Years back I found this on the web. However I cannot find it any longer. This address brings no results but maybe you’ll have better luck. http: www. prfelr0.f2s.com/prfelr0/hardness/hardness2.jpg
If you have a drill press this rig works OK. Also you will need a 7/16 dia. Ball bearing. I made a stem to hold the bearing and secure the stem into the chuck of the drill press. The drill press is never turned on. It is simply a way to easily apply pressure to the sample. Another item of need is your bathroom scale. Adjust the table of the drill press to allow room between the ball bearing which is mounted in the chuck and the bathroom scale which sets on the drill press table. Put your sample alloy on the scale and lower the press handle to apply 200 lbs. of pressure. Dwell about 3 seconds and let off. Measure the indent diameter with your calibers. Cross this measurement to the chart for your BHN . On the low end a diameter of .190 equals about 4.7 BHN and on the high end a reading of.090 equals about 21.7 BHN. I have compared samples to other commercial testers and would say this is accurate.
My copy is faded but as far as I can tell this is what it reads. Close enough for me. Hope you enjoy.