Since a cutting edge is simply an intersection of two surfaces, and it is impossible to overflatten one surface to make up for the other one. The first step to having a truly sharp chisel or plane blade is to flatten the back of the tool. Chisels should be flat over 70% of the surface, and at a minimum it should be completely flat at least an inch back from the cutting edge. Plane blades do not need to be flat over the entire surface, and this can be addressed using the ruler trick to be discussed later. Rob showed a quick and easy way to check for flatness.When looking at the polished surface, look at the reflection of a fluorescent light tube. If it's not flat, the mirror image will bend off to one side or the other, following the profile of the surface. A good chisel should be prepped first on a 1000 grit diamond stone. If it's in really bad shape you may have to go down to 300, and at that level Rob recommended just scrapping the tool and finding on in better shape. I've got a set of Craftsman chisels that I bought right after I started woodworking, and now that I know what to look for I realize how horrible they really are. When I was getting them ready for the dovetail class, I flattened the backs to what I thought was a reasonable level and then put them in the Veritas MKII jig. After polishing up to 4000 grit on the secondary bevel, I noticed the bevel looked slightly crooked. At first I thought it was the fault of the MKII, but upon further investigation I found that the sides weren't even flat enough to register in the jig correctly. I think I might try a set of Stanley Sweethearts, but I hate getting rid of tools so I plan on flattening and tuning the Crapsmans just to practice. Here's a video of Rob in action:
I know it seems odd jumping from 1000 to 16000 grit like that, but I can attest that we were able to make paring cuts on the end grain of some southern yellow pine with absolute ease. I think that's probably all you folks want to read at one time, so I'll try to follow up later this week with some illustrations to show whats physically happening with a secondary and tertiary bevel approach, as well as some of the ergonomic cues and stances he used during honing operations by hand.
Thanks for stopping by!