Hardness Tester Calibration - Need Help

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

I'll be very interested in seeing the responses from those who actually have and use these machines. That said, the measurement from machines like this is (I think) a generally linear function within the measurement range. You show different result at one point along that range, but that doesn't say anything about the rest of the range. The presumed error could be a result of either an offset, or a different slope; these two types of errors could have different sources (and different repair required).

I suggest you try the same sampling technique with your friends and another group of samples, say around 45 Rc or so, and see what that says. Ideally, you could have a set of two different hardness samples, say in the low 40s, and mid 50s, and test them both with equipment in other shops, and get a very good idea of how your instrument is misbehaving. You could (perhaps) also offer the results to the other shops that are doing you a favor, so they can understand where their hardness testers fall into the statistical range.

Good Idea, 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

Try measuring the movement of the penetrator indicator directly with your test indicator. Each point on the Rc scale with a Brale penetrator is equal to .00008".

For what it's worth, I've been in a shop or 3 with Phase II testers, and they all seem to have notes to compensate for the difference between what their machine reads and what the test samples are marked, usually about 2 points or so.

Mud, you made me look further and I found something interesting, the weights are hollow and filled with steel balls like the ones used for blasting:



This is one of the weights on the back that are multiplied by the lever to reach the 150kg at the indenter. I saw that if I increment slightly its weight by adding steel balls the hardness reading also increments slightly, so I will play with this weight until I reach a consistent reading to my standards/blocks.
I think I'm on the right path now! Adjusting the tab I highlighted in my previous post did nothing to correct the readings.


Pablo

Test it at the low end of the scale as well, like 30 Rc. Make sure it reads close at both ends and the center of the scale to be sure the adjustment is correct.

Wilson told me that their test specimens have a range, because the metal is not homogenous and can read slightly differently at different spots.

As far as having lots of indentations on your test specimen, that won't hurt as long as they are all on the same side, and you are not close enough to an indentation to cause a variation.

=> before messing with the weights, please write down their weight as you found them.

=> please get a brand new indentor and test block, they are not THAT expensive

=> please rub down the anvil and the contact surface between anvil and spindle with a piece of printing paper, continue til the paper stays clean

=> prior to measuring, carry out 4-5 full load cycles as there might be oil in the load bearing surfaces


THE BEST SOLUTION WOULD BE TO ASK A COMPANY LIKE WILSON TO COME OVER AND CHECK ON THE HARDNESS TESTER!

benedict leighton said:

=> before messing with the weights, please write down their weight as you found them.

Click to expand...

Done, took a photo of the only weight I changed

=> please get a brand new indentor and test block, they are not THAT expensive

Click to expand...

This one looks fine under a 10x loupe, no cracks, nothing wrong that can be seen

=> please rub down the anvil and the contact surface between anvil and spindle with a piece of printing paper, continue til the paper stays clean

Click to expand...

Will do, good idea!

=> prior to measuring, carry out 4-5 full load cycles as there might be oil in the load bearing surfaces

Click to expand...

I always do 3 loads before the actual readings, then average the next three

THE BEST SOLUTION WOULD BE TO ASK A COMPANY LIKE WILSON TO COME OVER AND CHECK ON THE HARDNESS TESTER!

Click to expand...

I want to avoid this due to high price they ask.
I use the hardness tester to measure the custom knifes I make, these measurements are not dealbreakers for my customers, I just want to know the heat treatment worked as specified by the steel manufacturer datasheet.

Now for an update, after playing with the weight first I substracted small amounts and then added about 8 grams so the readings match within 1RC tolerance the standard blocks.

Now I will send the tempered blocks to be measured with at least 3 hardness testers and then recheck mine.


Pablo