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Apr 22 2008

Edge Tool Sharpness and Flatness, The Fast Track.

Published by at 3:05 pm under Hand Tools,Sharpening,Skill Development

…Or: How I Learned to Stop Worrying and Love the Honing. 😀

Ok, this is a little longish, but there is no substantial way to provide a sharpening primer in a sound bite. I’ve tried to write about what will work well overall, without getting too focused on too many particulars in any sharpening media. No matter which way you choose to go ahead with sharpening, this advice should be helpful to you overall. It’s a reasonable primer that will put you on the road with usable sharpening skills. So grab a snack and a drink, and settle in for a bit. If you really want to learn to sharpen, reading this will likely be worth your time. Your Questions and Comments are invited as always!

When it comes to sharpening, abrasives are abrasives the world around. They may have particular idiosyncrasies you need to pay attention to, but they all abrade metal. Once you choose the abrasives you feel will work best for you, you will establish your own routine for working with them. All paths are means that will lead to a similar end. Waterstones, oilstones, ceramics, particulates, sandpaper, various styles of machine sharpening etc. The steel does not care; the abrasives don’t care either, as long as the grit equivilents of abrasiveness are appropriate to the goal. Sharpness.

For the sake of this discussion, I am referring to the abrasive grits, as they correspond to the grits common to waterstones. I do this simply for the reason that waterstones are very popular, but I am in no way advocating that waterstones are the best abrasive. Most all abrasives will sharpen, and it is up to the end user to investigate the pros and cons of the various abrasives to determine the best paths for themselves. For cross-reference please refer to this cross reference chart to derive the equivilent grit for the media you choose.

edge_honed.jpg

It is important to keep in mind that the goal of sharpness has stages.

Coarse grits are for grinding, heavy material removal, bevel forming, flattening. Initial flattening and bevel angle forming are the biggest jobs and to aid getting the job over with, the coarsest grits should be used to get the bulk of these tasks done.

Fine grits are for honing and polishing. Once you have established bevels and flatness on the backs, you will want to polish it. Removing coarse scratches in steel with finer ones is what creates finer sharpness. Sharpness actually is where the intersection of the two planes formed by the bevel and the back meet. The finer they are polished, the sharper they will be. the act of creating the wire or feather edge happens when the bevel side of the iron or blade is abraded until the dullness has been honed away. This is required to establish a fresh edge on the tool, and can be done with any number of the different honing or grinding grits.

It is up to the sharpener to determine how dull the tool is, and select the coarseness or fineness of abrasive grit needed to restore the edge to sharpness the fastest way. This means, it comes down to how much steel needs to be removed on the bevel side to form the wire, or feather. You must determine the condition of the edge, and the fastest way to restore it. If only a lttle honing is needed to restore the edge, don’t select coarse abrasives when you begin. If a lot of honing is needed, don’t select fine abrasives when you begin, but realize you will have to polish all the way up through the grits to the fine abrasives to restore the sharpness.

It is important to get a feel for the finish your honing equipment will give you as a finish result at each stage of the work. It will aid you to learn to evaluate what is needed, where to start, how long to hone, and when you have reached what was needed. Knowing this simplifies the task and helps you save time. This is experiential– it is learned by using the sharpening tools you have on your edge tools. It is getting to know one another. Call it sharpening intimacy if you will.

A little about flattening the back. Many people take this as meaning they must flatten the entire back of a chisel or plane iron to properly complete this step. Nothing could be further from the truth, but you can be happy if you’ve the mind to. Try thinking of how a knife has a bevel on each side of an edge. Remember how we hone both sides to make a knife sharp? We are simply trying to hone both sides of a chisel or plane iron too, but we want to keep the flat side in plane with itself for the most part.

Flattening the back really only means that you only need a planar surface near the location where the bevel is, and the width of flatness on the back, or in other words how far away from the edge on the backside need be no wider than the bevel is, but you are welcome to flatten more of the back if you like, because sometimes it is easier to hold the tool on a wider surface.

Flattening doesn’t have to kill you or be drudgery, just buy a Kanaban plate and some silicon carbide grit or diamond paste and get it over with. There are a number of places that sell those items, so in all fairness to them, please use a search engine for pricing. That is the fast track to flat backs. It is more important for backs to be a planar surface, flat, than it is to have a mirror finish, but the mirror finish is what we often wind up with eventually anyway.

flatback.jpg

The bevel side too, the bevel’s actual surface, is a narrow flattened plane. It is the intersection of these two planes where the angle forms. The flatness of these two planes are what help them succeed at being very sharp, at the angle point. In fact it is difficult, maybe impossible to achieve a high degree of sharpness if this planar-flat surface isn’t present on each surface that makes up the bevel.

