Why of Rc testing?

K

Ken H>

Joined
Dec 31, 2011
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Hello all, I've used a HR-150 Rc tester for several years now and there is something I've always wondered about. Normally when the loading handle applying weight the needle will stop somewhere outside 20 to 30 range. Sometimes the needle might settle all the way up between 10 to 20 during the loading but not often. Then when the unloading handle is pulled the needle will settle in the 60 Rc range (or where it should be) where we wish it to be. When the loading handle is pulled I've sometimes seen the needle settle all the way to zero, or even more in the CCW to 90, but still when the unloading lever is pulled it will still read in the 60 Rc range. This morning I think I saw the most movement of all with the needle going CCW all the way around to about 80.

Rc-Set-jpg.jpg


After the unloading lever is pulled the needle moved CW to 66 Rc - about where I expected it to be with a 26C3 strip being tested.

Rc-Final.jpg


I know the theory of testing, the diamond penetrator presses a depression in metal with 150 kg force applied (loading). When the unit is unloaded the "magic" happens and it measures the depth of depression to determine hardness. It's really the diameter that's being measured, but with a set penetrator size the depth will give the diameter. I do the same thing to measure hardness of lead. I've got a 1/2" size steel ball that presses into the lead, then under a microscope I measure the diameter of depression with calipers. This allows me to calculate the hardness of the lead.

Can anyone explain to me why the needle will sometimes move so far CCW during the loading move? It seems to happen more often with really thin material. Perhaps there is some bending happening that skews the loading numbers? I'm open for suggestions.

Ken H>
 
I have not paid attention to differences where the needle ends up, I think it's around 20/30 but the speed of the needle on load seems affected by the temp in your shop which changes oil viscosity in your oil dampener...but interested in replies, I have not had this tester long but accuracy is great.
 
Doing a bit of testing I took a .040" strip of 26C3 that had a nice bow putting the bow so the diamond pressed it to the anvil. During loading while the diamond was pressing bow down solid against the anvil the needle made two CCW revolutions before settling down. Interestingly, when the unloading lever was moved the Rc reading was 67 Rc. Turning strip over so the bow such the strip was solid against the anvil, the needle settled in the 20 to 30 range during loading, and after unloading the needle showed 66 Rc. Close, but not the same.

I suspect when we see the needle move CCW to zero or more during loading, what is actually happening is the blade isn't perfectly flat and is flexing a tad.
 
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that makes sense. could you try grinding the bow out of the strip so it sits flat on the anvil?
 
Well, since there is a good 1/4" bow in the strip there's not much chance of grinding it out. By laying the strip with bow down so there is no give in the strip during loading that should give about the same results as grinding it flat.
 
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