sharp/dull blade drawing Radiused irons - concave abrasives small map
Finest abrasives.
Microbevels front and back.
Use a jig.
Copyright (c) 2002-15, Brent Beach

Introduction

On page 3 of an article here, Christopher Schwarz demonstrates honing a radiused blade using a fid. This use of the word fid is not in the online dictionaries, but I will use it here. In short, he uses a scrub plane to cut a matching groove in a board, glues abrasive to the groove, then uses this fid to hone the blade.

A longer explanation of the steps involved:

  1. end view, board with groove planed in Use your scrub plane to plane a groove along a piece of softwood. I used a 24" long board with straight grain and no knots to make 3 8" fids.

    You must clamp a batten to the board, so the plane follows the same line each time.

    Because the scrub plane sole on either side of the mouth prevents the plane from dropping into the groove, you must advance the blade after each pass. Don't try to take really thick shavings. The resulting groove should be smooth and free of tearout since it will be the base for the abrasive -- so you must use a decent piece of wood. If each shaving is 0.002" thick, you will be done after 75 passes.

    I used a Stanley #27 transitional plane for this step. The plane's mouth was 1/4" - which is enough to allow the plane to eject shavings even when the blade set was 0.15". Metal bench planes with narrower mouths did not clear the shavings - I had to pry the shavings out with an ice pick after each pass.

    Continue until the groove is the full width of the blade - if possible. If you are using a bench plane, a plane with a cap iron, start with the cap iron about 1/8 inch from the edge. When you can increase the set no further, move the cap iron back another 1/8 inch from the edge and continue. You might have to set it back 1/8 inch again to complete the full width groove. Making a groove the full width of the blade will take some time - 30 minutes if you have all sorts of problems (first of 3 tries), 10 minutes if you have done it before and your blade is reasonably sharp (third try).

    I was quite surprised that a transitional plane could produce thin shavings with almost no skipping or tearout on this board, even with the cap iron 1/4" from the edge and the edge .15" below the sole. I did take quite thin shavings though. In this usage, the plane was not acting as a scrub plane. It was more like a moulding plane.

    You may have guessed, looking at this picture, that the groove is not circular. If you begin with a circular blade, the resulting groove will be elliptical. The long axis of the ellipse is horizontal and equal to the diameter of the original circle. The short axis of the ellipse is vertical, and about .7 times the original. That is, the groove is shallower than the corresponding section of the circle (see picture below).

    blade set Surprisingly, the #27 was able to plane the groove into a Douglas-fir board with the blade extended to expose the entire radiused portion. There was a little chatter at the start of some passes, and some tearout at a bad grain reversal, but the groove is very smooth.

    There is a chicken and egg problem here. If you don't have a fid, you can't use a honed blade when making the fid. While finish quality is not a concern with normal Scrub plane use, in this case you need a smooth surface for the abrasive.

  2. PSA backed abrasive Put PSA backed abrasive into the groove, forming a shaped hone.

    This could be a sensitive operation. You want the adhesive to bond to the wood, but not have any air gaps that will affect the shape of the honing surface and could result in early abrasive failure if the blade hones the bump away. When working with glass, getting the glass very wet gives you time to position the adhesive and squeeze out air bubbles before the adhesive bonds to the glass. I did not want to get the fid wet, since it is unfinished wood.

    I was a bit worried about this step, but it turned out not to be a problem for either the 15 micron or the 5 micron abrasives (this is the 5 micron abrasive). I cut a strip of abrasive to size, removed the backing material, put one edge along one side of the groove then, with the abrasive squeezed from the other edge a little to get the right shape, slowly lowered it into the groove. Perhaps because the abrasive does not stick to wood as well as it does to glass, there was no problem with air bubbles. I used a wine bottle to roll the abrasive down against the groove.

  3. stone vice, fid, spacers Use the resulting fid as a hone.

    Chris puts the blade in a vice, then uses the hone freehand at (about) the desired angle. I have my bench stone holder which turns out to be a fid-holder as well. [I cut the fid to length using my Goodell-Pratt mitre box, so the ends are square in both dimensions.]

    There is a slight problem with positioning - I want the base, not the top of the groove aligned with the surface of the vice. The solution was to measure the depth of the groove, make up a 12" slip this thick, saw it in three, and use the resulting slips to raise the jig above the table surface, while the fig remained on the surface. This gets the bottom of the groove aligned with the glass surface and makes sure the fid is horizontal.

    Three slips seem to be required -- two keep the wide jaw level, one to raise the narrow jaw to the same level.

Problems

blade in groove As mentioned above, there is a problem of circles and ellipses. If you begin with a blade with a circular edge, then the groove will be elliptical.

The relationship between the original circular blade and the resulting elliptical groove is completely determined by the bedding angle of the plane. With a plane bed at 45 degrees, the resulting elliptical groove will always have depth .7 times the blade camber.

blade at honing angle in groove It is the opposite problem when when you use this groove to hone the iron. A groove planed with the blade bedded at 45 degrees then used to hone with the blade at 30 degrees is too deep. Now the blade only meets the groove at the corners.

Solutions

What if we use this fid to hone the blade? It is clear from the picture that the resulting blade would be more sharply curved. Lets go ahead and see what happens.

What follows is a sequence of honing and grinding steps. Honing identifies the high spots, grinding reshapes the primary bevel just at the high spots.

scan, bevel side, shows two very small honed bevels I used the extension calculator in the usual way to find the extension needed to get a 30 degree microbevel. I put the blade in the jig and set the extension using my combination square in the usual way. Then, using the fid with the 15 micron abrasive, I lightly honed the blade. As usual, I began with pulling motions to avoid cutting the abrasive.