It is geometric, and has to respond to a couple different things all at once to be most effective. Just like you would hone both sides of a knife to restore its edge, It just happens that on edge tools, the back makes up half of the beveled edge. It is the straightness of these two planes which form a line along the intersection of these flattened and honed surfaces, and that makes this edge. It is also often helpful if this honed edge is square to the side of the blade, unless skew (an angle that is not 90 to the edge) is desired.

bevel_flatness.jpg

Guides are useful for grinding specific angles. Please feel free to use them for the heavy work. Honing and polishing are something often done quicker, which is more useful and easy if you can learn to do it free hand. Free handed, the side sharpening method is likely the easiest to learn and use. The steel does not care, the wood does not either. If you cannot do this well enough, please, feel free too use the honing guide.

I’ll add here that chisels seem great candidates for free hand honing, and I recommend it. It’s quick and keeps the tool really sharp as you work with it. Use the guide to grind though because it is faster. Avoid micro bevels on chisels to aid freehand sharpening. Never back bevel a chisel either, because the back of the chisel is important for registration in the work.

Plane Irons are another matter. I feel micro bevels can really help you here because there is a lot more metal to remove, and the honing process is more involved while getting blades in and out of the plane. I recommend using the guide all the time on plane irons, if really sharp sharpness is important to you while planing.

Back bevels can be useful for getting sharp faster on plane irons. Use them. They have other benefits on plane irons as well but I’ll cover that another time.

Remember, forming a wire or feather only means you have ground, honed or polished past the dullness, depending on how long you allowed things to dull. It means you can now stop grinding and start honing and polishing.

Honing up through the finer grits is how we remove the wire edge created by honing past the dullness. If you hone and do not create this wire or feather, then you have not honed past the dullness. The dullness is the wear on the edge that you want to remove. The wire edge is what you want. It is the indicator that you have abraded the steel enough to have ground away the dull parts. Once you have achieved this all the way across the edge, you can then begin working both sides of the bevel with the grits appropriate to the level they are currently polished, alternately, to hone the edge to working fineness.

When honing off the wire, always hone the backs of the tool with the finest grit you have previously honed the back with. It often means alternating stones from the bevel to the back, but why scratch up the back if it is already polished? Hone both sides on the appropriate grit as you work up through the grits.

wire_chase.jpg

While honing or polishing, never push the edge tool into the stone, always drag it. Breaking the wire edge off is easy to do and pushing the edge will increase the risk. Braking the wire edge off is equivalent to dubbing the edge. Instant Dullness. We want to hone the feather or wire off, not break it off. The end result is vastly different. Pull the blade while honing and you’ll be fine.

If you examine your backs and bevels after honing to a mirror finish and see a glint of light right at the angle point, that is a flaw in your work. It should be a complete surface with no glints of light sparkling at you, especially from the bevel angle point. Glints of light indicate dullness.

In most steels, once you have honed through the grits to the 8000 grit stone, you have honed finely enough to pare end grain pine. End grain pine is the most difficult wood to pare without crushing; it really is the toughest task an edge tool for use in wood will ever see. (Japanese tools often use finer steels and traditionally were used in softer woods, so there may be some benefit in honing good Japanese steels slightly finer if for use in soft woods)

side_sharpening.jpg

There are no recognized tests of, or charts with “scales of sharpness”. The time honored test for sharpness has been and always been; if the edge is sharp enough to do the required work in the material required, to the level of result desired, then sharp was sharp enough. I personally contend that with the wide ranges of tooling, sharpening media, and woods to use them on, this test for the woodworker is still good enough to be true.

The only major corollary to this adage is concerning edge-wear. Can the sharpness last a reasonable quantity of time, so as to bolster productivity? That would depend on the environment. Is it one of production or for that of the hobbiest? Harder tooling is wonderful in a production setting where the sharpening media can match the task of maintaining it, but can the hobby woodworker go toe to toe with the price tag needed to buy in? For most of us, common tooling is just fine, and the results of adequate sharpening as shown in the work have always sufficed, once learning what is needed so as to make results repeatable has been established.

You do not have to shave off your hair! Many people contend hair popping sharpness is an adequate test for sharpness. The truth is, it isn’t. Hair is not wood, and shaving a hair from the surface of skin is is a completely different set of circumstances and dynamics happening, that actually do not require the sharpness required to take a shaving from wood.

