Adding metal to the pot during casting

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labop
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Adding metal to the pot during casting

Post by labop » Mon Jan 18, 2021 8:51 am

I’ve seen posts occasionally about temperature variation caused by adding sprues and reject bullets back to the pot during casting. The variation I see on my PID set-up with a 20-pound pot appears small but the issue did make me curious as to what the effect on lowering the pot temperature really was. I asked an engineer with thermodynamics expertise to do the calculation using 20 pounds (9072 grams) alloy in a pot at 730°F (387.8°C) and 500 grain (32.4 gram) bullets, one at 70°F (21°C) and one at 400°F (204.4°C). My routine, which only takes a few seconds is to drop the sprue back in the pot from the cut so it is still hot, drop the bullet on a towel, roll it looking for defects, check the base and either keep it or drop it back in the pot.

The bottom line is that putting a cold bullet back in the pot drops the temperature 3.42°F. A bullet cooled to 400°F only drops the melt 2.29°F. So if you want to maintain a temperature ±10°F and your PID is holding the pot at ±3°F then one bullet and sprue will not be a problem. Hopefully your bullet quality is high enough that you don’t have to consider this often. You can extrapolate from these numbers for multiple bullets, a smaller or larger pot with different weight bullets. labop
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DeadEye
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Re: Adding metal to the pot during casting

Post by DeadEye » Mon Jan 18, 2021 9:49 am

Labop, you're having fun your way and that's good I guess. I read your post and the image of some buffalo hunters sitting around a campfire working out your formula on a piece of Birch bark with a hunk of charcoal came to mind. For me that's not what this is about. This is about doing old things the old way and having fun. I get more than enough exposure to new tech all day every day. I have to ask what difference you think 3° is going to make at 800 yards? You can't hold this sport to that kind of tolerance because you can't control the wind or the light or the rain. If you miss what are you going to blame? Shoot, shoot, shoot and learn. Ten years from now you might be ready to apply your formulas in a meaningful way. In the meantime I worry that you may frustrate yourself and we loose a shooter. BTW, what's a PID? :lol: :lol: I know what it is I just don't have one but I still win my fair share of matches.

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Kurt
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Re: Adding metal to the pot during casting

Post by Kurt » Mon Jan 18, 2021 10:57 am

You read a lot about PID's are necessary to cast a bullet to hold the variances down. Maybe you do or don't, I don't know, I don't have a PID.

Holding the variances down has more to do with how you cast. When I get the alloy and mould up to temperature, my bullets are from light to the heaviest less than a grain and I get this using a Dutch oven over a turkey burner or using a electric lead pot. I like the electric because I can cast in the basement where it's warm in the summer and cold in the winter. :D

I cast between 700º and 750º. The higher the tin in the alloy the cooler I cast.

I judge the alloy temp watching the frosting showing up under the cut sprue plate. When the galvanizing look gets excessive I cool the mould with a quick dip in a coffee with the bullet still in the mould into the can sitting next to me on the floor with a mix of Dawn because I like that smell better than Jungle Jake :D

This does two things, It cools the overheating mould and also puts cover of soap on the mould that keeps the lead from sticking to the mould. I cast pretty sloppy and dirty.

Keep the spigot on the ladle clean so the dross don't get into the cavity. I use a rag that has lube on it and that lube covers the outside of the ladle keeping it clean when you dip it in the pot.

I fill the ladle with as much as it will hold and mate the spigot to the sprue plate with it on the side and rotate the mould upright holding the ladle tight to let a slow cavity fill than I lift the ladle off the plate slow still letting the ladle empty slowly with it 5"-6" above the hole.

This does two things, It puts more pressure on the fillet cavity and also makes a vibration settling the alloy to make a complete fill.
When you do this you will see a very small string dimple on top of the sprue or none at all. I can tell when a bullet will drop light by just looking at the sprue on how deep the dimple is.

Yes this is a messy way to cast, but I get consistent weight bullets.

Another thing that will keep the variances down besides a slow mould fill and that is a tight sprue plate especially the way I cast. A loose plate will get raised up as the cavity fills.
When I see vent hairs starting to show up my mould is getting to hot and it gets a quick dip.

I cast 50 and put the cut sprue back in the pot, stir the alloy skim the dross and take a small break putting the cast bullets in a pistol bullet sleeve they come in that I find at the range and stir again and start casting.

Now I'm not saying to not use a PID, it makes no difference to me if one does or don't use one. If it helps you use it.

Kurt
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Re: Adding metal to the pot during casting

Post by powderburner » Mon Jan 18, 2021 11:58 am

Labop. Sounds like you had fun checking it out. Thanks for sharing. I also add my sprues back sometimes and sometimes not. Depends on the mould i use. I drop my sprues opposite where the ladle is and give the pot a stir after i return the ladle to the pot. This gives the sprue a chance to melt and get spread around. I can see the temp drop one or two but it is back on average by the time i pull the next ladle.
An interesting thought none the less. Fun to read them formulas even when i dont get em.
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Don McDowell
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Re: Adding metal to the pot during casting

