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Blanking small slugs from fully-annealed C260 brass


Jul 22, 2021
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?
Are you using a pressure ring to hold the blank down and stop material flow, you may even need a impingement ring.. Material clearance can be up to 12%. A harder material may give a better result. Have you backed off the die to give a shorter die land. Measure your slug thickness and check it is still the same as prior to blanking.
No pressure rings or impingement ring, just using hard urethane as a hold-down/punch stripper. I figure 1/2-hard would probably behave better but the parts need to be dead soft. I've cut the blanks in half to measure the thickness and they're the same thickness as the parent material. I'll grind the punch further and see if it eventually starts to behave. I never needed to go this far with brass and thought something strange is going on...
Ok, re-read this, and I'm as baffled as you are. Never saw anything like this. I'd try a fixed stripper plate, as I'm thinking that's all we did for blanked parts of an aspect ratio like this.

Otherwise, a spring, or urethane, loaded stripper plate.
I rolled several yards of material to thickness and then used the ASM heat-treating handbook to guide the annealing of samples at different times and temperatures. I don't have a hardness tester that works on thin brass, but the idea was to leave the material in a somewhat harder state (the ASM handbook says with these brasses you can work-harden and then anneal to the desired hardness, or vice versa--didn't know that before a few days ago.) With the brass somewhat harder--and I know I need to quantify this for consistency--I was able to get the burnish-break diameters nearly identical. It's still weird that it's not larger on the bottom, but I can live with this for now.

Hi Borealis:
Is fineblanking them out of the question?
re: fineblanking...yeah, it's a good thought and would surely deliver the geometry but because of the process that immediately follows blanking, it's not an option in this case.
When you inspect the edge of the slug what is the percentage length of the sheared edge with regards to materials thickness. Typically you want 1/3 shear and 2/3 break.
Also how flat is the slug. Is there any dishing.
Yes, it's dished about 0.005" (The "thickness" of the blank measures ~0.032" while the material before blanking is 0.027"). When I had less punch-die clearance, the blanks were flatter, probably dished more like 0.003" iirc. Yesterday I punched out about a thousand parts and all of them after the first hundred started blanking normally, with the break line being about 0.001" smaller diameter than the burnished edge. Under the microscope you can see the burnish-break ratio is about the same as before, about 75%/25%. I have no idea what changed, but this is behaving much better now.
Grinding burr on the die? I used to scrape them off with a scrap strip of Be-Cu I had laying on the bench. More for avoiding the debris from scoring things up.

And I kind of remember parts coming out better after a micro radius wore on the edges of punch and die. And let it run for 250k parts.
Your material may have slightly age hardened. Take a close look at the strip also when checking for causes of problems. The softer material will automatically be drawn down into the blanking die more than a harder material at the beginning of the blanking process. Think of it as if you placed a sheet of plastic over a ring and pressed down on it with your finger, if you then replaced the plastic with steel you would get less deflection. If you're material deflects under blanking it will spring back after leaving the die and the fractured edge diameter may now be slightly larger.
Two things. Under the scope, the die did already develop a small radius, well under 0.001, but it's there. Second, I do think the hardness of the material is more important than I had estimated at the outset. A new batch of material yesterday was behaving differently from the stuff I had annealed a few days ago from the same lot of parent material and using the same annealing recipe. The age hardening may be important. I'm going to mark this material and let it sit for a few days and try it again.
Are you hardening and tempering in a controlled environment or with a torch. Is your punch and die alignment perfect. You shouldn't be seeing wear so soon with such soft material. There is a procedure for setting up a roller guide die set properly in a press. If you aren't aware of it ask and I'll go through it.
Insert die.
Clamp spigot or screws or what ever clamping system you have lightly.
Run die through cycle by hand a few times.
Cycle to bottom of stroke.
Tighten spigot.
Cycle again and stop at bottom.
Tighten hold down clamps or screws.

On really tight clearance dies I will paint the punch or die with marking pen and cycle the press by hand to make sure there is no contact.

It's also a good idea to have parking and setting stops on the die rather than just laying the die on its side with the punch and die disengaged during movement storage.

I have had to regrind far to many dies where the die was slightly cocked at full height during clamping and the cutting edges clash.

Check platen and ram are dead parallel.
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Are you hardening and tempering in a controlled environment or with a torch. Is your punch and die alignment perfect. You shouldn't be seeing wear so soon with such soft material. There is a procedure for setting up a roller guide die set properly in a press. If you aren't aware of it ask and I'll go through it.
Thanks for sharing the setup procedure. FYI, I'm doing the annealing in a PID-controlled heat-treating oven.
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.