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I am making a die for cutting arbitrary shapes from thin (0.1–0.5 mm) copper foil/sheet using a manual arbour press. The product will be an oak leaf shape, approximately life-size, for use as part of a sculpture project.

I am pretty experienced at general fabrication and metalwork and have access to a well equipped fabrication workshop including TIG welding, lathe, manual die grinding, manual forging and heat treatment, etc. (but not CNC milling). However, I don't know what would be an effective and reasonably low cost way to make a die for this sort of process. Is there a reasonable way to make this in-house or is it better to just get a die made by a specialist?

Accuracy of sizing isn't that important, the main priority is aesthetically smooth and well-finished edges. Visual appearance and minimal finishing are the main goals. I will be cutting in batches of around 700.

The outline will be a stylised oak leaf, so a continuous loop with various lobes, where my first priority is smooth transition between radii rather than exact tolerances.

The options I've considered so far are:

  • A knife-type tool (perhaps pressing onto a rubber block)
  • A shearing tool with a positive and negative die (like a hole punch)—in this case I am particularly interested in how to work out the optimum tolerances between the top and bottom tools for a given sheet thickness (probably 0.1 mm annealed copper)
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  • $\begingroup$ What sort of tolerance do you need on your finished part? $\endgroup$ – Ethan48 Apr 5 '16 at 16:11
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    $\begingroup$ Quality is more about getting a visually accurate part with burr free edges than geometric tolerances. I can tolerate a lot of variation in size as long as lines are smooth and edges aren't too rough or sharp (ie safe to handle) $\endgroup$ – Chris Johns Apr 5 '16 at 16:16
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I'd recommend approaching this as a two-step process of cutting the shape and then preparing the edges. In my experience the cut edges of 5 mil annealed copper (0.005", about the size you're looking at) are always sharper than I want to be casually handling, even without burrs. It's also a bit of a delicate edge to ease because of how malleable thin, annealed copper is.

Cutting

I suspect die-cutting is the way to go here. Most projects I've seen or done with this material have been smaller-scale, where it's practical to cut one to a few pieces using a variety of hand snips. These snips give you basically a zero-clearance shear and make the cleanest cut (rotary cut-off wheels, especially hand-held, are by far the worst) but the edge still demands attention. I expect this would be comparable to the positive/negative die cutting method you've considered. With a thinner foil, you might be able to get results with a knife edge but 5+ mil is thick enough, and copper ductile enough, that I'd expect more of an embossing effect if you tried it here.

The challenge is that you're going to want very little clearance between the positive and negative dies. For example, using this die clearance calculator suggests 5% of thickness as the "regular" clearance for any reasonable set of parameters (C110 H01 being the closest material option). That would be 5–25 µm clearance on your die. I'm hardly an expert on die-cutting but this doesn't sound like something you're going to achieve in-house.

That doesn't mean you need to resort to specialty fabrication, though, especially if perfection is not the goal. My advice, since your stock is inexpensive, is to make a prototype in-house ASAP with as reasonably tight a fit as you can achieve between your positive and negative dies, using your available tools and whatever method you're most comfortable with. It's very possible that will serve your purpose. (You'll need to keep your negative die aligned on the press, of course.)

Edge Preparation

Depending on the results of your cutting process, your edges might get rolled a bit. If this happens, I'd try placing the cutout concave-down on a sheet of medium-fine grit sandpaper and giving it a few strokes with light pressure. This might be sufficient, since a workable die shouldn't produce any burrs, and it's "quick and dirty" enough to not be infeasible for a batch of several hundred cutouts.

If you don't get any dishing, that seems like a bit of an issue, as your edge isn't exposed all along one plane. But believe it or not, you can do quite a bit with something as simple as a scouring pad, when working with a material as soft as annealed copper. If you need a bit more abrasive power that will still fit nicely into the nooks of your shape without creating any sharp nicks or dents, try a sanding sponge. If you need to get into smaller nooks or you're worried about bending/crimping your cutouts using a pad or sponge, a small round file can also do the job, though likely more painstaking labor with more aggressive action (you could accidentally cut a notch in the edge).

The possibility that a bit of die-roll would actually help in the edge preparation step might seem to imply that more clearance is actually helpful. To some extent that may even be true; however, the more stretching-deforming action you have along your edge, the less likely you are to get a clean cut without burrs. As the edge rolls, it will become work-hardened. The irregular shape of your oak leaf cutout won't help here, as stresses may be less consistent along the path of the cut, and you may start to move more in the direction of tearing/ductile fracture. So I would still start by trying for the smallest-clearance dies you can quickly prototype.

Then again, it's possible that you'll decide the rough edge that results from your cutting method isn't too sharp for your purposes after all. It doesn't sound like you're producing a piece that will be handled very frequently or by very many people after it's finished and you're going to have to handle the assembly with care simply by virtue of working with such thin copper. Yet another reason to start by prototyping and then address problems as they appear, rather than ahead of time.

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    $\begingroup$ I'm not a die expert either, but I think with something as thin and soft as .5mm copper, for these fairly low quantities true die cutting is feasible. I believe you only need a negative die for punching where as cutting is typically done with a shark edge and a soft backer behind the material (wood or rubber.) These cookie-cutter-like- dies could more plausibly be manufactured by hand then matched dies and punches which are typically precision ground and heat treated. One other consideration is that for these quantities CNC laser cutting may be cheaper than creating the tooling. $\endgroup$ – Ethan48 Apr 9 '16 at 14:49
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In the interest of anybody else who has a similar problem I have been experimenting with rule die cutting which seems to be quite a promising approach.

I am using 12mm plywood for the die form, cut with a fret saw and various thicknesses of cold rolled steel shim stock for the cutting edge. In initial tests this works OK on 0.1mm copper foil and produces acceptable edge finish. In fact using soft wood or rubber to cut onto it rolls the edges over somewhat which is an advantage in this application.

It also gives clean cuts on paper.

I initially used 0.6mm shim stock ground with a single beveled cutting edge. Although this is nowhere near as hard as heat treated tool steel it is at least easy to hone to a polished cutting edge. I used diamond and then ceramic stones to achieve this. The 0.6mm shim is perhaps a bit on the thick side for this and is difficult to bend to radii less than about 40mm.

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I'm new here, however at this sort of business for a long time. Yes die cutting is great and has low $ cost for short and med. runs. I have even made a blanking die (steel rule or clicker) for 5 parts and smacked them out with a hammer and block of UMHW. There are jewelry grade prototype systems (kits) see RioGrande for blanking small parts from thin material. For limited runs I have sent my drawings out for acid etch blanking. Just remember to let them know what piece you want back, so you don't get a really a really nice sheet full of fancy holes. The acid thing is also great when it comes to the tough materials.

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