Upgrades to Homemade BHN Tester

[mtngun]

Recently I've been experimenting with alloys and heat treatments that reach 40 BHN. The problem is that at 40 BHN the indentation made by my homemade tester is tiny and that makes it difficult to read accurately, even with magnification. Also I had been using a lathe-turned 10mm hemisphere to indent the bullet, and while the lathe-turned ball was pretty darned good it wasn't 100% smooth at the tip, and those 40 BHN bullets were using the tip of the ball.

So ..... I did a couple of things to make the indentation bigger and rounder.

First I added more weight to the tester, enough to increase the force (including the press leverage) from 100 kg to 150 kg. The "official" BHN test calls for a 10mm ball and 100 kg weight, but you don't have to use the "official" weight or diameter, it just makes your results look more "official."

Second I modified my indenter to include a smaller 5/16" ball bearing. In theory the smaller ball should sink into the lead a little deeper, and the ball bearing is smooth and perfectly round. I can choose either the 5/16" ball or the 10 mm ball just by flipping the indenter around.


Result: the old setup created a 0.074" indentation in a 36 BHN bullet, while the new setup creates a 0.090" indentation. It doesn't sound like a big gain and it's not, but it helps.

[mtngun]

FYI while my homemade BHN tester works fine now that I've got it set up and fine-tuned, if I had to do it over again I'd try a different approach.

I haven't actually tried this, but if you have a drill press and a bathroom scale, you could set the bathroom scale on the drill press table, set the bullet on the scale (or actually set it on a piece of flat steel to spread the weight), and then use the drill press to push a ball bearing into the bullet. Hold a steady pressure on the bathroom scale for 30 seconds, then measure the indentation and plug it into the standard BHN formula.

If the bathroom scale method works, it would be easy and free assuming you already have a drill press and a bathroom scale. I don't have a bathroom scale so I haven't tried it yet.

[mtngun]

Today I was having a particularly tough time measuring a tiny indentation, even with a magnifying lamp, so I broke out the USB microscope. This guy measured 0.079" - 0.081", for 45 BHN.

[mtngun]

I ran into some issues using the smaller diameter indenters to measure BHN, so I'm going back to the drawing board.

First up, my original 10mm indenter had been turned on a lathe, and while the lathe did a pretty darned good job, there were some microscopic imperfections on the tip of the indenter. The imperfections don't show in this photo so you'll have to take my word for it. Anyway, I splurged on some 10mm ball bearings and epoxied a ball bearing into a 5/8" thingamajiggy that fits into my push-thru die holder.


I used to measure the indentations with calipers and a magnifier, but then someone pointed out that Gimp (and some other free photo editing programs) have a "measure" tool. Take a photo of the indentation with some sort of scale in the picture, then you can zoom in on the photo and measure the indentation with accuracy limited only by the resolution of the photo.

A USB microscope works well for taking the photo, but some cameras and even phone cameras are capable of taking decent closeups.


Load the photo in Gimp. Measure the scale (this one is marked in tenths of an inch). In this example, the measurement across 0.300" on the scale was 569 pixels. In other words, the scale of the photo is 569 pixels per 0.300 inch, or 1897 pixels per inch.

Next I measured the diameter of the indentation. I actually measured in several different places: 257 pixels, 257 pixels, 261 pixels. Well, the cat was chewing on my arm when I took the last measurement! Average of the three measurements = 258 pixels. To convert that to inches, divide 258 pixels by 1897 pixels per inch, = 0.13618 inch.


The load applied was 150 kg. for 30 seconds, more on how I apply the load later. Plug the numbers into the formula for BHN (a spreadsheet crunches the numbers for me) and we get 15.6 BHN. Should we believe that number? Well, the alloy was 50% lino and 50% clip-on wheelweight, and I was expecting 16 BHN. That's pretty close.

In the next few days I'll try to post measurements for all my alloys including "known" standards like pure lead and linotype. Then I'll repeat the tests with different indenters and different loads and see how the results stack up.

Most people buy store-bought BHN testers and that's fine, but like many casters I enjoy making my own stuff whenever possible. I'm not going to run out and buy a store-bought tester, but if anyone wants to loan me their store-bought tester, I'll be glad to include it in my BHN shootout.

[mtngun]

More data using the 10mm ball bearing and a 150 kg load.

The first set of linotype measurements were made without filing the part line off the bullets -- and the mold that I use to cast the 45 caliber cylindrical test slugs does have a distinctive part line. Well, the part line seemed to cause a lot of variation on the high side, so I filed off the part line and tried again, and got much better results.

In general it is looking like the homemade tester does a bang-up job as long as I use a 10mm ball and as long as I file the part line off hard bullets.


