Lapping Technique

[les winter]

I have read that the more efficient way to lap a plane iron or chisel back flat is to alternate grits, going from coarse to medium and fine and back to coarse rather than staying at the coarse grit. Can anyone tell me why this should be so?

thanks.

[Robert Hazelwood]

I can't say I have heard that, and it doesn't make much sense to me as an intentional strategy. However I have found that judging flatness from the consistency of the scratch pattern can be tricky on the lower grits. Often the surface will look perfectly flat with an even scratch pattern at 80 grit, and then when you switch to 150 grit you start to notice some lingering low spots. If removing those low spots is taking too long on the 150 grit you might go back down to 80.

[Andrew Pitonyak]

I have heard the same, but never gave it much thought...... until you asked....


Enter total speculation mode, so, take it for what it is worth

So I have a bunch of really rough pointy sandpaper, what does the surface look like after I use it for a bit? Many deep grooves. OK, so what if I still use that rough sand paper? Adding more grooves.

If I move to a less aggressive paper, it will likely quickly knock off those high spots. Think about using a rake and you want to move material off. First you have some rows, and then maybe a grid... It might help to take down the high spots (which should be pretty fast, and then you can go deep again.

I have zero evidence to back any of this wild speculation. But it seems plausible.

[Ted Phillips]

I've heard this as well and didn't give it much thought. But coarse grits can create "fuzzies" where the scratches are ripped out of the sole of the plane. Maybe these prevent the sole from fully registering on the flat surface? When you grind them down with finer grits, you smoothe away those fuzzies, obviously...

I wonder if SNOPES.COM has anything to say about this urban myth?

TedP

[michael langman]

Les, lapping is a finishing operation, and the idea behind doing it efficiently is to leave as little material as possible to remove before lapping.
Most surfaces are surface ground before being lapped for a desired flatness and finish.
If a surface grinder is not available then files should be used before lapping to get the desired near flatness and finish desired.

Lapping with a courser grit produces deeper scratches in the surface of the steel. The larger grit becomes smaller and thus cuts slower as it is being used. Cleaning off the surface and then using the coarser grit will be faster then going to a finer lapping compound, if a lot of material is needed to be removed.

A sharp flat bastard mill file will remove steel much faster then any lapping compound and if used to knock down the high spots in an irregular surface first will be the most efficient way to finish a planes sole.

If the steel is hardened an 8"-12" diamond file will work most efficiently to remove the high spots. If no diamond file is available then coarse grit is the fastest way as long as you keep wiping off the old compound when it becomes dull and broken down.

[lowell holmes]

I lap the back of my chisels on a fine stone or sandpaper. I do lap the back bottom inch of the chisel.

[William Fretwell]

After flattening the back for a while on one grit it all looks the same, but is it? Use a finer grit for a while & you can see what is going on. That's when you see it's not really flat and go back to the coarse grit. I don't think it's better or faster just more informative.

[Bill McNiel]

After flattening the back for a while on one grit it all looks the same, but is it? Use a finer grit for a while & you can see what is going on. That's when you see it's not really flat and go back to the coarse grit. I don't think it's better or faster just more informative.

Exactly, IMHO.

[les winter]

Thanks for the input. I note that Deneb Puchalski of Lie-Nielsen says it is faster. I believe he states so on the chisel sharpening video.