Surface grinding a Rockwell test block back to new condition?

Laverda · Aug 24, 2021

I have a few Rockwell hardness test blocks that are running out of areas to test upon. I was wondering if I could surface grind them back to new? As they are scrap if I can't do this, what do I have to lose? If I get the exact same hardness reading before and after grinding, then I should be OK? I am assuming they are through hardened and if I keep everything cold when grinding, I don't see the problem if I am only grinding off a few thousands. Yes I know the test surface is polished but I think I can do this too on a surface plate. Eventually the block will get too thin after being ground many times, but I think I will be dead long before this happens.

What do you think? It seems a shame to scrap these blocks which are not cheap to purchase. I was getting ready to order some new blocks when I had this idea.

My Brown & Sharpe No.2 grinder is ready and waiting if the consensus is this may work?

jccaclimber · Aug 24, 2021

Laverda said:
I have a few Rockwell hardness test blocks that are running out of areas to test upon. I was wondering if I could surface grind them back to new? As they are scrap if I can't do this, what do I have to lose? If it measures Rockwell 60 before grinding and 60 after grinding, then I can expect I did OK? I am assuming they are through hardened and if I keep everything cold when grinding, I don't see the problem if I am only grinding off a few thousands. Yes I know the test surface is polished but I think I can do this too on a surface plate.

What do you think? It seems a shame to scrap these block which are not cheap to purchase.

My Brown & Sharpe No.2 grinder is ready and waiting if the consensus is this may work?

Assuming it's through hardened (HRC 60 example) the concept seems plausible, and I agree that it's silly to throw something out that you can re-use. That said, as a customer who requires NIST traceability on all measurements I'd expect you to have them recertified* at a capable facility. I'm not sure what the cost for that is, although I'd like to think it's not awful. On the other hand, time on a grinder, time to polish, sending out and paying for a fresh cert. $100 to replace something that can easily be used 500 times doesn't sound so bad.

It's a bit like a granite table. For my own personal use I don't bother to get the slab checked up on because it's close enough for most things and I can validate a local spot for rare times it matters. For commercial work when it often matters I just pay the experts and don't worry about it.

*I'm assuming if you felt you were qualified to certify them yourself you would have just ground them down and tried to validate them by now.

Hammerhead800 · Aug 24, 2021

Hi I’m sorry for having to do it this way. Can someone tell me how to start a new thread? I can’t find anything on this site to do so.

gbent · Aug 24, 2021

I think if I was going to do it, I'd grind the side that is used, then flip it over and use the other side. If both sides are full buy a new one, then don't ever use the back side.

johnoder · Aug 24, 2021

Hammerhead800 said:
Hi I’m sorry for having to do it this way. Can someone tell me how to start a new thread? I can’t find anything on this site to do so.

You are in the GENERAL section. The illustration below is for the ANTIQUE section

EPAIII · Aug 24, 2021

You are in a particular topic/thread page here. There is no button for a new thread on the page(s) of a topic. You need to be on one of the "Forum" pages, like "General" or "CNC Machining" or "CAD / CAM". On those pages you will find a "Post New Thread" button at both the top and bottom of the page, on the left.

You can get to those "Forum" pages either from the "Quick Navigation", drop down box at the bottom of this page or from the location line at their top. At the top of this page you should see a line that says, "Forum -> Manufacturing Today -> General" You can just click on "General" to go to the "General" Forum page.

Hammerhead800 said:
Hi I’m sorry for having to do it this way. Can someone tell me how to start a new thread? I can’t find anything on this site to do so.

Laverda · Aug 24, 2021

No, I don't need NIST traceability or certification on the reground blocks. All the blocks running out of room to use are in the Rockwell "C" 55 to 65 range. They cost anywhere from $70 to $90 a piece and although they last a long time, I have a problem with throwing away something I can fix for next to nothing. The back side of the blocks are not polished and would still need to grind the front side to make it flat. I was thinking they may be case hardened which is why I have not tried grinding them yet. But I am gonna try it now. Keeping my fingers crossed as I was going to spend over $400 on getting new ones.

jccaclimber · Aug 24, 2021

In they case grind half and see if it matches the unground half.
Might not be a bad time to find out how much the polish/lack thereof matters in your specific case. A good grind might be good enough.

Conrad Hoffman · Aug 24, 2021

I'm pretty sure they tell you you can't do it, but if you check the hardness before and after, it seems reasonable. The finish does matter, so try to get same.

Ray Behner · Aug 25, 2021

Seems like the displaced material will go far beyond the width and depth of the grind. That upset material may not have the same consistency as new. Just guessing here,

eKretz · Aug 25, 2021

I was going to say the same as Ray. The steel beneath where the penetrator has pressed into the part may have been altered in hardness for some ways below the surface, so it might take more than just a cleanup grind to get to where the results will be consistent. And grinding much off may result in a softer surface since quenching happens from the outside in. Only way to find out for sure is to try it. Grind and lap one and see what happens.