The sharpness required for shaving is not as sharp as required for paring wood and as such is not an adequate test for pairing wood. I can shave hair with knives coming off 1000 grit abrasives. Since this is true, what if you can shave hair, should you stop honing well before you are sharp enough to do the work you need to? I was able to accomplish hair popping sharpness with spit on carborundum stones as a young kid, so I am not personally impressed with hair popping sharpness.

For woodworking, shaving hair is the equivalent of a neat card trick. Good for show, not much go. This is a long way from the sharpness we need to push a blade through end grain pine with least effort, so as to pare it. I am saying, we are aiming for and achieving a much higher level of sharpness. If you have sharpened to 8000, you are well past the sharpness needed to shave hair. (Read this as, being able to shave hair can fool you, I hope I have made that clear.)

04/jig_honing.jpg

If you visually examine the edge and see no glints of light reflecting from your edge, and you have polished to the 8000 grit level, no tests are necessary. At 8000 and after a little stropping on leather, the tool is as sharp as you will ever need, I assure you. Honing beyond here is a lot of work spent honing with a diminished work time in wood. In other word, the honing takes longer than the dulling does in this range. You can strop on leather with a little honing compound if you like for a slightly finer edge, which is sometimes temporarily helpful in softwoods. Again, if you feel the need to test, the pine is fine.

fine_honed.jpg

Much has been and continues to be written about sharpening, and I encourage you to study this subject, and pay attention to your own realizations as you sharpen. This here is at the heart of what you really need to know. It isn’t rocket surgery. Once you get the heavy work done, (flattening and bevel shaping) it is done forever, and “maintaining” sharpness should generally take no more than 30 strokes on most any stone as you hone up through the grits. This means, If it is taking more that 30, you chose to hone with a stone to fine for the work you need done.

Don’t let your tools get too dull before you touch them up. You can easily maintenance hone as you work. It can take less time to keep your edges in working condition if you become a fastidious maintainer. If you have to rebuild edges every time you sharpen, then you have likely waited too long, and that is a lot more work than 30 strokes of maintainence sharpening.

I need to touch on one last thing here. Bevel angles. Commonly, woodworkers like to bevel their tools so as to be easiest to push through the work. I feel there are some hard fast rules that need to be understood.I am going to touch on a common for instances.

In a nutshell, not all steels are created equal. There are trade-offs we have to learn to live with. Here are a couple.

A-2 like any Steel, has a particular molecular structure. In A-2, the hardening process forms carbide particles in the steel which has a high wear resistance. It will stay sharp longer than that of other steels, but it will require you to sharpen it at usually no less than a 35 degree bevel angle in order to maintain an edge that won’t fail. In other words, If you attempt bevels of 30 degrees or less with A-2, the effect will often result in the edge failing and crumbling. This is due to the very carbides that form to make it wear resistant. It also takes longer to sharpen than High Carbon, or O-1 Steel. As such, this steel is not the best choice for low bevel angles where paring is desired. A-2 is far better lasting where the tooling will be struck with a hammer like in mortising, or for people who prefer a lot of chopping with their bench chisels, or when used to plane in abrasive woods like many tropical hardwoods.

O-1 and high carbon steels are considered finer grained and do not form these carbides in them in the same way A-2 does. As such, these steels are able to hold a shallower bevel angle than A-2 commonly can without edge failure, they sharpen faster, some feel they sharpen finer, and lend themselves well to shallower bevel angles that works well with paring and lighter impact work that is common with many american hardwoods.

in any case, watch your edges. If you find them failing it is usually some combination of the steel type and wood hardness coming to loggerheads with the style of work you are performing and the bevel angles you have. Prepare to adjust the bevel angles accordingly.

My overall sense of this as well as my recommendation to you is this. A-2 Steel really prefers most usually to have a 35 degree bevel ground on it for best outcomes. To go shallower than 35 degrees with your bevels is something you may find works, but please don’t have high expectations. While these angles are not good for paring, they are great for rough service, so mindfully purchase O-1 or high carbon steels for the paring tools. O-1 Is not going to hold a lasting edge is really rough service. Steepening the bevel angles will help, but it still preforms better for finer work. Asking one steel to be all things to the various woodworking tasks is not going to happen. The same is also true of the tooling itself, some things simply find it difficult to interchange. Generally Speaking, Rough service bevels are in the 35 degree range, General purpose bevel angles are in the 30 degree range, and light service or paring bevel angles will be in the 25 degree range. The steel you have may require slight adjustments, just realize your steels can not be all things to all situations and you will be well serve when reaching for the right tool for the job.

I hope these tips help get you on the path to maintaining your tooling with the least effort possible!

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