Hardness Tester Calibration - Need Help

[PEU]

I have a Rockwell C hardness tester that I think needs calibration, when I use the standard provided with it (64.5 +/-1 RC) it consistently reads 62, but since it was indented so many times I had doubts about its accuracy.

So I purchased some round O1, made 8 slices, hardened and then surfaced both sides so they are super smooth (they attach one to the next with a simple rub) and then distributed a few of these "soon to be standards" to friends with RC testers, all of them returned to me with the same readings 64~65RC so Im pretty confident about their accuracy now.

With one of these I keep reading no more than 62~63RC no mather what I do.

I inspected the indenter with a loupe (triplet) and I don't see any defects or cracks and is smooth to the touch.
The lever on the side when I apply the load moves slowly as it should, meaning the hydraulic spring on the back below the weights works fine (when I purchased it I refilled it because it was almost dry and the lever went from preload to full load in a snap)

I would like to tap the forum knowledge pool to help me calibrate the tester, a lab asks a lot more than I prepared to spend

I tried to adjust the reading by untightening the center screw and the knurled retainer, moved it and then retightened the knurled piece, but I keep reading 62~63RC so I guess this is not what I need to adjust.

Thanks in advance.

Here are photos of my hardness tester:

and a view from the top

Pablo

 

[timgunn1962]

I can't help on the adjustment, though I seem to recall needing to adjust some tiny screws at the indicator end on mine (different make), somewhere off the right-hand-side of the second and third photos. It was a while back and it's all a bit hazy now.

Do any of your friends with hardness testers live close enough to be able to easily do a test with your indenter and their "known good" indenter, on the same test piece, with the same machine? It would eliminate one possible problem area.

It is also probably worth repeating your DIY test-block exercise with at least one lower hardness, perhaps 2, as multipoint calibration is usually better than single-point. Using Ground Flat Stock is easier than slicing round and the as-ground surface finish seems adequate to me.

 

[PEU]

I'm tempering some samples to reduce hardness to 60RC and 55RC and do the rounds again, in the meantime I have a 43.5RC standard I used for the following test.

Removed the front and using a digital comparator I made a readings table with four columns: Load applied, Measurement, Hardness and linearity.
Linearity is the correlation of the digital instrument readings.

Here is the setup I used:

These were the readings:

The only adjustment I can make to the hardness dial is this tab, which slightly adjusts the reading

I don't have the means for reading if the load applied to the indenter (should be 150kg) is correct or not

Any idea will be appreciated.


Pablo

 

[aarongough]

When I had to adjust mine it involved undoing the two screws in the top bar that goes to the dial, the ones on either side of the big 'thumbwheel' screw, and moving that little plate forward/backward on the bar.

First though: double check that your tester is setup perfectly level, if it's off level by even a little bit that will throw off the readings.

Try googling for a manual for the machine, or call the company that makes it to see if they can supply a manual or other instructions.

-A

 

[logem]

Peu,

If your probe is reading a lower Rc value than those of your local associates, than the only reason for this (in my opinion) is that your tester is applying more force than it says it is applying to the test sample.

I don't know how to resolve this, if it is the case. Just thought that it should be considered.

Mike L.

 

[aarongough]

Peu,

If your probe is reading a lower Rc value than those of your local associates, than the only reason for this (in my opinion) is that your tester is applying more force than it says it is applying to the test sample.

I don't know how to resolve this, if it is the case. Just thought that it should be considered.

Mike L.

I don't believe this is correct. In my case my hardness tester was reading 2.5 points low when I bought it (new!), after calibration it works perfectly with all the Rockwell C test standards that I have (63.7HRC, 48HRC).

Since levelling it, and calibrating it, it has held it's calibration perfectly.

 

[Don Hanson III]

Peu, that screw top center is for adjusting the pre-load I believe. You really need to get a manual to show how to calibrate it.

 

[PEU]

If your probe is reading a lower Rc value than those of your local associates, than the only reason for this (in my opinion) is that your tester is applying more force than it says it is applying to the test sample.

Spot on!

A little history before, when I purchased the Hardness Tester the old owner showed me that he removed the weights for shipping and marked the ones (3) that he used, but there was a fourth weight.
I checked the weights and they were marked with a number, now I understand them, if you add three numbers you get the magic number 150 which I guess is the weight at the indenter. The problem I was having is that one of the weights was wrong!!

After changing and now the three sum 150 my readings began to make more sense, my only problem was they were off by 2RC, upon more inspection of the weights I found they are hollow and filled with little steel balls like the ones used for sand blasting.

So after taking this photo for the record I started adding/removing weight until my reading average was coincident with the calibration standard.

Them I took the standards I made (65/61/55) and gave them to a friend who measured it at his work and yesterday the same blocks were checked at another friend, both measured within 1RC of my readings!!!

So now I consider my tester calibrated


Thanks for all the help!!


Pablo