Portable Rockwell Hardness Testing 

Its easy but you need to check that the tool is tight and in good operation. I found these to be very delicate but did the job. Training is imperative

whats the difference between standard and Superficial testers

Good question. That is tiny. I tried it with a 6-40 because that is the smallest screw we had and it worked fine. You must have a flat end on the screw for it to work. However, if the end of the screw is not flat you could grind it flat to test.

can this be used for knife steel

So, I guess you'd have to cut off a small part of the screw shank, then to hold it, probably embed it in a solid epoxy holding medium, to hold it in line with the penetrator, correct?

Instead checking the diameter, test the ends or the shank. Yes, the Ames Tester will do the job.

This screw is a small 5-40 machine screw, how would you hold something that small to get a reading from the end of the screw?

Can this be used to test the hardness of various plastic filaments, such as ABS and PLA, used for 3d printing?

Does this video show two different results for the same piece of metal?

Good question. That is tiny. I tried it with a 6-40 because that is the smallest screw we had and it worked fine. You must have a flat end on the screw for it to work. However, if the end of the screw is not flat you could grind it flat to test.