How to do a concave (parabolic) grind??? Help!!!

I

Ian Sayers

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Sep 14, 2015
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I am trying to figure out a way to do a concave parabolic grind. Note that I am not trying to do a hollow grind. I want to grind shallow hemispherical (bowl-shaped) depressions with radii ranging from golf ball to cantaloupe sized. So far, I haven't seen any grinding products that would work. I see there are concave cutting wheels for cutting sink holes out of marble and granite, and I saw a mushroom-shaped grinding wheel for snag removal, but it was too small. The closest thing I found were diamond grinding wheels with a slight radius, that cost about $100 for a 6" wheel, but they won't really work for me because the radius of the wheel face doesn't match the wheel diameter, so I would get rounded slots instead of spherical depressions. Does anyone know a good way to do this short of CNC milling?
 
I don't follow. Is this knife related? What would possibly need a "hemispherical depression" the size of a cantaloupe?

What exactly are you trying to achieve?
 
Are you trying to make "graton" depressions in a blade bevel?

For fully spherical depressions, there are two methods. The first is a spherical cutting disk/wheel, which as you discovered are rare and expensive.
The other makes a fairly reasonable hemispherical depression. It is done by woodworkers all the time. The cutting device is any wheel or disc that will cut the material. The trick is that between the wheel and the work, one is stationary, and the other is rotated. In woodworking, a radial arm saw blade is lowered the right amount to achieve the desired depression profile in the wood. The arm is locked, and the motor and blade are rotated. In production setups, the blade is stationary, and the work rotates on a table or spindle arm. This created a perfect ( more or less) hemispherical pocket in the wood. In stone or metal, it is done similarly, and the cutting blade/wheel ... or the work ... is turned 360° to make the pocket. A similar or more eliptical gouge is made by swinging either the wheel or the work in an arc.
 
Thanks for the advice Stacy. It did occur to me that I could use that approach with an angle grinder and a jig, but I think it'll require a ton of hand finishing to make it look good. I want to make a blackened steel knife blade with the flat and ricasso covered with these depressions to mimic the appearance of a blade knapped from obsidian. The concept has been rattling around my head for a while, but it's challenging to pull off. Depending on the micro structure of the obsidian, the flutes created by knapping are often thin and elongated rather than spherical, and I could accomplish that look with small contact wheels, but small depressions will make a washboard surface and affect the cutting performance, whereas I am looking for more subtle depressions with larger radii, which should be comparable to scallops in terms of how they affect cutting performance.
 
OK, now I know what you want.

Use a small wheel fork on the belt grinder. I use a 3/4" or a 1" wheel. Press the blade against the wheel at varying angles. By tilting and twisting the blade, you can quickly "knap" the surface. I do it fairly often. I will look around for a photo. Others do it and there are usually several images online.

An alternate way is to use your regular contact wheel, and run the belt just slightly off the right side. Press the blade against the edge at an angle and make the scallops. This is very fast ands makes a random "flint knife" look. It do it with 120 grit, then go back over with 400 grit. You can leave it crisp at 400, of buff with black compound for a more worn look.
 
A dremel seems to be the way a lot of purely stock removal guys get all kinds of texture.. Use your imagination! ;)

~Paul
My YT Channel Lsubslimed
... (It's been quite a while since I've posted any new vids)
 
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