Knifemaking Math Problem

Frank Aguirre

Frank Aguirre

Well-Known Member
I was never really a super mathematical genius. I got a problem I am trying to figure out. I am working on a dagger and what I am trying to figure out is the angle of the grinds. The dagger blade will be 4" long and 3/4" thick. I would like it to have a flat surface between the grinds and not have them meet in the middle. The steel is 1/8" thick 1080 steel. I have sat here for a while trying to figure it out and the answer I came up with was that I am a big dummy.:bud::D I know that there are some far more talented mathematicians on here that could help me figure this out. Thanks Dogs!
 
Frank Aguirre said:
I was never really a super mathematical genius. I got a problem I am trying to figure out. I am working on a dagger and what I am trying to figure out is the angle of the grinds. The dagger blade will be 4" long and 3/4" thick. I would like it to have a flat surface between the grinds and not have them meet in the middle. The steel is 1/8" thick 1080 steel. I have sat here for a while trying to figure it out and the answer I came up with was that I am a big dummy.:bud::D I know that there are some far more talented mathematicians on here that could help me figure this out. Thanks Dogs!
Click to expand...

Frank, I need more information. To figure out an angel I needed the width of grind or the width of flat you want.
 
Drawing it up in CAD software, I get 11.89° per side (or 23.78° included) using a 5/23" flat between the grinds. Using a 12° angle will result in a flat of 0.162" width.
 
JC in SC said:
Drawing it up in CAD software, I get 11.89° per side (or 23.78° included) using a 5/23" flat between the grinds. Using a 12° angle will result in a flat of 0.162" width.
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JC you are exactly right. I did rounded it up .010 thousands of a degree, to 12 Degrees which makes your grind 3 thousands of an inch too shallow. Frank I am sorry. :D
JC will be glad to tell you how to set up the 11.89 degrees. Then he will be glad to inform how to hand grind it to maintain 11.89 degrees. Then he will explain to you how to sharpen it maintain that .297 wide grind. Then he will tell how to accurately measure the final grind. :D
 
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Jim Adams Customs said:
Try this Website to figure out an angle
Click to expand...


So let's say you were using Fred's bubble jig on this calculator but no dagger just a standard blade. What are specs one would need to enter into each parameter? In other words in the first field we want to calculate what and what? Then we would input stock thickness and blade width (I'm guessing) in which fields?

I'm horrible with this stuff as well so I need the "for dummies" version :eek:
 
I believe opposite would be blade thickness divided by 2 (need angles on both sides of blade), hypotenuse would be the bevel (full flat grind), and the adjacent would be blade width.
Correct me if I'm wrong. Been awhile since I've taken a math class. :)
 
I meant that as a confirmation of the number you had provided as opposed to a recommendation that he grind at 11.89°.
 
Frank Aguirre said:
Fletch the bubble jig comes with a 12 degree setting. That is perfect!
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Yeah I know but I'm wondering about how to go about using that calculator to determine all the angles for whatever different size blade someone is making. I'm guessing that chart someone posted can also be used but how familiar is anyone with the accuracy of that? If that gives the same calculations as the calculator then everything is peachy. :p
 
Fletch Helical said:
Yeah I know but I'm wondering about how to go about using that calculator to determine all the angles for whatever different size blade someone is making. I'm guessing that chart someone posted can also be used but how familiar is anyone with the accuracy of that? If that gives the same calculations as the calculator then everything is peachy. :p
Click to expand...

I'm familiar with that chart...It assumes a zero grind to the edge, but as shown, the calculations are so fine, that it is not usually a factor.

Try it out and see

This formula is in Excel and uses Excel calculation features try by copy and pasteing it into a cell
=DEGREES(ATAN((0.5*StockThickness)/BevelHeight))

yes, you are dividing the stock thickness in half with .5 x StockThickness

if you want real accuracy subtract the edge thickness first
=DEGREES(ATAN((0.5*(StockThickness-Edgethickness))/BevelHeight))

so how far is that chart off?

1/8 and 1 1/4 is 2.9 degrees
1/8 and 1 1/4 with .030 edge thickness is 2.2 degrees

1/4 and 2 is 3.6
1/4 and 2 is with .040 edge thickness is 3.0 degrees

Is that close enough ?

if we redo that chart what edge thicknesses would you want?
 
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12345678910 said:
I'm familiar with that chart...It assumes a zero grind to the edge, but as shown, the calculations are so fine, that it is not usually a factor.

Try it out and see

This formula is in Excel and uses Excel calculation features try by copy and past it into a cell
=DEGREES(ATAN((0.5*StockThickness)/BevelHeight))

yes, you are dividing the stock thickness in half with .5 x StockThickness

if you want real accuracy subtract the edge thickness first
=DEGREES(ATAN((0.5*(StockThickness-Edgethickness))/BevelHeight))

so how far is that chart off?

1/8 and 1 1/4 is 2.9 degrees
1/8 and 1 1/4 with .030 edge thickness is 2.2 degrees

1/4 and 2 is 3.6
1/4 and 2 is with .040 edge thickness is 3.0 degrees

Is that close enough ?

if we redo that chart what edge thicknesses would you want?
Click to expand...

I love excel. Your formula is spot on.
 
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I'm horrible with excel so to be honest I'm just going to take your word for it that the chart is fine. So thanks to whoever posted that link and thanks to those who have verified it's accuracy.

Last question: When it's referring to the bevel height is that basically how far up the blade the plunge would be? For example take 5/16 stock.... Let's say you want plunge to be 1" you would grind at a 8.9 (9 degree) angle until you hit a 1" plunge? Or am I totally misunderstanding that chart?
 
Fletch Helical said:
I'm horrible with excel so to be honest I'm just going to take your word for it that the chart is fine. So thanks to whoever posted that link and thanks to those who have verified it's accuracy.

Last question: When it's referring to the bevel height is that basically how far up the blade the plunge would be? For example take 5/16 stock.... Let's say you want plunge to be 1" you would grind at a 8.9 (9 degree) angle until you hit a 1" plunge? Or am I totally misunderstanding that chart?
Click to expand...

That's it
 
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