Chamfering the Parting Line

[justindad]

I have applied a chamfer to the top parting line of a few mold blocks to sharpen the bases of my boolits, and it works. I have an iron mold that is having a particularly hard time fully venting the base; I’ve lightly chamfered with a moderate grind stone twice, but it’s still not where my other molds are. I am going to do a third swipe to increase the chamfer further, but I don’t have enough experience here to know if I am about to go too far - maybe my issue is something else.
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Have you over-chamfered a mold block? Any guidance? Thanks in advance!
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[HWooldridge]

It will flash if over-chamfered, which is probably not a problem unless you go crazy and leave a .065 fin (I'm exaggerating).

A .010 break is probably enough to vent the bases and not create a "leaker". You can make a pretty good stab at measuring it if you have something to compare, like a micrometer or caliper that can be set to the desired size, then make a visual comparison.

[justindad]

I suppose it’s probably enough to keep the chamfer half the size of a typical vent line - I’m no where near that right now.

[longbow]

I do the same thing with moulds like Mihec moulds which are so precision machined that the sprue plate almost seals to the top of the mould blocks. A light chamfer allows better venting. Also, lossening the sprue plate a bit helps. I don't like a tight sprue plate anyway so I make sure mine almost swing free and that can help too.

I keep the chamfers pretty small so vent line size or smaller is likely a good guideline, you don't want flash there!

Longbow

[bobthenailer]

I have done the venting process to all of my moulds about 30 moulds in all,, workes from 2 cavity to 8 cavity

[Sam Sackett]

That’s one on the first things I do on a Lee mold. That and a few other things make them run real well.

Sam Sackett

[justindad]

I ran the grind stone over the edge again, but I had a firmer grip and I cut away a tad more then I meant to. I can see the chamfer when the mold is closed, my finger nail falls into the valley, and it is about the same size as the other vent lines… but no finning. I got 70 good boolits with sharp edges in one hour. So far, so good.
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I think I still have a problem with venting, but I think it has something to do with all of the venting. I can watch the lead slowly sink into the cavity, after I shut off the fill valve on my bottom pour. If I am real careful to build a thick sprue puddle over one cavity, and then move to the next cavity, I can get two good boolits. If the run-over from filling the first cavity makes it’s way to the second cavity, I won’t get good fill out.
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When this mold arrived new from Midway, I adjusted the alignment pins so I could only get a 0.002” feeler gage between the two halves. I am going to push the alignment pins out until I can get a 0.004” feeler gage between the two halves when the mold is closed. Let’s see what that does.
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Here’s the worst example I have. The two boolits came from the same pour. My mold was plenty up to temperature, lead spilled off the hot sprue plate like water, and I had already cast about 40 good boolits.

[Cap'n Morgan]

One way to improve venting is to use a checkering file to (carefully) add venting lines across the top of the mold.
A thread repair file is cheaper and will also do the trick.
Clamp some sort of guide on the mold top to rest the file against to prevent it from wandering.

The method can be used on the mold faces as well, but most venting problems occurs between the mold top and the sprue plate.

[HWooldridge]

I ran the grind stone over the edge again, but I had a firmer grip and I cut away a tad more then I meant to. I can see the chamfer when the mold is closed, my finger nail falls into the valley, and it is about the same size as the other vent lines… but no finning. I got 70 good boolits with sharp edges in one hour. So far, so good.
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I think I still have a problem with venting, but I think it has something to do with all of the venting. I can watch the lead slowly sink into the cavity, after I shut off the fill valve on my bottom pour. If I am real careful to build a thick sprue puddle over one cavity, and then move to the next cavity, I can get two good boolits. If the run-over from filling the first cavity makes it’s way to the second cavity, I won’t get good fill out.
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When this mold arrived new from Midway, I adjusted the alignment pins so I could only get a 0.002” feeler gage between the two halves. I am going to push the alignment pins out until I can get a 0.004” feeler gage between the two halves when the mold is closed. Let’s see what that does.
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Here’s the worst example I have. The two boolits came from the same pour. My mold was plenty up to temperature, lead spilled off the hot sprue plate like water, and I had already cast about 40 good boolits.

