Flat/Scandi Jig and Grizzly modification, lots of pictures.

PetrifiedWood

PetrifiedWood

Well-Known Member
Those of you who have been following my thread about uneven grinds will know that I decided to try a jig to help get the results I'm after.

I found a post on a British forum about a jig similar to this, but I added some adjustment screws to the bottom to allow it to fit a range of different steel thicknesses.


It starts with some aluminum angle and plate, 1/4"X2". I wish I had more precise methods for cutting it out but a hacksaw and file, along with my belt grinder were all I used. The edges aren't perfect, but they don't have to be for my purposes.
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I coat one side of the angle in layout dye.
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Measure and scribe a line for the holes. (I only use a cheap harbor freight caliper for scribing like this. you should use the right tool and this is most definitely not it! :D
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Punch my centers. The holes wind up a little off, but I'm not overly concerned with that.
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Tap the holes for 1/4"-20.
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The next step involves drilling and tapping the flat clamp plate for some 10-24 screws, and some 1/4" holes to pass the 1/4"-20 screws. This is where my design differs from the one I copied. In the original design, the guy used a small piece of steel attached to the bottom of the clamp plate the same thickness as the blades he's grinding. This means each time you grind a different thickness of steel, you need a new clamp plate. My 10-24 screws can be adjusted in or out to allow the clamp plate to seat flat on a wide range of different thicknesses of material.

Here's some pictures of the jig holding a blade.
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And a detail shot of how my adjustment screws work.
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Ok, excuse the mess! :eek:

Here's the grinder as-is. Tiny little table unsuitable for grinding with a jig.
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So I drilled 4 holes in it...
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Drilled and tapped a piece of 1/4"x4" aluminum stock.
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Put it all together and now I've got a nice big table to guide the jig.
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Set the angle of the table to 10 degrees from the platen for a 20 degree inclusive grind.
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About 15 minutes later I have a near perfect scandi grind on my very first try!
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So it has really been a confidence booster and now I can focus on aesthetics and heat treat for the time being instead of being frustrated by messed up grinds.
 
Very well done. I use one like it, only mine is made from a flat piece of stock for the bottom, with the blade clamp mounted in the middle, that way all I do is rotate the jig to grind from one side to the other, no need to remove the blade and stick out of the other end of the jig. I do full flat grinds also. I have a mathmatical formula to figure the degrees of the bevel, if anyone is interested I would be glad to post it here. You use the sceintific calculator in Windows to figure it.
Dale
 
JDW said:
Very well done. I use one like it, only mine is made from a flat piece of stock for the bottom, with the blade clamp mounted in the middle, that way all I do is rotate the jig to grind from one side to the other, no need to remove the blade and stick out of the other end of the jig. I do full flat grinds also. I have a mathmatical formula to figure the degrees of the bevel, if anyone is interested I would be glad to post it here. You use the sceintific calculator in Windows to figure it.
Dale
Click to expand...

This does interest me. I have been using one of those "angle finder" dials. I hold it against my platen and adjust the arm of the grinder until it reads 90* so I know the platen is vertical. With that set, I then use it to angle the table into the platen to 10* for a 20* inclusive grind.

But this does intrigue me. If you use half the angle (in my case, 5 degrees instead of 10), would you expect the grind line to go twice as high up the blade? It would seem that if that's the case it would be relatively easy to calculate how high the grind would go for a given thickness and width of stock.
 
Nice looking set up. Well thought out. Post some pictures of the knife when yo get it finished.

Free
 
Thanks guys I appreciate the comments! I will definitely post some pics of that knife when it's finished. Just ran a heat treat cycle on it today.
 
Here is the formula to calculate the angle. It works from three dimensions, stock thickness, desired edge thickness, and grind height. I got this from someone else, don't remember who for sure, but they said that I could share it anytime. It took me severial tries to figure out how to do it, but if you use the example given and just plug in your numbers in place of the ones shown, it will work. For someone who knows how to do trig and other formulas, it should be no trouble for them, but for me it took awhile to soak in. Also using the Windows calculator, you must have it set to the sceintific veiw to do formulas.
Hope this helps out. If you have any questions, i will be glad to try and help.

Blade grind angle formula
A = arctan((S - E)/(2*G))

Where:
A = angle of flat grind per side
S = stock thickness
E = edge thickness
G = grind depth desired (maybe I should call this grind width
instead -- it's how wide the grind will be when done)

Note that A is the angle *per side* that you are looking for
(naturally enough); but S is the *total* stock thickness, and
E is the *total* final edge thickness desired after the grind.
arctan is the same as an "inverse tangent" or sometimes you'll
see it as written as atan

Let's say you have a blade that is 1/4" thick, and you want a grind
1 1/2" wide on a 2" wide blade (note the formula doesn't care about
how wide the blade is total -- only the actual desired grind width), and a final edge thickness of 0.040":
So
S = 0.25"
E = 0.040"
G = 1.5"

so the angle we want is:
A = arctan ((0.25 - 0.040) / (2*1.5))
which is
A = arctan (0.21 / 3) = arctan (0.07) = 4 degrees

If you're using the calculator in Windows, you need to click on the
"Inv" box then hit the "tan" button to do the arctan. Also, make sure the box under the number display has "Degrees" selected.
 
Mike928, that is a great job, thanks for doing that. I had always thought that there should be a way to just plug in the dimensions and have it done. Much easier than going through the whole formula.
 
You are both welcome. If you'd like, you can download it and to your PC so that you don't have to use Google for it and you can save your measurements.
 
levernut said:
How do you grind the belly of the blade? Do you tilt the jig?
Click to expand...


No, since the tip of the blade is lower (while grinding edge up), the angle will place it farther from the belt, just as the spine is farther from the belt. To grind the belly to the tip, all you have to do is draw the handle of the blade away from the belt and allow the edge to pivot on the belt itself as you grind. If you build the jig, you can make a few passes with the belt turned off so you can get a feel for it. But the jig works so well that even if you get an uneven spot on the grind, you can usually correct it by applying pressure along the parts of the blade that need more material removed on your subsequent passes.
 
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