Blade smoothness and skate rocker radius gauges"IceSkateology's rocker radius gauge measures curvature from 3 points, with a total spacing of 1 3/8". That means the difference in post height of the center post to the outer posts is only about 9 microns different between 7 foot radius and 8 foot radius.
Derivation: If R is the radius in inches, and d=1.375 inches is the distance between the first and 3rd posts, the Pythagorean theorem shows that the middle post height should differ from the outer post heights by R-sqrt(R^2-(d/2)^2).
For 7' rocker, we get 7*12 - sqrt((7*12)^2-(1.375/2)^2) = .00281347718 inches post height difference.
For 8' rocker, we get 8*12 - sqrt((8*12)^2-(1.375/2)^2) = .00246178287 inches post height difference.
The difference between those two values is .000351694313 inches = 8.93 microns.
Does anyone believe that the blades are actually smooth to that size? For an 80 grit wheel, if I found this right, the grit size is around 200 microns, though grit size isn't quite the same thing as surface roughness.
That's why I like the idea that a gauge should ideally do a quadrature (least square) fit, rather than a 3 point fit.
I planned to estimate radius from a scanned image of the blade, using quadrature fits - but I'm not sure that scanners are accurate to that level either.
Maybe surface roughness is part of why radius gauges show so much variation on skate blades. I know the edge definitely isn't that smooth - under a strong magnifying glass, you can often see edge roughness. I'm not sure how to judge sharpened hollow center line roughness, which is what you probably want the gauge to measure.
A legit question is whether 9 microns or so of surface roughness affects blade performance much. Maybe it doesn't matter, and we should ignore detailed radius gauge measurements? As well as blade companies that try to claim other company's blades are awful because rocker radii vary - maybe they just initially polish the hollows of their blades a bit better, which won't last.
Of course, 9 microns might affect blade performance. Diffraction measurements suggest that the lubricating water layer on ice at skating rink temperatures is only about .04 - .05 microns thick. Only a precise empirical test that would be hard to do could answer performance questions.
BTW, I tried to do a cheap version of Bill_S's measurement. I taped a blade to a ruler, and used the depth gauge of a calipers to measure the distance of the edge to the ruler at various points along the ruler. Alas, tape isn't very stable - I got far too much inconsistency in measurement, even for one point. (I also tried use a micrometer, but the variation due to tape movement was still too large.) Sigh. This is harder to do than it seemed.