Problem flattening a chisel

[jarhead]

I have been working of flattening a Marples chisel for a very long time. There seems to be a low spot on the tip as you can see in the picture. I drew some lines with a permanent marker and used a 50 grip sand paper for a while and still the lines would not disappear. At this point I have taken off a lot of metal and wonder if I should consider this chisel defective. Some have suggested to try using a belt sander. Any other tips?

[vinht]

Double check your flattening surface is truly flat. Also, be sure your technique is correct. You should only need to flatten the first inch so concentrate on keeping the leading inch of the chisel flat to the surface, no need to do the entire back.

Vinh

[jarhead]

quote:Originally posted by vinht

Double check your flattening surface is truly flat. Also, be sure your technique is correct. You should only need to flatten the first inch so concentrate on keeping the leading inch of the chisel flat to the surface, no need to do the entire back.

Vinh

Vinh,
I am using a 3/8" glass as a flattening surface. I doubt that the glass is not flat. As far as my technique... I had a woodworker show me the flattening technique - I use 3 fingers to apply the pressure to the area being flattened and gently support the rest of the chisel with the other hand. Initially, I was flattening about 2", but tonight started concentrating on the first inch.

[Mainemarc]

Hi Jarhead:

I've always suspected that my personal flattening and sharpening bugbears have more to do with technique than the flatness of the substrate.

Depending on the direction you're moving your chisel, as well as how you're holding the chisel handle, you can subtly favor one side or the other, leading to uneven wear. Definitely switch directions and your grip for a while to see if that affects your results.

Also, abrasive paper has a tendency to bubble up ever so slightly just ahead of the chisel as it moves across the surface, which can also lead to one spot maddenly resistant to wear.

I don't even try for a full inch across the back any more. If I can get even 1/4" behind the edge mirror smooth across the full width of the back, I'm happy.

[jdkenyon]

I have read a lot about sharpening and flattening chisels but I don't fully understand the reasons behind it. What is the purpose of flattening the backside of a chisel?

[LarryG]

quote:Originally posted by jdkenyon

What is the purpose of flattening the backside of a chisel?

First of all, although pretty much everyone, myself included, calls it the "back," it's really the face.

Whatever you call it, if both it and the bevel on the opposite side are not perfectly flat, the resultant sharpened edge will be curved, not straight, which means the cut will not be uniform across its full width.

[Mainemarc]

Really just a riff on what Larry said, but if you think about it, the cutting edge of a chisel is merely the intersection of two surfaces: the chisel back and the bevel. The ideal edge is where those two surfaces come together to form a straight line of single steel molecules lined up, one after the other, across the entire width of the intersection.

Of course, that single molecule edge thickness is only theoretical, but the closer the two surfaces are to being perfectly flat, the closer one can come to that exquisitely keen edge.

[jdkenyon]

Thanks guys, that makes a lot of sense.

[Woodnut]

quote:Originally posted by Mainemarc
<snip>Of course, that single molecule edge thickness is only theoretical,</snip>

I can only imagine the amount of wood I would waste (and destroy) with a mono-molecular chisel edge...

[LarryG]

quote:Originally posted by Woodnut

I can only imagine the amount of wood I would waste (and destroy) with a mono-molecular chisel edge...

Probably a lot less than I wreck with my multi-million-molecule edges, which don't so much cut the wood as beat it into submission.

One of these days, I gotta learn to sharpen ...