Honing chisels, getting rounded corners...

[john bateman]

I bought a set of MHG chisels on sale recently:
http://www.hartvilletool.com/shared/images/products//medium/Bench-Chisels.jpg
and began honing them using one of these Veritas guides:
http://www.highlandwoodworking.com/ProductImages/sharpening/027004.jpg
I'm using silicon carbide sandpaper on a granite surface block. I started by flattening the backs with 300grit, then honed the bevels with 300, 400, 600, 800, 1000grit. They are coming out very sharp, but the corners are getting slightly rounded in the process.

It seems to me this would occur if the roller guide were rocking slightly, though I can't detect this happening. I'm trying to maintain consistent hand pressure to keep the chisel flat to the paper as I hone.

Anyone have any suggestions to improve my technique?

Thanks.

[Robert Rozaieski]

Could be the sandpaper wearing faster in the middle than at the edges, which results in more aggressive cutting at the outsides. This happens even faster with the lower grits, which ironically cut the most aggressively. This is a drawback of sharpening with sandpaper. The solution is to change the paper frequently, especially the lower grits. Also, if you previously used a sheet to flatten a chisel back, definitely start with a fresh sheet (or at least a fresh area of the sheet) when you move to the bevel.

FWIW, this happens with stones too. It's called dishing and it's a result of the middle of the stone (or paper) seeing more use than the outer edges. The difference with stones is that 1) the grit continually wears away and exposes fresh grit and 2) stones are typically flattened frequently to keep the dishing to a minimum. Paper has neither of these qualities so instead one needs to change to fresh paper frequently, even before the previous sheet is completely worn. This is why paper sharpening is good for the short term but is an expensive long term solution.

[Frank Drew]

John,

As Robert notes, check to see if your stones are flat, that you haven't worn a slight groove in them because that could cause your corners to get slightly rounded. But that would also slightly crown the cutting edge, so check the chisels, too. Ideally, a chisel should be dead straight across, or even ever so slightly higher at the corners, which helps in positioning the tool precisely on your cut line if you're chopping mortises, for instance.

[Phillip Pattee]

I think Robert has hit the nail on the head. I found that with scary sharp I had to change out the paper far more often than one would normally believe to achieve consistent results. When you go back and forth over nearly the same spot, you do wear out the grit in the surface and as move slightly right or left you hit new abrasive on the edges of your blade. This is something to pay attention to when you are flattening a plane sole as well. Move the chisel around all over the paper, stop and change directions, etc. Make sure you try to use all of the surface. If you freehand, move the blade in a figure eight pattern over the whole surface.

[Adam Cherubini]

You may be pushing too hard. Try a lighter touch and see if that helps.

I would draw on the edge with a Sharpie, then got back to your finest paper. Try a light touch and see what pattern you get on the bevel.

Rounded corners can also happen because the back is rounded. That's easy to do when honing side to side. The chisel can roll, or the media can create a "bow wave". It also happens just because the grit hits the near side, then tucks under the tool. Pretty sure electric sharpeners (belt sanders, Worksharps, etc) do this. Theorhetically, that back has to be perfectly flat( straight) side to side for the edge to be straight across (provided the bevel is straight/flat) Honing side to side can be disadvantageous for many reasons.

Adam