Learning to Sharpen #9: Grinding to set a new bevel for a plane iron

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Blog entry by Mark Colan posted 09-23-2013 10:09 PM 1224 reads 0 times favorited 4 comments Add to Favorites Watch
« Part 8: Flattening the back of plane irons Part 9 of Learning to Sharpen series no next part

The Problem

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.

My Equipment

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 Mistakes

Here are some of the errors I made and what I learned by making them:
  1. 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.
  2. 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.
  3. 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

4 comments so far

View Don W's profile

Don W

15747 posts in 1349 days

#1 posted 09-23-2013 10:41 PM

Grind the bevel on the grinder, then learn to free hand on the paper ( eventually moving to a stone). Then you can follow the hollow grind and there is no worries about a minor discrepancy in angle. Plus its faster and cheaper.

My honest opinion, you’re putting to much thought and precision into a process that does not need it.

-- Master hand plane hoarder. -

View Mark Colan's profile

Mark Colan

209 posts in 1627 days

#2 posted 09-23-2013 11:07 PM

Thanks for your suggestion and opinion.

I am an engineer by training, and thought and precision is how I learn things. After I learn it, and acquire the required skills, I can see about doing it more simply. But I actually enjoy the thought and precision.

-- Mark, hack amateur woodworker, Medford (greater Boston) MA

View Don W's profile

Don W

15747 posts in 1349 days

#3 posted 09-23-2013 11:24 PM

I’m A different kind of engineer, but appreciate your attention to detail. I hope I didn’t offend. I guess the point I was trying to make was in this process the difference between a 24 degree and 26 degree doesn’t mean much.

I watch a lot of new sharpeners try to make everything perfect and the only thing it needs to be is really sharp. Save perfection for the dovetails.

-- Master hand plane hoarder. -

View Mark Colan's profile

Mark Colan

209 posts in 1627 days

#4 posted 09-24-2013 01:22 AM

No, no offense taken! I do appreciate your opinion.

I’m not certain that the difference in angle was as little as 24 vs 26 degrees. The big difference was the amount of hand grinding to get to sharpen the leading edge: I was mainly grinding the heel of the bevel with the first tries at hollow-grinding via electric grinder. Regrinding on the bench grinder with a smaller angle made a big difference in the amount of time I spent grinding by hand.

I suspect I could learn your more ad-hoc method of sharpening quickly if I was working with you. Without hands-on training, I have to muddle along, making mistakes, figuring out how to correct them, and improve slowly. It will certainly go faster next time. Of course, next time, I don’t expect to have to flatten the back much.

-- Mark, hack amateur woodworker, Medford (greater Boston) MA

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