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| Sharpening - Grind then hone |
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| Finest abrasives. | ||
| Microbevels front and back. | ||
| Use a jig. | ||
| Copyright (c) 2002-15, Brent Beach |
In these pages I describe a sharpening sequence that uses coarse abrasives to shape the tool. I try not to let the coarse abrasive actually reach the edge. This is the grinding step. Once the tool has the correct basic shape, I use a series of finer abrasives, starting at the edge with each new abrasive. These are the honing steps.
So, two different approaches, each with its own name. Grinding - coarse abrasive away from the edge. Honing - fine abrasives starting at the edge.
Assuming a good edge geometry with no nicks then, I usually adopt an optimistic view and assume I can just hone the blade. So, I set up for honing and do a couple of test hones on the 15 micron abrasive.
Where did the abrasive scratch the bevel?
| Scratch location | Diagnosis | Action |
|---|---|---|
| Well back of the edge | The primary bevel angle is greater than the honing angle. | You will have or grind the primary bevel. |
| At the edge | The primary bevel angle is less than the honing angle, all the way to the edge. | Only minor honing with 15 micron required, then continue with finer abrasives at higher angles. |
| On an existing microbevel that is wider than 1/64" (0.5mm) | Repeated honing has produced a wide first microbevel. | It is time to reform the primary. This page takes you through the grinding process, preparing for honing. |
| Unsure? | You need more explanation than this short table. | Go through the page in which I grind then hone a factory fresh blade, with lots of scans and explanation. |
When a woodworker talks about a sharp edge what they really mean is an edge with the correct included angle, with smooth surfaces. The "correct" angle depends on the tool, the wood, and the way the tool is used. Usually we cannot change this angle. What we can do is get the best possible - smoothest - surfaces at the edge.
Now smooth is a relative thing, since all surfaces are bumpy at some level. Each size abrasive puts scratches in the surfaces. The size of those scratches depends on the size of the grits in the abrasive. Nicks in the edge are formed from these scratches. The edge therefore has a non-zero size - related to the depth of the scratches. If you design your sharpening sequence so that each abrasive removes all the scratches left by the previous abrasive, the thickness at the edge is limited by the size of the grit in the final abrasive.
Only the last few thousandths of an inch of a plane blade actually come in contact with the wood. This is the only part of the blade that need be smoothed with the finest abrasive.
How fine should the last abrasive be? I use 0.5 micron 3M abrasive for this final smoothing. Is this fine enough? Too fine? I have done a few tests in which I stopped after the 5 micron abrasive (with the same final included angle). The results were inconclusive - continuing on to the 0.5 micron abrasive might be required. Stopping before the 5 micron abrasive is a mistake (at least 1200 grit USA, 5000 Japan, P4000 Europe). Going beyond the 0.5 micron abrasive will not change edge quality in a measurable way.
In summary, for any given included angle, sharper really means smoother. An abrasive somewhere between 5 micron and 0.5 micron abrasives is the right final abrasive for woodworking tools.
Knowing what abrasives to use is only half the battle - you must also use them correctly. Correctly means you must focus your honing efforts starting at the edge when using very fine abrasives. If you try to smooth too large an area, it just won't happen - you can't remove all the scratches left by previous abrasives. You will remove some of the tops of the scratches, but will not get down below the bottoms. With very fine abrasives you must use a honing jig.
Grinding - use coarse abrasives - prepares the part of the bevel away from the edge. This is very important.
This is a composite image (225X) of a blade before and after grinding through the edge with a coarse Silicon Carbide bench stone. The image on the left shows the dull blade. This blade was sharpened using my standard procedure then put through an edge durability test until it would no longer plane easily. The narrow bright line along the edge of the dull blade is the wear bevel that we are trying to eliminate by sharpening the blade.
I then ground the blade at the same angle as the final microbevel (about 32 degrees) on a Silicon carbide bench stone. The image on the right shows the resulting edge. It is clear that there is a lot of honing now required to remove all the scratches left by this coarse abrasive. Much more than if the grinding had stopped short of the edge.
How could the ragged blade take shavings when the apparently smooth blade could not?
The answer lies in the shape of the blade at the edge. While smooth even at 225 times magnification, the blade on the left won't plane because its shape is wrong - it has a very narrow wear bevel on the downward facing side. This wear bevel acts like a boat hull, causing the blade to climb up the wood and plane along on the surface. The rough blade has no lower wear bevel. It is able to cut into the wood. So, both the shape of the tool right at the edge and the smoothness of the upper and lower bevel, determine the sharpness of the edge.
If a blade as rough as the blade on the right works, why bother with honing with fine abrasives? That question is discussed in the page on grit and edge durability.
The is one more unexpected result - it will take less time to hone the dull blade. Again, the dull blade has the wrong shape but smooth surfaces, while the sharp blade has the right shape, but rough surfaces. It is easier to rehone the dull blade because the edge is defect free. Once we start to hone the blade on the right we will find chips at the edge that will take quite a bit of honing to remove.
This is a scan of (the middle third of) a blade made of a steel type called CPM 3V. This metal is close in toughness (wear resistance) to High Speed Steel. The primary bevel was prepared using a bench stone in a couple of minutes. The primary reaches with less than 0.01" of the edge, uniformly across the blade. This blade has no metal damage at the edge and will take just a couple of minutes to hone to the usual brilliant, durable edge.
The key to grinding is to bring the primary bevel up to, but not through, the old edge. Grinding is not meant to destroy the edge, it is meant to prepare the tool for honing.
My grinding with a benchstone page shows you how to do this.
I have found that a 1" belt sander can be used fairly well to grind up to the edge. This sharpening tutorial was based on a belt sander and sheet abrasives on glass.
I have used the belt sander for years to grind plane irons. Occasionally I still make a small error and blue a corner of a blade. After discovering the coarse Silicon Carbide bench stone, I now use it except for radiused blades, reshaping an edge, or for a badly chipped blade.
Next page of the FAQ - The geometry of Honing
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