Thread: How to sharpen & polish a convex chisel?

Dengy, so it 's got a bend, longitudinally?

I only flatten the first inch any way. So derust, which ever way you like, the polish the back first inch. I would never use it for fine work, due to the bend, but for rough work it will be ok.

Yes Pat, it had a longitudinal bend. It is convex or bellied, such that if you lay the back of the chisel on a flat surface it will rock along its length.

The method I was shown years ago involved cutting the 1 mm thick "bulge" out of the chisel's back using a coarse grinding wheel mounted in a Dremel.
Do you have access to a Dremel and coarse grinding stone?
No need to do the entire back of the chisel or even grind to a consistent depth the width, it's actually better to leave narrow shoulders on the "bulge" which you will remove by flattening the back using your coarse diamond stone.

Thanks for this Ian. I tried a test on a narrower convex chisel. I marked spot on the top surface a half an inch from the cutting edge, and pressed heavily at this point on the 220 grit diamond plate to wear a flat spot. Not unlike what you are suggesting doing with the Dremel and a grinding wheel.


After about half an hour or so of rubbing this point flat over the full inch, I’ve got what is a respectable flat surface from the cutting edge back about 1 inch.

I then moved the pressure point another half inch away from the cutting edge and repeated the effort, eventually getting a 2” flat surface. As usual, that last 10% of flattening near the cutting edge involved 80% of the effort.

But goodness, what a lot of effort that involved. To speed things up, I did try scraping along 60 grit and 90 grit wet and dry paper on a sheet of plate glass glued to a block of wood, but I found the the scraping with such coarse paper was difficult for short lengths, as my finger tips will attest. It also tended to cut away sections near the cutting edge that I did not want removed, and involved a lot of work on the diamond plate to restore that uniform surface.

Also, rubbing the chisel along the length of the paper and subsequently at right angles to that resulted in different wear that needed to be restored. For example, after getting the scratch pattern nearly uniform right up to the cutting edge by scraping longitudinally along the length of the blade, I found that when I subsequently scraped in a direction at right angles, across the blade, it would adversely affect the area near the cutting edge, leaving it worn away. I found it best to stick to scraping in the one direction, along the length and not across the width of the blade.

I did try flattening on the side of the wet grinding wheel on my som’s old Tormek Supergrind 2000, but found it physically too hard to control. Even after dressing the side of the wheel. Even with a foot switch. The slightest inattention could gouge the blade surface, usually along the edge of the blade. It still required a lot of work on the diamond stone afterwards to clean up and get a consistent scratch pattern along that first 2” of blade.

I was reading an online article about flattening of chisels, and the advice given for a convex chisel was simply to throw it away. I can see now how they came to that conclusion.😀

Originally Posted by Dengue

Thanks for this Ian. I tried a test on a narrower convex chisel. I marked spot on the top surface a half an inch from the cutting edge, and pressed heavily at this point on the 220 grit diamond plate to wear a flat spot. Not unlike what you are suggesting doing with the Dremel and a grinding wheel.

by "coarse" I meant something like a 24 or 36 grit Dremel stone.

220 grit diamond is far too fine a grit for what you are attempting to do.

The advantage of using a Dremel is that the grinding wheels are small so that material removal is localised and you can periodically check your progress using your 220 grit diamond plate. (though a 120 grit, or coarser, stone would be a better choice when shaping the back of a "bellied" chisel)