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The Newbies Arena Are you new to knife making? Here is all the help you will need.

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  #1  
Old 08-28-2012, 10:14 PM
SharpEdge0913 SharpEdge0913 is offline
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Rockwell Hardness Testing Help

So I invested in a Grizzly hardness tester and have been playing with my new toy. After learning how it functions I am struggling with accuracy. I do 4 tests, throw away the oddball, and then average the remaining three. All four tests are really close, the oddball is not really that odd. It just seems that all my results are too high, even on the test block that came with it.

For example, I just heat treated 440C (1880 soak for 20 minutes, plate quenched and tempered 2 hours at 325). My average was 61. That just seems high. The test block is 62 and my average is 64.

I am thinking of buying a certified block online and testing that. Lets say I get a certified block online, run it through, and my average is still 2-3 points higher. Is there a way to calibrate the machine to make it right, or do I just subtract 2-3 points from each test going forward?

Thanks for any insight. I can't find much on these machines online, and what I do find seems to confuse me even more. On a positive note, I tested some annealed steel and the reading was really low, so I feel my blades are getting hard from my heat treat, which in a way validates my recipe a bit. So that is exciting. Just more fine tuning.

Last edited by SharpEdge0913; 08-29-2012 at 08:01 AM. Reason: Removed O1 test; can't confirm it is valid
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  #2  
Old 08-29-2012, 06:48 AM
WBE WBE is offline
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Going by your temper heat of 350? on the 01, a 62rc reading may be a tad low.
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  #3  
Old 08-29-2012, 08:56 AM
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Ray Rogers Ray Rogers is offline
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I totally agree with WBE, 325 F is way too low so the reading you are getting is probably about right.

I have a Grizzly hardness tester as well and you can calibrate it yourself if needed. You said you had a test block but then mentioned that you might buy a certified block off eBay. A certified block is the only block you should be using and it should be within one point of the hardness you plan to test for most often.

Hardness testers cannot be calibrated for say, 61, and then be expected to read accurately at 55. That's a million miles away in Rockwell land. So, if most of your blades will be Rc 60 then calibrate as close to 60 as you can get (within 1 point if at all possible). When you calibrate, set the machine so that it reads as near exactly as possible to the hardness marked on the test block. Generally, you should be able to get within a tenth or two of the rated hardness.

To calibrate, you'll need a small screw driver and a tiny wrench. You'll see the required sizes when you pop the lid off the machine. You should see a screw and a locknut marked with some red paint. That's it, twiddle that thing.

Your testing process is good. To get consistent results you'll need to test at the same temperature (reasonably close) as 35 F in the winter and 80 in the summer will give different results. The machine needs to be on a rock solid base also. If it must sit on a wooden floor then you may need to mark the floor to know where your feet are when testing as shifting your weight can change the reading. The needle can stick so I tap the side of the machine to jar it loose - this can change the reading by a half point or so.

In Rockwell land, being a point off is not 'close'. Each Rockwell C scale point is represents TEN times the hardness of the previous point. The Griz is a good machine, more than adequate for our needs as knife makers, but it isn't laboratory equipment so don't get too hung up on getting ultra precise readings.....


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Old 08-29-2012, 05:07 PM
SharpEdge0913 SharpEdge0913 is offline
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Thanks for the information, much appreciated. I think that when I finally make a knife worthy of my makers mark, I may have to put a footnote that says *with the help of The Knife Network lol.

I tossed out the heat treating results as I am still new at that, and I very well could have messed something up with that process, so I stuck with the test block. I focused on consistency, made sure everything was the same. One thing I noticed is that I was messing around on the table with something else while it was working, so I quit that. That may be it, because I did 2 more averages, and got within a half point each time. It is a sensitive bugger!
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Old 08-29-2012, 05:20 PM
WBE WBE is offline
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Excellent. Good for you. Seems like new equiptment is always smarter than the operator, but only for the time it takes to learn it's particular quirks. By the way, with 01, a 425? temper should give you about 60/61rc, which is about it's ideal combination of strength and edge holding. Less and you begin losing edge retention. More and you begin losing strength.
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Old 08-29-2012, 08:08 PM
SharpEdge0913 SharpEdge0913 is offline
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I made a note of the 425 temper and will try that with my next blade. thanks for the info!
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