to hang it from the top rim of the platen. This is home made by the way. I don't know of any commercially produced.

[Bladepeek]

Thanks Lin. I might just try that. I did glue some kind of carbon foam on once, but it got just a bit too concave. Actually I wanted a flat grind, but was just trying to take some of the bounce out of lap jointed belts at the time. Bought better belts and quit using it.

[Lin Rhea]

If you want a softer belt on a flat platen you might try the Trizac "gator" belts. I really like em.

[Bladepeek]

Yeah, I like them too. They really cut a long time too and the finer ones give you almost a brushed finish that is really nice.

Lin, could you explain,or (for those of us who are short of imagination) show what a rotary platen is?

[tippit]

I guess I'm kinda weird in that I flat grind my blades with only two belts an aggressive 36 belt to almost finished...but I also forge to almost finished.  Then I'll take a 100 Trizac gator belt to remove the 36 grit marks.  Finally I'll use the 100 Trizac on my Rotary Platen to a finished blade & edge.  This blends and smoothes everything out.  Plus this is Before Heat treat!

Before heat treating, I'll take the edge off to less than 1/2 the diameter of a dime.  I've had no problems with edge warping.  After heat treating, I'll re-roll my edge with 100 Trizac belt & hand finish.  This makes finishing is so easy for me.  Grinding is now the quickest thing I do.

Bladepeek...

KMG Rotary Platen from KMG site...

The Rotary platen attachment is universally designed to fit a variety of belt
grinder machines. The attachment uses a reinforced flexible belt that rotates
around a series of grooved rollers for the purpose of providing support to the
abrasive belt. The attachment can be rotated into different positions which to
utilize performance variations due to different roller spacing as seen in the
photos above.  The grooved rollers restrict the reinforced flexible belt from
any side to side movement which allows the abrasive belt to be tracked
independently.  The belt tension is fully adjustable.


   

   

[Lin Rhea]

I pulled this pic off the KMG site. I have one and use it a lot. It has a rubber belt that backs the sanding belt to provide varying degrees of soft backing.
   

[Lin Rhea]

I was typing at the same time Jeff. Sorry.

[Bladepeek]

Two posts saying the same thing sure reinforce the credibility of each other    

Ok, I can picture it (pun intended) now. I do something similar at a very primitive level on my Burr King by using the space between my 2" and 5" wheel and varying the belt tension. The spacing is a little less than the longest presented in the KMG picture and by varying the belt tension from "tight as I dare get it" to "fairly loose", I can do something like that.

I've been flat grinding before heat treating; cleaning it up on the flat platen; then hand rubbing; then slack-belt blending to a a sharpened edge.

Wish I was starting all over and buying my first belt grinder again. I've seen several that off a bit more flexibility than mine. Oh well, I'm still no where near out running the quality of my tools   :(

[Lin Rhea]

A couple things Ron. Just things to bear in mind.

One thing that the rotary platen does that a slack belt cant do even if the span is short is to support the belt from cupping. The rubber actually counters that a little allowing you to push it straight. When you are trying to blend a guard to the softer handle material, this is invaluable. Where a slack belt will tend to dig in the handle material, the rotary platen will help you keep that on the same plane. As you roll the knife around and see it working you can just kiss whoever came up with the rotary platen idea. You'll appreciate it that much.      

Another thing about the sequence of steps. I will roll the edge and test the blade before I complete the hand sanding. If for some reason the heat treat is not just right based on my testing I can tweak it or redo it. After I am happy with the heat treat I will finish out the blade and proceed with the handle. Again just something to think about.      

[Bladepeek]

Thanks Lin. Not too hard to figure out what the "M" stands for in "MS". I look at your latest knife with the bronze liners and everything is so square and perfect. Sure is nice to look at.

I don't see a rotary platen in my foreseeable future, but I know what you are talking about with the belt wanting to cup and dig in at the edges when going from hard to soft. I'm just marveling at my new variable speed and wondering why I put up with funning "flat out" for 18 years  

[tippit]

I bought mine originally for handles because of the slack belt digging out the handle material and not grinding the metal.  Now I don't know how I could get along with out it for my rolled blade.  I tried all sorts of ways to get a stiff slack grind without satisfactory results.  Since Bladesmithing is my passion/hobby/therapy, I have two...one in the Aiken winter forge & one in the Boston summer forge.