What Size Wheel for Hollow Grind?

redsquid2

redsquid2

Rockabilly Interim Pardon Viscount
Knifemaker / Craftsman / Service Provider
Joined
Aug 31, 2011
Messages
3,125
So, I keep looking at this pattern that I worked up, and I keep thinking it needs a hollow grind.

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...but the 2 X 42 is my one and only grinder. The lower wheel measures 4 3/4" o.d.

Here is a practice piece that I worked on last night, with a 80 grit zirconia belt:


11207853043_fcf913e23b_c.jpg




11207870656_2141823f1b_b.jpg




I guess I will go back to the 2 X 42 tonight and see what it looks like when I grind it down thinner.

What do you all think? Is this a big enough wheel for a hollow grind?

Thanks.
 
Looking at the grind shown by the example, I would forget about the hollow grind and just do a sabre/sacndi grind.

You can make a hollow grind that simulates a much larger contact wheel by tilting the knife at an angle to the wheel. This takes rock steady hands and some practice. I would not recommend starting on this project.
At 90° to the belt ( horizontal) the grind is the same as the wheel diameter
at 45° it is like a much larger wheel
At 0° ( vertical) it is a flat grind.
 
bladsmth said:
Looking at the grind shown by the example, I would forget about the hollow grind and just do a sabre/sacndi grind.

You can make a hollow grind that simulates a much larger contact wheel by tilting the knife at an angle to the wheel. This takes rock steady hands and some practice. I would not recommend starting on this project.
At 90° to the belt ( horizontal) the grind is the same as the wheel diameter
at 45° it is like a much larger wheel
At 0° ( vertical) it is a flat grind.
Click to expand...

bladsmth, thanks for all the information. I'll skip the hollow grind for this one. Meanwhile, I can practice the technique you describe, on scrap.

Hesparus, good question. With 1/8" thick stock, I would say 1/2" to 9/16". I drew a line on the pattern, where I want the top of the bevel to be. However, I didn't really think about what the knife would be used for. I just cut out the pattern, then put that black line on there, and thought, "Looks cool."
 
So I did some math: assuming 1/8" stock, leaving 0.01" at the edge before sharpening, and a "perfect" hollow grind that leaves a slope of 0º at the edge, your wheel should make a grind that's just a hair over 1/2" wide (if my math is correct, that is, and I'm not 100% sure it is). If you're comfortable with that, then your wheel is big enough.

- Chris

BTW, I like your pattern.
 
I'm assuming your 2x42 is like mine and has an aluminum wheel on the bottom. I don't think you can get a consistent grind on that wheel. You need something with a little give to it. The "bump" on the joint alone would make it very hard.
 
Most folks calculate a hollow grind wrong.

First, you have to allow for some metal between the hollows on each side. About .020" is as thin a web as most dare go.
Second, the grind is an arc. If you make the edge the thinnest spot, you are going to only use half the arc chord.

So, with.125" stock you have .105" usable thickness to work with. Half that is .053" ( segment height). To make a .500" high hollow grind ( 1.0" chord), the arc radius would be 2.5"......so grind it with a 5" contact wheel.
 
A .599 grind height would be achieved with a 10" using the rest of the above stats computed by the widget.?
 
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