The preparation of natural and synthetic stones for knife blade sharpening is connected with the knife sharpening itself. Obviously, to properly make a knife blade sharp, the surface of the abrasive must be flat and clean.
There are actually a number of reasons why natural stones and bars may not be flat, and wearing out is the main problem with any abrasive. Every whetstone gets a dished out area in the center, which makes it impossible to give the proper flat surface to the secondary bevel of the knife blade edge and make it sharp.
One of the reasons to level a knife blade sharpening whetstone or a bar may be the dished out areas caused by the industrial or manual production of the abrasive stone. This applies to Apex type stones used in benchtop household sharpeners with a guide of various levels of complexity and grinders.
Besides, you can change the level of grit of natural stones. Running a little ahead, we should say that if you give some natural sharpening stones an almost mirror-like surface with the intention of using them for a super fine finishing, they may not work. In the future, such natural stones will bring back their working grit level no matter how hard you try to make them finer.
Some aluminum oxide based, diamond and CBN based artificial stones should be broken in before the first use and occasionally refreshed during use.
Breaking in means removing a small layer of material from the abrasive bar. It is most common to do with artificial abrasives. This should be done, for example, with Boride T2 abrasives, because the actual working area of the abrasives is on the sides, not on the plane with the name.
The refreshing is similar to breaking in, in purely mechanical terms, but serves to refresh the abrasive grain structure if the surface side has become too smooth. In the case of diamond and CBN stones, this process also serves to remove the excessive bonding layer, which prevents the abrasive grains from working.
One important knife blade sharpening step is the abrasive care. After each use, it is essential to clean the individual stones or sets of stones from clogging. This is a process in which tiny metal and coolant particles get into the hollows of the abrasive and prevent the grit from working and renewing properly.
Lapping is also one of the processes used to preapare abrasive stones. Lapping is a fine finishing process for the finest abrasive stones. It is usually done with diamond stones on a steel plate using diamond pastes. Afterwards, such stones are used to create micro secondary bevels on the knife blade edge.
Natural stones can be lapped using their own slurry, when water is added to the working surface of the stone and several strokes are made with a fine whetstone with a closed structure, in order to "knock out" the grit from the stone. Due to the lapping on the clean glass, the whetstone gets its natural grit level.
The two following points are extremely important when working with abrasive during preparation and in knife blade sharpening in general.
Most people believe, that when they buy a new sharpning whetstone or bar, they can have confidence that it will be perfectly flat. It is likely to be true only if this is a truly high quality product that comes from a trusted manufacturer. It is better to doubt it than to try to fix mistakes in the knife blade edge sharpening process. This can cause damage or distortion of the secondary bevels surface side, If you do not check the flatness of the abrasive.
In mass production, the manufacturer's quality control department cannot completely rule out some small tolerances in the manufacturing process and the material itself.
For this reason, you should always check the condition of the working surface side of the abrasive before use.
How to do this? You must have some sort of calibration tool, with at least one perfectly leveled plane.
In the best case scenario, such tools would be length gauges that come in sets with special alignment rulers. However, this is an exception to the rule, because such tools are very expensive and it makes sense to buy them only if you really need them.
A good and much cheaper tool for checking the flatness can be a caliper or at the very least a metal ruler.
You can check the flatness of the tool itself by placing a caliper or ruler against glass or against each other. If this test does not show significant deviations, then your tool is straight and you can use it to check your sharpening stones.
Abrasive hygiene is a very important aspect of the abrasive whetstone preparation process. It is very important to remember that abrasives of different material removal rate are used to preapare the stones. For this reason, when switching to finer abrasive powders, the surface of the whetstone itself and the plane on which the whetstone is levelled should be as clean as possible to avoid damaging the contacting planes.
A more detailed step by step description of the process is given below.
The mechanism and basic tools used to preapare knife blade sharpening stones, regardless of the reasons described above, are almost the same.
Silicon carbide powder is the main abrasive agent used to prepare the free-grit abrasive stones and grinding bars.
Silicon carbide crystals are usually colorless and shiny. Black silicon carbide powder is the most common, but there is also green powder. The green powder is more pure, more fragile, and crumbles faster, so it has a higher abrasive power.
Such powders are used for industrial finishing of metal surfaces and metalworking tools of high strength.
