All of my plane irons have a bevel. Presumably it is close to the original 25-degree angle. But after flattening the backs, the next step, grinding and honing the bevel, wasn’t working.
The problem I was having seemed to be that the tip of the edge was not getting honed. After a good bit of grinding with a coarser grade paper, I noticed that the other edge of the bevel (let’s call it the “heel”), opposite the tip, had the grinding marks. I surmise that this is because the angle of the bevel is greater than the angle used by my sharpening jig.
For abrasives, I am using #320 paper glued to glass, and then a variety of papers set on top of the #320 for grinding and honing, the way I was taught in the sharpening class I wrote about a few entries back. For the jig, I made one with 1.73” height from Brent's sharpening jig instructions.
Technically, the two layers of paper changes the geometry slightly, thus the recommended blade extension from Brent’s calculator could be thrown off by the additional height. But while that is true, I think the amount of the change is small, and I note that the accuracy of Brent’s calculator is only to the nearest 1/32” of blade extension, which is potentially a bigger error.
I am using a Porter-Cable PCB8575BG variable-speed 8” grinder with its original wheels. The specified speed range is 2000-3450 RPM.
I also invested in a Veritas bench grinding tool rest some time ago. While I was nervous about setting a new bevel for my blades, mainly because plane irons are considerably thinner than the chisels I have set bevels for in the past, with trial and error I improved my technique and got a suitable new bevel.
Learning from My MistakesHere are some of the errors I made and what I learned by making them:
- It was easy to overheat the blade, resulting in black/blue marks at the edge where it contacted the grinding wheel, which represents places where the blade has lost its temper, thus getting software. There are three ways I know of to address this. First, dip the blade into water to pull out the heat. But for a wide blade, the blade does not get into the water fast enough. Second, noting that the most common location of the burn is at the corners, move the blade more quickly as I start, and as I finish, to reduce the time this area is in contact. Third, noting that it was the grinding time and repetitions that caused the burning, use the coarse wheel instead of the medium one I was using. Because fewer grains of abrasive contact the blade, it does not heat up as fast; also, it does not take as long to work, so I can move more quickly and use fewer repetitions. My grinder has a minimum speed of 2000 rpm. For grinding blades, a slower speed would be good. See below.
- Having achieved a bevel, I used a square to check the blades squareness, and by drawing a thin pencil line, found that it was not square. Realign the grinding tool rest to the wheel, and make sure that the blade is snugly against the guide pins, indicating proper alignment to the jig.
- Having achieved a square bevel, I found that the heel of the bevel, not the edge, was getting ground when using the jig. This is because the grinding angle is greater than the angle using Brent’s jig, and requires adjusting the tool reset to a narrower angle and starting over. Though I used the angle jig included with the Veritas grinding support, it gave me angles that were too steep, so I adjusted freehand until I found an angle that works.
It took a long time, but eventually I got the ducks lined up – a setting of the Veritas grinding support to get a square edge and a narrow enough angle, and a technique that ground the blade with minimal burning of the corners of the bevel. I knew I was there when I started honing with Brent’s jig and saw that the tip was being honed instead of the heel. It was down hill from there.
A Slower Grinder?
Now, what about the speed? My variable-speed grinder has a minimum speed of 2000 RPM, which is still fast for what I am doing. I am trying a couple of solutions.
First, I ordered a speed control from Amazon. I’m not convinced this will help, as this control only works for motors with brushes, and I am not sure my grinder has that kind of motor.
Second, my original grinder was one I found on the street, not far from someone’s trash: what appears to be an old washing machine motor with a grinding wheel attached to it. No switch, and power attachment points are exposed! I got that and an assortment of wheels by just taking them. It appeared to be a giveaway (I do this from time to time myself). Now I think that motor DOES have brushes, so the speed control should work on it. I am thinking of building a specialized grinding setup using this motor and its wheels, and a shop-built stand set to a fixed angle of 25 degrees. If/when I do, I’ll report in this blog.
Last, I found a vintage human-powered grinder on ebay, and got it for about $28 with shipping. Since I am working with hand tools, a human-powered grinder seems appropriate. It has not arrived yet, but I’ll blog about it later.
Help me out!
I would welcome comments on this entry, whether what I think I learned was correct, and whether there are better solutions than what I have tried. Thanks for reading!
-- Mark, hack amateur woodworker, Medford (greater Boston) MA