Cutting Thin Rings: Homebrew Punches

The Totally Featureless Clock is back for a refit: its preferred location turned out to have essentially no RF at all, so I must move the antenna out of the clock case on the end of a cable.

Drat!

I put the ferrite bar inside a length of PVC pipe, turned down to make it less ugly, with white plastic end plugs. Rather than fiddle around with complex mountings, I cushioned the fragile bar in closed-cell foam, which meant I needed some way to cut a bunch of foam rings.

Some rummaging produced a thinwall brass tube with about the same ID as the PVC pipe. A brief trip to the lathe put a reasonably sharp edge on one end.

Sharpening the brass tube

Sharpening the brass tube

That edge is more keen than it looks; while it’s not razor-sharp, it’s plenty good enough. I didn’t use it as a punch, just grabbed it in a rag to cushion my palm and rotated it through the foam against a plywood scrap.

That produced a bunch of foam cookies.

Foam cutouts

Foam cutouts

The bar diameter was close enough to a standard hole punch that I didn’t have to make one. Centering by eye and rotating by hand turned the cookies into donuts.

Punched holes

Punched holes

And then they fit just fine…

Cushioned ferrite bar antenna

Cushioned ferrite bar antenna

I made more donuts to swaddle the bar from end to end inside the PVC tube. I slipped the antenna in from the left, then pushed the donuts over the bar with Yet Another Brass Tube. The end result is an antenna compression-packed in foam, which ought to keep it in good condition through at least a minor oops.

Finished antenna housing

Finished antenna housing

The screws pass through the end plugs to hold them against the pressure from the foam cookies at the bar ends. The holes are slightly counterbored on the top to blend the screw heads into the curve of the tube. There’s a 3/8-inch flat along the bottom that will eventually settle against the underside of a shelf.

  1. #1 by Raj on 2010-07-29 - 09:11

    Nice job Ed. I did a similar job on a foam sheet and came up with headphone pads for the ancient Sennheisers. Those ones we used to get in the 70’s.. 1000 ohms and 50mW to pain threshold. They had colorful cushions that would slowly break off.

    • #2 by Ed on 2010-07-29 - 09:40

      cushions that would slowly break off

      That seems to be the common failure mode for headphones: everything works, but you can’t wear the things!

      Our daughter had developed ears with black flecks from her favorite headphones as the cushions shed their thin plastic coating. I’ve suggested that she sew up cloth cushions to no avail.