In knife blade edge sharpening, such powders can be used to prepare almost all sharpening stones and grinding bars. An exception to this would be a high-strength, closed-type bonding agent. For example, Idahoner Fine and Ultra Fine sharpening stones have a closed bonding type.
Sharpening stones are processed on the free grain - silicon carbide powder in our case, because it is the refreshing and crushing of the abrasive grain that provides the necessary material removal of the sharpening whetstone.
To level out various bumps and different materials, you need the silicon carbide powder of various grit sizes. The best one for roughing and quick removal can be used starting from 80 FEPA units. Further you can use 220, 360, 600 and 800.
Powder change step is approximately a factor of 1.5 to 2, but this is just an advice.
For powders with a FEPA value above 800, many experts have different opinions. The reason is quite simple. There is no point it trying to level out a sharpening whetstone or bar to an almost mirror-like surface. After the first knife blade edge sharpening use the surface side will get more coarse.
Besides, if you still manage to polish a hard whetstone, it will not work as expected. In addition, such fine powders are quite rare.
It is necessary to know one important feature of sharpning stones and bars - there are a number of stones with an open structure - soft and coarse abrasives and for their leveling and finishing it is recommended to use silicon carbide powder of twice the grit. For example, leveling a small dished out stone of 600 JIS (25 µm) grit aluminum oxide stone can be started with 120 grit powder, continued with 220 grit, and completed with 360 grit, which corresponds to the grit size of the stone itself. The use of finer powders is no good.
The glass should be tempered and have a sufficient size. The size depends on the length of the knife sharpening stone or bar that you want to level. It is not important if the glass is rectangular, square or round. The thickness should be about 8-10 mm and the chamfers should be removed.
The same glass for levelling stones of different grit levels is not appropriate. The reason is - imagine you were leveling coarse abrasive on 80-120 grit powder, it would not be reasonable to work further on 200-300 or even 400 powder grit, because the glass surface side is too rough after 80-120 grit and will not give any further results. But taking into account that the glass has two sides, the use of it on the other side can save you some money.
It is best to use a separate, independent surface side for each grit of silicon carbide powder.
The technique of moving the bar across the glass is quite simple - it can be an infinity sign or a zigzag motion or even a circular motion. It is a good idea to change the position of the bar as you work, so that the material is removed evenly, and you should not put pressure on the bar, just hold it in place.
You need water for the powder to work properly, and a 0.75-liter plastic spray bottle. The bottle for spraying flowers or houseplants is the perfect solution. At the hardware store you are sure to find something. You can also use an empty bottle of household chemicals for cleaning windows.
Having a spray gun is necessary because water in the form of a small cloud covers the fine powder and prevents it from spreading. Be especially careful with powders smaller than 300 grit, because they tend to spread through the air and this is bad for your health to inhale them.
A rubber or silicone mat can be very useful to keep the glass from sliding on the work surface.
During work, the silicon carbide powder crushes and creates a fairly thick slurry and will dry out as it works if it is not washed away.
To keep slurry from leaking from your workplace, place the mat with the glass in a plastic tray and occasionally drain and flush all waste slurry into a separate bucket using a spray bottle.
You should also thoroughly rinse off all waste slurry and powder residue with water after you finish using one of the glass surfaces to avoid damaging the prepared surface of the sharpning stone.
You will need a regular pencil to draw a grid or just oblique lines on the surface of the bar. This is required to see how much material is removed from the bar and whether you are reaching the dished out spot.
When you are approaching the dished out area in the leveling process, it is best to also check the thickness of the abrasive on both ends of the bar. It would be best to do this with an electronic micrometer. That way you will know exactly how flat your stone is over the entire area.
It is necessary to prepare all accessories and consumables and proceed as follows:
If you follow the above instructions, you will have a sharpening stone ready to go. If you want to do it properly all these recommendations will come in very handy.
The more you practice doing this, the better you will get each time you do it.
Remember that after you finish levelling the surface of the sharpening stone, there will be a sharp edge on each side of the stone. If you leave the edges as they are, they will come in contact with the knife blade secondary bevel and can cause considerable damage that will be difficult to repair. It is extremely important to remove each edge by chamfering them. To do this, you can use a 400-600 grit powder.
You need to take the stone in your hand and put it to the glass at a 45 degree angle and without pressure run a few times over the thin water slurry of silicon carbide. You will immediately notice a thin chamfer on the edge.
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