19 May 2010

The 'Bump'

No, it's not what you're thinking!

After some protracted effort, I finally managed to finish off my Japanese chisels, recently acquired from Workshop Heaven. It's been a bit of an uphill struggle to sort out the backs...the bevels were fine, but the backs caused a problem. I was told by Matthew that the heat treatment of the steel causes a minute 'bump' to form just behind the edge, shown in the ringed circle. It's easy to see it by holding a rule against the back and then squinting along with a light behind. I tried 100g sandpaper glued to glass and mdf but it was still there and try as I might, I couldn't seem to get rid of it. The 3M papers on float glass were excellent in refining the surface, but the grades I had available were too fine so I thought I'd give it a shot with a couple of coarser papers, 60 and 40micron PSA. I stuck these to some more 10mm glass and within around fifteen minutes, the problems I'd encountered were gone...a dead flat back was produced.

What's interesting though is that these chisels are really quite crudely made, showing the legacy of hand forging and grinding. If you look at the bevel on the 30mm Orie Nomi, you'll see that it's not been ground square by the maker as the bright shiny bit is where I prepared it on the 3M papers with the Kell III guide, which is guaranteed to produce a square edge. (The black triangle is just a marker pen so I could see progress)

Purists though, may well be gagging into your morning coffee if you have a look at the section of steel behind the blades...that's been ground away as well! This has been done for two reasons, firstly to give clearance when mounting in the Kell III and secondly to maintain a dead flat surface as the blade is moved back and forth over the float glass. If it hadn't been ground, then as the heel of the chisel meets the edge of the glass it'll lift the blade (or chip the edge) further exacerbating an already existing bump...

...definitely not required!

1 comment:

John Walker said...

Hi Rob...

I am following this chisel post closely. I don't think I shall go completely to Japanese chisels, but I am tempted to buy a few dovetail versions. So I am learning here!
Thanks again.