I needed an insulated feed through for an antenna lightning arrestor I'm building for shortwave radio. The STLs as presented are specific to my application, but I am including the FreeCAD file as well, in case you need to modify this to fit your own use.

As is, the feedthrough is designed to these specifications:

• Screw size is m4.
• The mounting hole is drilled out to 11/32".
• The material is 2.5mm thick.
• The shoulder is 18.75mm in diameter.

To modify this for your own needs, you only need to change the parameters in Sketch under bushing. The four constraints you need should be red. There are two construction lines - one for the thickness of the walls and one for the diameter of the screw. The shoulder diameter and the material thickness are modified on the actual drawing.

If you are interested in the lightning arrestor itself, it's just a scrap piece of aluminum angle into which I've drilled and tapped three holes appropriate to the spark plugs I'm using. It doesn't really matter much what type spark plug you use. I'm just using three used NGK BMP4ES plugs that I pulled out of mowers I've worked on. Wrap a solid wire from one screw (fed through the feedthough) around each of the three plugs and back down to the other feedthrough. You can solder the wire to the plug caps if you want, but as long as you wrap it tightly, I don't think it's really necessary. The whole assembly is then mounted and somehow grounded. I plan on killing two birds with one stone. I'm going to mount the block to a steel pole that I will drive into the ground. Your antenna comes in and attaches to one side while your radio attaches to the other side.