Mistake on alloy calculator, or my mistake?

Please read and understand the entire post before responding. My problem here is very pointed, and has nothing to do with the Brinel hardness of pure lead!

I greatly enjoy using the alloy calculator I downloaded from this website, but there is either a mistake or I don't understand something. ??

The Rotometals formula used in the calculator is: 8.60 + ( 0.29 * %Tin ) + ( 0.92 * %Antimony)
This was copied directly from the spreadsheet calculator.

formula from Rotometals website: 8.60 (Antimonial Lead) + ( 0.29 * Tin ) + ( 0.92 * Antimony )

The value of Pure Lead in the calculator is listed as "5". I have no problem with that number for pure lead, as that is not my problem here. As can be seen from the formula on the Rotometals website, the BHN of 8.6 is for their Antimonal Lead, not Pure Lead. The Rotometals Antimonal Lead already has 3 to 5% antimony in it. So, should the formula used in the calculator for our purposes not use the value of 5 instead of 8.6 to start the formula, since the base metal intended for the formula for our use is pure lead, and not antimonal lead? I checked, in the fx bar of the spreadsheet, and the actual formula as entered does use the value of 8.6 as already described.

Or, I am missing something? The reference to the Antimonal Lead in the formula (in parentheses) was copied directly from Rotometals website. It is their reference, as I did not edit that into the formula.

This formula description is on the first page of Rotometals "Bullet Casting Alloys" page, just under a picture of a boolit and the American Flag. I would give a link, but I fear running afoul of the "links rule".

Confused or not, something does not seem right to me. But, I greatly enjoy using the calculator. Thanks to any and all who have provided this for us. I just cannot remember who posted this for us.

Thanks,
Bob

I believe the alloy calculator formula originated from Rotometals and Bumpo put it in a spreadsheet to make it easier to use.

there are some blends of alloys that create a new metal that is extremely difficult to calculate. The calculator will get you close to where you need to be.

I'm not sure you understand my question. It does not matter where the mistake (?) may have originated, there is a mistake I firmly believe with the starting value. The starting value appears to be for Antimonal Lead, even by Rotometals. We generally do not start our alloys with Antimonal lead, but we mostly use Pure Lead I would assume.

Sorry, but your response did not deal with my question. But, thank you anyway.

Edit: Rotometals states specifically what alloy they start with: Antimonal lead. I am sure very few of us are using Antimonal lead as a start to plugging in alloy values. This problem adds a duplicity of added antimony, and a correspondingly wrong answer.

Edit 2: I stated an assumption that we use pure lead to start. Even though we generally do not use pure lead to start, we list out the amount or types of lead we use that breaks down the alloys used in our "batch". I appears this formula is already adding in around 5% antimony, though it is not visible due to the value of 8.6 to start with. I a person was to say our alloy has 5% antimony and inputs that in the formula, the end result of the formula would give credit to the alloy of between 8 and 10%, as 3 to 5% appears to be buried in the value of 8.6 as opposed to the use of 5 in the formula.

The problem is, when mixing various elements such as lead, tin, antimony, etc., the BHN number of the various alloys is not linear. So, the 8.6 number associated with lead in the formula is a compromise to get the closest to the actual BHN in most cases. If you change the formula and replace 8.6 with 5.0 (which is the true BHN for lead) and enter 20 pounds of lead, it comes up with the true BHN of 5.0 for lead. Now, if you add 1 pound of Tin such as would be the case for a 20 to 1 alloy, instead of getting the true BHN of 10, you get 6.4. Essentially, the formula is a compromise for which no one formula works in all cases. Hope that helps.

Don

I think the formula will get you close but casting and testing the finished product will tell the tale as there are many variables including time before testing hardness, air vs water drop, homogeniety of the liquid melt etc. For practical purposes, a few 10ths of a percent one way or the other or even whole percentages is not worth worrying about I think.

There is no such thing as pure lead, pure antimony, pure tin. All have some contamination to an extent however small as does your reloading equipment. The more componants you add into the equation the more variation in the final product you will have. The real question is does the extent of the outside contamination influence the final product to an extent that it renders the product unviable. Unless you are in a lab setting then the answer most likely is no as there are so many things involved like powder, primers, alloy, air temperature, barrels etc.

Is this a practical concern or just a "wondering" type of thing?

Answering your question, yes, put in whatever the starting point is of your alloy in place of the 8.60 for antimonial lead.

As mentioned above it won't be that accurate the further you move away from the 'known' value.

Actually, you are better off leaving the 8.60 in if you are looking for the most accurate BHN's, that's why they use it.

Don

Not if you are starting with wheel weights or some other alloy. Pure lead from Rotometals is BHN 5, Antimonal lead is 8.6. Put your starting alloy in the equation. That's why Rotometals lists it that way.

I built my own version of the alloy calculator a couple weeks ago and pondered this very question for some time. I decided that the formula has errors, you’ve flagged one, that largely offset each other in the range of typical bullet casting alloys. It doesn’t work for pure lead, or pure tin, or even pewter or solder, but when the mix is close to a bullet casting alloy it’s pretty good.

FWIW, it’s rarely very far off from the weighted average of the real hardness of the components.

The 8.6 listed here was due to wheel weight as being the ubiquitious alloy of the day; a day that is passing or, in my area, has passed.

Cast on!

prs

Quote:

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Originally Posted by USSR

Actually, you are better off leaving the 8.60 in if you are looking for the most accurate BHN's, that's why they use it.

Don

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Okay, I understand the situation much better now. The 8.6 value is more of a fudge factor than the exact value of a given alloy to start with (e.g., pure lead or antimonal lead), even if it might have been early on.

Thank you for all responses.

Bob

Fudge factor, couldn't have said it better. Kinda like trying to convert CUP to PSI, only worse.:)

Don

Right, rather than a derivable equation it is interpolated from testing data and observations (empirical data, just like the equation we civil guys design your bridges with). Only good within the limits of what it was intended to be used for. “Good enough for government” work as they say.

Yep fudge factor is a good way to put it.Picture a graph where the fist little bit of alloy makes the hardness go up fast then it flatents out when you ad a bit more alloy.the 8.6 is a good figure to use for the range of harness we use in bullet casting.Once you get to about 9bhn the curve flattens out.I am sure it likly gets off a bit again when you get up to a certain BHN on the hi side.I found when i got a harness tester {LBT}the foumula works well.I used 5 when i first started mixing my bullet alloy.I allways ended up about 3 bhn harder than I was shooting for.

I think you are using the tool wrong. You have to use the tool to degermine the compaositon of your ingredients and then calculate the composition of you final alloy by your own calculations then the tool can tell you the hardness of your final alloy.

Say you have some almost pure lead, some linotype and some wheel weights and you want make an alloy with a BHN of 16 you pick a composition that has a BHN of 16 say like Hard Ball. So you try to figure our how to get to 2, 6 and 92. You could try wheel weights plus two percent tin but you don't have tin you have linotype so maybe you think about 50/50 linotype and pure and you end up with and boom you are there. That leave you without using your wheel weights. But say you have a lot of wheel weights and you want to use them. What happens if you reduce the lino and add wheel weights. You will be lower on tin but can still hit the hardens target if you reduce the pure.

Tim

My understanding is the relationships are not linear. The formula is calculating as if they are. It is probably than a SWAG for small changes.