working out correct angles

[baggins]

Just about to make a start on a new blade but have been thinking more about the actual angle of the bevels. On my last few blades, i've used 3mm steel, and used a scandi grind which runs to a depth of 10mm on one and 15mm on another. Having now spend a silly anount of time on the net refreshing my trig skills, i've worked out that the 15mm bevel has left me with a very fine angle (now my maths is bad at the best of times) but, according to my calculations, i now have an angle of 2.76 degs on each bevel so a total cutting angle of 5.5.
Now is it me or does that sound really low. I was aiming to get to 30 odd degrees min.
By my calcs to achieve anything like what i'm after, i need to be creating a bevel of only about 5.5 mm which doesn't seem alot and looks wrong.
Any ideas or thoughts please.
By the way, for the math jockeys out there i used the following equations.
A (the angle)= inverse Tan X/Y (X being the centre distance from the edge to the bottom of the bevel and Y being the width of the bevel itself)
Cheers Guys
Baggins

 

[FGYT]

Could do the maths But i normally use Autocad and draw it

heres a 28deg on 4 and 3 mm stock

 

[Biddlesby]

I would agree that a 5.5mm bevel (if by this you mean the distance from the cutting edge, along the bevel) with steel of a thickness 3mm would lead to around 30 degrees internal angle at the cutting edge.

But if the thickness is 3mm and the bevel is 15mm then surely the angle is 2*arcsin(1.5/15)=11.5deg. Using a tangent function leads to much the same answer (this would imply "a 15mm bevel" means the distance from the cutting edge, parallel to the flat face of the blade).

But I may be misunderstanding your description. A diagram would speak a thousand words.

 

[mick spain]

Could do the maths But i normally use Autocad and draw it

heres a 28deg on 4 and 3 mm stock

Yep that's how it's done on 3mm I always think the grind face looks to narrow so I usually give it about a 23 degree bevel 11.5 a side

 

[baggins]

nice one guys, thanks. i just wanted to get close to that ellisive 30degs as that seems to be a good all round cutting angle. I have noticed that my earlier knives did take alot of maintenence and wanted to make something a little more robust. Am glad to know that my maths isn't as bad as i thought (that of a very sick, numerically challenged, glow worm). The decision now is, form or function. But at least i am now in a position to choose.
If i could workout how to get my notepad arty picture up, i could show you calcs.
Ah well..
Cheers Guys
Baggins

 

[billycan]

I just used a protractor to draw two lines with 29.something degrees between them which is supposed to be the optimum. Then whatever stock thickness i was using e.g. 4mm, measure the gap between the diverging lines till you reach a point that measures 4mm. Then measure the distance from there to the point and that will give you the bevel length.

Not sure that will make sense, but it is very easy to understand on a piece of paper.

but roughly: 5mm bevel on 2mm stock
6mm bevel on 3mm stock
7mm bevel on 4mm stock

 

[FGYT]

I just used a protractor to draw two lines with 29.something degrees between them which is supposed to be the optimum. Then whatever stock thickness i was using e.g. 4mm, measure the gap between the diverging lines till you reach a point that measures 4mm. Then measure the distance from there to the point and that will give you the bevel length.

Not sure that will make sense, but it is very easy to understand on a piece of paper.

but roughly: 5mm bevel on 2mm stock
6mm bevel on 3mm stock
7mm bevel on 4mm stock

Do this on some graph paper and you can scale it up ie 10mm =1mm
or 1" =1mm

Duncan

 

[Longstrider]

I just did a little practice run on some of the off-cuts that came about when I cut the profile out and judged the angle for my knife from there. I ended up grinding the bevel at about 24 degrees overall, but there again I was using 5mm stock !

See the knife at http://www.bushcraftuk.com/community/showthread.php?t=19407 I 'm pretty pleased with the way it turned out for a first attempt at Scandi grinds