Convex edge on long blades

ferguson

ferguson

Joined
Feb 21, 2001
Messages
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On my 25" Sirupati and Kobra I was having a hard time getting a good edge using traditional methods. I had done some serious polishing on a buffer and the edge was buffed off. Like Yvsa and others I prefer a convex edge for toughness. I was in the process of glueing some dense foam rubber (a mouse pad) to a thin board, over which I would fasten some fine sandpaper. As stated in the sharpening FAQ, the foam will "give" as you stroke the blade over the sandpaper, giving you a good convex edge. Suddenly I realized that I already had a tool backed with firm foam rubber at my disposal. I had bought an electric straight-line finishing sander at a yard sale for $10. I carefully clamped it upside down in my vise. It was shaped so that I could do that without breaking it. Then I fastened some 600 grit wet-or-dry sandpaper and turned it on. Moving the blade lightly over it just as though I was using a stone, I had a burr in just a couple of minutes. Turning it over, I lightly sharpened the other side. It should be noted that I removed very little metal. Then the paper was changed to 1500 grit and I lightly polished both edges. A couple of minutes with my leather strop loaded with white polishing compound, and I had the best convex edge that I have ever been able to produce. When I went to test it on my arm, the hair jumped off screaming at the sight of the big blade approaching!
In a few places the mirror polish was made satin where I got the angle of the blade too low, but I was able to take care of that with the strop. When I try my Kobra I'll probably start out with 1000 grit so that I take off the minimum amount of steel possible.

This method may not be for everone, but I thought It might be of interest to other novices like myself.

Steve
 
"I had bought an electric straight-line finishing sander" Steve

Forgive my naivete, what exactly is a straight-line sander?
Is this the older "back and forth" model I remember from the 60's and 70's? as opposed to the new orbital sander?

And a question for Steve and Yvsa: Can you do this same thing with a belt sander? an orbital sander?

Thanks
 
Originally posted by Pragitam
[BForgive my naivete, what exactly is a straight-line sander?
Is this the older "back and forth" model I remember from the 60's and 70's? as opposed to the new orbital sander?

And a question for Steve and Yvsa: Can you do this same thing with a belt sander? an orbital sander?

Thanks [/B]
Click to expand...

The sander I have has a lever to switch from orbital (circular motion) to finishing (back and forth strokes). I had it set on finishing, but from the marks where I touched the side of the blade, it was still making tiny circles. So an orbital would work. A portable belt sander would be way too aggressive in my opinion. The belt would be stretched too tighly, and it has a steel plate behind the belt. Knifemakers like Jerry Hossom use their 2"x72" belt sanders with no backing (slack belt) to put a convex edge on a blade. That is, just a belt between 2 rollers running at a slower speed with nothing behind it. When the blade is touched to the belt, it "gives" resulting in the convex shape. This is a great way to do it, but 2x72 grinders are out of the reach of many hobbiests. You can pick up an orbital sander for nearly nothing. Just be sure that it has a rubber pad that will deform to give you the convex shape.

Steve
 
Steve
Great info!

I am constantly amazed at how much one can learn on this forum.

Especially for a newbie like me.:cool:
 
Great idea!!

BTW, I've found that I can use a figure-8 motion with the sandpaper/mousepad combo without tearing up the paper if I'm careful, so I 'spect the orbital would work. A LOT faster :)

Is it easy to obtain a replacement rubber pad for these sanders if one really screws up and cuts through? I'd think it would be a little tricky not to nick up the edges of the pad on the recurve.
 
Well it seems that the back and forth works alright for Steve so I'm saying it would be okay.
The orbital sander kinda bothers me although it may be fine as well, but for someone buying their 1st sander I would opt for the back and forth or a portable belt sander with a rubber pad under the belt if such a thing exists.
If the belt doesn't run way to fast an option may be to use some sticky glue, like the abrasive disks are held on with, and glue a piece of rubber to the base under the sanding belt.
The rubber needs to be resilent and fairly dense so as to not give to much or wear away to quickly.
A variable speed belt sander would be ideal since it could be slowed way down.

Another way of possibly grinding a blade this way would be to clamp the blade down securely and then run the belt sander over the edge to both sharpen and maintain the convex edge.

A Caution!!!! If anyone decides to try this you must be Very, Very careful to NOT OVERHEAT the blade!!!!
A bucket of cold ice water and a spong to wipe the blade down with between passes would be very advisable.
I have a plastic/rubber wastebasket that I keep almost full of water setting next to my Grizzly Knife Grinder to keep even soft steel cool as it's my opinion that carbon can be burned out of steel if it's gotten to hot.
It's also important to keep an aggressive cutting belt on no matter what the grit when working with heat treated or hardened steel as a lessor belt is more apt to burn the steel quickly.
If the steel turns any color at all it's been too hot, even a straw color although a straw color may not mean it's ruined, just compromised.
If the steel turns blue then it will need to be rehardened!!!!
It's a good idea, but care does need to be taken, especially on a blade that's already hardened!!!!
The same advice would apply if you were getting a 2" X 72" grinder.:)
 
sander protection
try cutting a piece of mouse pad ,then sandpaper, then clip both onto the sander.that way it will give more and protect the sander rubber if the sandpaper cuts.;) :)
 
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