Tuesday, December 11, 2012

Sweeping Plunge

May 2011, I signed up for a Pay It Forward charity build off started by Lorien Arnold (Great Knife Designer). It is now an annual charity. Lorien would present 3 knife designs. The participants would choose one of the designs. Make the knife, sell it and the proceeds go to your choice of charity. Here is the original PIF thread CLICK ME if you want more detail. 

I chose this design:

The biggest area that concerns me is that sweeping plunge of the primary grind. I reached out to Matt Bailey one of my favorite makers and got some very helpful advice on how to do the sweeping plunge. after some practice (a lot of practice) I felt confident I could pull out some consistent grinds of this type. The trick is to grind a standard plunge to the apex point of your sweep. In this instance where the grind goes off the spine. 

You then need to hang the belt off the platen approx 1/4" so that it can climb over the plunge line edge you just created while working your way back toward the guard keeping the grind angle the same. This means your knife will lift off the platen a small degree until you get the grind where you want it. One of the critical tips I found was to keep decent edge pressure then as you near your target sweep check every pass to decide where you need to apply pressure. I even take my eye off the edge/belt contact and check where the belt is cutting near the spine (which I never do on standard Plunges). I also found out  you need to use a stiff belt for a smooth curve, the flimsy thin belts wrap around the platen edge too much and the platen cuts into the plunge. 

My belt process: 
Rough grind: Blaze 36 Grit, 120 Grit, Gator 300 Then Heat Treat

The Plunge is set with the Blaze belts, the Gator belts do not need to be hung over the edge of the platen too much, I hang it around 1/8" just so the belt will not cut into the steel too much. Light pressure with the gators so you don't ruin your sweep.

I have done hundreds of sweeping plunges now and I don't have to stay as rigid as I did during the first PIF knife. I can do the entire sweeping plunge without breaking it into two grinds as shown above. I still mess them up and spend a lot of time trying to get both sides to match.

Here is a quick video I made, sorry it is not that great and has some obstructions on one side but helps demonstrate the process.