Chisels

Chisels.

You can see a dull edge on a chisel as you can on a knife blade. If you hold a chisel with light shining directly on to the cutting edge and then rotate it slightly with the edge towards you, a dull edge will reflect light but a sharp one will not.
You want to remove any dullness or roundness from the cutting edge of the instrument. This can be done on a powered grindstone but should be done by removing the least amount of metal without overheating the metal and losing its temper (Hardness).

Orthopaedic and dental chisels, unlike carpentry chisels, are made of very soft steel and will need sharpening quite frequently.   They should be done frequently, possibly after each use. Not that they get blunt after each use but they are worth looking at, better to look at them and do a minor touch up than wait till they get very blunt and the surgeon complains and the operation is compromised. This is best done with a normal sharpening stone.

A short piece on sharpening stones.

There are two bevels or angles on a chisel, the first is the main one that you can see this is called the ground angle and should be about 25 degrees, or some say it should have a length of 2.5 times the thickness of the chisel.
The second angle is called the honed angle and should be about 30 degrees, the ground angle can be done on a revolving grindstone but is better done on a flat stone and should only occasionally need re-doing. The honed angle is done on a sharpening stone to give you the razor-sharp edge, I like to do this on a black Arkansas stone then you get a really good edge. See a link to my article on sharpening stones above.

How and when to re-grind.

When a blade becomes chipped, damaged or curved from a lot of use you will have to re-grind it; that is the end is squared up and the angle of 25 degrees put back on to it.

First check with an engineers or carpenters square or by eye that the cutting edge is at right angles to the sides. If it is not or if it is chipped or damaged grind it square either on the power grinder or on an oilstone, do it very gently so as not to grind too much away. Hold the chisel at right angles to the stone.

Having squared up the end or got rid of the chip or damage you will have the 25-degree with a flat end, now you will have to put back the honed angle, the final cutting edge.

If you have had to grind off a lot of steel you may have to redo the 25-degree bevel then put on the honed bevel.

To restore the ground angle of 25 degrees, rest the chisel on tool rest and touch it gently on the wheel at the same angle as the remaining part of the bevel. On a flat stone feel the chisel on to the stone so you know that the bevel is lying flat on the stone.
If you are using a powered wheel do not let the blade overheat – keep it cool by frequently dipping it in water, and if you hold it with your finger near the sharp end you will feel it getting hot, if the heat makes you jump then it is getting too hot.

If the edge turns blue then it is too hot and will have lost its temper, grind that part off and start again because you will never get a good edge on it.
If you do all this on an oilstone then you will avoid the problem of overheating.
Check that the length of the ground edge is 2.5 times the thickness of the chisel.

Continue this until the bevel is right down to the end of the chisel. Check again that the end is square.

You are now ready to put on the final honed bevel of 30 degrees onto the chisel.

To do this you need your finest stone, a natural stone is best, an Arkansas, Belgian or Japanese water stone, if you have one. Oil your stone or water it if it is a Japanese water stone.
Then hold the chisel at about 30 degrees to the stone with your index finger right on the sharp end almost touching the stone, then draw it back towards you in one smooth movement keeping the pressure and angle even. Sometimes this can be done with one stroke or maybe two.
Look at the end and you should see a neat 30-degree bevel, test the edge for sharpness with your thumb, Look at the end under the light you should not see any light shining off it as described at the beginning of this article.
once you are happy that the honed bevel is square and sharp, If you must now feel the flat side of the chisel at the end you are sharpening you may feel what is called a burr across the width, this is a tiny ragged sliver of metal left behind when the end gets really sharp, to get rid of this you will have to rub the back of the chisel on your stone, if you do you MUST keep the chisel absolutely flat to the stone or you can do it on a piece of very fine sanding paper that has a little water on it.
You may have to repeat this once or twice to get it really sharp but with practice once should be enough. You can feel when it is sharp because it will catch the skin on your thumb when you feel it and not reflect light.

You can also do the final honing process above with a fine wet and dry sandpaper, say 800 grit. Find a thick piece of plate glass or a granite surface plate. Run your paper under the tap then place it on the glass or surface plate, it will stick by surface tension.

The above text applies to the ordinary flat chisel, you will come across the gouge, that is a chisel with a rounded blade. These are more difficult to sharpen.

First of all, you need a small round stone, that is one which is the same diameter or of a slightly smaller diameter than the gouge diameter and the ordinary flat sharpening stone.

Start by making sure that the end of the gouge is square to the length as above for chisels, once you have got this right you can start to sharpen it.

Here you are going to redo the honed angle to get it sharp again.
Start with a little oil or water on the large flat stone, begin by feeling the gouge on to the stone so the original honed angle is followed.
Now begin to move the chisel up and down the stone in a figure of eight motion, rolling the chisel to and fro across its width as you go up and down, keep the chisel at the same angle as the bevel that is already on it, it is rather difficult to describe, but by rolling it you are sharpening it across the whole of the width of the bevel.
Continue to do this till the gouge is sharp, the bevel on a gouge is much steeper than an ordinary chisel and as a result you will probably not get it quite as sharp. Again feel it with your dumb to test its sharpness.

Once you are happy it is sharp use the round stone, with a little oil on it, to make sure that the inside curve of the gouge is quite flat and there is no burr on it, but the stone MUST be kept absolutely flat against the gouge on no account must you put any angle on it.

When you have got your chisel or gouge sharp, give it a good polish on a powered buffing wheel, do not polish the very end that you have just made razor-sharp as you will probably blunt it, not that polishing it will make it any sharper but it does look good.

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