Upgrades to Homemade BHN Tester

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Re: Upgrades to Homemade BHN Tester

Postby mtngun » Sat Jul 09, 2016 7:42 am

A method suggested by the NRA years ago was to sandwich a ball bearing between an ingot of pure lead and an ingot of unknown lead, and then squash the sandwich in a vise. The BHN would then be 5 BHN * (D_pure / D_sample)^2.

But that only works on ingots. How would you use it to measure bullets?

This is what I came up with.

I use a 45 caliber cylindrical slug for BHN and density measurements. In any event, 45 is a popular caliber so many casters will have a mold for a flat nose 45.

I cut a piece of 1/2" pex pipe that was slightly shorter than 2 cylinders and the 10mm ball. The pex serves to hold the cylinder sandwich together during the test. Put the sandwich in a vise and squash them.
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That was easy! :) However, the pure lead slug obturated so much that it was stuck inside the pex. :( To get it out, I ended up slitting the pex with a utility knife. That's OK, the slitted pex still will serve to hold the cylinder sandwich.
Image

Results, as measured with a USB microscope:
D_pure = 0.299"
D_lino = 0.1535"

5 BHN * (D_pure / D_lino)^2 = 19.2 BHN

That compares to 21.7 BHN for the 10mm / 150 kg test. At least it's in the ballpark.

I'll run this system through its paces with other alloys and we'll see if it continues to give ballpark accuracy.

Advantages:
-- cheap, cheap, cheap !
-- if it indeed gives reliable ballpark accuracy, that's more than you can say about some hardness testers. :D

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Re: Upgrades to Homemade BHN Tester

Postby mtngun » Sun Jul 10, 2016 3:01 pm

Following up on the "cylinder sandwich", here are the results. Besides linotype, I included one medium-hard alloy and one extra-hard alloy in the test so that the full spectrum of BHN would be covered.

Since the hardness of COWW and reclaimed shot are age-dependent, I retested a single sample of each on my homemade 10mm / 150kg tester. The 6-day old air cooled wheelweight was 15.3 BHN, and the 6-day old oven treated reclaimed shot was 42.7 BHN.
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I thought the accuracy of the "cylinder sandwich" was decent -- better than some storebought testers -- providing you take an average of several measurements, providing the pure lead that you use for comparison is really pure lead (my pure lead was store bought and supposedly 99.99% pure, or something like that), and providing you have a way to measure the indentations accurately.

Considering that it only costs $8, the "cylinder sandwich" is a viable alternative to store bought BHN testers.

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Re: Upgrades to Homemade BHN Tester

Postby mtngun » Tue Jul 12, 2016 7:19 pm

I received the loan of a new toy -- er, I mean tool -- to play with. :lol:

The Lee tester uses a spring loaded 5/32" ball. According to my calculations, the spring exerts 59.2 pounds force on the ball when compressed as per the instructions. That's pretty darned close to Lee's claim of 60 pounds. 8-)
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To my surprise, the 20X graduated pocket microscope included with the Lee tester was of decent quality. The scale shows up clearly in the scope, and when I held the Lee scope up to a stainless steel scale, they matched up within 0.002" over 0.100". My only complaint about the pocket microscope is that it shows you a mirror image, so if the view shows that the scale is to the right of the indentation, it's actually to the left. This is confusing when you are trying to get the scale lined up with the indentation.

But .... I've already tested a 4mm ball (nearly the same as 5/32") and a 60 pound load on my homemade BHN tester, and the results were goofy. Can the Lee tester somehow do a better job than my homemade 4mm / 60 pound test? :?:

The Lee tester leaves itty bitty indentations on hard alloys. Nonetheless, the pocket scope is accurate enough to measure them, though it can be difficult to maneuver the pocket scope to get the scale lined up with the indentation. After satisfying myself that the pocket scope gave good readings if you tried hard enough, I switched to using the USB microscope. The USB microscope & digital measurement in GIMP gave the same measurements as the pocket scope, and it was a lot easier to do.
Image

To my surprise, the Lee test results were mostly in the ballpark and the variation from measurement to measurement was reasonable.
Image

I have more to say about why the Lee gave good results while my homemade 4mm / 60 pounds test gave bad results, but it's been a long day so that will have to wait until tomorrow. In the meantime I give the Lee hardness tester good marks providing you use digital measurement.

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Re: Upgrades to Homemade BHN Tester

Postby mtngun » Tue Jul 12, 2016 7:39 pm

Data for my homemade 10mm / 150 kg tester has been update to include "J.R." brand reclaimed shot.
Image

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Re: Upgrades to Homemade BHN Tester

Postby mtngun » Wed Jul 13, 2016 7:25 am

Re: why did the Lee 5/32" / 60 pound test give decent results while my homemade 4mm / 60 pound test gave goofy -- and mostly high -- results?

For a while I suspected that anything other than a 10mm ball might skew the results, but the Lee tester proved that a smaller ball can work OK.

The bullet does not know which brand of tester is pushing on it, the bullet only knows that a certain diameter indenter is pushing on it with a certain force.

There is no dispute about the diameter of the indenter(s).

I used the same digital measurement system to measure the indentations with both the Lee tester and my homemade tester, and I have confidence in those measurements.

What I can't be confident about is the applied force. That is the weakness of any BHN test (other than Frnkeore's method, where the applied force does not matter because it cancels out of the equation).

There are always friction losses that we cannot measure. The friction losses subtract from the applied force and cause the indentation to be smaller, which makes the BHN appear to be higher than it really is.

In the case of my homemade tester, I have not been taking into account the weight of the moving parts. That's because they don't weigh much compared to the load. The moving parts weigh about 2 pounds, while my 150 kg test uses a 52 pound weight. Compared to a 52 pound weight, errors due to 2 pounds of moving parts or a few pounds of friction are small.

