After posting my previous blog post and film on my method for sharpening wood chisels and plane irons using a flashlight, here I received a question from a fellow 'Instructables' member EmcySquare, asking if this same technique could be adapted for sharpening knives. Here it is and here am I using it to sharpen my Swiss Army knife.
This method is to enable the beginner at sharpening to get a feel for the correct angle by using a home-made low-cost guide. After using it for the first few strokes, you should begin to get an idea of when you are achieving the correct angle but you can always refer back to your guide for confirmation. The flashlight method works very well with the wood chisels and plane irons because they have sufficient length to cast a shadow onto the guide lines I drew on the 'screen'. The problem when trying to apply this to a knife, is that the blade is unable to cast a useable shadow. However, having given this some thought and with a minor modification, this is no longer a problem.
Unlike the chisels and planes, which are usually sharpened to an angle of 30° on one side, knives are sharpened at a more acute angle on both sides. Manufacturers specify the sharpening angle for their products and this angle may alter depending on the duty the blade is to undertake. Here are a couple of useful sites which detail the different angles and explain the sharpening process in depth: http://www.sharpeningsupplies.com/selecting-bevel-angle.aspx and http://sharpeningmadeeasy.com/index.htm. In my case, the two knives I wanted to sharpen both needed an angle of 20°.
The Sharpening Process
The main principle is to hold the knife on the whetstone at the correct angle throughout the sharpening strokes. The method I created uses a flashlight to project the shadow of the item being sharpened onto a vertical screen onto which the sharpening angle had been drawn.
The first stage was to mark the angle from the point on a horizontal base line, using a protractor and then the line was drawn from the origin.
Once the screen was in position the flashlight was mounted in the holder, as described in the previous blog and the whetstone was placed in between them.
To determine the required blade angle, I held a small steel rule flat onto the side of the knife blade at right angles to the knife edge.
When the light was turned on and the edge of the knife was placed on the stone, the shadow of the rule was seen on the screen and the knife was inclined until the shadow of the rule was coincident with the line drawn on the screen. This angle was checked each time the knife was drawn back to its starting position.
There were two alternatives to sharpen the other side of the blade. First when the blade was at the opposite end of the stone the blade was flipped over and the rule held on the opposite face of the knife but this time the rule was angled towards me. The angle was checked by the alignment of the shadow with a second line on the screen drawn at 20° from the horizontal in the opposite direction. The disadvantage of this was that my arm was now blocking the light so it was not as easy to view the correct angle.
The alternative to this was to turn the knife over so it was held in the other hand. Being right-handed I found this to be a little more difficult at first, but practice makes perfect!
And of course the broader the blade, the easier it is to hold the rule in place.
So now you can sharpen to any angle you wish. If you are sharpening to the same angle with every knife you will soon become used to attaining this without constant reference to the guide. Happy sharpening!
Now have a look at the film to see the process in action.
At the end of the film, I asked for comments and suggestions and here are some great tips I have already had and put to good use:
"Small variation: After pushing the blade away from you with the handle in your right hand, switch the handle to your left and hone the other side. That way your arm doesn't block the flashlight."
and from fellow- 'Instructables'
"you could benefit of a simple way to fix the straight angle to the blade, like a magnet or some screws. Strong falt magnets can be found in old computer hard drives.
- for the back panel where the shadow is cast, I'm going to use a cutting mat, since it has all squares and parallel lines. You can keep the board vertical and incline it the amount of angles you need. This way you have multiple parallel lines to use as a reference even while you slide the blade back and forth."
"Simple solution for countering the shadow: Flip the blade over to your other hand.
In the beginning this will feel a bit off, but your get used to it soon enough."
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All the best, Andy
© Andy Colley 2014