Call me masochistic, a slow learner, what ever.....I have a problem with wanting to reinvent the wheel.
The acrylic showed some promise, but maybe can be improved? This time I used a file to prep the surface, leaving a scratch pattern that was perpendicular to the primary axis. Side-to-side, as it were. I was thinking that maybe the grit would set into the substrate more evenly.
A sprinkle of gems...
Then set, using a hardened steel roller. A large roler bearing, mounted on a handle is what you SHOULD be using, but.....
This time, I just sprinkled the grit loose, no glycerine. I had a chip-breaker/back blade/secondary blade (laminated construction, just like the main blade) that I had shortened, so am setting a whole new bevel here. Well.....I hog out most of the material with a grinder first. This is refinement, I guess. Babbling..... Sorry.
I rinse off the accumulated swarf and any loose grit that still remains, then have at it again.
Yep, it works. The action is peculiar, like the blade is riding in tracks, on rails, so I try to incorporate a swirling or erratic stroke. This can speed up the sharpening process (think "random-orbit" sander vs the older vibrating palm Sanders), but it is hard to maintain a flat bevel this way. That's why the scratch pattern looks all screwy.
Maybe acrylic isn't the best material, or maybe this stuff was too hard. The material that is commonly used for a lapping plate varies. Balsa wood, copper, brass, even lead, it all depends on what you are lapping and what you are trying to achieve.
I liked this MUCH better. The grit bedded down well, possibly too well? The scratch pattern tamed down quickly, probably because the larger grains bedded more deeply during use. This combination felt more like a sharpening stone, but still not particularly fast or aggressive.
Derek Cohen has a great writeup on his blog, about making lapping plates from old cast iron plane bodies.
A cast iron clutch plate. Cast iron is made in a whole variety of hardnesses and densities. I think that a "soft" iron would be nice to try. This clutch plate is pretty hard.
It works. It feels like the diamond is mostly rolling around, and only a portion of the grit is getting embedded into the cast iron. If you look at the bevel, you can see a division between the soft iron and hard steel laminations, just like you would see if you were using a waterstone. It is the loose grit, tumbling around and putting microscopic dents into the iron, that gives the characteristic "hazy" look that is so pronounced with a laminated blade.
It works dry (see above)...
.... And wet. The viscosity of the fluid used to lubricate and flush away the swarf can have an effect on the "feel" of the lap, and the appearance of the blade itself. This is using a (relatively) thick oil, 10w40 synthetic motor oil.
I liked this combo for the "push". It felt like the high viscosity slowed down the rolling action of the grit. It left a more even haze to the iron. Compare the even, gray look of the soft iron to one of the pictures of embedded grit/acrylic substrate. The embedded grit leaves a very shiny/scratchy finish. I think that it's soooo cool that the same material (#120 diamond grit) applied differently, can give such different results.
This is using a thin oil, mineral oil cut 50% with mineral spirits. Is about the consistency of WD40.
It feels different, looser, light, and very rumbly/roly/crunchy. And VERY messy. The appearance of the iron is more scratchy and not as fine as when used with the thicker oil. It still feels like only a small proportion of the grit is getting embedded into the cast iron substrate. This is a great combo (a cast iron lap w/diamond grit) for finer grit, something like a #400 and up. This coarse...... Not so much.
And a piece of maple plywood.
The diamond grit embedded well into this material, as is evidenced by the uniformity of the bevel face. There is almost no visible line between the soft and hard metal. That means that the grit stayed anchored and actually sheared the metal, as opposed to rolling. This combination felt the most similar to an actual diamond sharpening stone. That said, the aggressiveness still quickly diminished as the diamond grains got pushed further into the substrate.
My ideal would be this stone.....
... just a bit harder. Super fast, sharp grit, and fun to use (and it looks like a rock! I love that!), but too soft. It dishes quickly and gives up too much slurry, which gets in the way. I need to get back out to the beach, and find some new "real" stones. Like this one, but harder.