|Brent's Pages - Site Map|
|Microbevels front and back.|
|Use a jig.|
|Copyright (c) 2002-15, Brent Beach|
I do have experience sharpening more than plane irons and chisels though. I will add some thoughts and ideas on this page as time permits.
These scrapers all use a soft steel blade with a rolled burr. The blade is softer than a plane blade or a chisel. The blade must be of soft metal or rolling a burr would fracture the edge. The downside is the blade dulls faster.This is a sketchup model of the rolled edge. The blade is shown in the orientation in which it is used. The bevel is down, sharpened at 45 degrees.
Cabinet scrapers usually tilt the blade a little more than in this model.
Typically the burr is only 0.001" or so. Some people say that the blade above the burr acts like a cap iron, to turn and fracture the shaving, thus reducing or eliminating tearout. In fact, the burr is better at breaking a shaving than any cap iron. First, the burr is only 0.001" or less. Try to get your cap iron that close to the blade edge. In most planes, if you set the cap iron within 0.001" of the edge of the blade, the shavings would jam in the plane mouth. Second, the angle between the burr and the blade is steeper than the leading edge of most cap irons. Finally, there is no danger of the shaving getting caught between the burr and the back of the scraper blade!
The scraper blade in this model is about 2 1/2" long by 2 7/8" wide. Some people hone all four edges of the blade. My blades are only sharpened on two opposite ends. The #81 above came with a Disston marked piece of steel - it could be a Disston scraper blade or part of a Disston saw - that is only sharpened on one end.
I put a 35 degree primary on both ends. The first microbevel is 40 degrees. This can be done with a 4" slanted jig, with included angle 50 degrees. The extension in this case is 1 1/4".This is a quickly made honing jig that matches the dimensions of the model, with the blade from the #80 ready for honing. (In fact, rehoning - you can tell from the bright lines along both edges that it already has honed back bevels.)
Rather than try to flatten the back of the scraper blade, I just use the back bevel ability of the jig. This geometry - assuming a thin jaw thickness of 1/8" - puts a 6.5 degree back bevel on the blade with no slip. That is lots. I use slips when honing the front, but not when honing the back. It does not get you as good an edge (the back bevel is too wide to be quickly honed by the very fine abrasives), but the slips put on too large a back bevel. Spend a little longer on the lower grits when doing the back.
You can see that the back of this blade has not been flattened. You can also see the thin bright line at the edge - the honed back bevel. Flat where it counts!
After honing, you can use the blade as it is, or turn a burr in the usual way. With no burr, you take very light shavings. With a small burr at 5 degrees to the front bevel, you take more aggressive shavings. With a 10 degree burr you almost have a fine smoothing plane. It takes just light pressure to turn a burr.
When you come to hone the back, remember the burr (if you used one). If you begin with a push stroke, you will almost surely rip the abrasive. Start honing the back with light pressure and pull strokes until you have removed the old burr.
You might avoid a full rehone by trying to re-establish the burr without honing. First, turn the burr back by using the burnisher on the back of the scraper blade. The burnisher should be parallel to the back bevel. Then turn a new burr from the front.
The spine of the straight razor is thicker than the rest of the blade. When you rest the razor on a hone, the thickness of the spine determines the honing angle. You cannot hone below that angle, but you can hone above that angle by adding the equivalent of a slip under the spine. The easiest slip is a bit of tape. Most people do not use a second honing angle, choosing to hone and strop at the same angle.
Because the jig is built into the razor, you are not actually freehand honing - although it sure looks like it.
The blades in disposable razors are honed with several bevels, the last of which uses a very fine abrasive. With straight razors, people typically use natural stones and then strop on leather (smooth face, no stropping compound). Since stropping helps in this case, the honing abrasive must be quite coarse.
There are a number of very good web sites that deal almost exclusively with straight razors - all facets of honing and use. There are also a number of good discussion forums.
The web site of Belgian discussion forum for users of Coticule hones contains some interesting discussion on hones and honing.
Knives are used in many different ways - there is far more variety in how knives are used than in how plane irons are used.
This variety of form and use means that there are many different correct ways of sharpening a knife. There are also many more ways of testing a knife, depending on how that knife is going to be used.
My use of knives is more or less restricted to kitchen knives. Even there the variety of shapes and uses means there is no single formula for sharpening.
