|Radiused irons with a belt sander|
|Microbevels front and back.|
|Use a jig.|
|Copyright (c) 2002-15, Brent Beach|
Scrub planes use highly cambered irons. Edge shaping of scrub irons is covered in the main page on using a belt sander for grinding.
Having gotten the desired camber, grind the primary bevel with the 40 grit Alumina Zirconia belt, but with the tool rest at 25 degrees.
Again, you should be rotating the blade constantly. Even where there is an obvious high spot, don't simply hold the blade at the spot, but rotate the blade back and forth, with some extra pressure at the high spot.
You can see that most of the sparks are under the blade. I am grinding near but not at the edge. If you let the 40 grit abrasive reach the edge, you will have an edge that will require lots of additional honing. This technique of grinding up to, but not reaching, the edge is discussed at length in the From New to Sharp page.
In this picture, you can see that the primary does not quite reach the edge. It looks like the primary is concave, especially near the left side, but the primary is flat.
By not grinding the actual edge, you preserve the edge quality prior to grinding the primary. In this case, the edge quality was bad, since we just shaped the edge. However, most primary grinding is refreshing an earlier primary after a few microbevels (honing cycles). In that case, the edge is still pretty good -- better than if you now hit it with 40 grit. You can see the size of the abrasive particles on the 40 grit belt in the background.
Now using 600 grit, with the tool rest at 30 degrees, hone the secondary bevel.
Again, you should be rotating the blade constantly. I use very light pressure here - there is very little metal to remove.
Because we are honing at a larger angle than the primary angle, first contact with the abrasive is at the edge. As you hone the microbevel widens away from the edge. You have very little metal to remove, so work with very light pressure.
Sparks were flying during this operation, but not at the same level as the grinding operations.
This blade is ready to use. In fact, I used it to make the jig described in the section on hand honing. I was able to plane a groove in a piece of Douglas-fir .15" deep, by gradually increasing the set on the blade.
You do not need to have a scrub plane to use radiused blades. In this test I used a transitional plane - a wooden bodied plane with metal depth adjust. These planes usually do not work well for fine work - the almost certain misalignment between the metal frog and the wooden base seems to ensure poor performance. However, they can be used for rough work like scrubbing.
The blade is a laminated Stanley Sweetheart iron. [Aside: I think this is the best blade value around, especially when you can often get it complete with a plane for about the cost of a new after-market blade. My tests have shown that these blades, while not as durable as new blades using A2 or HSS steels, are easy to sharpen and retain a much smoother edge during use than the A2 blades. Not that a smooth edge is that important for a scrub plane!]
I reshaped the blade with a 60 grit AlZ (Alumina Zirconia), holding the blade perpendicular to the belt. The 2" blade has a 2-1/2" radius of curvature.
I shaped the primary bevel at 25 degrees using 120 grit AlZ. This first picture, at 10 times magnification, shows the entire bevel. The arc midway across the bevel is the lamination boundary. (It looks like the high carbon steel is 45% of the thickness of the blade.)
Increasing the magnification to 60 times, the lamination boundary is still visible near the right edge (it shows up as a change in the texture of the image).
The edge itself looks pretty rough.
At full magnification, 200x, the cutting edge looks pretty ragged.
The uniform brightness away from the edge indicates a flat bevel. The dark areas at the edge probably represent a wire edge -- thin steel pushed back by the abrasive belt but not yet broken off.
The width of the bevel visible in this picture is about 1/32", the part in focus about 1/100".
Still working at 25 degrees, this time with 180 grit AlO (Aluminum Oxide).
This time with 220 grit AlO, at 25 degrees. I am using a very light touch with these grits, just trying to improve the main bevel slightly. I rotate the blade on the rest, with very light continuous pressure on the belt, very little sparking. The blade does not warm up in the few passes I make here.
The darkness at the edge indicates the wire edge is still present.
With 320 grit AlO, at 25 degrees. Light passes, cool blade.
There appear to be fewer problems near the edge, the uniform brightness almost reaches the edge now. The edge is still not very straight.
To direct the smoothing effort specifically to the area of the bevel near the edge, I increased the angle 30 degrees, using a 600 grit belt.
You can see the margin of the new secondary bevel towards the right side of the picture. This secondary bevel is about 1/50" wide.
The edge itself is getting better, but is still uneven.
Again at 30 degrees, this time with a 15 micron 3M belt. 600 grit and 15 micron grit appear to be about the same.
I buffed the bevel, front and back, very lightly using a honing compound on a hard felt wheel. The level of scratching left by the honing compound is consistent with results obtained in other tests of honing compound on leather and hardwood - a bit better than 600 grit.
Notice however that buffing appears to have removed the wire edge completely. The edge is now much smoother and much straighter.
In use, the plane (a Stanley transitional #35) worked pretty well. After about 5 minutes of use, I removed the pitch from the blade and took this picture.
Notice the very thin "wear bevel" along the edge (the continuous thin, slightly darker line right at the edge). It looks darker because it is at a steeper angle to the light used to illuminate the blade, so does not reflect into the microscope the same amount.
The width of the wear bevel is consistent with wear bevels in my earlier tests of edge durability of bench and block planes. The quality of the worn edge - still straight and smooth, with no drop outs - is consistent with earlier observations of Sweetheart irons.
To test how buffing affects a worn blade, I buffed the blade again. This time I held the blade a little differently, so the scratches produced are angled to the edge.
Buffing did not remove the wear bevel.
Buffing is useful in the removal of the wire edge created during grinding, but cannot remove the wear bevel.
This blade is clearly not as well sharpened as those blades sharpened with 3M abrasives on glass. The scratches left by the honing abrasives are much larger than those left by the 5 micron and 0.5 micron abrasives. If you use this system you should expect that the blade will dull a little faster than a blade sharpened in my usual way.
I have read claims that hot grinding - using a belt sander or a grinder - heats up the edge and leaves a burr made up of bits of abrasive and bits of melted steel. Cool grinding - sheet abrasive, oil and water stones, Tormek type systems - in contrast does not leave this burr. Part of the claim is that this burr produces a rougher surface on the work, then breaks off leaving a rougher tool edge.
The roughness apparent in the pictures at lower grits could be this burr. After buffing though, I see no signs of such a burr or damage from its removal. After-use comparison of this edge and edges sharpened using by abrasives on glass, shows little difference. Buffing appears to be able to remove any burr without damage to the edge.
I suspect that wood turners could use this combination of a belt sander and a buffing wheel to get sharp, durable edges on radiused skews and gouges.
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