How Flat is Flat Enough

[Frank Drew]

I've never used magnifying glasses or microscopes or micrometer calipers to prepare my edge tools, but I like "sharp" tools and prefer a "flat" back to a chisel for the work I would mostly use a chisel for -- chopping mortises or dovetails, etc.; for those operations a convex back would be a hindrance, IMO.

I'm not going to engage in a discussion on whether or not flat or sharp are real or mythical, though, beyond saying that of course we can't achieve perfect levels of either condition.

[Adam Cherubini]

(I agree that 1/64 is too thick, but I was trying to come up with an example that wasn't way off at one extreme.)

I think we need an actual scenario to make this more real.

I was think along the lines of paring the shoulders of a tenon or the flat ends of the "tail" side of a dovetail joint . I agree that about .005 will be trimmed off at a time, and usually there will be a few shavings taken to sneak up on the gauge line. It seems like the wood does care about the reference face, but in both of these examples, the grain tends to be oriented perpendicular to the cut. Might not be a good example. What scenario are you thinking of?

With you 100% (which is why I wrote "depends..."). Have to look at individual scenarios. I use chisels for tons of things. Probably more than any normal woodworker. I love chisels. Removing a tiny step of end grain is different from paring long grain. End grain can create enough force to cause an undercut. I know because I've done it. But I think this is the basic operation that many guys think of when this subject comes up. And I agree a flat chisel seems like a requirement for this to work.

I'm going to do some more work on this subject. I want to know how far out of flat my chisels are in inches. I suspect that the chisel that doesn't develop even scratch patterns or the OPs that you can see light when checked against a straight edge COULD be out less than .001". My "out of flat" chisels, won't develop even scratches on a Shapton, but I sincerely doubt many guys would have any trouble using them or even know they weren't flat.

So i agree Jamie, we need specific scenarios and real measurements of flatness. I can borrow some inspection tools from the machine shop and see what I can come up with.

[John Coloccia]

I suspect that the chisel that doesn't develop even scratch patterns or the OPs that you can see light when checked against a straight edge COULD be out less than .001".

Without writing a treatise on this, I will say for a fact that I can easily see fractions of a thousandth under a straight edge. Now, I have some machining experience, and you start to develop an eye for this sort of stuff, but I think ANYONE with reasonable eyesight can candle a joint to far less than .001", especially if you're candling against a thin straight edge.

My criteria is that I flatten on a course stone (a diamond stone) until I have a consistent scratch pattern. I polish on a ceramic stone (a Spyderco) until those scratches are gone. I polish further on an ultra-fine Spyderco until the scratches are gone. Then I strop. As I use the edge, I strop frequently....very frequently. I have several strops kicking around on my bench pretty much at all times. That keeps them ultra sharp for a LONG time.

IMHO, the straight edge has no real use for sharpening other than possibly the "ruler trick". IMHO, there is a lot of money to be made with sharpening equipment, and I'll admit I went down the rabbit hole and have spent thousands over the years on sharpening contraptions, but it's just not necessary and it's not that difficult. That said, the only thing that gets you there is practice, and sharpening contraptions help you get there in the beginning. It's helpful to hook up with someone that can show you the light.

But toss out that straight edge. For sharpening, you should be monitoring the scratch pattern. If you're not rocking and creating a convex surface, the scratch pattern will tell you everything you need to know about the quality of the edge.

Just my opinion.

[Jacob Reverb]

Build something!

HERETIC!!!

[Jim Matthews]

If you really want to bend your noodle around the idea, have a browse through
Paul Sellers videos on YouTube.

His technique seems to work, and should be readily repeatable without special tools.
Of note, he uses simple Marples bevel edged chisels for nearly everything.

That means he need only flatten the back of 4-6 chisels for the majority of his work.

His speed at putting an edge on his chisels and plane irons depends on developing a convex (!) bevel.
As mentioned by John Coloccia above, judicious stropping keeps things hopping.

It's not the method I use, but it is compelling.
His demo slicing paper has me flummoxed.