But my homemade 4mm / 60 pound test uses a 9.4 pound weight. Compared to 9.4 pounds, friction losses and the weight of the moving parts may be significant. I can't feel much friction, but the goofy results with a 4mm ball suggest that friction is there whether or not I can feel it.

So .... I may have mentioned in a previous post that I was thinking about building a new homemade tester based on a "nutcracker" design. The simplicity of a nutcracker design means that it could be made to have minimal friction, and the weight of the moving parts could be easily accounted for. As time allows, I will try to build the nutcracker tester and see if it will work with a 4mm / 60 pound test. If not then I'll probably buy a Lee tester, but I'm betting the nutcracker design will work.

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Re: Upgrades to Homemade BHN Tester

Postby mtngun » Thu Jul 14, 2016 6:02 am

A USB microscope is very nice for photographing indentations, but many ordinary cameras are capable of taking decent closeups.

This was taken with a Canon S120 point & shoot, set to "macro" mode for close-ups. The resolution after cropping was only about half the resolution as my cheap USB microscope, but nonetheless it was sufficient to get the job done.
Image

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Re: Upgrades to Homemade BHN Tester

Postby mtngun » Thu Jul 14, 2016 5:39 pm

Re: new homemade BHN tester to fix the shortcomings of the original homemade tester. The goals were:

-- minimize friction
-- account for the weight of any moving parts
-- increase the leverage

I used a simple "nutcracker" design. There is only one pivot point so friction is minimal. It has 7.3X leverage in 4mm mode and 11X leverage in 10mm mode. The weight of the lever arm is included when calculating the applied load. There's not a whole lot to go wrong with this simple design, so it oughta work. :)

Sorry for the cluttered photo, I'll post a better photo after the tester is completely finished.

The tester clamps in a bench vise. When not in use it will hang on the wall.

The load consists of a bucket containing scrap metal. It was weighed on a digital postal scale and metal added until the desired weight was reached. A notch in the lever arm makes sure the bucket sits in the right place. The load is applied for 30 seconds as per standard procedure for any BHN test.
Image

Results with a 4mm ball and 59.3 pound load. While not in 100% agreement with other tests, all results were at least in the ball park, and fairly consistent. I can live with this level of accuracy for measuring small caliber bullets.
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Tomorrow I will try to test the "nutcracker" with its 10mm ball and perhaps a 200kg load. Assuming it passes that test, I need to make adjustment knobs for the indenters, and then give the assembly a coat of paint, or perhaps powder coat.

On another note, I've been wondering "what if I replaced the Cabine Tree's conical indenter with a 4mm ball?" :geek:

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Re: Upgrades to Homemade BHN Tester

Postby mtngun » Fri Jul 15, 2016 4:51 pm

Testing the second homemade tester (HT2) with a 10mm ball and a 200 kg load. My original homemade tester used a 10mm / 150 kg test, because 150 kg was about the practical limit without pulling my reloading bench over. :lol: The thought behind increasing the load to 200 kg was to make a bigger indentation that would be easier to measure.

Well, the 200 kg indentation was a little too much of a good thing, because it smushed a 45 caliber pure lead sample like a pancake. :lol: Oops. It was kinda noisy on air-cooled wheelweight and didn't settle down until lino. In summary, 10mm / 200kg worked great on heat treated samples, but was too much for all around use.

Image

This has been a fun project and I've learned quite a bit. The two biggest improvements were 1) using digital measurement rather than calipers and 2) eliminating friction losses and accounting for the weight of moving parts.

As of this moment I would be equally happy using either the Lee tester or homemade tester #2 with 4mm / 60 pounds -- as long as I measure their tiny indentations digitally. Third place goes to my original homemade tester with 10mm / 150 kg. Frnkeore's method using a "cylinder sandwich" is also a viable option, though slower because twice as many samples and twice as many measurements are required.

The Cabine Tree tester works well on softer alloys but accuracy is not-so-great on harder alloys. If time allows I want to try making a 4mm ball indenter for the Cabine Tree, and see if that helps.

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Re: Upgrades to Homemade BHN Tester

Postby mtngun » Mon Jul 18, 2016 1:12 pm

The experimental indenter for the Cabine Tree tester, with a 4mm ball.
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As best I could measure, the Cabine Tree's spring has a spring rate of 146.5 lb/in. The initial preload is 0.316", which generates 46.3 pounds force. Turning the screw one turn as per the instructions can increase that force to up to 60.9 pounds, though in practice the force will always be less because of the indentation taking up some of the screw movement.

The hope was that the 4mm ball would be more sensitive to changes in BHN than the cone indenter. But in fact the 4mm ball was less sensitive.

Chart shows the actual dial readings, not BHN.

For example, with the 4mm ball there was only 0.004" difference between the air-cooled reclaimed shot (14 BHN with the Lee tester) and the oven treated reclaimed shot (35.9 BHN with the Lee tester), while the cone indenter had 0.0150" difference between those alloys.
Image

I also tried testing both indenters with 2 turns of the screw, in addition to the normal one turn. I was hoping the extra force might improve sensitivity, but it didn't.

So I have to admit defeat at improving the Cabine Tree tester. :roll: It is what it is. It's very well made, it's fast and easy to use, and it gives good results with soft alloys. If you only shoot black powder, the Cabine Tree would probably serve you well. But if you shoot a lot of harder alloys, the Cabine Tree is not the best tester. :cry:

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Re: Upgrades to Homemade BHN Tester

Postby mtngun » Tue Jul 19, 2016 8:52 am

This chart may be helpful to Cabine Tree owners. Let me know if you can't see the chart -- I'm still trying to figure out my new-to-me photo host. :lol:
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