|Radiused irons - flat abrasives|
|Microbevels front and back.|
|Use a jig.|
|Copyright (c) 2002-15, Brent Beach|
Other pages in this series show how to sharpen scrub plane blades in various other ways.
This jig is a simplified slanted jig. I use slanted jigs to increase the range of motion. This jig is 2" tall with a 65 degree angle at the top front. It can be used for both grinding a 25 degree primary bevel - extension 3 8/32" - and honing microbevels starting at 29 degrees - extension 2 19/32".
The jig is simplified in its construction. Rather than use a T-nut in the thin jaw, the simplified construction uses a machine screw that threads directly into the thick jaw. This page describes the making of a simplified square jig. The simplified slanted jig is built much the same way.
This jig also uses the modification for highly cambered irons - a pointed base rather than a straight base.
This jig is made out of Douglas-fir - a hard softwood. Woods that are too hard - most tropical hardwoods for example - don't hold the blade as well and are quite brittle. The harder North American softwoods work well for these jigs.
Here the iron is at the extension for the 25 degree primary.I scanned this blade several times during the grinding and honing. In each case the iron was still in the jig, the bevels down on the scanner glass.
This was the condition of the blade when I started. It looks like I had ground and honed it using my belt sander last time. The blade has camber of around 0.09" in a 1 1/2" wide blade (Stanley #40 1/2 type blade). This blade will take a shaving around 1/16" thick (0.063").This scan was done part way through the grinding. The primary bevel is now within 0.015" of the edge across the blade.
You have to check regularly while grinding. Look to see how much of the old honed bevel is still present. Look to see if it is wider in some parts than others. Always immediately work the area with the thickest remnant honed bevel.
I found that holding the back of the blade with my right hand - thumb on top - and pushing down on the blade near the edge with one or two fingers of my left hand allowed my to keep rolling the blade back and forth to keep the primary bevel even. As well as rolling the blade, you have to work back and forth across the abrasive.
The black line on the left side of the blade in this photo is actually a flaw in the blade. This is an older Stanley blade - standard for this plane. It is a laminated blade and the line shows that the blade as very slightly delaminated.I continued grinding the iron until the primary almost reached the actual edge. On the left, it might have. The closer the primary is to the edge, the smaller the first honed microbevels will be.
You can see in these pictures that the primary bevel is quite wide - 0.25". The blade is 0.12" thick - quite thick for a Stanley blade of this era (late Sweetheart).
This picture was taken at the end of the honing process - with the jig on the thick slip using the 0.5 micron abrasive.
This is a scan after the 15 micron abrasive.
Works like normal honing, but you have to continuously roll the blade to keep the honed bevel even.
I cut the paper a little when I started. It turned out the outside corners were a little rough. If you start by smoothing the outside corners a little using only pulling motions you can remove any roughness without damage to the abrasive.
A close look and the first microbevel looks pretty uniformly wide. Does this indicate great skill on my part?This is a scan after the 5 micron abrasive.
Only a very small portion of the edge is in contact with the abrasive. This means you do not need very much pressure at all. With small area and light pressure the force per unit area is still quite large.
Again the second microbevel looks pretty uniformly wide. Does this indicate great skill on my part?This is a scan after the 0.5 micron abrasive.
Again, very uniform microbevls. Does this indicate great skill on my part?
In fact, the honing process naturally creates uniform microbevels, even on these highly cambered edges.
The cutting rate of the abrasive depends on the width of the microbevel. So, parts of the edge that have a wider microbevel will have less material removed, those with narrower microbevels will have more metal removed. The tendency to uniform microbevels is quite strong.
That does not mean you should not check regularly and when you see narrower areas, work those areas a little more. Not exclusively, keep the blade rotating, but work a little harder to widen the narrow areas.
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