I honed just until the honed microbevel was 1/32" wide.

Oops! Look at how small that microbevel is. You can't see it? Well, it is the small triangular smooth bit out by the right side of the blade. It turns out it is only about 1/8" side to side!

A little discouraging, clearly suggesting a move to plan B. Fortunately, the board in which I planed the groove for my fids was long enough to make 3 fids. I put some 40 micron abrasive into the third fid and continued.

Continuing meant first regrinding the primary just at the honed microbevels, nowhere else along the edge. With the belt sander tool rest set at 25 degrees, I ground the primary under these two very small microbevels so the primary reached almost to the edge again.

photo, bevel side, showing current microbevel I repeatedly honed and ground. Each grinding operation spread the primary back toward the edge in those parts of the blade with honed microbevels. Each honing operation creating a new microbevel that curved a little farther around the blade.

Hone again. Notice that the microbevel is spreading inward toward the middle of the iron. The apparent microbevel on the right side of this picture is left over microbevel from the previous sharpening of the blade before making the fids. Only the shiny secondary bevel at the side is the newly honed microbevel.

This is the fourth honing. Progress is slow even with 40 micron.

photo, bevel side, showing current microbevel Grind again, hone again. Only grind the area of the honed microbevels, not the middle of the iron.

Fifth honing.

photo, bevel side, showing current microbevel Grind again, hone again.

Sixth honing.

photo, bevel side, showing current microbevel Grind again, hone again.

Seventh honing.

photo, bevel side, showing current microbevel Grind again, hone again.

Eighth honing.

I am not sure why this is getting faster, but the microbevel is spreading across the edge.

photo, bevel side, showing current microbevel Grind again, hone again.

Ninth honing.

photo, bevel side, showing current microbevel Grind again, hone again.

Tenth honing.

photo, bevel side, showing current microbevel Grind again.

Tenth grinding. To show how the grinding comes close to but does not actually hit the edge. I could have gotten a little closer right at the side, but it is better to do a little less than do too much.

photo, bevel side, showing current microbevel Grind again, hone again.

Eleventh honing.

This is a Stanley laminated blade -- you can see the lamination line in this picture.

photo, bevel side, showing current microbevel Grind again, hone again.

Twelfth honing.

photo, bevel side, showing current microbevel Grind again, hone again.

Thirteenth honing.

Showing the whole edge here, since the microbevel almost goes side to side.

photo, bevel side, showing current microbevel Grind again, hone again.

Fourteenth honing, and the microbevel spans the blade.

While the honing/grinding sequence to take the blade from the radius used to plane the groove down to the radius created when honing in that groove may appear to pretty long, you only have to do it once. Subsequent honing will just require 30 seconds on the 15 micron, then 30 seconds on the 5 micron. If the honed bevels are too wide, regrind the primary almost to the edge and then hone. The total time should be just a few minutes.

If you decide to grind another blade to this new radius, begin by tracing this shape on the new blade and grinding it to shape as shown above. Then form the primary bevel by grinding at 25 degrees. Then use the fids to hone that blade.

Finer Abrasives

micrograph - 15 micron bevel With a 40 micron secondary bevel the full width of the blade, it is time to move on to finer grits. In this case I chose to regrind the primary, again stopping just short of the edge, then honed with 15 micron abrasive.

The left half is the microbevel at 30 degrees using 15 micron abrasive, the right half is the primary at 25 degrees using 120 grit on the belt sander.

micrograph - 5 micron bevel I used a 0.01" slip and 5 micron abrasive next.

Using a slip increases the angle and hence changes the shape of the fid, from the point of view of the blade. Even with this slight increase in bedding angle, the blade meets the fid only in the middle of the blade. That is, by using a slip we isolate the abrasive action at the middle of the curve. This is exactly what we want of course, since we will only using the middle of the blade when scrubbing, as opposed to making a fid.

In this picture of the middle of the blade, the 5 micron microbevel has completely removed the underlying 15 micron microbevel. The left half is the microbevel at 32 degrees using 5 micron abrasive, the right half is the primary at 25 degrees using 120 grit on the belt sander.

The fact that raising the jig on a slip makes the radius of curvature of the blade appear smaller, thereby allowing you to hone just the middle of the blade rather than the entire edge, means honing with the finer abrasives takes even less time than for a straight edge. A few swipes and you are done.

Honing on slips works so well that I now regret not making 4 fids so I could use 40, 15, 5, and 0.5 micron abrasives.

Target Radius

scrub before and after radii compared The resulting edge is elliptical, not circular, with radius 2", not 2.5".

This picture has a scan of the blade after honing superimposed on a scan of the blade before honing - the shape it had when it was used to plane the groove.

Double click on this image to see the full sized scan, click to get back to the thumbnail.

The lesson here is that to make a fid that hones to particular radius, you must plane the fid groove using a blade with a larger radius. If you want a 5" radius of curvature on your scrub blade, you must plane the groove with a blade whose radius of curvature is about 7". I started with a blade with a 6" radius, ended up with a blade with a 4.3" radius.

Back Bevel

Unfortunately we cannot simply flip the jig over and do the back bevel. With the back bevel having a honing angle of 2 degrees, a fid with a 0.15" depth would put a camber of 4.3".

Working the back of a cambered blade is probably best done with the blade flat on the abrasive. Raising the blade even a bit would isolate a very small arc of the blade on the abrasive, quickly resulting in flat areas at the edge.

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