BHN Measurement Revisited

If you've been following my 6BR thread you may be aware that I ran into some issues using smaller diameter indenters to measure BHN with my homemade tester, so I'm going back to the drawing board.    First up, my original 10mm indenter had been turned on a lathe, and while the lathe did a pretty darned good job, there were some microscopic imperfections on the tip of the indenter.    The imperfections don't show in this photo so you'll have to take my word for it.   Anyway, I splurged on some 10mm ball bearings and epoxied a ball bearing into a 5/8” thingamajiggy that fits into my push-thru die holder. I used to measure the indentations with calipers and a magnifier, but then someone pointed out that Gimp (and some other free photo editing programs) have a “measure” tool.   Take a photo of the indentation with some sort of scale in the picture, then you can zoom in on the photo and measure the indentation with accuracy limited only by the resolution of the photo. A USB microscope works well for taking the photo, but some cameras and even phone cameras are capable of taking decent closeups.Load the photo in Gimp.   Measure the scale (this one is marked in tenths of an inch).   In this example, the measurement across 0.300” on the scale was 569 pixels.    In other words, the scale of the photo is 569 pixels per 0.300 inch, or 1897 pixels per inch.     Next I measured the diameter of the indentation.   I actually measured in several different places:  257 pixels, 257 pixels, 261 pixels.   Well, the cat was chewing on my arm when I took the last measurement!     Average of the three measurements = 258 pixels.    To convert that to inches, divide 258 pixels by 1897 pixels per inch, = 0.13618 inch.   The applied load was 150 kg. for 30 seconds -- more on how I apply the load later.    Plug the numbers into the formula for BHN (a spreadsheet crunches the numbers for me) and we get 15.6 BHN.    Should we believe that number?   Well, the alloy was 50% lino and 50% clip-on wheelweight, and I was expecting 16 BHN.   That's pretty close.     In the next few days I'll try to post measurements for all my alloys including “known” standards like pure lead and linotype.     Then I'll repeat the tests with different indenters and different loads and see how the results stack up.     Most people buy store-bought BHN testers and that's fine, but like many casters I enjoy making my own stuff whenever possible.    I'm not going to run out and buy a store-bought tester, but if anyone wants to loan me their store-bought tester, I'll be glad to include it in my comparisons.