Home-Made Hardness Tester

How to make a homemade hardness tester.   My tester uses an iron RCBS reloader special reloading press, but you can use other presses if you are willing to do a little math.

  • 1/4″ ball bearing
  • 2 ea. 1/4″ flat washers
  • 1 ea #10 flat washer
  • magnifying glass
  • calipers
  • RCBS iron reloader special press

 

I tried epoxying the 1/4″ ball bearing to the washers, but it didn’t hold, so I just smeared some alox on the washers and that sticks just fine.   A little alox holds the ball and its washer on the shell holder while another washer is stuck to a seating die with more alox.

The seating die is adjusted until the press handle is perfectly horizontal.   Sometimes you have to remove the shell holder to get it right.

I filed a notch on the press handle exactly 12″ from the hinge pin to mark where the weight will be hung.

The handle must be horizontal because the leverage varies as the handle moves through its arc.   I calculated the correct leverage for the horizontal position.   On this reloader special press, the weight applied at the notched spot is multiplied by 6.38.   In other words, if you hang 100 pounds on the notch, 638 pounds will be applied to the ball bearing.

Originally, I used a bucket of lead ingots (you can weigh the bucket on a bathroom scale).   Now I using a barbell weight, just because it is handy.   Set the weight on gently and let it sit for 60 seconds.   Because of the press leverage, the 20 kg weight results in a 127.6 kg force on the ball bearing.

Normally I use a magnifying glass when measuring the indentation.

OK, now we do the math ….. or better yet, let a spreadsheet do the math.

BHN = P / {pi * D * 0.5 * [D – sqrt(D*D – d*d)]}

where:

P = the imposed load in kilograms (127.6 kilograms in our example, and it should be approximately one to five times D*D)

pi = 3.14159

D = the diameter of the ball in millimeters ( 0.25″ or 6.35 mm in our example)

d = the diameter of the indentation in millimeters (0.086″ or 2.1844 mm in my example)

If you plug the numbers in, this bullet’s hardness is 33.   It happens to be oven treated wheel weight, no tin added, about 2 weeks old.

So what if I have a reloader special press but I don’t like to do math?

Relax, I’ll be glad to send you a Works spreadsheet that does all the conversions and calculations for you, providing you have an iron reloader special press just like mine.

So what if I have a different press?

    You’ll need to calculate the leverage when the press handle is horizontal.   I originally did the calculations back when I was an engineering student, but there is an easier way.

    1. position the press handle so that the handle is about 1″ above horizontal, and measure the distance from the end of the handle to the floor with a tape measure.
    2. without moving the press handle, use your caliper’s depth gauge to measure the distance from the die seat to the ram, and record this number
    3. move the press handle down exactly 2 inches, using the tape measure as a guide.
    4. without moving the press handle, again use the calipers to measure the distance from the die seat to the ram.   Subtract this distance from the original distance to determine exactly how far the ram moved.
    5. the press leverage is the distance the handle moved divided by the distance the ram moved. On my press, the ram moved 0.332″ as the handled moved 2″ about the horizontal position. 2 divided by 0.332 = 6.024 leverage. Not perfect, but close to the exact answer of 6.38.
    6. if you like, let me know your leverage or your measurements, and I’ll modify the spreadsheet to suit your press.
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