Re: How rasps are made, hardness, sharpening
Tom Huett
>I had the pleasure to tour the Nicholson File plant in Cullman Alabama about two months ago. When this discussion of files began I thought it interesting that most people really don't know how files are made. SO, I emailed the manager of engineering at Nicholson File with some questions and here is the text of his reply:
First, let me say that Nicholson Files are the best files in the world. We are the bench-mark for all other brand files. We continuously conduct comparison test with world-wide competitor products. We are consistently #1 in "stock" removed. Files are placed in an automatic file testing machine that is used to remove material from a test block of known hardness of steel. The test measures the grams of steel removed from the test block after a series of strokes.
Specifically, how does the Rockwell hardness vary in the body of a file? Is it consistent from one end to the other, or is the hardness greater in some areas of the file (body as opposed to tang)?
The hardness of a file should be of uniform hardness from the point (the working end of a file) to a location near the last tooth (the heel). The tang or handle should be soft to prevent breakage. If you find a variance in hardness the process is out of control. This means there is a problem in temperature of quench. It could also be a problem with alloy content or loss of carbon.
Also, does the hardness vary from the surface to the center of the file?
A properly hardened file will have a small inner core that is softer than the surface. This effect prevents breakage in normal use.
What specifically is the process called which imparts the hardness to a file? Is it case hardening?
We call this process "hardening." The blank is elevated to above 1400 degrees F. The actual temperature is file shape and steel alloy dependent. After achieving the desired temperature, the file is quickly quenched to provide a file with a uniform outer shell and a softer inner core.
Has the process for hardening files changed over the years?
Yes, we are in our third generation of hardening. Originally we used molten lead as the heat medium. We then tried salt. We now use the environment friendly Bismuth. Additional we use induction hardening on the symmetrical products, example: mills, flats, rounds, etc.
Our process is in constant evolution. Market, suppliers, competitors, and government compliance issues impact the methodology. We are constantly changing processes in response to customer demands. We actively seek new customers and develop new products.
Note that he had to reply while trying to do a yearly inventory, but I think he addressed some of the issues. If you ever are near Cullman, Alabama you should stop by the Nicholson File plant and see the worlds biggest file (about 30 inches long and about 250 pounds!).
Tom