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Blanking small slugs from fully-annealed C260 brass

Its coming up on 40 years since I walked out ...

But I remember getting stacks of coils 'slit' from the roll, and some of them just gave us problems. I suspect the edges were work hardened a bit more than the center, from the deflection of the rollers. It was measurable in thickness. Mostly Kovar or Alloy 42.

We'd have a coil that would crack on a formed section, and I'd fiddle with the die. Foreman would give up and load another coil and it ran fine.

Infuriating.
We used to order .3mm tinned coil that we would slit ourselves. Probably 15 2m wide coils at a time. Slit the first coil and run it through a high speed progression with a pre draw pierce then final draw and rollover plus a few additional ops. First coil split on final draw. Load the next one, same thing. Come pick up all your material and replace it. I tried to introduce a draw ability test with samples before delivery. No luck eventually just gave up. My current deep draw process I am working on will be. Sample to be chemically analysed with a draw ability test and hardness test before I even let the supplier send me the material. Keep a sample strip including one each of the many draw stages for future reference if anything goes wrong with the material supplied.
 
I've needed a bunch of round slugs as the starting point for some small parts. I made a punch and die out of A2, hardened and tempered back to HRC58, then ground to size. The slugs get punched out of fully-annealed C260 brass that was custom rolled to 0.027". The punch and die are fixtured in a roller guide die set installed in a 5-ton punch press. The die diameter is about 0.280. With the punch, I started out with just 1% clearance based on material thickness on each side (~0.0005" on diameter). Here's the issue: Usually, the burnished edge of a slug coming out of the bottom of the material is LARGER diameter than the fractured edge at the top of the sheet. Here, it's just the opposite. It's a small amount, but that burnished edge is about 0.002" smaller diameter than the fractured edge on the top. I've been grinding the punch little by little to increase the clearance thinking this will cause the fracture line to go the other way but no dice. Now it's at about 8% clearance (~0.004" on diameter) and I'm still seeing the same thing. It just turns out for the next fixturing step, the burnished edge needs to be the biggest diameter on the part. I haven't done a lot of punch/die work but I've never seen this, and all the books on die design say it should be going the other way. Any ideas about what's going on or what to try next?
The P&D clearance should be Five Percent on a side of the thickness of the material or .0027 total Punch & Die clearance. If the clearance is tight a double break can exist. Ideally 1/3 cut, 2/3 break. At .008 there is too much clearance. Urethane stripper is questionable without a steel surface against the brass. It more than likely is allowing the brass to flow into the urethane.
Roger
 
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