Chisel backs getting out of flat with sharpening over time

[David Keller NC]

Fitzhugh (Do you go by "Fitz"?) -

You don't say what grit you're using to flatten the backs/hone the bevel on your chisels. This matters, particularly if you're putting a lot of pressure on the blade. My guess is that you are since you mention needing to change your technique to keep your fingers from locking up.

You can use a coarse grit of SiC paper such as 220 as the initial stage of back flattening/honing a nick out of the bevel, but you won't get a good result if you're putting a lot of pressure on the blade. Unlike a water or oilstone, the grit on SiC wet/dry comes off of the backing fairly easily when used to hone steel. And that ridge of material, along with putting a lot of pressure on the blade and using a forward stroke (where the edge is being pushed into the paper rather than pulled back toward you) will dub the edge.

This effect is magnified if you're using glass that's less than 1/2" thick, particularly if the surface the glass is setting on isn't flat (like a countertop). It may not sound like much, but having that glass flex by 10 thousandths of an inch over about 6 inches is going to do precisely what you describe in your first post.

There are several possible fixes - one is, as you note, to use side-to-side motion rather than back & forth when flattening the back. However, you can also develop a belly on the back of the blade from side-to-side rather than along its length, though this is less likely than a belly along the chisel's length. Another fix is to use a piece of float glass that's at least 1/2" thick (you used to be able to get pieces of this from Lee Valley, though I think they've discontinued it in favor of their cast iron lapping plate) so that it will be less susceptible to flexing as you use it.

But the best fix for this problem is to order an inexpensive granite surface plate - one of these can be had from Woodcraft for about $35 (the 9"X12" size). An even better fix is to use a coarse sharpeing stone in conjunction with the granite surface plate for flattening chisel & plane blade backs and honing bevels. One uses the granite surface plate with wet/dry sandpaper to keep the sharpening stone within a couple of thousandths of flat, and the sharpening stone won't flex in use.

[Jack Curtis]

...The solution is to make that belly area concave - sort of like a Japanse chisel....

Huh? Do you think Japanese chisel backs are concave? Or are you talking about the hollows?

[Steve Friedman]

Huh? Do you think Japanese chisel backs are concave? Or are you talking about the hollows?

Referring to the hollows - not convex along the edge. I was just trying to point out that it's OK to create a concave area where the belly was in order to make it easier to get a flat area along the edge. Getting that flat is impossible as long as any part of the bellied area is riding on the abrasive. I once watched Deneb at L-N try to remove the bellied back on a spokeshave blade of mine and it was exhausting just watching him.

Steve

[Jack Curtis]

Referring to the hollows - not convex along the edge. I was just trying to point out that it's OK to create a concave area where the belly was in order to make it easier to get a flat area along the edge. Getting that flat is impossible as long as any part of the bellied area is riding on the abrasive. I once watched Deneb at L-N try to remove the bellied back on a spokeshave blade of mine and it was exhausting just watching him.

Seems to me this is what grinders are for. Apply to back judiciously.

[Ryan Baker]

You can flatten a back moving side-to-side, front-to-back, in figure-8s, etc. -- and they all can work fine (some are easier than others). It is hard to judge without being there, but it sounds like most of the problems are either technique (not keeping the chisle down flat) or the paper bowing up (not stuck down well enough). Working more slowly may help.

Are you holding the chisel by the handle? That creates more leverage and will cause more trouble keeping the chisel flat. Hold just the blade, as close as possible to where it is on the paper. One hand should be pressing just down flat at the tip while the other moves the blade.