Flat grinding.

S

Sword and Shield

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Apr 3, 2004
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I've made my first knife, and used a convex grind. Works great, but I'd like to do flat for my second, as the blade overall is smaller and the edge profile should be thinner.

However, I've hit a brick wall. My belt grinder keeps giving me convex, and I'm grinding against the platen as it is. Would sliding the platen forward help the issue? I'm using one of these- Delta SA180

I've tried drawfiling, and again keep getting convex. Any ideas?
 
Practice, Practice, and Practice some more! If your getting a convex edge on a platen then you would have to be adjusting the angle of your grind as you go, perhaps a grinding jig would help. I don't suggest becoming dependent on the jig as it will hinder your abilities in the future. If you hold the file at the same angle while you draw file you will get a flat grind, it just takes practice and patience.

-Dan
 
I'll have to keep playing with it. I'm working on an interpretation of an Ahti pattern, and would most prefer to do a flat grind.
 
I have the same grinder-the platen will flex if you really get on it and may account for the convex grind as the belt will drift away from it.
 
I think I may build a jig for this one, and then practice flat work on scraps.

My idea is simply to screw the blade down to a piece of 2x4 that I've cut a bevel on, probably about 10 degrees. Would that work? I'm also going to round the end so I can keep the angle steady for the belly of the blade.
 
If you're trying to flat grind on that skinny little wimpy platen in front, you might be there awhile(like, forever) getting a flat grind. You'd be better off using the disc on the side, IMO.
 
The only thing that would concern me would be keeping the blade flat, but I can get the platform rigged up again. :) I'll give it a try this weekend.
 
go to a scrap yard and see if you can get a piece of thick angle iron cut to match the one on the delta. I put a ceramic platen on the platen it came with , didnt do any good , that platen is to flimsy to flat grind on. As Danbo said , try the disk , that will flaten those grinds right out.
 
I have a KMG and thought I could flat grind pretty good. Then I made a disk grinder. The disk show me every little bit of dip or wave in my flat. I am now going from heavy grit on belt to next grit on disk and back and forth. Use the disk
 
Steven Penner said:
S & S

You can calculate your angles - 10* the way you describe is 10 per side and 20* overall.
depending on the size of the knife- might be a bit steep - chopperish

See the link below - you can use the chart i posted to determine the angles ahead of time.

http://www.bladeforums.com/forums/showthread.php?t=562593
Click to expand...

I'm trying my darndest, but I can't quite parse your math.

Granted, you don't have 5/32" stock on there, but a full flat grind- 1" in this case- should be 9 degrees per side. Yours has it around 9 total.
 
I could have buggered something up – lets see.
Math is not my strongest skill

5/32 spine thickness and 1” bevel height

5/32 .156 25 is the spine thickness and the smallest side of an Isosceles triangle.

74px-Triangle.Isosceles.svg.png


In order to use trigonometry, imagine we split the blade down the middle of the spine…to get a pair of right angled triangles.

HALF the stock thickness is the “opposite” side.
Opposite:
.156 25 / 2
.078 125


Bevel width is the “adjacent” side
Adjacent:
1.000”
Tangent of an angle is the ratio of the length of the opposite side to the length of the adjacent side.


Tan = Opposite / Adjacent

180px-Trigonometry_triangle.svg.png

I ripped this photo from wiki - it's not scaled right for our example


.078 125 / 1
= .078 125

Inverse Tangent of .078 125
= 4.47 Degrees

For ONE side of the blade
Set platen 4.5 degrees off of perpendicular.

Total angle is about 9 Degrees.

Anyone else ??? if this is the correct formula, I can add 5/32 to the chart
 
Last edited:
I reworked it- my math was in error. I was using 5/32" as the opposite instead of 5/64.

Just did some hardcore grinding with a 60 grit disc. Now I'm getting somewhere!
 
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