I just posted a comment on an old link regarding some hardness testing I have done and one of the members suggested I create a new thread with the information.
I've been building saws for about two years now and have become interested in the technical aspects of saw design. In my reading I have found that Disston at some point or other decided that the optimal hardness for saw plates was Rockwell C 52. The saw building community uses blued spring steel in sheet or rolled sheet form, often 1095 in building custom saws for the boutique market. My nature is to question things, especially benchmarks that were established a century ago like Disstons Rc52, and thus I set out to learn about the hardness of saw plates.
I recently acquired an Ames 2-S hardness tester. This instrument measures superficial hardness on the Rockwell N and T scales. This device works by measuring the depth made on the test article when a measured load is applied to a point called an indenter. On the N scale it uses a specially shaped diamond point and on the T scale it uses a 1/16" carbide ball. For hard materials you use the diamond and for softer you use the ball. The N and T scales specify three different loadings, 15, 30 and 45kg for different thicknesses of test materials. This links to a table giving the appropriate loading and indenter for various test article thicknesses. http://www.continentalht.com/userfil...sion-chart.pdf See the Hardness vs Minimum Thickness Chart at the top left of the first page.
Here's the instrument.
Ames 2-S hardness tester 2.jpg
And here's the data.
The conclusions I draw are:
1) 1095 steel is a consistent product. Despite the fact that the test materials were purchased from different sources over a period of several years the hardness values for the products were all very similar with a remarkably low standard deviation of 0.34 Rc points. Plus these tight results show you that both my technique and my hardness tester are working well.
2) Of the custom makers, only one is using 1095 steel. The other two makers represented in my collection are using softer material. Maker 3 is using very soft metal, 40% softer than 1095, - likely because it's cheaper and/or easier to work with. Maker 2 is using something that is about 7% softer. The highly significant t values (t < 0.05) for these measurements indicate a high degree of confidence that the measurements are not due to chance.
Maker 2's saws work just fine for me, but I'm not using them for production work. Makers 2 and 3 are names that everybody in this area knows well so I won't reveal them.
3) Disston's products varied in hardness over the years. The Disston saws in my collection were manufactured in the period spanning post WW1 to post WW2. The variability is not too surprising because the concepts of industrial quality control were in their infancy in the early 20th century. Disston also used steel that is harder by just under 4%, or 2 Rc points, than is 1095 but the difference is on the edge of statistical significance (t = 0.04).
Sharpening old Disston saws is going to be harder on your files than is sharpening modern saws. If you're really worried about the longevity of your files get some saws from Maker 3 and your files will last a long time but you may not get much wood cut.
Further, modern custom saws made with raw 1095 steel are not as hard as are older Disston saws. If the plate hardness is important and if Disston optimized his products we've actually taken a step backward in our unquestioning use of 1095.
I'm gonna go lick my wounds now.
Originally Posted by David Dalzell
