Putting an edge on a Wood Carving Knife
Sharpening a Wood Carving Knife
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This information refers to putting the edge back on a carving knives, but if you have dinged, nicked, or a badly damaged your blade, you need to see someone who can restore the edge of the knife to working condition. This information is intended to keep your knife edge razor sharp.You need to know a little about steel in order to be able to sharpen properly.STEELS
STAINLESS STEEL - Stainless steel is not appropriate for carving instruments, you may have heard of a 440 C stainless steel that is used for scalpel blades and assumed that it would be good for carving. If you think about it however you will see that a scalpel is used once to cut something that is softer than wood and is then thrown away. The main reason that they use this steel for scalpels is not that it is a good cutting instrument but rather that it does not contain elements that react with blood to promote infection like other steels would. Many surgeons are now using glass scalpels since the healing of wounds made by glass is even better than those made with 440C. There is no reason that the stainless feature is needed in carving since carving tools are not subjected to conditions where they are likely to rust anyway.
CARBON STEEL - Mild steel cannot be made hard enough to take an edge. It is the presence of Carbon in the steel that allows the steel to be made hard. It has always seemed odd to me that you can harden a piece of steel and make it into a tool that can then be used to cut the piece of steel that the tool was made from.
Keep the following in mind about carbon steels:
If the steel is too hard, the tool may snap or break since it is not capable of bending very far without breaking. Steel in this state is much like glass. In addition to this any stress placed on the cutting edge, found when hitting a knot or when prying with the edge may cause a crescent shaped chip to be knocked out of the edge.
If on the other hand the steel is too soft, the edge will become dull very quickly and will need to be re-sharpened more frequently.
In relationship to sharpening a blade that is harder to sharpen, the longer it takes to sharpen it, the harder it is, and the longer it will stay sharpened.
OPTIMUM HARDNESS - The debate over optimum hardness is one that will go on forever. Optimum will depend on many factors, including the characteristics of the alloy of the steel, and even more important the use to which the blade is being put.
To talk more about hardness we need a reference. The reference that is used is the Rockwell C scale. (A and B scales are used for softer stuff like plastic) The scale we want goes from 50 to 65 with 65 being the harder end of the scale.
Commercial Knives are as a rule about 57 on the Rockwell C scale. Some Swiss Army knife tested and it was 57. Manufacturers have decided to err on the side of toughness since you would send a knife back to be replaced if the blade snapped off but not if it got dull.
WHICH TYPE OF STEEL SHOULD YOU USE?
There are actually hundreds of types of steel available. This is another area for endless argument As a general rule, the more carbon a steel has in it, the harder it can be made. The hardness is caused by the formation of crystalline structures in the steel that are frozen in place by the quenching in the hardening process.
Here are some of the common types of steel:
ID USE PERCENTAGE HARDNESS
1095 Clock Springs .95%
0-1 .90% 62 - 57
L6 Saw Blades .70% - .90% 63 - 55
5160 Leaf Springs .56% - .64% 62 - 55
D2 Dies,Cold Chisels 1.50% 61 - 54
W2 Old Files .60% - 1.40% 64 - 50
In making knives use O1 tool steel because it has a high enough carbon content, it is readily available, and you can be sure of consistency from one piece to another.
HEAT TREATING STEEL
a) Annealing (softening) - heat the steel red hot and then allow the steel to cool slowly. Heat it in a fire and cover it in the ashes and leave it overnight. Can use vermiculite or a box of ashes to put the item in to let it cool slowly.
b) Hardening - Heat to the critical temperature ( steel becomes non magnetic at the critical temperature) and then quench. Varying steels use different quenches. O1 uses oil. W1 uses water. A1 uses air.
c) Tempering - Lowering the hardness to the useful state for the application. Use the tempering charts that come with the steel.
Tempering colors - as steel is heated it oxidizes in the atmosphere. This process causes the steel to change colors. You can use these colors to temper steel using the following guide:
Straw (light yellow/brown) RC 60
Dark Brown RC 58
Magenta RC 57
Blue (Spring) RC 56
As a general rule don't use power equipment to sharpen carving tools.
Sometimes I do use them in order to save time.
With machines it is very easy to heat up the blade to the tempering temperatures that we were talking about just now. When the blade turns blue it has been softened to the point that it will no longer hold an edge as well as it would originally. It is very easy to do this at the tip where the tool is thin. Once done if you carefully polish of the color, you can no longer see that the blade has been ruined. Do not suggest that you let other people use power to sharpen your tools unless you are sure they know what they are doing. If you find that a knife that used to hold an edge, no longer will especially at the tip then this might have happened.
SHARPENING CARVING TOOLS
Carving Knives, unlike pocket knives and hunting knives, do not have bevels at their edges. The sides of the knives go straight or with a slight convex curve to the edge. New tools especially the less expensive ones will need to be checked and may need considerable work to be done to change their profile to one that is useful. For this you need a coarse stone or a grinder. If using a grinder keep the tool wet to keep it cool and go slowly. When using belt grinders remember that an old belt will cut less and heat more. Use new belts to sharpen tools.
