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Thursday, January 26, 2012

More on hollow grinding

Yesterday I talked a little about hollow grinding on one of my block plane irons. Here's a little more detail, with some super-high-resolution CG. Er...scanned pencil drawings.
Top is hollow ground, bottom is flat. The shaded area shows the part of the iron that's removed when applying the primary bevel. The dark area is what's removed during honing. The hollow grind offers more relief behind the secondary bevel, and gives more time between regrinds.


For free hand honing, you can grind the primary at the same angle that you intend to hone. This allows you to balance the blade on the stone. Essentially, you get two points of contact (top and bottom of the hollow). I'm a guide user, so I attack it differently. I hollow grind at an angle 5 degrees less than I intend to hone. That means that I'm only working the very edge of the iron.
When you use a flat bevel, the secondary can creep up faster, then it's back to reapplying the primary. On thinner irons, like what comes in a Stanley plane, this isn't a big issue. When you have irons that are 3/16"-1/4" thick, you can save a lot of time (not to mention wear on your stones) by grinding a hollow. It will greatly reduce the time and effort required to sharpen.
This is obviously not a full-on dissertation on grinding and sharpening, but I hope it's a little helpful.
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4 comments:

  1. Tico VogtJanuary 26, 2012 at 7:38 AM

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  2. Tico VogtJanuary 26, 2012 at 7:39 AM

    Nice drawing. What is the lowest angle you have ever hollow ground a blade?

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  3. kgooldJanuary 26, 2012 at 9:35 AM

    Thanks for more info. The only reason I sand my first and second bevel is because I have a cheapo bench grinder with a wimpy tool rest on it. Mabye I should invest in one of those veritas grinding rests.

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  4. markJanuary 27, 2012 at 4:27 AM

    Tico: One of my chisels is ground in the 15 degree range. However, that was because I wasn't paying attention. The cutting edge is honed at 25 on that one. I know Rob Cosman talks about honing at about 17 degrees for chisels used in soft wood. If I was going that low, a 15 degree grind would be appropriate.
    Kelton: Cheapo bench grinders are the reason I rebuilt the suicide grinder and got the Veritas tool rest. I had planned on making my own rests, but I have had other projects going and didn't feel like milling one. IMHO, it's money well spent.

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