The basic principle of grind then hone still applies. As with plane irons and chisels, the grinding step shapes the knife back from the edge. The honing step shapes the edge.
I intend to do more work with knives and will be putting that work on this page or on a page of its own. In the mean time, here are some photomicrographs of knife edges.
|Note - This jig was designed in response to a question from a visitor to these pages. It has not been built.|
The model blade has a primary angle of 20 degrees. This bevel can be produced by grinding using a belt sander. It would be very difficult to produce this bevel using only hand techniques because of the width of the blade (narrow so hard to hold), the thickness of the blade and the length of the blade. The result is a primary bevel that is large (over 1.68 square inches) compared to primary bevels on plane irons (0.47 square inches), chisels, normal knives. I have found that hand grinding time increases with bevel area in a greater than linear fashion. That is, a tool with a bevel that is twice as big takes more than twice as long to grind. This may be contrary to observed grinding results but it has been my experience. So, while you could hand grind this blade using a jig similar to the one shown below, it would take a lot of time.
Grinding on a belt sander is much more practical. Practical, but not easy. The blade is narrow and long. There is no way to hold the blade so that you can do a complete pass of the primary bevel past the belt. If your hold on the blade is too far from the belt, the blade could tip. Keeping consistent pressure as you move the blade past the belt is difficult. You can practice before grinding actual blade if you make a wooden model of the blade. That means putting a 15 degree angled face on your wooden model before grinding the 20 degree bevel! When you can consistently grind (sand) an even bevel on a wooden model it is time to try grinding the blade. [Of course, if your blade has an appropriate bevel already then there is no need to grind. If the bevel angle has gotten too large from repeated honing, you may have to bring it back to a lower angle as indicated here.]
Here is the same blade sitting on a honing jig. You bolt the blade to this jig the same way you bolt the blade to the splitter. You would have to drill holes through the bottom of the jig appropriate to your particular blade.
This jig is dimensioned to let you hone a 25 degree bevel. The dimensions were obtained from the extension calculator. This calculator lets you go from angle and blade extension to jig height. In this case, the blade extension is the width of the blade. If you want some other angle or have a different width blade, that calculator will size your jig for you.
This the the blade and jig flipped over to hone using a piece of sheet abrasive on glass. The blade width means that the distance between the blade edge on the abrasive and the jig edge on the glass is short - just under two inches. It is very hard to use a honing motion square to the blade edge with such a short range of motion. In this case you would probably be better to use a side to side honing motion. With a blade edge almost 11" long, to get a honing motion of 6 or 8 inches, the abrasive would have to be up to 20 inches long. You could use two 10" pieces, end to end.
If you would prefer to use the normal honing motion (across rather than along the edge) then you could make a slightly different jig. This jig allows a range of motion when honing of over 4 inches - enough for good results. Here is the larger jig on a suitable sized piece of glass.
Sizing the abrasive and the glass is based on your blade. If you use the bigger jig, then the abrasive length must exceed the length of your blade. That is the only requirement. If you use the smaller jig, the abrasive length must exceed the length of your blade by 4 or 5 inches to allow the side to side motion.
The glass must be big enough to hold the abrasive and supply the sliding surface for the jig. For the larger jig, which has a motion across the edge of over 4 inches, the abrasive should be over 4 inches wide and the glass over 4 inches wider than that. For the smaller jig, which has motion along the edge, the abrasive should be 2 inches wide and the glass 2 inches wider than that. In both cases, add another inch to the glass width so you won't keep falling off the edge. Size the glass to the next larger half inch - no need to try for 64ths.
The sizes of the pieces of wood in the jig are also only examples. The thickness of the pieces of wood do not affect the honed angle. The two crucial lengths are the height of the back piece - 1 49/64" in the large jig - and the distance from the blade edge to the front side of the back piece - 3 51/64" in the large jig. Of course, you will use different sized stock.
It is also important that the blade edge be parallel to the back piece. In the case of the smaller jig, where the blade spine rests against the back piece, getting the edge parallel is easy. In the case of the larger jig, you should consider using a temporary piece of wood between the spine and the back when bolting the blade in place. Once the blade is in place, the distance from the blade edge to the back piece is the width of the blade plus the width of this temporary piece. Use that number in the extension calculator to see what angle you will be getting. [Important adjustment about numbers to use in the extension calculator below.]
This is a second example of a wide jig, showing how you can start with a piece of wood for the bevel jaw and build the rest from that.