Sharpening a knife that has never been sharpened. Start on one side and begin to hone. On stones, diamond, synthetic, ceramic or natural always push the blade into the stone edge first. When moving the edge keep the portion of the edge that is in contact with the stone at right angles to the direction of travel. This may mean that with a curved blade you will have to change the angle as you move. The angle that you hold the blade at should start with the side of the blade lying flat on the stone and you should lift until the stone is taking steel at the edge of the tool. To help with this at first you can use a magic marker to color the side of the blade. You can then see where the stone is taking steel off of the blade. As you get more experience you can look for the scratches that the stone makes as you sharpen. You want to avoid holding the blade at too steep an angle so that the tool is not too blunt. As you get down to the edge of the tool you will start to form a burr along the edge of the blade. You should be able to feel this burr with your finger. You must keep working until the burr is felt all along the edge. Once the burr is complete you go to the other side and repeat the process. This will force the burr to the opposite side where once again you will feel it all along the edge. Without this burr you cannot sharpen tools properly. Now you move on to a finer stone and repeat the process.
How many stages? Starting with a finer stone will take you longer but it will work. Starting with a coarser stone will be faster but you run the risk of taking off more metal that you need to take. Experience is the way to learn about this. If you have a larger blade or a lot more steel to take off before you get to the edge then use a coarser stone first.
Strop - The last step in sharpening is to use the strop. A strop is a piece of leather on a support that is used to polish the edge. On a strop you move the tool so that the edge is moved backwards along the strop, so that the edge does not cut into the leather. The strop is usually charged with an abrasive material, like Green buffing compound. This is a chrome oxide abrasive in a wax suspension. The abrasive is the equivalent of a 50,000 grit sandpaper (if such a thing should exist) This abrasive is hard enough to be able to remove steel from the edge of the tool. Strop, with lots of pressure. Make several passes on one side of the blade, and then do the same on the other. Gradually change so that you end up with one pass on each side of the blade and then gradually lessen the pressure. Test the blade and then repeat.
At some point the burr comes off of the edge of the tool as a wire edge. Often this is a very small piece of metal but if you watch carefully you will see it. It often comes off on a fine stone or at least starts to, but will most often be seen on the strop.
When are you done? On method of testing the edge is to see if it will shave. Another method is to make a sweeping cut on the end grain of a piece of basswood. A sharp knife will leave a glassy smooth cut. If there is a dull spot on the blade it will leave a lighter colored streak on the cut. You can use this to identify the portion of the blade that still needs work. There is a possibility at this point that a portion of the wire edge has not been removed and instead has been folded around the edge of the blade.
KEEPING TOOLS SHARP
Once a tool is sharp you should only need to strop it to keep the edge in perfect condition. You should keep your strop handy and strop your tool every 20 minutes as you work. Also be aware that carbon tools will rust and that the very sharp edge will likely rust first. This means that you should strop your tool just before you begin to use it every time you get it out. You should only have to go back to the honing stage if A) you drop the tool or chip the edge of the tool or B) if you need to thin the profile. Keep your tools in containers that will avoid the chance of the edges coming in contact with other pieces of metal as this is where most carving tools are damaged.
Many tools that have much blunter profiles than they should for woodworking. This may be caused by tools being held at too steep an angle while stropping. Using power to strop can cause this to happen very quickly, which is another reason to not to sharpen with power.
To correct a blunt profile you need to remove steel from the side of the blade but not the edge. This can be done with a coarse stone or power, however, if you use power you must be very careful not to touch the edge to the belt and to keep the tool cool.
Note: Sometimes you can see a white mark on the blade if you look at the edge under a bright light. This would show a flat spot that needs to be sharpened.
Other edges - Chisels and gouges are sharpened in the same manner. Outside edges on curved edges can be rolled as you hone the edge. Inside edges are harder to deal with. One option is to use small stones. Round and angled stones, called slip stones are used on these insides. If you do not have these there are other options. Do most of your sharpening on the outside edge. To reverse the burr that is on the inside you can use emery paper glued or wrapped around a wooden form. (180 or 240 emery paper to start, 320 or 400 for fine) Another option for Dremel fans are the rubber wheels with abrasive compounds in them. These are good to use on the inner edges. Again be careful not to overheat these. Stropping is easily done by making channels or forms in a piece of basswood and drawing the tool backwards through the channel after charging the channel with the green compound.
Note 1 - Laminated blades. Swedish Solyd knives (woodworking utility knives) are made with a very hard steel core that has soft steel laminated on both sides of the blade to give it strength so that it will not break. These knives while they look like sheath knives are sharpened without bevels as they are designed to be used on wood.
Note 2 - Differential tempering refers to tempering different parts of the blade to different hardness. This is most famous in Japanese swords which have very hard edges with soft backs. Commercial knives are made with uniform temper as it is too expensive to vary the temper.