Laverda · Aug 25, 2021

The results of grinding the block:

The hardness tester was sent to the calibration lab on July 9th.

The block was marked 46.3 HRC. Tested the block three time and all measured about 46 HRC.

Next I tested the back side of the block which was not polished like the front and it measured 45.8 HRC. It appeared to be blanchard ground and although perfectly flat was not that smooth of a surface. So the polishing of the block made a very small if any difference.

OK, now time to grind the polished side in my Brown & Sharpe grinder. The block was put in the freezer overnight (-10 degrees F) and I had a spray bottle of ice water for grinding. The grinding wheel that was in the grinder was 120 grit aluminum oxide so that was what I used. I only needed to grind off a few thou to clean it up. After lapping it on the surface plate with 600 grit and then back to the hardness tester, it measured 46 HRC. I checked in four places and all measured the same.

The block I use the most is 62 HRC and it having very little usable area left on it is what started this. The indent on something this hard is not much so will require only a tiny bit of grinding.

I agree that quenching happens from the outside so it may be a little bit softer in the center but if I ever need to regrind a block again and it does not measure the same, it goes in the trash.

Now I have six more blocks to grind. The over $400 I saved will buy a lot of beer!

Dave G. · Aug 25, 2021

I don't see why you couldn't regrind the hard block for non-certified work. Just a couple of points - On regrind you need to remove all traces of the work hardening from the first set of indents. It might be as much as 8X the original penetration depth - which is not that deep on the HRC 46 block. There is also a minimum depth of material - it's 8 or 10X the depth of the indent. So it's not much for the hard blocks. Finally, there must also be a minimum block thickness to prevent warping from repeated imprints on just one side.

TGTool · Aug 25, 2021

I'm not persuaded that you'd find the block softer in the middle. Sure, the temperature change initiates from the outside on quenching, but it should propagate fast enough (the critical rate) for all the molecules to be "frozen" in their changed state. It will be at maximum hardness for that alloy. The piece is then held in the draw oven (tempering or drawing the temper) at a temperature chosen for the desired final hardness where the hardness is drawn back from maximum. It really ought to be the same all the way through, allowing for possible local aberrations caused by the compression of the brale.

eKretz · Aug 26, 2021

Depends how thick it is and the cooling rate etc. It definitely happens. And that's why I said "may..."

I would expect the one tempered back to be less likely to have much difference. The near full hard one more likely.

Sounds like the softer one is going to be fine. It will be interesting to see how the harder one compares.

dian · Aug 26, 2021

what are the blocks used for? just wondering, as you have a hardness tester, right?

TGTool · Aug 26, 2021

dian said:
what are the blocks used for? just wondering, as you have a hardness tester, right?

It's like a standard for a micrometer to verify that the hardness tester is correctly calibrated. He mentions that he has several standards for different hardnesses.

I was taught that you only ever use the top face for checking. I can imagine that if the backside has any raised areas on it that might interfere with testing accuracy. In the shop I was in there were a few marks on the back side, which is a little like finding your machine tool has drill divots on the vise left by some ignorant or ham-fisted operator.

Laverda · Aug 26, 2021

I ground the HRC 62 block today and it still reads at 62. The indent on blocks this hard looks to be only about .002" or so. So a very quick fix for a $70 block of steel.

Dian, to expand on what TGtool said. If our are heat treating parts, there are times when you need to know exactly how hard the part is when its done. The piece I have in the lathe at the moment is made out of 52100. After heat treat it needs to be at least HRC 62. The worn out part I am copying measured at HRC 64. If it's less than HRC 62 it will eventually fail. Also making a shaft out of 8620 and it also needs to be at least HRC 62.

CarbideBob · Aug 26, 2021

TGTool said:
I'm not persuaded that you'd find the block softer in the middle. Sure, the temperature change initiates from the outside on quenching, but it should propagate fast enough (the critical rate) for all the molecules to be "frozen" in their changed state. It will be at maximum hardness for that alloy. The piece is then held in the draw oven (tempering or drawing the temper) at a temperature chosen for the desired final hardness where the hardness is drawn back from maximum. It really ought to be the same all the way through, allowing for possible local aberrations caused by the compression of the brale.

Not a very thick piece so not so much variation would be expected if as said get well below the old points and the added distortion level
Get up in the inch or two and there a difference if you section the part.
It is still hard but certainly not a calib standard.
What would seem simple is to grind and check against a new one... but did you check a used spot or a new spot?
For the work involved and the price of new and the wonder if correct..... Would you grind down a gauge block from one size to another if used up?
Bob

Laverda · Aug 26, 2021

Bob,

I am not worried about testing on new or used areas of the block after re-grinding. For a Rockwell C value of 0 the diamond is only penetrating .007" maximum in to the metal. At a Rockwell C value of 62 it is .00266". If I grind off .006" on the HRC 62 block, I have removed any and all deformed or work hardened metal.

Were I making parts for NASA, I would not do this. But I am not!

Surface grinding a Rockwell test block back to new condition?

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