This is normal if you have overflow. I pour the first one with the nozzle pushed into the pot spout then pull away slowly and let the sprue fill completely, then repeat on the second cavity. If I get an accidental leak on the first cavity and it flows into #2, I just stop and accept I only got one good bullet on that pour. The short shot winds up in the sprue pile and gets remelted.

[ascast]

Just to be contrary - If I saw a rifle mold, long rang BPCS done like that I would consider it junk. Too big a chance of getting fins there which may ruin any hope of hit past 100 yds. Casting technique generally will fill out the base edge. With pistol bullets it's not so critical.

[Dusty Bannister]

You mention that when you fill the first cavity and it over flows into the second cavity, you end up with a venting issue. Perhaps the secondary pour is not hot enough to "melt" through the already solidifying second cavity and the smaller hole traps air or due to slow fill, and cool sprue plate reduces the cavity of the slush formed and stops the flow into the cavity. This has nothing to do with a correctly vented mold, but poor management of the pour. Ladle or Bottom pour?

No mention of tracing the mold vent lines to be sure there is no debris from original manufacture, but that might not be a factor if the mold was casting well and something changed. Ventilation is needed, but be aware of the extra heat loss and cooling of the mold blocks and sprue plate. Perhaps a hotter casting temp might help with this issue.

[uscra112]

One thing I learned from the writings of Charlie Dell is to keep that sprue plate HOT!! Spill a lot of lead over it with every pour. I no longer do the "connect the spout" trick. I dump lead through the hole quickly and let it overflow. With a 2-cavity I hold the mould so the spill flows away from the second cavity. Well-filled-out bases come naturally.

Has never even occurred to me to chamfer a mould like that. Stone the faces flat is as far as I go.

[MT Gianni]

How tight is your sprue plate? Does it swivel freely when you pick your mold up? Have you tried reversing the order that you fill in?

[justindad]

…poor management of the pour. Ladle or Bottom pour?

Thanks Dusty - this comment led me to the answer. My production rate last night was twice what it ever had been before.
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1) My RCBS bottom never had the fantastic drip-free valve I was hoping for - I’ve tried to clean the valve a few times without success. The valve has been particularly bad lately, giving a turbulent stream, the flow rate changes, and it recently started sticking. I emptied all the lead and did a better job cleaning the valve than I have before. My flow is constant, the stream is straight & smooth, and it’s no longer sticking
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2) In an attempt to keep the lead liquid on top of the sprue plate for an extended period of time (to give more time for venting), I was pressure pouring with the valve deep into the sprue holes. I realized I could be blocking air that can vent out through the sprue hole while the stream is flowing. So now I’m keeping the top of the nozzle 1/4-1/2” away from the sprue plate.
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3) I decided to cast with an analog thermometer in the pot. I got best results around 740F. Above 750, and the edges were not as sharp. It got down to 710 when my pot was too low, but results were still good. Casting fast with a moderate temperature gave better results than pouring hotter lead that required a longer cooling time before I cut the sprue.
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My bases had good edges, but not so sharp that I nearly saw flashing at the base. I want to push this further until I do see flash at the base, and then dial the process back just enough to remove the flash. Maybe changing my COWW +1%Sn to 2%Sn is the easy button.

[justindad]

Thanks to everyone for your thoughts! I will think about them more this week.

One way to improve venting is to use a checkering file to (carefully) add venting lines across the top of the mold.

I have a sprue plate I can sacrifice for the sake of boolits. I might cut vent lines in that, so I don’t have to worry about the process forming burrs in the cavity. I’ll report back if I do.

No mention of tracing the mold vent lines to be sure there is no debris from original manufacture, but that might not be a factor if the mold was casting well and something changed.

Vent lines are clean.

I no longer do the "connect the spout" trick. I dump lead through the hole quickly and let it overflow. With a 2-cavity I hold the mould so the spill flows away from the second cavity.

I will be more vigilant about keeping the two cavity sprues separate from each other in my next casting session.

How tight is your sprue plate? Does it swivel freely when you pick your mold up? Have you tried reversing the order that you fill in?

My sprue plate almost falls freely when I hold the mold sideways. In one session where my set screw loosened, the plate went from tight to loose, and I didn’t notice a difference.
Reversing the order of cavity fill is mandatory with this mold. Filling the closest holes first for 1/2 to 2/3 of the time works well.