Protecting Electronics in Metal Military Surplus Ammunition Cans- Part 2, by J.T.

I am sharing a better solution for protecting sensitive electronics stored in metal military surplus ammunition cans made into a Faraday cage. In the Part 1, I talked about the ammunition cans that can be used. We then began the instructions for building ammunition can gaskets by listing the materials and tools required. Now, let’s move on.

Making and Assembling the RF and EMP Shielding Gasket

The following instructions are for a 20mm M548 ammunition can, which is my preferred can size. If using another ammunition can, you will need to make adjustments in measurements within these directions to adjust for your ammo can’s size. I’ll give some guidance along the way for doing so.

1. Cut the sheet metal into a 7” by 16¾” rectangle, for the 20mm can.

But always start out by taking a good measurement. You know the old saying of “measure twice, cut once” still applies here. (See the description below as to how to measure and cut the right sheet size for the gasket.) For other can sizes, cut the sheet metal accordingly. Make sure that the sheet metal slightly overlaps the rubber seal but isn’t too big that it overlaps the exterior dimension of the lower part of the can or affects or interferes with the inside edge or lip of the ammo can.

2. Test the fit of the rectangle piece of sheet metal.

It should be centered and equally spaced or all sides must be equidistance from the inside edge of the lid. It’s important to cut to size to slightly overlap the rubber seal, but the sheet metal must lay inside the “impression” on the rubber seal of the ammo can. Make sure that you test the fit. If the rectangle sheet doesn’t fit the inside dimensions of the can, then trim accordingly. The closer you can fit the sheet metal to the inside dimensions of the can, the better the final gasket. The perimeter of the sheet metal should have about one quarter inch of space between the sheet metal and the impression on the rubber seal.

3. Take the rectangle sheet metal and burnish a 1” perimeter around the sheet metal with steel wool or sand paper.

Clean the surface to remove any oils or coatings using alcohol or acetone, but put this flammable material away while you may be soldering.

4. After you’ve burnished the perimeter, take the rosin paste flux and place a thin coat around the perimeter of the sheet metal.

This thin coat of flux will also help to hold the copper mesh shielding tape in place once you start to solder.

5. Place the electrical shielding tape on the perimeter of the sheet metal rectangle gasket.

Overlap the edge of the metal sheet by ¼” to ½” so that you have at least ½” or more of the mesh tape that will provide the seal and shielding effect over the rubber gasket. The mesh tape will be soldered to the sheet metal and must be wide enough to overlap the outside dimensions of the ammo can by at least 3/8”.

6. Lay the tape in such a way as to give you at least 2” of extra tape at each corner of the rectangle.

From past prototypes and experience, this extra length of tape will be important to form a corner that will be durable and tightly sealed when the lid is closed and latched tight. This will also ensure that the corner won’t become damaged over time when you remove and reseal the lid.

7. You should have the tape laying on the edge of the sheet metal and with about ¼” on the sheet metal.

Don’t stretch or deform the mesh tape during this step. Start the tack soldering of the tinned copper mesh shielding tape to the sheet metal using the soldering iron and solder. (See the final result in the picture.) Tack the opposite corners first, then put a small 1/8” tack of solder every ½” inch across one side. This requires you to first heat up the sheet metal and the mesh copper tape with the soldering iron at the same time. Hold the end of the soldering iron against the sheet metal and tape while making sure that you don’t have a heat conducting material under the sheet metal. Use something like an upside-down metal pan or cooking sheet tray that’s placed under the sheet metal gasket. Then continue to heat until the metal and iron will melt and capture the solder. (See the soldering tips and links below.)

8. Continue to tack the shielding tape on the other three sides of the metal gasket.

Remember to keep the tape at least 2” longer at each corner.

9. Once you’ve got the shielding tape tacked down, now take the corners and overlap the tape.

Fold back one end over one side, and then do the same with the other side so that you end up with a nicely squared corner with an extra layer of mesh tape. The perimeter of mesh copper tape should be the same width around the outside of the sheet metal gasket.

10. Test to see if the gasket and the tape will fit inside the lid.

It’s okay to have some excess tape around the perimeter. This excess will be pushed under the edge of the lid (between the rubber gasket and the edge of the lid). This makes for a nice seamless and tight fit. In an early prototype, I successfully used a very small bead of silicone caulking adhesive to keep the mesh tape in place. However, this enhancement is a “belt-n-suspender” and not necessary if you are careful in removing and replacing the lid and occasionally check the integrity of the copper mesh shielding tape.

11. If the gasket (the sheet metal rectangle and the mesh tape) fit inside the ammo can lid and the mesh tape easily covers the “impression” on the rubber gasket and lip of the ammo can, then you’ve got the first important step.

You also have your quality assurance test done in the construction of the gasket.

12. For the other corners, the tape has been folded back over to form a nice corner without any rough or ragged edges.

Tack solder the corners with your soldering gun. You should now have a nice, durable, double thickness tape mesh corner. It will not fray or be damaged upon removing or resealing the ammo can.

13. To “belt-n-suspender” the gasket, you may also take another strip of mesh tape and solder another layer of tape to the gasket.

You do this by overlapping the first layer of tape to get a better shielding effect. The copper mesh tape is sufficiently thin that a second layer can be added. This doesn’t impact the rubber seal or make it too difficult to latch the lid to the ammo can.

14. Test the gasket and make sure that it fits inside the lid and the copper mesh tape covers the impression on the rubber seal of the lid.

This is a critical step and is a quality assurance test. The shielding tape must cover the outer edge of the lip of the can by at least 1/8” when the lid is placed on the can and latched tight. If the tape does not cover the outer edge, leakage may occur and may compromise the effectiveness of the shielding.

15. Once you’ve got the right fit, the gasket must be centered on the lid and with each edge equidistance from the inside of the lid and then marked for the locations of the pilot holes for the screws that will be used to attach the gasket to underside of the lid.

The sheet metal gasket and the copper mesh tape will be placed face down. That way the solder tack points should be not visible. (See picture.) You’ll want to screw the gasket into the metal ribbing that’s on the underside of the lid. Remember: don’t drill the pilot holes through the lid. You will need at least six #8 or #6 self tapping screws that are ¼” long to secure the gasket on the underside of the lid. Screws that are 3/8” long will work if you will be using the finishing washers to accommodate these longer screws.

Tomorrow, in Part 3, we will continue with these instructions.

See Also:

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  1. I have heard that micro-wave ovens are ready built Faraday Cages! Any truth to this? If anybody can check it out I would be very interested in hearing about it!

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