Garrett Hack replies: If your stones are well worn it will take a lot of abrasive action to flatten them. A coarse diamond plate can do it as will silicon carbide powder on plate glass with lots of kerosene as a lubricant. Even better is inexpensive electroplated diamond abrasive paper stuck to thick plate glass. I use 400 for all my stone flattening and it’s really impressive.
Dan Faia replies: Using a coarse diamond stone is the easiest and most convenient way to flatten oil stones. It can also be used as a coarse stone alternative when flattening chisels, plane irons, etc. Diamond stones will wear in time, but if used properly, they should last for many years. Another alternative is to use a coarse Carborundum powder (80 grit equivalent). The powder must be used in conjunction with a flat plate (I have used a piece of angle iron that I flattened on a sandpaper lapping plate). Adding a touch of water to the powder will create a slight mud that will prevent the powder from just being pushed off the plate. Lap the stone until flat and then rinse under water to clean away residue.
Bill Taylor replies: Did the old timers ever try to flatten a stone or just cut a new on— just wondering? For us in the present day, we are fortunate we can purchase a large diamond plate to resurrect the oil stone. I say large depending on your oil stone size or the intended purpose of the plate.
Once you own a diamond plate you will find new uses all of the time. I especially like it when sharpening my old thickness planer blades. I don’t own carbide planer blades but use a small diamond plate to sharpen my carbide router bits all of the time.
I believe most of the present day Arkansas oil stones are inferior to older ones because the quarry is depleted or the quality stones are costly for the same reason. I say this because during a trip to Arkansas some years back I recall having read this.
If you are able to find an old dished out stone and flatten it on your new diamond plate you may very well have a superior quality stone to anything you can purchase today. With this said the modern steels in many of our tools are too hard for old oil stones. For instance would you try to sharpen a carbide tool on an oil stone. Nope—it will just gouge the stone! During that Arkansas trip I did purchase a stone as a memento. There were not an abundance of stones available to purchase.
I have never attempted to use a lapping plate in any of my woodworking endeavors. I did use lapping processes way back when I was a technician for flattening silicon wafers which then became silicon chips or integrated circuits. The machines had a rotating black plate (maybe carbide) and a pump system to keep the slurry fresh. We used a glass plate for flatness and some abrasive if we hand lapped. This is old timely stuff and I am sure the semiconductor industry has flown by this procedure.
Phil Kinsler replies: I made the investment in a diamond plate and have never regretted it. Another way is to use lapping grit on a glass surface. Veritas includes a glass lapping plate on their Stone Pond product.