DIY formed sheet metal punch

[Jim S]

Here's a punch for aluminum sheet I recently finished.  It is a test case for a larger one with similar curves and there is a forming operation after the punching to bend the edges that I will work on next.  Cut the halves from steel on a CNC.  It's not hardened as I don't plan to run a lot of units.  The blocks are 3.5" x 4".  Material to punch is 0.032" thick.

And the punched part.  Edges came out smoother than I expected.

[Nathan Schrenk]

Jim,

This is neat. Thanks for sharing.

Nathan

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[Myles Farrell]

Jim, just curious what kind of clearance did you aim for between the two dies?

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[Keith Rogers]

Looks great!

Sent from my mobile device

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[Jim S]

Clearance is 0.004" (double that if you measure diameters but on irregular shapes like this that doesn't make much sense to me).

On Monday, January 8, 2018 at 11:00:15 AM UTC-6, Myles Farrell wrote:

Jim, just curious what kind of clearance did you aim for between the two dies?

On Mon, Jan 8, 2018 at 10:59 AM, Nathan Schrenk <nat...@schrenk.org> wrote:

Jim,

This is neat. Thanks for sharing.

Nathan

On Sun, Jan 7, 2018 at 10:31 PM, Jim S <jimski...@gmail.com> wrote:

Here's a punch for aluminum sheet I recently finished.  It is a test case for a larger one with similar curves and there is a forming operation after the punching to bend the edges that I will work on next.  Cut the halves from steel on a CNC.  It's not hardened as I don't plan to run a lot of units.  The blocks are 3.5" x 4".  Material to punch is 0.032" thick.

And the punched part.  Edges came out smoother than I expected.

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[EschewObfuscation]

That's pretty neat, and in a similar vein to a project I've been contemplating (after all the other half-finished ones ofc...) But I don't know enough to do mine properly. Can you suggest any sources of design rules for such punches, so we could say "for A linear inches of edge in B material, we'll need about C force, and the dies should be at least D thickness" etc?

[Jim S]

There are a number of places on the net to get information about punching force.  Some assume round holes but from that you can calculate the length of sheer and pressure per unit length.  Here is one:

https://www.unipunch.com/Tech-Support/Tonnage-Calculator

If the force is too high for your press you can taper the edges so it doesn't all sheer at once.  I didn't do that but am considering it for the full size one.  As for thickness there are calculators that give deflection for a given force on a beam.  This will give a fairly good estimate of deflection and this needs to be minimized but is never zero.

[Jim S]

I just ran the numbers on my punch.  The total perimeter is 13 inches.  For aluminum material 0.032 thick it gives 5 tons pressure needed.  Seems reasonable as I did it in a 20 ton press with some force but nowhere near the max.  My final punch will be almost 4 times bigger so putting some angle on the punch face "punch sheer" is something I should look at doing.  That can reduce the force as much as half according to their table.  That would be about 10 tons for the large punch which should be very comfortable for a 20 ton press (it's an import so I'd like to keep the tonnage well below the max to reduce the chance of bending it and for ease of use).  The concept is shown here:

https://www.unipunch.com/Tech-Support/How-To-Reduce-Tonnage

Here are some videos I just found that look helpful but I haven't had a chance to watch them yet.

https://www.unipunch.com/How-The-Tooling-Works/UniPunch-Seminars

No affiliation with unipunch - just a site I found with info.  Whitney sites (several variations on the name combinations with other names) also has some good info as well as several other punch makers.

[EschewObfuscation]

Well 50 ton bottle jacks are available if you can fab an adequately strong frame for them to work against... but actually, I was thinking of just placing a dozen 3/4" bolts with some greased washers around the perimeter of the dies, and torquing them down in rotation. Either way should work; making the dies is more a challenge than the press, I'd think, as long as one's not looking for a huge production run of parts.

In any event, I had better clear some other projects before starting yet another.