Post by Don McDowell » Mon Jan 18, 2021 12:12 pm

I might drop the first 1/2 dozen or so bullets cast when getting the mould up to working temp, but after that I just drop the sprue's and any reject bullets out of the mould into a coffee can, and then use that for starter in the pot for the next batch.
Seems that dropping the sprue and bullets back in after things get to going give to much trouble with oxides floating on the top and you end up skimming to often, breaking the rhythm.
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charlie young
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Re: Adding metal to the pot during casting

Post by charlie young » Mon Jan 18, 2021 12:40 pm

I start casting at 780 and turn the temp down at about 810 so I hang around 800, and if my mould is preheated correctly, I almost never discard a bullet. When I get them to the reloading room and weigh them, out of 100 bullets, there might four or five that aren't in the 1 grain variance, if I had a good session. I've tried casting at different temps and get almost no significant variance once you get everything to temp. Sprue drops into a 1/2 bushel metal bucket. I do the same thing as Kurt as finishing off the fill with the ladle held a few inches above the hole. Makes for a better fill.

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Don McDowell
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Re: Adding metal to the pot during casting

Post by Don McDowell » Mon Jan 18, 2021 12:44 pm

Did a little simple test. Set the first 1/2 dozen bullets aside, when weighing up after the session was done, sure enough, those bullets while looking great fell out side the 1 grain variation.
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bruce m
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Re: Adding metal to the pot during casting

Post by bruce m » Mon Jan 18, 2021 3:26 pm

what don says has proven true.
all my sprues do not go back to the pot until finishing a session, when the whole mix is fluxed again.
which raises the point of the added scum that appears on the pot when adding sprues as you cast.
maybe the added variance in weight did no affect the bison shooters who cast on a campfire in the wind, but when shooting at 800 to 1000 yds what is going on between your ears can help or hinder you.
as far as history goes, we have to remember that no one from the west turned up at creedmoor.
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DeadEye
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Re: Adding metal to the pot during casting

Post by DeadEye » Mon Jan 18, 2021 3:41 pm

And what we also have to remember is that BPTR has a hard time drawing 50 shooters while the Quigley the last few years has been over 600. Not saying one is better or more authentic but just two different approaches. Take your pick.

Paul
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Re: Adding metal to the pot during casting

Post by rdnck » Mon Jan 18, 2021 3:45 pm

But we did have one guy from Creedmoor venture out west in the person of Milton Farrow, no less. He nearly got his ass killed by a buffalo. His guide saved him.
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VBull
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Re: Adding metal to the pot during casting

Post by VBull » Mon Jan 18, 2021 4:14 pm

I've always put my (hot) sprus back into the pot as I'm casting. I also weight my bullets as I'm casting by putting them on a Lyman balance after dropping the spru back into the pot. The pot is an RCBS bottom pour with PID controller that has been calibrated for that particular pot. Weighing after each bullet drop shows you when the mold and alloy have reached equilibrium. Once you develop a cadence the scale will often remain plus / minus .5 grain. Many times less. Mold temperature is critical.

I have to use a bottom pour as I'm missing all but my thumb on the right hand and watching me cast with a ladle is not a pretty sight. I do have to adjust the flow as the pot level goes down. I also notice the weights tend to be more consistent if the pot is only filled about 1/2 way (10 - 12 lbs).
The digital readout on the PID controller shows a rapid change when sprus and or reject bullets are added. I'm seeing a rapid recovery of only a few seconds after the temperature dip.

I typicly don't shoot silhouette but shot a lot of BP target rifle matches until a few years ago. My wife and I have set a lot of records using bullets cast this way. I think it comes down to what you are comfortable with. There are only a few absolutes in this shooting game and many different methods to get these rifles to work.

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Tasmanian Rebel
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Re: Adding metal to the pot during casting

Post by Tasmanian Rebel » Mon Jan 18, 2021 4:26 pm

I do same thing as Frank. I put the hot sprues back into the pot. Works for me.
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Re: Adding metal to the pot during casting

Post by Kenny Wasserburger » Mon Jan 18, 2021 4:46 pm

I do the exact same thing Frank and Doc do. My hot sprue goes right back in the pot

I also preheat my mould pour the first one, chuck it and the sprue back in the pot. I run 60-70 bullets a session, I have no whiskers, or galvanized look to my bullets from start to finish. Anyone that does needs some lessons in casting. I use a cadence count method and never get my moulds too hot someone that continues to do so, is a slow learner.

Oh btw those 60-70 bullets have a .3 of a grain weight variance. And I also don’t own a PID.

I do have the 5@200 PP record for group, actually I have won two of those plaques, and a few scope National titles to go with. I am not saying my method is best, but I find it hard to argue the results.
Others methods work just as well.

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charlie young
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Re: Adding metal to the pot during casting

Post by charlie young » Mon Jan 18, 2021 5:53 pm

Paul, you are on a roll my friend. Sounds like the old USA is gonna have caravans coming up from the south. We'll just have a little one coming from the North about spring time in June right thru the Port of Raymond!!

DeadEye
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Re: Adding metal to the pot during casting

Post by DeadEye » Mon Jan 18, 2021 6:25 pm

:wink: :wink: She'll be a sight to see Charlie.

For the record I'm with Kenny and Frank and Keith, cut 'em and put 'em back in the pot. I start casting when the thermometer hits 700° then when it hits 800° I start to keep them but I don't use a hot-plate to warm up the mould.

Paul
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