Need your brain power !!

Brendan

Brendan

Joined
Feb 13, 2002
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How do I work out how high a point has to be from a wall to be a certain angle:

ie - if I place a knife 12 inches away from a wall, how high do I need to have the edge so if I run a file along the edge parrallel to my workbench I will be sharpening the knife at a 20 degree angle !!

:o
 
This would be a good question for the Engineers on this forum, but for the life of me I don't know why it was posted in this forum. :confused:
 
I just built a workbench and fitted a rotating vice. I was thinking about a sharpening system I saw a while back and it got me to thinking that I could make my sharpening alot better by applying some angles and accuracy, this is the hardest part of hand sharpening, so if I could get the angle as accurate as possible I would get sharper results. I just guestimated and angle tonight on the rig and got the knife hair popping sharp. So I thought if I could get a precise angle it would be even better.

The vice can be placed any distance from the back wall and if I have the knife at say 4 inches high and 12 inches from the wall for example I would have an angle of 20 degrees - I don't know if there is an equation for this and that is what i want to know !!

I'm confusing myself !!
 
is there an equation to work out different distances to get different angles !!
 
Hey there,

I might be mistaken, but I think the equation you are looking for is the "tan" function from trigonometry. It's been eons since I did this in school, but if I'm not mistaken, for any right triangle,

tan (angle) = opposite leg (height) / adjacent leg (distance from wall).

Therefore, just multiply the distance from the wall by the tan of the angle you are looking for. The answer will give you the height.

Thus your "guestimate measurment" was essentially correct.

ie. 12 inches * tan 20 = 4.36 inches

Let me know if this makes sense.

_pete_
 
ok - thanks !!


I just did some measuring - optimum blade edge distance from wall for working comfort is exactly 49.9 centimetres. At this distance the blade edge is 14.2 cm (this should be an angle of 0 degrees) off the surface of the bench exactly. Lets take the most popular edge angles, what should the blade edge heights be ??:

10 -
15 -
18 -
21 -
24 -

I have used cm as the accuracy is better !! This is taking the grind into account so the height of the blade edge is taking the blade edge as a flat edge so the knife is in the vice at almost pointing back at me !!

Wish I had of listened at school !!

I hope it works too !!

thanks mate !!
 
edited to change length from wall to 50.1 !!

just easier to setup quickly like this !!

I think I'm going crazy !!
 
and the killer is it's different for each blade - a slightly bigger blad ethe same distance from wall gives a blade height of 14.6 cm !

this is pointless !!

excuse the pun !!
 
Brendan,

Pete's exactly right with that equation. It's easy to figure out the height for any angle. I assume that you're fixing the blade so that it's horizontal to the work bench. Here's the heights you wanted for the angles you listed. You'll just have to "add" the height of the blade to each of these distances. Good luck.

10 = 8.83
15 = 13.42
18 = 16.28
21 = 19.23
24 = 22.31

Art
 
Rat Finkenstein said:
I dont really get what you are saying. but, I would highly recommend buying a protractor.
Click to expand...

I bought a machinest's protractor. I'll see if I can dig up the link.

Rob
 
The answer lies in trigonometry.

Too much to go into here. That is why the Lansky-style sharpeners are popular, they've already done the work for you...:thumbup:
 
From 0 degrees (from the edge and the middle of the blade)
· 13 cm back.
· 59 mm up from that point
· = 20 degrees.

Lock the handle of the knife in the vice.
Put up a standing piece of wood 13 cm back from the cutting edge.
Lay a piece of wood on the blade, messure the point on the standig wood
Messure the thickness of the blade, half that. (let us say 3 mm). Go to the standing point of wood, go down 3 mm from the point you made.

Messure 59 mm up from this new point and make a new point. This point is now 59 mm above the middle of the blade and 13 cm behind the edge = 20 degrees. Drill a hole above this point, or, cut of the wood in this height.

Put a guide rod on your sharpener, or make a holder for your sharpener and make a hole for the guide rod in the holder. Lay the sharpener on the edge and start sharpening…

The knifes handle is locked in the vice, that means that you can work on the complete blade.

If 13 cm is to small for you, draw this on a paper and make the lines longer.

Thomas
 
Thomas - that makes things much easier - thanks buddy !!

I will let you know how I get on !!

How about for other angles ??
 
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