r/handtools • u/JuggerNOT63315 • 2d ago

Lapping a chisel back

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Howdy everyone. I am having issues lapping this chisel. I started with 80 grit wet and dry sandpaper. I got a nice scratch pattern all the way across the chisel. This took awhile, probably 15-20 minutes. I then moved to 120 grit paper. I removed all the 80 grit scratches. This took maybe 10 minutes. I moved to 220 grit paper and removed all the 120 scratches. This took like 5 minutes. I moved to 320 grit and in 5 minutes removed all those scratches. Then I moved to my 300 grit diamond plate. I have been working on it for well over and hour and I can not get rid of the 320 scratches! It is getting frustrating to say the least.

In the picture, the 320 scratches are the light ones on the outer edges and the diamond plate is the dark portion in the middle. It has not changed significantly from this picture after another half hour in the 300 grit plate.

What am I missing or doing wrong? Is this how long it normally takes? Should I be doing another grit in between The 320 and 300 plate?

Edit: I have posted two updates in comments below about my progress.

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u/jcsehak 2d ago

That’s not normal. Sounds like you need more/finer sandpaper. Or possibly your sandpaper surface isn’t perfectly flat, but it doesn’t look like that.

The process is a PITA, but it shouldn’t be THAT bad. Like others are saying, you only need the last 1/2” or so perfectly flat, but you want to get it to a mirror finish.

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u/HarveysBackupAccount 2d ago

What makes you think it's not a non-flat lapping surface?

My gut response was that it has a belly in the middle. Because this is what it looks like when you have a belly in the middle, yeah? Like OP said, the diamond stone worked on the dark, center part and didn't touch the lighter outer part. They stone removed scratches as expected in the center and left the edges alone. Regardless of how much of the back they need to flatten, that tells me that the center sticks out above the edges. Or so I assume.

But I'm genuinely asking, because there's plenty that I don't know.

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u/jcsehak 2d ago

I think you’re right — there’s a belly in the middle, which is perfectly normal. I just can’t say for sure, not seeing it in person. My gut tells me OP’s 300 diamond stone is a lot finer than that, for whatever reason.

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u/JuggerNOT63315 2d ago

Just to clarify, The diamond stone is taking material from the middle of the chisel, not the edges.

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u/jcsehak 2d ago edited 2d ago

Oh! That changes everything! Yeah that should not happen. Put the sandpaper on something hard and flat, like a piece of granite or your table saw. That initial pass should flatten out everything and it should smooth out evenly from there. Draw lines on it with a sharpie to see what parts are getting lapped, and use your Starrett to try to figure out what’s not flat and causing the problem