Scraper Science: The Role of the Burnisher
OK I’m sorry to be a bit of a tease about all this stuff with sharpening scrapers. It’s not that I’m trying to string you along, it’s just that I still have more questions than I have answers. However, with some research and a phone call today, I have one less question and one more answer.
First, the question: Of the 14 scraper-sharpening techniques I tried, eight of them recommended burnishing the flat face of the scraper several times before burnishing the edge to turn the hook/burr. The explanations for why you burnish the flat face of the tool were varied: It is to soften the metal, to harden the metal, to consolidate the metal, or to warp the metal over the edge so you can turn it into a burr.
So I did what any mind-muddled journalist does: I called an expert.
Ron Hock runs Hock Tools and is one of my favorite metalheads in the business. He spends as much time selling excellent replacement irons as he does dispelling the myths about metal that are spread by woodworkers. When I explained the problem, he chuckled.
“That,” he says, “sounds like what happens sometimes when woodworkers talk about metal.”
After some discussion, here’s what Hock concluded. Burnishing the flat face of the card scraper does two things: It work-hardens the steel by compressing the crystal structure of the steel. The burnisher is harder than the scraper. Burnishers will typically be of a Rockwell hardness of 58 to 60. Modern scrapers are typically Rc 48 to 53. The harder burnisher will compress the steel in the softer scraper making it harder and probably more durable in use. This would be an even more useful trick when burnishing older scrapers, which would have a Rockwell hardness that was much lower, more like in the mid-40s, Hock said. (Scrapers were typically made from old sawblades in the early days.)
The other thing that the burnisher does is to draw the steel off of the face of the scraper. Essentially, it moves the metal so the steel makes a small point where the face meets the edge. Why is this important? It makes the scraper’s burr much easier to turn when you burnish the edge of the tool. You can turn the burr in one stroke and without much pressure.
Hock’s points about steel fit in perfectly with my experience during the last 10 years I’ve sharpened card scrapers. Must you burnish the face to get a burr? No. But if you don’t burnish the face, the burr is more difficult to turn, and you must use more pressure or more strokes. Using more strokes or pressure can introduce error and create an irregular burr.
Point two: Burnishing the face creates (in my experience) a burr that lasts longer. Hock suggests that this is because the steel has been work-hardened by the burnisher before turning.
So I got the burnisher figured out today, but there are still a couple questions I’d like to get answered. Such as: Why do card scrapers work when no other plane will?