Fair Woodworking

January 22, 2012

How do you know your stones are flat?

A good friend of mine has suggested that I’m a little anal retentive about my stones and sharpening.

He’s wrong. I’m anal retentive about just about everything.

He really should get his facts straight before making such a comment.

With that in mind, lest I appear the hypocrite, I’ve been attempting to confirm that there is at least some factual information behind my radical opinions, and apparent war with popular opinion. (Say that five times fast!)

There are smart people somewhere in the world with fancy equipment that can test for flat to increments that are unfathomable to people like me, and despite my warm and honest looking face, I will ever be able to borrow some for use in my shop. So what is the common man’s best method?

If I was testing a piece of wood I’d use a straight edge. Just place the edge on the wood and move one end. It will pivot on the highest point.

I went out and got a 12″ straight edge for this application. It didn’t take long for it to dawn on me that my stones are abrasive, and rubbing against it would quickly make my straight edge not so straight.

By the way, if anyone is looking for a 12″ swerve edge I may be able to help them out.

My next idea, didn’t come to me as an idea, it was a problem.

Flattening the backs of blades is a job I don’t like doing. The only thing I like less is flattening planes. Flattening blades can be a slow process and gives you much time to think about what you are doing.

Your coarse (1000) stone has two jobs. Remove the scratch pattern from the milling process, and define “flat” for the rest of the job.
This can take a while but it’s worth it.

The next stone (4000) has one job. Remove the scratch pattern of the 1000.
This I’ve found can take forever! The problem I’d have is that it would initially only remove the scratches from part of the blade. By the time the 4000  had removed all of the 1000 scratches it would take 2, 3, maybe 4 times as long as the 1000.

When I’d get to the 8000 I’d be near the point of tears as it would take even longer still.

What the heck was going on!?

It seems my stones were not as flat as I thought they were.

What I’ve come to accept is that your stones do not make your blades flat. They make them into the mirror image of the stone. If your stone is concave at all, the blade will come out convex. Same thing it the stone is convex. When you move to another stone, if it is not the same shape as the first stone, it will first need to re-establish its version of flat before it can remove the previous scratch pattern.

At this point I’d discovered that the 4000 and 8000 actually had 2 jobs. Remove the old scratches and not screw up the shape determined by the 1000 stone.

Essentially the 3 stones need to be virtually identical in shape to “flatten” your blades quickly and easily, but this has nothing to do with true flatness.

But the problem still remains that we don’t have a way to confirm that the stones are flat.
Back in the days that I used sandpaper on glass to flatten, I read about the idea of rubbing two stones against each other. On a whim I tried it out. I took two matching 1000 grit stones that had been freshly lapped on the sandpaper, marked a bunch of pencil lines on either and rubbed them together. Because they were both shaped by the same sand paper, they would be the same shape. When facing each other they become opposites. Concave facing concave does not mate, and the same with convex. Only if they both were truly flat would the pencil lines wear off evenly because the opposite of flat is still flat.

As it turned out, my glass was not flat, because the pencil lines would consistently wear off faster in some places than others.

But now I could always just rub the two together until all the pencil lines were gone and then they would be flat. Right?

Well no.

Any error in hand motion could cause one stone to round, just a little, and the other to dish out to match. And besides, this seems a little backward to have to work your stones out of true so you can flatten them (hopefully) against each other.

Well this is really getting long winded, so I’m going to wrap up today’s post with the closest thing I can find to the every day man’s version of a flatness test. It’s not a test you will want to do every day, as it could get expensive.

Get yourself a new Veritas plane blade (preferably one that fits a plane you own. Or even better, buy a plane that fits the blade!!!). They are lapped to ±0.0002” across the blade back and really are as flat or flatter than I’d ever expect to get my stones. What I’m saying is that if your 1000 stone does not remove the old milling marks evenly, your 1000 stone is not as flat as the blade was. It’s that simple. Unfortunately, you will need a new blade for every test, so as I said, it could be expensive.

Two posts in and I still haven’t provided what I believe is a viable solution to the flatness issue. What is worse, is that I still haven’t given a halfway good reason why you should be this anal about flatness.

I’ll get there, I’m just slow…

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