The tentative plan is to collect a bunch of data with both the 10mm ball bearing and with a 4mm ball bearing, and then see if I can come up with a correction factor for the 4mm results.

[mtngun]

From "Engineering Properties and Applications of Lead Alloys" by Sivaraman Guruswamy:

The takeaway is that he recommends 10mm / 125 kg for 3 - 39 BHN, or 10mm / 250 kg for 5.6 - 78.8 BHN.

[mtngun]

One of the goals of this project was to try to find a way to get decent results with a 4mm ball, so that I'd have a way to measure 22 & 243 caliber bullets. Also I wanted to try a 4mm / 60 pound combination because I read somewhere that the Lee tester uses 4mm / 60 pounds (actually 5/32" but that's nearly the same as 4mm).

Here's some results for 4mm / 60 pounds, and as you can see, they stunk. Not only were the BHN's goofy, but the variation was awful. I.e. linotype measured anywhere from 28.3 BHN to 75.5 BHN. I've never used a Lee hardness tester so I can't comment on how well it works, if indeed it uses 5/32" / 60 pounds, but I'm just not seeing it. If anyone wants to mail me their Lee tester on loan for a week or two I'll be glad to run it through its paces.


So then I tried increasing the weight on the 4mm ball to 50.3 kg (111 pounds). No joy, it still gave nonsense numbers with lots of variation.


I increased the weight on the 4mm ball once more, this time to 71.8 kg (158 pounds), but the BHN numbers were still nonsense.


Maybe I'm missing something, but I haven't been able to get a 4mm test to work decently. I'm going to give up on 4mm for the time being.

Continued in the next post ...

[mtngun]

The good news is that I continue to get decent results using a 10mm ball bearing and 150 kg load. The numbers for known alloys like pure lead and linotype look good, and the other alloys are at least in the ballpark. Repeatability is fair.

One thing I learned while doing these tests is that imperfections on the the surface of the bullet can affect the results quite a bit. I got into the habit of filing the surface to remove any part line and to make the surface reasonably flat, but the file leaves microscopic striations and even a filed surface sometimes is still not flat. Striations or non-flatness seem to result in BHN variation on the high side.

FYI "R2RS" = Batch #2 of Rotometals Reclaimed Shot, and "HTR2RS" = Oven Treated Batch #2 Rotometals Reclaimed Shot.


I've been trying different webcam software to operate my cheap USB microscope. For this photo I used "Cheese", with the resolution set at 1600 pixels wide. That gives you some respectable resolution for measuring even small indentations like on this heat treated reclaimed shot.


Another thing I noticed is that if I measured a particular indentation with digital photo measurement, and then checked the measurement with calipers under a magnifying lamp, the calipers would always read 0.003" - 0.009" larger. Which is the correct measurement? I'm not sure, but I'm going to stick with digital photo measurement rather than relying on my not-so-good eyesight to read calipers.

A customer kindly offered to loan me his Cabin Tree tester for this evaluation, and when it shows up I'll run it through its paces and see how it compares to my homemade tester.

Otherwise, my plan is to stick with the 10mm / 150 kg homemade tester. That means I will not be able to test small caliber bullets, but oh well.

I'm thinking about making another homemade tester, based on a nutcracker design, that will have more leverage so it can use 200 - 300 kg load, for a bigger indentation on heat treated alloys.

[mtngun]

I added 20:1 alloy to the chart for the homemade tester with 10mm ball & 150 kg load.

Most sources put 20:1 at 10 BHN, and my homemade tester averaged 9.6 BHN.


It looks I'll be getting the loan of a Lee hardness tester so it'll be interesting to see if Lee has better luck with a 4mm ball than I did.

[mtngun]

The borrowed Cabine Tree tester showed up. I was impressed with the heft and the quality.


The Cabine Tree does not use a ball indenter, rather it uses a cone with a small meplat. Similar to a Saeco tester, it doesn't really read BHN, it just reads "something" and that "something" is supposed to correlate to BHN.


It was extremely easy to use the Cabine Tree and the tests went very quickly. The dial indicator eliminates most of the human error.

The instructions suggested filing a flat on the surface to be measured, and I agree with that. I also found it helpful to lightly file the other end of the bullet, to knock off the sprue bump. If you don't, the sprue bump might compress a little bit during the test, and that will throw the reading off.

A supplied chart is supposed to correlate the dial indicator results to BHN


I measured each alloy at least 5 times and took an average, results below.


I had high hopes for the Cabine Tree because it looked well built -- and it is well built -- and because the dial indicator removes most of the human error. However, like most storebought hardness testers, it uses a small indenter rather than the standard 10mm ball. That's understandable because a 10mm Brinell tester would be big and heavy and expensive. The problem is that it seems to be difficult to get good results with a small indenter. Look at the numbers and draw your own conclusions.