3D printed coil forms

[Big Bad Bob]

(this is my ObHack)

Some time ago I started messing around with LF band radio, specifically
WWVB which I wanted to hook up to a microcontroller. I'm doing it the
"old school" way, and that means adjustable coils.

The thing is, you just CAN NOT BUY THEM any more. They were hard to get
anyway, usually purchased ni bulk by radio and TV makers back in the
day, and still are I guess [for front-ends]. But even HAM coils are
hard to get. And the coil forms? Nearly everything is "some coil we
grabbed out of equipment that you can dismantle and use as a form".

And they are EXPENSIVE!!!

SO I've been working on 3D printing the coil forms, and using "whatever
is available" ferrite rods. I order big ones and 2 of them break in
transit. I still have some small ones (not threaded) on order, to
arrive next week.

In the mean time I 3D print a plastic form that works pretty good for
the large cores, with a nominal 100uH for 1 layer (62 Turns) and at
least +60/-80 uH adjustment range.

I expect the smaller ones to be useful for HF and VHF bands, as well as
IF transformers (both AM and FM).

The real hack is the adjustment: 1.5 inch nylon 4-40 screws glued to
the top of the ferrite. So far I made some plastic-printed screw
threads and holder, 9.5mm in diameter, which would glue to a 10mm rod
[cut to size of course] Ferrite is difficult to cut without breaking it
but I'll use a carbide wheel, should be ok. I am able to file and grind
it ok, as one of the broken pieces is about the right length and I
ground and filed the ends to be flat(ter).

So it's basically a long plastic tube with doohickeys to tie wires to so
when you wind them you can anchor the 'far' end so it doesn't unravel on
you when you layer it.

Here are the current versions [frequently being updated]
https://raw.githubusercontent.com/bombasticbob/3DObjects/master/coil_form.scad
https://raw.githubusercontent.com/bombasticbob/3DObjects/master/coil_form2.scad

(the 2nd hasn't been tested yet)

these can be modified as you see fit to suit a different core size

Basically the 'yoke' at the top supports a 1.5" nylon 4-40 screw. You
put 2 nuts [on either side of the yoke top] and a screw there, slightly
tighten the nuts for tension, and hot-glue in place. Then you use one
of the "slug top" 3D prints [or try gluing directly] glued with epoxy to
the top of the proper sized slug [filed as needed so it can spin
freely]. I apply glue to the threads after so it doesn't come
un-threaded, though the "slug top" design for 10mm has strong enough
threads that you should be able to tighten it.

The 10mm slug top is here:
https://raw.githubusercontent.com/bombasticbob/3DObjects/master/slug_top10.scad

It takes a while to render...


So, in short, you can NOW wind replacement adjustable coils if you
purchase the ferrite slugs, and glue screws to the top of them in one of
these coil forms.

(for as long as ferrite cores are available, anyway)

NOTE: for VHF and higher you might want to use powdered iron cores,
rather than ferrite. Apparently they have less inductance, but also
fewer eddy current problems than ferrite.


--
(aka 'Bombastic Bob' in case you wondered)

'Feeling with my fingers, and thinking with my brain' - me

'your story is so touching, but it sounds just like a lie'
"Straighten up and fly right"