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The Folding Knife (& Switchblade) Forum The materials, techniques and the designing of folding knives. |
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#1
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Hardness tester
I bought one of those China made rockwell tester that was damaged during shipping, I have it repaired and I am getting results that are 1 point lower than my calibration blocks on the 45 and 65 hrc I get a consistant 1 point lower.
Should I try to adjust/calibrate the machine or just simply add 1 to my readings? It doesn't seem to me that it would be a problem but I am not familiar with these machines. J. Prater |
#2
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Jeff, do you have the manual with instructions for calibrations? I've calibrated testers but it's been so long ago I've forgotten just how to do it.
I wouldn't worry about the 1 Rc difference as long as it remains constant. By the way, to save using up your calibration block get yourself a ground HSS lathe tool bit blank and use that for checking consistency. It will read over 61Rc. Just record the third reading you get on the tool bit and recheck it once in awhile. Always check hardness 3 times in approx. the same area on a work piece. The 3rd. reading should always be the highest and that is the accurate reading. |
#3
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hardness tester
Don,
The manual is just a few pages of info that is poorly written in English, it doesn't say anything about adjusting the calibration. I had done a search and read in a past post your idea for saving the test blocks and I will certainly do that. When you say do three tests do you mean for example: do three consective tests on a blade in the same general area and use the last reading? What does it do for the machine, you must move the blade a little each time. J. Prater |
#4
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Hey Jeff , where did you get the tester from ?
Dwane |
#5
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Does the ring on the dial turn on those machines to make slight reading adjustments?
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#6
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Quote:
This helps remove slack between the stage screw and nut and squeezes out any oil. Mike, yes, the dial is free to move to zero. |
#7
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I bought it on ebay, it was a new unit that had some shipping damage, you turn the dial to 0 each time you start a test and there isn't any adjustment. In the top of the machine there is a balance beam with a weight that is marked with two red dots from the factory, I am guessing that if you move that weight you will change the calibration. No instructions with it that tells you anything other than to put oil in the speed control.
By the way how many seconds should it take from the time you start the test for your handle to stop moving. The instruction seem to say 4- 8 seconds. |
#8
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Yep, you move the weight to change calibration.
In the back you will find an oil pot that must be kept full of oil. That dampens the weight falling. It should take a few seconds for the handle to stop moving. Check the oil level in the pot. Last edited by Don Robinson; 01-01-2007 at 08:49 AM. |
#9
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I did three consecutive on a calibration block and the third test was the highest reading less than 1/2 point off.
I found a hss lathe cutoff tool that measures 60 and will use it to make sure I stay in the right range. Don you were right the third test is usally the highest sometimes the second and third are very close. Thanks, J. Prater |
#10
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Quote:
For an accurate test the piece must be smooth, flat and both sides parallel. If the diamond hits a scratch it affects the reading. Sounds like your tester is calibrated within a half point then, huh? |
#11
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The cutoff tool isn't tapered but I found a cobalt 1/4 square
lathe tool that has a much better finish, it tests 65 would that be a better for future reference. The tester is so close on the hardness scale I would not want to try any calibration, about how many seconds shoud it take for the loading handle to completely stop? The dial indicator stops within 4 or 5 seconds and the handle will keep moving for about 20 seconds when I release the main weights. Do I have enough pressure to dampen the falling weight? |
#12
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I have one of those Chinese testers. Yours sounds like it is working fine just as it is. At different times of the year when the room temperature varies from pretty warm to fairly cold the amount of time it takes for the handle to stop moving varies also. Nothing to worry about, just wait until it stops and then wait a few seconds longer. I like to gently tap on the side of the tester with my finger tips after the lever stops - often you'll see the needle 'settle' a little farther over when you do this.
It is not unusual for a hardness tester to read dead on at 60 Rc and then be off by one or two points at 45 Rc. That's just the way they work. That's why they make hardness calibration blocks in the 60 range, the 40 range, and in other ranges.You calibrate for the range you expect to use. If you want your blades to be very near 60 Rc, then use a block that is as close to 60 Rc as you can get. Hardness testers of the Rockwell design do not really measure hardness, per se. What they are really doing is COMPARING the depth of a dent they make in one piece of metal (the test block) to the depth in another piece (your blade) and this comparison is neither perfect nor completely linear. Once you understand this, you'll have a much more reasonable expectation of what your hardness tester can actually do for you. That's the reasoning for the recommendation that you test three times and take the last measurement and that all three l measurements be done in a small area. Any slight change in metal density or surface angle can radically change the reading. I usually measure 4 times, add up the three closest results and average them to get the final answer. The single most important element in hardness testing is consistancy. Do your measurements exactly the same way every time. I even go so far as to place my feet in the same spot each time so that the floor doesn't flex and move the machine by a few millionths of an inch (because I don't have a concrete floor)... |
#13
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Quote:
My tester is located in an air conditioned room so it only takes about 10 seconds for the weight to fall all the way. I think your tester is working just fine. |
#14
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If your cobalt lathe bit is ground to a fine finish it should test around 65Rc. The HSS bit I use checks 63Rc.
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blade, knife |
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